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Site effects of the 1997 Cariaco, Venezuela earthquake

Article (PDF Available) inEngineering Geology 72(1-2):143-177 · March 2004with95 Reads
DOI: 10.1016/j.enggeo.2003.07.002
Abstract
During the July 9, 1997 Cariaco earthquake, the small town of Cariaco (located 10 km SW from the epicenter) and Cumaná (capital of the State of Sucre, located about 80 km west from the epicenter) were the most affected towns. The damage in Cariaco was essentially restricted to one-century-old dwellings in the downtown area, but also three rather modern buildings collapsed. A maximum intensity of VIII (MMI) was determined for the epicentral area with a clear orientation of the major damage along the strike of the El Pilar fault in east–west direction. The induced effects associated with this event are dominated by liquefaction phenomena and lateral spreading on soft sedimentary lowlands (along the shoreline of the Cariaco Gulf and riverbeds), as well as sliding at unstable slopes. Site studies were carried out in Cariaco, involving the geotechnical analysis of boreholes, seismic refraction studies and microtremor measurements to determine the characteristics of the Quaternary sediment fill in the area. From seismic refraction surveys, an interface separating sediments with S-wave velocity lower than 700 m/s from stiffer ones was located at 60–90 m in depth in the southern part of Cariaco. Further north it is supposed to exceed 90 m. Predominant periods of soil, derived from microtremor observations in Cariaco, vary between 0.6 and 1.2 s. The high percentage of damage in the center of Cariaco can be attributed to the poor quality of the dwellings combined with the presence of thick, poorly consolidated soils, and, in some particular cases, to liquefaction phenomena.
Figures
Site effects of the 1997 Cariaco, Venezuela earthquake
Jorge Gonza
´lez
a,
*, Michael Schmitz
a
, Franck Audemard
a
, Rommel Contreras
b
,
Antoine Mocquet
c
, Jesu
´s Delgado
d
, Feliciano De Santis
a,1
a
Venezuelan Foundation for Seismological Research, FUNVISIS, Apdo. Postal 76880 Caracas 1070, Venezuela
b
Centro de Sismologı
´a, Universidad de Oriente, Cumana
´, Venezuela
c
Laboratoire de Plane
´tologie et Ge
´odynamique, Universite
´de Nantes, France
d
UCV-CENAMB, Caracas, Venezuela
Received 28 April 2003; accepted 24 July 2003
Abstract
During the July 9, 1997 Cariaco earthquake, the small town of Cariaco (located 10 km SW from the epicenter) and Cumana
´
(capital of the State of Sucre, located about 80 km west from the epicenter) were the most affected towns. The damage in
Cariaco was essentially restricted to one-century-old dwellings in the downtown area, but also three rather modern buildings
collapsed. A maximum intensity of VIII (MMI) was determined for the epicentral area with a clear orientation of the major
damage along the strike of the El Pilar fault in east west direction. The induced effects associated with this event are dominated
by liquefaction phenomena and lateral spreading on soft sedimentary lowlands (along the shoreline of the Cariaco Gulf and
riverbeds), as well as sliding at unstable slopes. Site studies were carried out in Cariaco, involving the geotechnical analysis of
boreholes, seismic refraction studies and microtremor measurements to determine the characteristics of the Quaternary sediment
fill in the area. From seismic refraction surveys, an interface separating sediments with S-wave velocity lower than 700 m/s
from stiffer ones was located at 60 90 m in depth in the southern part of Cariaco. Further north it is supposed to exceed 90 m.
Predominant periods of soil, derived from microtremor observations in Cariaco, vary between 0.6 and 1.2 s. The high
percentage of damage in the center of Cariaco can be attributed to the poor quality of the dwellings combined with the presence
of thick, poorly consolidated soils, and, in some particular cases, to liquefaction phenomena.
D2003 Elsevier B.V. All rights reserved.
Keywords: Cariaco earthquake; Induced effects; Site effects; Soil characteristics; S-wave velocity; Predominant periods
1. Introduction
On July 9, 1997 at 15:24 local time, an M
W
6.9
earthquake struck northeastern Venezuela (FUNVI-
SIS, 1997), the seismically most active region in the
country. The shaking was strongly felt from the
eastern part of Venezuela up to the capital Caracas,
located in the center of the country. Building damage
was widespread in and around the town of Cariaco,
where 40% of the poorly maintained ‘‘bahareque’
dwellings, a traditional type of one-story adobe hous-
es with wooden frame structure, were destroyed.
Structural damage of reinforced concrete buildings
occurred both in Cariaco and in the 80 km distant
capital of Sucre State, Cumana
´(see Fig. 1). Damage
0013-7952/$ - see front matter D2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.enggeo.2003.07.002
* Corresponding author. Fax: +58-2-2579977.
E-mail address: jorgeg@funvisis.org.ve (J. Gonza
´lez).
URL: http://www.funvisis.org.ve.
1
Now at Ingenieros De Santis, Caracas, Venezuela.
www.elsevier.com/locate/enggeo
Engineering Geology 72 (2004) 143 – 177
resulting from induced effects was mainly concentrat-
ed on lifelines (i.e., road network and the water supply
system) and some housing areas characterized by self-
constructed concrete-framed masonry dwellings. No
seismic-resistance considerations had been applied
there, neglecting the ground conditions at riverbanks
and shorelines in particular. The earthquake was the
biggest event in eastern Venezuela during the 20th
century, which has a remarkable record of historical
earthquakes (e.g. Grases, 1990; FUNVISIS, 1994).
The second most important event was the 1929
Cumana
´earthquake that generated a significant
amount of damage in Cumana
´. A recent re-evaluation
of that earthquake by Mocquet et al. (1996) questions
the assigned magnitude of 6.9 because high damage
that occurred during that event can be explained by
the local soil conditions, which significantly amplify
the effects of even moderate earthquakes. They con-
clude that this earthquake did not release a significant
amount of stress in that region.
Both events were related to the El Pilar fault, a
right-lateral strike-slip fault that belongs to a con-
tinuous fault system of some 800 km length and
100 km width in northern Venezuela. It accommo-
dates the relative motion along the Caribbean
South American plate boundary (e.g. Minster and
Jordan, 1978). Two major geological units have
been recognized in the studied region: the meta-
morphic rocks of the Araya-Paria peninsula to the
north and mainly Cretaceous sedimentary rocks of
the Interior Range to the south (e.g. Metz, 1968;
Vignali, 1979), both roughly separated by the El
Pilar fault (Fig. 1). Neogene rocks are exposed in
Cumana
´, on the western edge of the Araya penin-
sula, northwest of Cariaco, and south of the El Pilar
fault. Quaternary sediments are exposed in local
basins, such as the ones where Cariaco and Cumana
´
are situated. The sediments that constitute the
subsoil of Cariaco belong to a basin controlled by
the El Pilar fault system. The basin accumulates
sediments from the sedimentary range to the south
and the metamorphic units to the north, which
causes heterogeneity within the sedimentary fill
(mainly of continental origin). Within this basin
the Cariaco River left several abandoned meanders,
which constitute the youngest sedimentary environ-
ment of the basin. These meanders, which are more
susceptible to liquefaction phenomena, are located
mainly to the south of Cariaco. Masaki et al. (1998)
suggest from the evaluation of microtremor meas-
urements that the deepest part of the sedimentary
basin would be north of Cariaco. However, such a
basement geometry could not be confirmed by the
results of seismic studies, which indicate that the
deepest part of the basin is located close to the El
Pilar fault on its southern border (Schmitz et al.,
submitted).
In this paper, we report the results of macroseismic
research, the evaluation of induced effects, and site
effects in the area. Geotechnical studies, microtremor
Fig. 1. Geological map of the region under study (simplified after Bellizzia et al., 1976), location of the El Pilar fault after Audemard et al.
(2000). The star indicates the location of the epicenter after FUNVISIS et al. (1997), Mm = Mesozoic metamorphic rocks, Cl = Cretaceous
limestones, Ts = Tertiary sediment, Qp = Quaternary sediments (Pleistocene), Qr = Quaternary sediments (recent).
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177144
Fig. 2. The regional distribution of macroseismicity related to the 1997 Cariaco earthquake with the estimation of the MMI intensities (above)
and zoom on the area related to the surface rupture (thick line; below). Observed intensities at individual data points (see Appendix A) are
symbolized by rhombs: MMI = 4; squares: MMI = 5; circles: MMI = 6; triangles: MMI = 7 and stars: MMI = 8. The dark star indicates the
location of the epicenter.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 145
measurements, as well as seismic surveys were per-
formed in order to investigate the site and induced
effects in the town of Cariaco. The structural behavior
of buildings, or damage is discussed in relation to
specific geologic site conditions.
2. Macroseismicity studies
In order to collect Modified Mercalli Intensity
Scale (MMI) data from eastern Venezuela, several
field reconnaissance commissions were spread all
over the region in the first 2 weeks after the earth-
quake. Almost 100 localities were visited and sur-
veyed for macroseismic effects. However, the density
and distribution of collected data depends on the
available road network. About 300 questionnaires
were analyzed for the generation of the macroseismic
map, where the maximum assigned intensity is VIII
(Fig. 2). Average values were determined for locations
with varying values. The map integrates data collected
and published by different groups (FUNVISIS, 1997;
Schwarz et al., 1998; Bonato and Herna
´ndez, 1999;
Mocquet and Contreras, 1999). Earthquake-related
site and induced effects have not been removed from
this intensity map, and are discussed in this paper.
The macroseismic map of the Cariaco 1997 earth-
quake (Fig. 2) clearly shows two interesting aspects:
(1) The damage is most pronounced along the El Pilar
fault. The area with intensity VIII stretches over
30 km in length from Chiguana to El Cautaro,
matching the extension of the surface rupturing
mapped by FUNVISIS (1997) and Audemard
(under review, a). However, several houses placed
right on the surface rupture underwent very light
or no damage at all, as reported by FUNVISIS
(1997) and Audemard (1999).
(2) The damage intensity decreases with increasing
distance from the epicenter, as expected, but not
in the same way in all directions. Intensity
attenuation is more important in north south
than in eastwest direction (Fig. 3). This might
indicate the existence of a strong anisotropy, or
directivity effects due to the fault orientation and
rupture propagation as well as by the general W
E alignment of the regional structure, whose
surface expression is the regional geomorphology
with a WE oriented sedimentary (soft soil) basin
and WE striking mountain chains. In general,
eastern Venezuela is characterized by major
ENEWSW trending structures (such as: folds,
Fig. 3. Decay of macroseismic intensities in E–W direction (solid line) and N –S direction (dashed line) from epicenter. The towns of Cariaco
and Cumana
´show average intensities of VIII and VI, respectively.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177146
thrusts and foliation), and east west oriented
faults, which favor energy propagation in such
direction and damp it across. As a deviation from
the general intensity pattern, the towns located on
young alluvial plains exhibit higher levels of
damage than neighboring settlements located on
harder grounds.
One example is the city of Cumana
´, where effects
of local site conditions and induced effects occurred at
a great extent (see section on induced effects in this
paper; FUNVISIS, 1997; Lang et al., 1999), and
intensities were higher than the average calculated
for Cumana
´(as displayed in Fig. 2). Cumana
´has
always been severely affected during most known
Fig. 4. Fourier spectra with the three components (above) and H/V values (below) for two sites in (a) Cariaco (c07) and (b) in the Cretaceous
bedrock south of Cariaco (cc2). The site within the town (c07; for location see Fig. 5) shows a peak at 0.8 s and the bedrock site (cc2) at 0.3 s,
considered as predominant periods.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 147
earthquakes in eastern Venezuela, as attested by the
repeated occurrence of induced effects, such as lique-
faction and lateral spread phenomena (for further
details, see Audemard, under review, b). This damage
concentration can be attributed to the thick sequence
of Holocene alluvial/delta plain deposits of the Man-
zanares River underneath the city (Beltra
´n and Rodrı
´-
guez, 1995). Studies carried out after the Cariaco 1997
Fig. 5. Map of Cariaco with the predominant periods from microtremor measurements. The circles indicate the soil sites where measurements
were performed (c07 from Fig. 4 is located northwest of line 7). The bedrock site (cc2 from Fig. 4) is located about 500 m south of the surface
rupture, which separates soft soil to the north from more consolidated sediments to the south. The location of the seismic refraction lines (black
lines with arrows indicating the direction of observation) as well as the composed profiles A and B (dashed lines) are indicated. 1 = Raimundo
Martı
´nez Centeno high school; 2 = Valentı
´n Valiente school; 3 = Bank of Orinoco; 4 = Brekerman Street; 5 = Las Flores Street; 6 = Bermu
´dez
Street.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177148
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 149
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177150
earthquake, pointed out well the susceptibility of
Cumana
´to damage, among those that deserve men-
tioning:
(a) Microtremor analyses using the Nakamura method
(Nakamura, 1989) performed by Abeki et al.
(1998). They claim that the sediment sequence
over large areas of the Cumana
´alluvial plain is
poorly consolidated since it shows long predom-
inant periods (about 1 s) and a large H/V peak
amplitude, implying the eventual occurrence of
unfavorable site effects in Cumana
´.
(b) Studies of soil structure interaction (Lang et al.,
1999).
(c) Estimation of the S-wave velocity at test sites in
Cumana
´yielding velocities as low as 250400 m/s
Fig. 6. Time–distance plots (a) and seismic sections (b c) for line 2 at the northwestern border of Cariaco (for location see Fig. 5). As an
example for data coverage and the control of the reciprocal times at each shot point, the picks of the first arrivals from and S-waves (above) and
P-waves (below) are displayed as time–distance plots (a). Shot-point locations 480 m (b) and 1080 m (c) with the observed seismograms with
picked arrivals (above), the observed (circles) and calculated (lines) travel times (center) and the calculated raypaths with the S-velocity model
(below). Raypaths with velocity model and calculated P-wave travel times of shot point 480 m are displayed as well (b, lower part). First breaks
of S-waves are partially overlain by signals of air blast (c).
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 151
Fig. 7. Seismic section along line 3 at the southern limit of Cariaco of shot-point location 480 m with the observed seismograms with picked
arrivals (above), the observed (circles) and calculated (lines) travel times (center) and the calculated raypaths with the S-velocity model (below)
and the P-velocity model (lower part). No disturbance by signals of air blast is observed along this profile.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177152
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 153
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177154
for the upper 25 m of unconsolidated sediments
(Kantak et al., 1999).
3. Site effects
Comprehensive geological and geophysical studies
were carried out in order to characterize the soil
conditions in an around Cariaco, including micro-
tremor and seismic refraction measurements focused
on the soft soils. The results of a seismic refraction
study for determining the basement geometry in the
town of Cariaco sedimentary basin are reported in
Schmitz et al. (submitted); here, we will focus on the
seismic characteristics of the soils in the town of
Cariaco. The derivation of the liquefaction potential
of the Cariaco subsoil was the main target of the
geotechnical investigations (see Section 4.2).
3.1. Microtremor measurements
Microtremor measurements were performed at 64
sites in the Cariaco area using a broadband sensor
(Guralp CMG-40T) with a natural period of 30 s and a
24-bit digitizer (Nanometrics Orion). The observation
points were selected in non-noisy areas avoiding
artificial noise sources. The sensors were installed
on natural ground with a variable distance of 200
500 m between points. Time windows of 300 s with a
sampling frequency of 100 Hz were recorded. For the
calculation of the Fourier spectrum, 3000 samples
(30 s) were selected at five time windows with low
coherent energy from near sources and from these the
H/V spectra (mean of horizontal components) were
calculated following the guidelines given by Bard
(1999). The characteristic peak in the H/V relation
was assumed to be the predominant period of the soil
site (Fig. 4).
The predominant periods within the town of Car-
iaco vary between 0.6 and 1.2 s (Fig. 5), whereas the
bedrock sites to the south show peaks at about 0.3 s
(Fig. 4b). The sites within Cariaco were divided into
six groups (see Fig. 5). Slightly higher values of 0.9 to
1.1 s are observed along seismic profiles A and B,
whereas lower values of 0.7 to 1.0 are observed in the
northeastern part of the town of Cariaco and close to
the fault rupture, but no distinct zones of equal
predominant periods can be separated within Cariaco.
3.2. Seismic properties of the soils in Cariaco
The S-wave velocity of soil is among the most
important geophysical parameters for the character-
ization and classification of a soil profile in order to
determine the acceleration levels to be incorporated in
the design of an earthquake-resistant structure. There-
fore, seismic refraction measurements were performed
in Cariaco during July 1998, also for the determination
of the thickness of the soft soil, which could exhibit a
strong amplification in case of an earthquake. A total
of seven seismic lines were recorded in Cariaco (Fig.
5). The individual seismic line lengths varied between
360 and 1080 m with maximum shot-receiver offsets
between 120 and 480 m; the geophone spacing varied
between 5 and 10 m for the different lines. The shots
were located at both ends and in the center (along line
2 shot points were located each 120 m). Pentolite
charges between 100 and 400 g at a depth of 0.5 to 1
m were used as energy source. A 24 channel Geomet-
rics StrataView seismic recorder and 14 Hz geophones
were employed. The data were processed and ana-
lyzed using the REFRA seismic refraction software
(Sandmeier, 1998) for identifying first breaks. 1-D
models were calculated using the intercept time
method (e.g. Palmer, 1986), including the control of
reciprocal times (Fig. 6a). Forward modeling based on
the results of the 1-D models was applied along each
seismic line using the 2-D raytracing RAYAMP pro-
gram (Crossley, personal communication).
The analysis of the first breaks of the P-waves
indicates that the velocity of the uppermost layer varies
between 400 and 1040 m/s and the depth between 4
and 14 m. Below, the velocity of the saturated sedi-
ments ranges between 1700 and 1900 m/s (Figs. 6 8;
Fig. 8. Seismic sections along line 1 at the eastern limit of Cariaco of shot-point locations 0 m (a) and 510/580 m (b) with the observed
seismograms with picked arrivals (above), the observed (circles) and calculated (lines) travel times (center) and the calculated raypaths with the S-
velocity model (below). Raypaths with velocity model and calculated P-wave travel times of shot point 0 m are displayed as well (a, lower part). A
deeper high velocity layer, which is inclined towards the north, is observed in the S-waves (700 m/s) as well as in the P-waves (2500 m/s). The
geometry of the top of the deeper layer is well confirmed by the two shot points at 510 and 580 m (b).
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 155
Table 1). On line 3, we identified a third layer at 32 m
depth with Vp = 2250 m/s, and along line 1, an
increase to P-wave velocities of 2500 m/s is observed
between 60 and 90 m in depth (Fig. 8a). For identify-
ing the layering inside the saturated zone we used the
S-waves. A problem for the recognition of the S-wave
first breaks is the superposition with the air blast
arrival (Fig. 6c). Generally, arrivals from the air blast
precede the S-wave arrivals some 0 100 ms and make
the recognition of later phases difficult. Despite the
existence of the air blast energy in some sections, data
quality is good and picks of first breaks can be done
with good accuracy. The arrivals of the S-waves
cannot be unambigously distinguished from possible
arrivals of surface waves. Nevertheless, the difference
in seismic velocity (the velocity of the Rayleigh wave
is 0.919 times the velocity of the S-waves; Sheriff and
Geldart, 1995) between both wave-types would result
in only slightly elevated S-wave velocities, if Rayleigh
waves were picked.
Along line 2 at the northwestern limit of Cariaco
(Fig. 6), an upper layer with an S-wave velocity of
200250 m/s overlays one with a velocity of 300 m/
s. The boundary is located at a depth of 10 m.
Below that depth, a velocity increase to 450 m/s is
observed with the depth increasing from 45 to 53 m
towards the NE. Arrivals from deeper layers cannot
be observed along this line, although the data is of
good quality throughout the seismic section up to
the maximum recording distance of 480 m. On other
seismic sections, high velocity layers (700 750 m/s)
at depths of about 8090 m appear at an offset of
250350 m (Figs. 7 and 8a). Therefore, we can
infer that the depth of a layer with a velocity
significantly higher than 450 m/s should exceed 90
m on line 2 (Fig. 9). At the southern border of
Cariaco (line 3, Fig. 7), the increase to 750 m/s is
observed at a depth of 80 m, whereas at the eastern
border (line 1, Fig. 8), the interface with an S-wave
velocity of 700 m/s is dipping from north (60 m) to
south (90 m). This dipping is well constrained by
the observations at shot points 580 and 510 (Fig.
8b). The same interface can be derived from P-wave
data and the underlaying layer has a velocity of
2500 m/s. Along line 5, which is located further
south and therefore closer to the El Pilar fault,
lateral variations are observed with a decrease of
the top of the layer with 500 m/s to 60 m in depth at
the southern end.
This velocity information of the Cariaco subsoil
was summarized in profiles A and B trending SW
NE and NWSE, respectively (Fig. 9), roughly
corresponding to the shorter and longer dimensions
of the town. Along profile A, which correlates across
seismic lines 3, 5 (projected), 6 and 1 (northern end of
this line), the velocities in the first three layers
decrease slightly towards northeast and the top of
the 700 m/s layer decreases in depth in the same
direction from 90 to 60 m in depth. This layer is
interpreted either as the weathered top of the Creta-
Table 1
P-wave and S-wave velocities for the seismic lines in Cariaco (for
location see Fig. 5)
Depth (m) Vp (m/s) Vs (m/s)
Line 1 0 – 8/14 1040 200
8/14 – 32/40 1790 280
32/40 – 60/90 1790 430
60/90 2500 700
Line 2 0 – 10 500 200/250
10 – 45/53 1750 300
45/53 – 450
Line 3 0 – 8 700 250
8 – 32 1900 300
32 – 80 2250 500
80 – 750
Line 4 0 – 7/10 800 200
7/10 – 35/38 1730 320
35/38 – 440
Line 5 0 – 4/12 700 200
4/12 – 30/60 1800 320/350
30/60 – 450/500
Line 6 0 – 4 600 150
4 – 12 600 250
12 – 40 1700 300
40 – 90 450
90 – 680
Line 7 0 – 10 700 200
10 – 45 1700 300
45 – 420
Generally, two strata are displayed for P-wave velocities, the
unsaturated sands and clays (400 – 1040 m/s) in the upper 4 – 14 m
and the water-saturated sediments (1700 – 1900 m/s) below. The
groundwater level (top of the second layer) is quite deep compared
to the observations right after the earthquake (see Section 4.2),
which is attributed to the dry weather conditions during seismic
measurements. From the evaluation of P-velocities, deeper layers
are only detected on lines 3 and 1 with velocities of 2250 and 2500
m/s, respectively, the latter one interpreted as base of the
Quaternary sediments. The distribution of S-wave velocities is
displayed in Fig. 9.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177156
ceous limestones exposed on the southern flank of the
Cariaco sedimentary valley, or as Tertiary sediments
exposed some 5 km west of Cariaco (Fig. 1). The
intermediate layer with an S-velocity between 440 and
520 m/s might correspond to coarse grained sand with
gravel deposited from the south, whereas the upper-
most layers with low seismic velocity are interpreted
as fine sand interbedded with clay.
Profile B, which is crossing Cariaco from NW to
SE, displays the information from lines 2, 6, 7 and 1
(southern end of this line). Here, an increase in
thickness of material with low seismic velocities
towards the center of the basin in the north can be
observed (Fig. 9b). The deepest layer with an S-
velocity of 680700 m/s is only observed on lines 1
and 6 (SE portion of profile B), at a depth of about 90
m. This layer could not be detected along line 2.
Therefore, we can assume an increase in depth to-
wards the north. If we regard the top of this layer as
the limit of the Quaternary sedimentary infill of the
basin, the total thickness of the Quaternary sediments
in Cariaco exceeds 90 m.
3.3. Importance of local site conditions during the
Cariaco 1997 earthquake to damage
Soil conditions determine the dynamic character-
istics of a site, meaning that strong variations in
Fig. 9. Composed profiles A (top) and B (bottom) located at the southeastern border and in the northwestern part of Cariaco, respectively. The
numbers represent the S-wave velocities for each layer. Solid lines: measured, dashed lines: interpolation. The type profiles indicate the
locations for which response spectra (Fig. 10) were calculated. For location, see Fig. 5; vertical exaggeration for profile A, 3:1; for profile B, 6:1.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 157
seismic response and damage distribution can be
observed between nearby sites, as evidenced for
example during the 1967 Caracas earthquake (Seed
et al., 1970). A strong correlation between surface
geology and damage distribution has been observed
for damaging earthquakes, especially in areas with
unconsolidated sediments (Rosenblueth and Ovando,
1991). For engineering purpose, unconsolidated sedi-
ments (or ‘‘soft soil’’) with S-wave velocities below
700 m/s are discriminated from consolidated sedi-
ments or bedrock (‘‘hard soils’’) with higher veloc-
ities (e.g. Seed et al., 1990). The new seismic
building code in Venezuela (COVENIN, 2001) refers
to the S-wave velocity as an important parameter for
the characterization of the soil profile. In this con-
text, civil engineers use the term soils interchange-
ably with terms like sediments and fill, referring to
any deposit, such as clays, sands, silts, or gravel
above bedrock.
The most severe structural damages on reinforced
concrete buildings occurred in the city of Cumana
´and
in the town of Cariaco. In Cumana
´, the Miramar, a
seven floor reinforced concrete building, collapsed
because of design problems such as the irregular
distribution of rigidities and the inappropriate struc-
tural design of the traverse steel elements (Bonilla et
al., 2000).
In Cariaco, the Valentı
´n Valiente School, built in
1958, had a reinforced concrete structure of two levels
with a rectangular plan, with frames only across. Big
horizontal displacements in the longer direction
caused the partial collapse of this structure. The U-
shaped building of the Raimundo Martı
´nez Centeno
high school, built in 1989, was composed of two 3-
level structures. The collapse of the first level of the
two main blocks occurred due to the inadequate
distribution of the masonry brick walls and big
horizontal displacements at the foundation level. The
Cariaco branch of the Bank of Orinoco was a two-
story structure, the first level of reinforced concrete
and the second level of steel beams. In this case, the
second level collapsed completely upon the first one.
This structure had been built in two parts without an
appropriate connection between the original structure
and the extension (Bonilla et al., 2000).
No distinct zones of equal predominant period can
be distinguished in Cariaco (Fig. 5), but a general
trend indicates lower values close to the fault rupture,
e.g. the southeastern limit of the sedimentary basin,
increasing towards NW. Most of the calculated pre-
dominant periods within Cariaco are in a relatively
close range between 0.8 and 1.0 s, which corre-
sponds roughly to a thickness in the order of some
70 to 110 m (considering the average velocities
varying between 280 and 350 m/s and applying the
simple relation of period equals average velocity
divided by four times the thickness), which coincides
well with the thickness of the unconsolidated sedi-
ments as derived from the seismic measurements. No
direct correlation can be done between the results of
the microtremor measurements and the seismic re-
fraction measurements (Figs. 5 and 9), as the varia-
tions of the predominant periods within the observed
range are stronger than the corresponding changes in
the S-wave velocities and thickness of unconsolidat-
ed sediments. Nevertheless, the locations of the
collapsed reinforced concrete buildings (numbers
1–3 in Fig. 5) and the damage concentrated between
the Brekerman and Bermu
´dez streets (46 in Fig. 5)
coincide with predominant periods of around 1 and
0.7 s, respectively.
Based on the seismic velocities, the response
spectra for three type-profiles in Cariaco (Table 2,
see Fig. 9 for location) were calculated (Fig. 10).As
no strong motion data were available from the Cariaco
area, an accelerogram from the 1979 Imperial Valley
at 10.6 km distance to the rupture was used as input.
This earthquake had a similar magnitude (Ms 6.9) and
rupture mechanism (dextral strike-slip) compared to
the Cariaco 1997 earthquake, and the epicentral dis-
tances match as well.
No disturbance is observed at periods below 0.1 s
at the surface (Fig. 10). In general, energy is absorbed
by the sediments to about 0.4 s, and amplification
occurs between 0.4 and 3 s for type-profiles 1 and 3,
and between 0.8 and 3 s for type-profile 2. For all
three soil profiles the acceleration values exceed 0.5
g, and reaches 0.8 g between 0.4 and 0.7 s for type-
profiles 1 and 3, although type-profile 3 has a smaller
soil thickness. The range between 0.4 and 0.7 s with
the highest values of acceleration do not coincide
well with the predominant periods from microtremor
measurements (Fig. 5). Therefore, we do not consider
the amplification effects as the main cause for the
damage that occurred in Cariaco during the 1997
event.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177158
Table 2
Input data of the Proshake 1.1 program (based on Schnabel et al., 1972) for the dynamic response evaluation of type-profiles 1, 2 and 3 (Fig. 10)
Layer
number
Material name Thickness
(m)
Unit
weight
(pcf)
Gmax
(ksf)
Vs
(m/s)
Modulus curve Damping curve
Type-profile 1, water table: 5 m; number of layers: 12
1 Clay (CL) IP = 16 5.00 99.88 1336.66 200.00 Clay-PI = 10 –20 (Sun et al.) Clay-average (Sun et al.)
2 Clay (CL) IP = 20 5.00 99.88 1336.66 200.00 Clay-PI = 20 –40 (Sun et al.) Clay-upper bound (Sun et al.)
3 Clay (CL) IP = 30 5.00 106.1 3195.48 300.00 Clay-PI = 20 –40 (Sun et al.) Clay-upper bound (Sun et al.)
4 Sand (SG) 5.00 106.1 3195.48 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
5 Sand (SG) 10.00 106.1 3195.48 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
6 10.00 112.37 8661.59 480.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
7 10.00 112.37 8661.59 480.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
8 10.00 112.37 8661.59 480.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
9 10.00 112.37 8661.59 480.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
10 10.00 112.37 8661.59 480.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
11 10.00 118.61 18,348.65 680.00 Sand (Seed and Idriss)-upper
bound
Sand (Seed and Idriss)-upper
bound
12 Rock equivalent Infinite 118.61 18,348.65 680.00 Rock Rock
Type-profile 2, water table: 5 m; number of layers: 12
1 Clay (CL) IP = 20 5.00 99.88 1336.66 200.00 Clay-PI = 10 –20 (Sun et al.) Clay-average (Sun et al.)
2 Clay (CL) IP = 20 5.00 99.88 1336.66 200.00 Clay-PI = 10 –20 (Sun et al.) Clay-upper bound (Sun et al.)
3 Clay (CL) IP = 20 5.00 106.13 3195.48 300.00 Clay-PI = 10 –20 (Sun et al.) Clay-upper bound (Sun et al.)
4 Sand (SM) 5.00 106.13 3195.48 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
5 Sand (SM) 10.00 106.13 3195.48 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
6 10.00 106.13 3195.48 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
7 10.00 106.13 3383.44 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
8 10.00 112.37 7612.73 450.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
9 10.00 112.37 7612.73 450.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
10 10.00 112.37 7612.73 450.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
11 10.00 112.37 7612.73 450.00 Sand (Seed and Idriss)-upper
bound
Sand (Seed and Idriss)-upper
bound
12 Rock equivalent Infinite 118.61 18,348.65 680.00 Rock Rock
Type-profile 3, water table: 5 m; number of layers: 12
1 Clay (CL) IP = 20 5.00 106.13 1420.21 200.00 Clay-PI = 10 –20 (Sun et al.) Clay-average (Sun et al.)
2 Clay (CL) IP = 20 5.00 106.13 1420.21 200.00 Clay-PI = 10 –20 (Sun et al.) Clay-upper bound (Sun et al.)
3 Clay (CL) IP = 20 5.00 106.13 2783.61 300.00 Clay-PI = 10 –20 (Sun et al.) Clay-upper bound (Sun et al.)
4 Sand (SM) 10.00 106.13 2783.61 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
5 Sand (SM) 10.00 106.13 2783.61 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
6 10.00 106.13 2783.61 300.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
7 10.00 112.37 6951.08 430.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
8 10.00 112.37 6951.08 430.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
9 10.00 112.37 6951.08 430.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
10 10.00 118.60 7336.46 430.00 Sand (Seed and Idriss)-average Sand (Seed and Idriss)-average
11 32.81 118.60 19,442.22 700.00 Sand (Seed and Idriss)-upper
bound
Sand (Seed and Idriss)-upper
bound
12 Rock equivalent Infinite 118.61 19,443.86 700.00 Rock Rock
Input data consider the geotechnical characteristics (Section 4.2) as well as S-wave velocities (Fig. 9). The shear module Gmax was
calculated based on the curves of Vucetic and Dobry (1991). Modulus and damping curves for clay after Sun et al. (1988) and for sand after
Seed and Idriss (1970).
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 159
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177160
4. Induced effects by the Cariaco earthquake
Induced effects associated with the Cariaco 1997
earthquake were very common and widespread be-
cause conditions were given for either mass instabil-
ities in areas of rough/unstable topography or
liquefaction and lateral spread in low-lying young
alluvial plains (Fig. 11). The epicentral region is
located on a poorly drained Holocene alluvial plain
(the Cariaco sedimentary basin with the Campoma
and Buena Vista swamps) at the eastern end of the
Cariaco Gulf. This plain is bounded to the south by
the Interior range, comprising Cretaceous sedimentary
rocks (Fig. 1), reaching a height of more than 1000 m
over less than 10 km (10% gradient). Further west the
range falls abruptly down into the Cariaco Gulf where
the existing flatlands along its coasts mainly corre-
spond to active Holocene alluvial/delta plains growing
at the major river mouths. To the west, induced effects
have been reported as far as the western seashore of
Cumana
´(Fig. 12), some 80 km west of the epicenter.
But to the east, these effects are observed only up to
the surrounds of Nueva Colombia, about 25 km away
from the epicenter. Only few induced effects were
observed on the northern coast of the Cariaco Gulf
and at the northern limit of the Cariaco sedimentary
basin (Fig. 11). Here, topography is much smoother
and the bedrock consists mainly of Mesozoic meta-
morphic rocks (Fig. 1).
4.1. Liquefaction and lateral spreading
Most frequent liquefaction features reported in
association with the Cariaco 1997 earthquake are sand
blows, and occasionally sand-vent fractures, but this
latter ones are generally related to lateral spreading
(Plate 1A and B). However, not all lateral spreads did
show venting. At the Piragua pool (north of Aguas
Calientes, on the southern limit of the Buena Vista
swamp), a northward lateral spreading towards a very
shallow creek unequivocally destroyed ground surface
though any sand venting occurred (Plate 1C and D).
The evidence to support this interpretation are the
following concurrent features: curved shape of wide-
open tension cracks affecting a rather flat topography;
considerable amount of opening across the cracks
indicates that some translation happened; tilting of
tops of slabs bounded by tension cracks also denotes
some rotation; the presence of a shallow (about 0.5 m
deep) water table; and topmost sedimentary sequence
observed in crack walls comprises a thick sandy layer.
Sand blows were observed between Laguna de
Buena Vista in the east and the eastern coast of
Cumana
´at Punta Baja (north of El Pen
˜on) to the west
(Figs. 11 and 12). All reported liquefaction features
are in active alluvial plains around the Cariaco Gulf.
Besides the evidence in the Buena Vista-Campona
region, the remaining sites exhibiting liquefaction
correspond to Holocene delta plains along the sea-
shore of the Cariaco Gulf and seldomly to sandy
beaches, such as Tocuchare and Ensenada Honda
(Fig. 11A). At the latter place, locals reported white
sand venting along an east west trending fracture
below the intertidal zone, as aligned sand blows, when
sea receded several tens of meters during shaking
which later induced local beach sinking.
On the southern rim of the Buena Vista swamp, in
the epicentral region, the pressure of the escaping
watersand mixture was high enough to unroot fully-
grown coconut trees. Sand venting was also reported
in ploughed papaya and sugarcane fields at Las
Manoas estate, and particularly at Campo Alegre (on
the northeast side of Cariaco) where 20 30 cm thick
sand blows were spotted (deduced diameter of sand
blows would be in the order of 2 to 3 m). There,
almost all dirt-road embankments were damaged by
lateral spreading, opening deep cracks that paralleled
road alignment. Perception of evidence of liquefaction
out of anthropologically modified zones was nearly
impossible because large areas are constantly flooded
and underwater, such as the Buena Vista and Cam-
poma swamps that cover about half of the Cariaco
sedimentary basin. However, it was common to ob-
serve slabs of riverbank that slid down to or laterally
Fig. 10. Response spectra (damping factor 5%) for three type-profiles of Cariaco: type-profiles 1 and 2 correspond to the representative structure
of profiles A and B (Fig. 9), while the type-profile 3 is located at the northeastern end of profile A, where the thickness of the Quaternary
sediments decreases (for details see Table 2). The thin and thick lines correspond to the response on the bottom and on the top of the sediments,
respectively. The accelerogram used is a horizontal component (recorded on stiff sediment at 10.6 km distance to fault rupture) of the 1979
(Ms 6.9) Imperial Valley earthquake at an azimuth of 140j, which had a rupture mechanism similar to the 1997 Cariaco earthquake.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 161
Fig. 11. Induced effects by the Cariaco earthquake around the Cariaco Gulf (A), and around its eastward continuation, the Cariaco sedimentary basin (B). T = Tocuchare,
EH = Ensenada Honda, C = road 9 (north of Cariaco), LC = Calzadilla beach, ACA = Aquacam C.A., AM = Atu
´n Margarita fish cannery, H = Veteran Hospital, Cumana
´.
Qal = Quaternary alluvial sediments, Ms = Mesozoic sedimentary rocks, Mm = Mesozoic metamorphic units.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177162
Fig. 11 (continued).
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 163
Fig. 12. Induced effects by the Cariaco earthquake along the shoreline in the city of Cumana
´: (A) in the harbor district to the west and (B) the El Pen
˜on area to the east; arrows indicate
the direction of sliding. Numbers coincide with the description of the locations in the text.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177164
Plate 1. (A) Aligned sand blows vented through an open-fracture related to sea-front relaxation on the Punta Baja beach, east of Cumana
´and NW of El Pen
˜on (for location, refer to
Fig. 11c); (B) detail of one of those aligned sand blows; (C) bird’s-eye view of northward lateral spreading of almost flat-lying ground of a coconut plantation at the Piragua pool, on
the southeastern edge of the Buena Vista swamp, next to the surface rupture of the Cariaco 1997 earthquake (arrow indicates the direction of sliding); and (D) detail view to the east of
previous locality exhibiting clockwisely rotated slabs over a shallow liquefied layer. Also notice water shallowness in the southernmost crack.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 165
moved into the creek bottom, regardless of bank
height, such as along the Cariaco River between
Cariaco and Terranova. Occasionally, where river-
banks were high, they toppled into the river. Most
embankments of small and narrow earthen canals
(generally less than 1 m high and typically less than
a couple of meters wide) along discrete portions also
cracked and moved laterally, either inwards or out-
wards. Lateral spreading also damaged national road 9
for a 500-m-long stretch just north of Cariaco (Fig.
11B). Here, earthen embankment failed because it
moved laterally to both sides towards two nearby
abandoned water-filled gravel pits dug during road
construction.
Within the town of Cariaco, vent fractures and
aligned sand blows were reported in the Brekerman,
Bermu
´dez and Las Flores streets (for location, refer to
Fig. 13). This southern sector of the town (paralleling
a very shallow creek located south of the town) was
strongly affected by lateral spreading, which had
associated liquefaction. Surface gradient was almost
flat (slightly inclined south), but houses within a stripe
of two blocks in width and few hundred meters in
length were severely damaged by water pressure that
cracked them all and broke up floor concrete slabs.
The affected houses were generally self-constructed
one-story concrete frame masonry structures, built
without any seismic-resistant design. Geotechnical
studies in this area identified the occurrence of alluvial
sand layers confined between clays of low plasticity.
On the Cariaco Campoma road, the road embank-
ment near a bend also failed by lateral spreading,
producing the typical road edge failure (Fig. 11B).
Getting away progressively to the west from the
epicentral area, liquefaction features were spotted at
Punta Cachipo (west of Chiguana) in association with
sea-front relaxation on the northern coast of the
Cariaco Gulf (Fig. 11B) and at several delta plains
Fig. 13. Map of Cariaco with the location of geotechnical drillings and their respective soil type percentages (for details of sand layers see
Table 3). Surface evidence for liquation has only been observed between Brekerman, Las Flores and Bermu
´dez streets in the south of the
town. The location of the geotechnical profiles 1, running from drilling P2 in the south to drilling P3 in the northwest and profile 4, running
from drilling P1 in the center to drilling P6 in the southeast (Fig. 14) is indicated.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177166
on the southern seashore, such as Punta Tarabacoı
´ta,
Punta Monte Cristo (NW of Marigu
¨itar), Punta Gua-
racayal, Tunantal and Punta Baja (Fig. 11A);this
latter just being east of Cumana
´. All these active
Holocene alluvial plains exhibited sand blows. The
largest sand blows were observed at the delta of the
Marigu
¨itar River (at Punta Monte Cristo, next to
Maigualida beach), measuring 4 5 m across at about
50 km west of the reported epicenter (Plate 2A and B).
On the Punta Baja sand barrier (corresponding to the
present beach) sand blows reached 1 m in diameter
(Plate 1A and B). But some few meters in land, within
the floodplain of the Manzanares spillway canal, their
size dropped below 30 cm. On the beach, an over 800-
m-long crack system paralleling the seashore
exhibited a very narrow (few centimeter wide) gra-
ben-like geometry (A in Fig. 12B). Frequently, those
cracks had smelly gray sand venting associated; and
occasionally exhibited aligned sand blows for over
few meters in length (Plate 1A), as those described by
Audemard and De Santis (1991). The graben-like
cracks resulted from sea-front relaxation that seems
to have eased formation of those 1-m-across sand
blows in comparison to those reported few hundred
meters away inland. At the auxiliary (artificially
induced) delta mouth of the Manzanares River, 30-
cm-across sand blows were spotted under brackish
water less than 10 cm deep.
Southeast of the auxiliary spillway of the Manza-
nares river, the fisherman village of El Pen
˜on, showed
damage to houses sitting on the former beach (B in
Fig. 12B). Parallel-to-the-shore cracking cuts across
about 10 houses and hydraulic fills next to the shore-
line, for a distance of the order of 150 m. All these are
fisherman dwellings that are self-constructed, con-
crete-framed masonry houses but with no seismic-
resistant considerations. Besides, fills were originally
hand-made that are bounded by walls of poor or
defective quality (mostly lacking good mortar).
The farthest most evidence of lateral spreading
were found on the El Guapo coast, south of the
seaport area (Puerto Sucre) on the western seashore
of Cumana
´(1 in Fig. 12A). Sea-front relaxation
damaged houses (of similar construction type to those
in Cariaco and El Pen
˜on) over a distance of 180 m.
The hydraulic fill of the southern part of the seaport
facilities (with some warehouses above; Puerto Sucre,
2inFig. 12A) also moved slightly seaward, inducing
surface cracking for almost 200 m in length. Further
north on this western seashore of Cumana
´, El Dique
sector (La Boquita) underwent riverbank lateral
spread for few hundred meters, damaging some 50
poorly constructed dwellings (3 in Fig. 12A). Indi-
vidual crack opening could reach 0.50 m and might
total over 1 m of lateral displacement across several
subparallel cracks. North of that, the earth embank-
ment of the fishery port was affected by lateral
spreading at two sites: (a) along an east west stretch
near the port abutment (4 in Fig. 12A) and (b) at the
tip of the port (5 in Fig. 12A and Plate 2C). Partial
collapse of the pier appears not to be influenced by
quality of construction. Conversely, it is very likely
that collapse of the rock embankment at the port tip
may be related to a huge submarine slump that
affected the mouth bottom of the actual Manzanares
River. This huge slide might be responsible for
damage to a dry dock on the right riverbank (6 in
Fig. 12A and Plate 2D) and to the ferryboat jetty on
the opposite bank of the Manzanares river mouth (7
in Fig. 12A). Likeliness of this mass-wasting process
is very high since the Manzanares River pours its
water into the very east end of the deep-marine
Cariaco trough that lies just west of the river mouth.
Along the Manzanares River inland, small sand
blows were seen near the Veteran Hospital (the
former Anti-tuberculosis Hospital of Cumana
´), locat-
ed both south of the river and southwest of the
Caigu
¨ire hills (H in Fig. 11A).
Though lateral spreading affected natural features
at many places (very particularly riverbanks and
sandy beaches but very rarely marine cliffs like east
of Chiguana (Plate 3A) in the northern coast of the
Cariaco Gulf), it also damaged cultural features very
frequently. Hydraulic fills along both Cariaco Gulf
coasts were heavily damaged, extending as far west as
Los Capotes (Calzadilla beach, LC in Fig. 11A;Plate
3B). Here, a sea viewpoint was affected by free-face
relaxation of the artificial fill. But the largest damages
with great negative economic impact by lateral
spreading were suffered by the seaside promenade
of San Antonio del Golfo and the fish cannery at
Marigu
¨itar (Atu
´n Margarita) on the southern coast,
whereas only the shrimp farms of Aquacam, C.A.
located east of Chiguana on the north coast (Fig. 11B)
were deeply damaged by lateral spreading (with
surface sand venting) of the southern earthen embank-
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 167
Plate 2. (A) Bird’s-eye view of sand blowing at the Marigu
¨itar delta, next to the Maigualida beach on the southern coast of the Cariaco Gulf (for location, refer to Fig. 11); (B) detail of
one of those isolated sand blows measuring between 4 to 5 m across; (C) collapse of rock embankment at the tip of the fishery port located on the western coast of Cumana
´and near
the Manzanares river mouth; and (D) sliding of hydraulic fill edge and rails at a dry dock induced by submarine slumping at the Manzanares river mouth (for location refer to Fig. 12).
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177168
Plate 3. (A) Small lateral spreading in few-meter-high Plio-Pleistocene cliffs on the northern coast of the Cariaco Gulf, nearby Chiguana (for location, refer to Fig. 11); (B) lateral
spread affecting small resting area sitting on artificial fill, at the Calzadilla beach, on the southern coast of the Cariaco Gulf; (C) bird’s-eye view of seaward lateral spreading of the
southernmost earth embankment of the Aquacam C.A. shrimp farm, east of Chiguana; and (D) sliding of travertine pools originally capping the El Pilar fault scarp developed on
Mesozoic mudstones at the Aguas Calientes farm. To the south, some unbroken pools can be spotted, whereas travertine blocks are piling up to the north (left). Arrows indicate the
direction of sliding.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 169
ments due to a free-face effect on the south (Plate 3C).
However, other sites displayed fracturing parallel to
seashore at lesser extent, such as Villa Frontado and
several localities on the western coast of Cumana
´
already mentioned.
From these observations, it can be stated that: (a)
liquefaction and lateral spreading induced by the
Cariaco 1997 earthquake occurred where all control-
ling factors concurred: in the active Holocene alluvial/
deltaic plains and seashore sand barriers; and (b)
liquefaction distribution during the Cariaco earth-
quake coincides with areas of larger intensities
(MMI VI to VIIII; compare Figs. 2 and 11). Addi-
tionally, it also reveals two relevant aspects: (1) since
liquefaction distribution is pulled to the west with
respect to the epicenter, the monodirectionality to the
west (or highly asymmetric bidirectionality) of the
rupture propagation proposed by Audemard (1999,
under review, a) and verified by numerical modeling
(Mendoza, 2000), and consequently of the energy as
well, appears to be supported from this unequal
distribution, though liquefaction-prone areas are also
more abundant to the west of the epicenter, around the
Cariaco Gulf; (2) although size and frequency of
occurrence of liquefaction/lateral spread features
diminishes away from the epicenter, as expected, a
great amount of lateral spreads has affected the either
natural or anthropologically modified seashore areas
of Cumana
´and suburbs in comparison with the
surroundings, which reveals that the Cumana
´subsoil
presents particular soil conditions.
4.2. Geotechnical characterization of the liquefied
soils in Cariaco
We will focus on the geotechnical conditions of the
southern part of Cariaco, particularly in the Bermu
´-
dez, Flores and Brekerman streets (Fig. 13) along two
profiles, one in the west and another from the center
towards the southeastern edge of the basin (Fig. 14).
In total, seven boreholes were studied regarding
lithology, compactness (NSPT), thickness and fine
content of the sand layers. The most important geo-
technical characteristics of the subsoil sand layers are
indicated in Table 3. Although superficial evidence for
liquefaction was observed only in the southern part of
Cariaco between the Bermu
´dez, Las Flores and Bre-
kerman streets (De Santis and Herna
´ndez, 1999),a
qualitative estimation of the liquefaction potential
following Seed’s methodology (Seed and Idriss,
1971), considering an acceleration of 0.3 g, was done
for all boreholes.
As no accelerogram from the epicentral region was
available, the liquefaction potential was estimated for
every single sand layer identified in the geotechnical
boreholes, using three different levels of seismic
acceleration: 0.10, 0.15 and 0.3 g, following Seed’s
methodology (Seed and Idriss, 1971). The results
indicate a liquefaction potential for all drillings at
different depth levels, except the P6 drilling, which is
located close to the basin edge (Figs. 13 and 14). They
also show that a higher number of sand layers than
observed should have liquefied if the acceleration
would have been above 0.3 g. The critical parameters
for the occurrence of liquefaction are a fine content
between 5% and 30%, 100% saturation with water,
and NSPT values below about 25 blows. Furthermore,
the calculations indicate that no layer would have
liquefied if the acceleration would have been lower
than 0.1 g. Therefore, we suppose that the seismic
acceleration in Cariaco could have ranged between
0.15 and 0.3 g. The recorded maximum horizontal
acceleration in Cumana
´, some 80 km west of the
epicenter, was 0.17 g on consolidated Plio-Pleistocene
sediments (FUNVISIS, 1997).
In Cariaco, liquefaction was observed in sand
layers belonging to sediments deposited in abandoned
meanders. These sediments represent probably the
youngest materials identified in the subsoil of Cariaco,
in areas near to Cumana
´and around the Cariaco Gulf.
Seed’s methodology for estimating liquefaction po-
tential should be used as a first approximation for
microzoning purposes, since it is a predictive method
of easy application relying on blow counts from the
SPT test, the fine content of sands and the maximum
acceleration expected in the region. Nevertheless, the
limitations of this method must be considered when
interpreting the results. Therefore, more refined anal-
yses are necessary for more reliable results for seismic
microzoning assessments.
In average, about 40% of all dwellings in Cariaco
were heavily damaged or collapsed during the earth-
quake. In the central region of Cariaco, over 60% of
them were damaged, whereas towards the southeast,
close to the surface rupture and on the more consol-
idated sediments, only 20% of the dwellings were
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177170
damaged (Ga
´mez et al., 1999). Nevertheless, we
cannot establish a direct correlation between the
predominant periods (Fig. 5), S-wave velocities (Fig.
9) and liquefaction potential (Fig. 13) and the dam-
ages in Cariaco, as many of the collapsed structures
were poorly maintained ‘‘bahareque’ houses, and the
damage distribution is strongly influenced by the type
and quality of construction.
4.3. Sliding
The hilly areas near the epicenter were extensively
affected by sliding but of rather small size. Excluding
the probable submarine slump that affected the mouth
of the Manzanares River at Cumana
´, slides are modest
in size and are rarely over a 100 m across. Displace-
ment is in the order of few centimeters, except for: (a)
Fig. 14. Profiles summarizing the geotechnical information in Cariaco crossing drillings P3, P1 and P2 from NW to SE (profile 1, top) and P1,
P4 and P6 from NW to SE (profile 4, bottom). The number of SPT counts are indicated for each drilling; CL = clay, SM = silty sand, SP = poorly
sorted sand, SG = sand with gravel.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 171
the slides affecting roads where cuts fell down on the
road and uncompacted fill of road bench slid down-
slope; and b) the earthquake-triggered slide that
destroyed travertine staircased pools at the Agua
Caliente farm where travertine blocks slid down along
slope of the El Pilar fault scarp (Plate 3D) and highly
calcareous milky-like waters were poured into the
Buena Vista swamp for several days.
Many roads were damaged by slides (landslides,
rockslides and rockfalls) in different degrees at several
places (for more details, refer to FUNVISIS, 1997 and
MTC, 1997): El Limo
´n(eastofRı
´o Casanay), El
Table 3
Borehole profiles in Cariaco indicating the most important geotechnical parameters of the sand layers with respect to their liquefaction potential
(for location see Fig. 13)
Sand layer
(depth)
Thickness
(m)
Classification
SUCS
N-SPT
(average)
% Fine Liquefaction
potential (Seed)
Saturation
(%)
Confined
by clays
Los Bloques (INAVI urbanization)—P3
1.55 – 8.55 7 SP 12 8.14 Yes 100 No
12.5 – 13.5 1 SM 29 12.78 No 100 No
14.5 – 16.5 2 SM 22 28.00 No 100 Yes
Medicatura (Preescolar Marı
´a Ange
´lica Lusinchi)—P7
0.55 – 2.55 2 SM 4 40.50 No Partial No
3.55 – 4.55 1 SM 8 31.50 Yes 100 No
6.55 – 8.55 2 SM 10 18.34 Yes 100 Yes
10.5 – 11.5 1 SM 18 27.00 Yes 100 Yes
14.5 – 18.5 4 SM 24 18.44 Yes 100 Yes
19.5 – 20.5 1 SM 29 31.00 Yes 100 No
Valentı
´n Valiente School—P5
0.55 – 1.55 1 SP 7 8.13 No Partial No
2.55 – 3.55 1 SM 9 30.49 Yes 100 No
5.55 – 6.55 1 SP 20 11.10 Yes 100 Yes
9.5 – 10.5 1 SM 9 19.17 Yes 100 Yes
Raimundo Martı
´nez Centeno School—P4
6.55 – 7.55 1 SG 6 5.86 Yes 100 Yes
9.55 – 13.5 4 SM 38 21.38 No 100 No
13.5 – 15.5 2 SG 28 11.57 Yes 100 No
Cariaco Major Office—P1
0.55 – 2.55 2 SM 8 39.55 No Partial No
3.55 – 4.55 1 SM 5 38.30 No 100 Yes
12.5 – 15.5 3 SM 25 22.67 Yes 100 Yes
19.5 – 20.5 1 SG 13 36.90 No 100 No
Brekerman Street—P2
1.55 – 2.55 1 SM 3 29.20 Yes 100 Yes
4.55 – 5.55 1 SM 20 11.10 Yes 100 No
5.55 – 8.55 3 SP 44 3.55 No 100 No
8.55 – 10.5 2 SP 14 4.75 Yes 100 No
Cumanagoto School—P6
2.55 – 3.55 1 SM 19 22.00 No Partial Yes
5.55 – 6.55 1 SM 19 21.00 No Partial No
6.55 – 10.5 4 SM 95 27.25 No Partial No
13.5 – 15.5 2 SG 100 11.86 No 100 No
Surficial evidence for liquefaction was observed only at drilling P2 in the Brekerman Street. The level of seismic acceleration considered for the
estimation of the liquefaction potential is 0.3 g. NSPT is normalized to 60% of blow counts from SPT tests.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177172
Salobre (south of Nva. Colombia), El Toporo (7 km
south of Rı
´o Casanay), Espı
´n (between San Antonio del
Golfo and Villa Frontado), La Ceiba, Panton
˜o-La Pica
(east-facing slope of the Garrapatero hill), Pericantal
(between San Antonio del Golfo and Villa Frontado),
Pericantal-Paradero local road, Rı
´o Abajo (Villa Fron-
tado-Blascoa road). Many of these roads run on top or
along steep slopes of hills and ranges. Also self-con-
structed ‘‘bahareque’’ or concrete-framed dwellings,
churches and schools were affected by mass insta-
bilities on hilly areas. Blanco Lugar and Juan Sa
´n-
chez are among those being damaged most. Sliding
was also reported on uninhabited hillslopes, such as
at: El Paraiso hill at Guaruta (7 km south of Rı
´o
Casanay) and near Periquito (6 km north of Rı
´o
Casanay).
4.4. Other effects
Two very common associated effects have been
mentioned and precisely described by locals but not
witnessed by authors of this paper except for very few
cases that suggest that reported facts are likely to have
occurred, namely sea-front retreat and water table
changes.
4.4.1. Sea retreat
Almost all along the coasts of the Cariaco Gulf,
inhabitants of fisherman or seaside settlements reported
that during the shaking the sea receded few tens (30 to
50) of meters and then came back rather smoothly to its
original position, and eventually went farther beyond
inland. In Cumana
´, where seawaters are shallow for
over hundreds of meters, people reported that water
retreated about 100 m. This allowed estimating that
waves were in the order of 1 m high.
This initial seawater retreat may be explained if
slip along the El Pilar fault during the Cariaco 1997
earthquake in combination with the relative position
of the water body of the Cariaco Gulf with respect to
the fault is taken into account. The water body is
located mainly north of the fault and the fault moved
dextrally (Fig. 1), thus pulling the gulf seafloor
towards the east and generating a water deficiency
on the opposite side (to the west). The sea retreat was
noticed very clearly west of San Antonio del Golfo.
Sea retreats have been evidenced during several
historical earthquakes in the region (see Audemard,
under review, b). Sea retreat during the Cariaco 1997
earthquake could not be imputed to the submarine
slump west of Cumana
´because the effects should
have been more local and not so widespread in the
Cariaco Gulf.
4.4.2. Water table changes
Many changes in water table elevation have been
reported. Some reports are reliable like the following
one: Audemard (1999) personally witnessed the lack
of running water at the Piragua pool (north of the
town of Agua Caliente) in the afternoon of Tuesday
July 15, 1997, which had returned next morning (1
week after the main shock). Also, a natural spring
broke up at El Cordo
´n de Cariaco, south of Cariaco
and at the foot of the limestone-rich hills lying to the
southwest (Fig. 11B). After elderly locals’ accounts,
this spring dried up during the Cumana
´1929 earth-
quake. To drain water from the flooded town and out
of their septic tanks, a backhoe-dug ditch was exca-
vated along the main street in July 1997. In April
1998, fresh water was still running across the town, in
the open ditch.
Furthermore, water flow dropped or dried out at
warm springs of Cachamaure-San Antonio del Golfo,
at San Antonio del Golfo and at two warm springs at
El Volcancito (both before the earthquake). On the
contrary, flow increased at Blanco Lugar where a
vertical water jet started 3 h before main shock. In
addition, that cold-water spring became warm, as
stated by local observers from Periquito.
5. Summary and conclusions
Permanent ground deformations were observed,
not only along the surface rupture (see Audemard,
under review, a), but also at sites affected by induced
effects like soil liquefaction, lateral spread, and slides.
Liquefaction and lateral spreading induced by the
Cariaco 1997 earthquake occurred where all control-
ling factors concurred, restraining them essentially to
the active Holocene alluvial/deltaic plains and coastal
sand barriers. The area of distribution of these phe-
nomena coincides with areas of larger intensities
(MMI VI to VIII). The induced effect distribution
shows a similar pattern to the isoseisms in the epi-
central area: a strong WE elongation along the
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 173
Cariaco Gulf and its eastward extension into the
Cariaco sedimentary basin (Figs. 2 and 11).This
directivity in the attenuation may be explained by
the regional, WSW –ENE to E –W oriented, structural
trend (folds, major faults, bedding and foliation) as
well as by the orientation of major geomorphic units
such as mountain chains (Araya and Paria peninsulas
and Interior Range) and sedimentary basins (Cariaco
Gulf and basin). Additionally, the induced effects
seem to extend farther to the west than to the east
(with respect to the epicenter), which might be
explained by an asymmetrically west-directed rupture
propagation as proposed by Audemard (1999, under
review, a) and modeled by Mendoza (2000). Sliding
was mostly confined to the hilly area south of the
epicenter and frequently damaged the local road
system. The slides rarely exceeded a size of 100 m
across, with the exception of a probable submarine
slump at the river mouth of Manzanares River in
Cumana
´. Other effects, like sea retreat or water table
changes, were frequent but did not generate serious
damage.
The soil conditions in Cariaco, the town most
affected by the 1997 Cariaco earthquake, were inves-
tigated in detail by different methods. The results of
microtremor measurements indicate predominant soil
periods between 0.6 and 1.2 s for Cariaco, without
clear recognizable patterns in the center, but decreas-
ing values towards the northeast and along the fault
rupture (Fig. 5). Two bedrock sites to the south
showed distinctly lower predominant periods in the
range of 0.3 s. In Cumana
´, predominant periods
exceed 1 s at coastal lowlands and close to the
Manzanares River (Abeki et al., 1998). The location
of the long period sites correlates well with the
damage distribution from the Cariaco earthquake,
some 80 km from the epicenter (Lang et al., 1999).
This paper presents results from a shallow seis-
mic refraction survey that was performed in Cariaco
in order to generate detailed input information for
the modeling of the dynamic behavior of the soil.
The soil sequence, which correlates to the Quater-
nary sediments, exhibits S-wave velocities below
700 m/s, exceeding 90 m in depth in great parts
of the town (Fig. 9). The boundary with the under-
lying Tertiary sediments (or the weathered top of
Cretaceous limestones), with S-wave velocities of
about 700 m/s (P-wave velocity of 2.500 m/s), could
not be defined for the whole town, as it could be
observed only on some seismic lines at depths
between 60 and 90 m, essentially in the southern
part of the town. We therefore assumed the depth of
the ‘‘hard soil’’ (or rock equivalent) at 60 and 90 m
for three type profiles, for which seismic response
spectra of the acceleration were calculated (Fig. 10).
As a principal result, energy is absorbed to about
0.4 s and amplification occurs between 0.4 and 3 s,
with acceleration values exceeding 0.5 g. The fre-
quency ranges with observed amplification do not
coincide well with the measured predominant peri-
ods. We do not consider the amplification effects as
the main cause for the damage occurred in Cariaco
during the 1997 event, as the one to two story
dwellings have a much shorter predominant period.
The high percentage of damage in Cariaco (about
40% of all dwellings were heavily damaged or
collapsed during the 1997 Cariaco earthquake;
Ga
´mez et al., 1999) is due to inappropriate con-
struction quality of self-constructed concrete-framed
masonry structures and/or poor maintenance of cen-
tury-old ‘‘bahareque’’ houses.
In general, the observed damage was due to an
inadequate behavior of the constructions because of
inappropriate design of the structural elements and
beam-column joints, as well as the high level of
acceleration, that introduced large displacements
according to the dynamic models. A strong correla-
tion between the soil characteristics and the damage
occurrence was reported for southern Cariaco. Here,
soil liquefaction occurred on recent sediment accu-
mulation from the Cariaco River, which flows along
the southern edge of the town. The geotechnical
profiles indicate sand layers being located within
low plasticity clays. Considering the thickness of
the soft soils reported for Cariaco in the order of
90 m, the 0.3 g value estimated for the maximum
ground motion using Seed’s method might be a
lower bound to the possible values, even more, if
we keep in mind that the records obtained in
Cumana
´, some 80 km from the epicenter and located
on consolidated sediments, already reported 0.17 g
(FUNVISIS, 1997). The new seismic building code
(COVENIN, 2001) will hopefully induce seismic
microzoning assessments in Venezuela, as local soil
conditions will be more important to the require-
ments made in this code.
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177174
Acknowledgements
Seismic refraction measurements were made pos-
sible by using the Geometrics seismic recorder of
Freie Universita¨ t Berlin (FUB)—special thanks to P.
Wigger and S. Lu
¨th. These studies were mainly
funded by emergency plan supported by PDVSA. We
thank A. Singer for encouraging most of the studies
reported here. FUNVISIS staff M. Bonato, L. Acosta,
L. Vasallo, C. Grima
´n, A. Herna
´ndez and N. Reyes
participated in the macroseismic evaluation. H.
Duque, G. Malave
´, J. Alvarellos (INTEVEP), A.
Pernı
´a and N. Reyes (FUNVISIS) contributed to the
seismic measurements and V. Rocabado (FUNVISIS)
to microtremor measurements and processing. We
thank M. Pen
˜a for her excellent china-ink drafting.
CAVIM did explosive handling for seismic energy.
Accelerograms used for dynamic modeling are from
PEER Strong Motion Database. Reviews of S.
Richwalski, P.-Y. Bard and one anonymous reviewer
contributed substantially to the improvement of the
manuscript.
Appendix A
Locations evaluated for the macroseismic map
shown in Fig. 2, labeled as in the map, indicating
the assigned intensity and the geographic coordinates.
Location # MMI LAT LONG
Agua Caliente 87 8 10.5 63.49
Agua Frı
´a Abajo 78 7 10.47 63.26
Agua Frı
´a Arriba 26 5 10.46 63.26
Aragua de Maturı
´n 15 5 9.97 63.48
Araya 31 5 10.57 64.25
Arenas 7 4 10.25 63.89
Barcelona 30 5 10.13 64.68
Blanco Lugar 89 8 10.49 63.36
Boca de Rı
´o 58 6 10.46 64.17
Caicara 3 4 9.82 63.6
Campearito 59 6 10.37 63.4
Campoma 90 8 10.51 63.61
Cangrejal 85 8 10.47 63.32
Cariaco 99 8 10.5 63.55
Caripe 27 5 10.18 63.48
Caripito 28 5 10.12 63.08
Carrizal de la Cruz 72 7 10.49 63.51
Caru
´pano 60 6 10.67 63.23
Casanay 100 8 10.5 63.43
Location # MMI LAT LONG
Catuaro 33 5 10.4 63.49
Cerro Campeare 75 7 10.5 63.32
Cumacatal 35 5 10.48 63.22
Cumana
´61 6 10.45 64.17
Cumanacoa 1 4 10.26 63.92
Chacopata 54 5 10.7 63.83
Chamariapa Afuera 55 6 10.54 63.53
Chiguana 96 8 10.49 63.68
El Cautal 92 8 10.51 63.3
El Cautaro 91 8 10.49 63.27
El Limo
´n 77 7 10.5 63.33
El Pen
˜on 24 5 10.42 64.23
El Pilar 56 6 10.55 63.17
El Salobre 93 8 10.51 63.31
El Toporo 81 7 10.46 63.34
El Vicio 25 5 10.58 63.57
Ensenada Honda 22 5 10.45 63.97
Fundo La Coquera 70 7 10.48 63.65
Garrapatero 57 6 10.45 63.45
Golindano 66 6 10.45 63.89
Guaca 63 6 10.67 63.4
Guacarapo 65 6 10.5 63.73
Guamache 18 5 10.65 63.82
Guanaguana 21 5 10.07 63.6
Guaruta 73 7 10.45 63.34
Guiria 20 5 10.58 62.28
Irapa 10 5 10.58 62.58
Juan Antonio 64 6 10.35 63.35
Juan Griego 4 4 11.1 63.95
Juan Sanchez 82 7 10.49 63.32
La Asuncio
´n 5 4 11.05 63.87
La Ceiba 71 7 10.51 63.27
La Chica 41 6 10.45 63.92
La Funcia 42 6 10.45 63.47
La Pica 50 6 10.45 63.45
Los Altos 19 5 10.23 64.47
Los Capotes 52 6 10.45 63.94
Mariguitar 53 6 10.45 63.9
Maturı
´n 6 4 9.75 63.17
Mochima 49 6 10.35 64.33
Nueva Colombia 84 8 10.52 63.31
Pampatar 23 5 11 63.78
Panton
˜o 94 7 10.48 63.45
Pinto de Punceres 11 5 9.92 63.3
Porlamar 12 5 10.95 63.85
Puerto La Cruz 8 5 10.22 64.62
Quiriquire 9 5 9.97 63.23
´o Abajo 13 5 10.43 63.61
´o Caribe 16 5 10.7 63.1
´o Casanay 69 7 10.52 63.33
San Antonio del Golfo 67 7 10.44 63.8
San Fe
´lix 34 5 9.95 63.65
San Jose
´de Areocuar 76 6 10.62 63.33
Appendix A (continued)
(continued on next page)
J. Gonza
´lez et al. / Engineering Geology 72 (2004) 143–177 175
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San Juan de Azagua 36 5 10.02 63.15
San Juan de Las Galdonas 29 5 10.72 62.83
Santa Ana 97 6 10.32 63.65
Santa Cruz 32 6 10.41 63.91
Santa Lucı
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San Vicente 39 6 10.23 63.2
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Soledad 51 6 10.59 63.54
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Tucuchare 45 6 10.44 64.01
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