Earthquakes in the Gulf of Mexico

Although the rate of seismicity in the Gulf of Mexico is relatively low, the Gulf of Mexico is not seismically quiescent.  Since the late 1800's there have been about 10 earthquakes large enough to be located which occurred in the eastern Gulf of Mexico.  These earthquakes are mostly small magnitude events (magnitudes 3-4) but so far in 2006, two events with magnitudes in the 5.2 and 6.0 have occurred.  All of the seisms occurred at very shallow depths beneath the Earth's surface in the deep water of the Gulf west of the Florida Escarpment.  The small magnitudes of these events are consistent with the absence of tsunamis in the recent historical record of the Gulf coast states, although even earthquakes with modest magnitudes (6.0) can produce a tsunami if they occur in the vicinity of unstable sediments deposited on a sloping surface.

           Figure 1, above, shows the locations of all known earthquakes which occurred in the Florida region since 1875.  The red and purple circles lie at the epicenters (e.g., on the surface) above predominantly very shallow (0-35 km) seismic foci.  The yellow triangles are reported and/or located earthquakes that occurred on land in Florida prior to 1975.  The large yellow star in the Gulf of Mexico marks the location of the September 10, 2006 magnitude 6.0 earthquake, which was felt by thousands of people throughout the southeastern U.S.  This is the largest magnitude Gulf earthquake we know about.  The Florida Escarpment is the west-sloping seafloor in the eastern Gulf of Mexico, visible in the Figure above as the long NNW-trending change in color from deep blue (deep water) to light blue (shallow water of Florida's western contiental platform).  Note also that one of the earthquakes occurred along the similar slope marking the shallowing of Gulf waters at the Yucatan Peninsula (lower left corner of the image).

           Figure 2.  Focal mechanism of the Sept. 10, 2006, magnitude 6.0 Gulf of Mexico earthquake, as determined by the United States Geologic Survey.  The focal mechanism "beach ball" shows a projection of the two possible fault plane orientations on which slip may have occurred during the earthquake.  This mechanism is a mixed type, indicating either right-lateral strike-slip on a very shallowly east-dipping fault plane, or nearly pure dip-slip on a steeply SW-dipping fault plane.  It is very likely that the second of these two planes was the actual fault plane.

          Figure 3.  Seismogams of the September 10, 2006 Gulf of Mexico earthquake as recorded at the IRIS GSN broadband seismometer DWPF, situated in the Disney Wilderness Preserve, Florida, near Orlando.  The three traces represent the motion recorded in two perpendicular horizontal directions (top two traces) and the motion in the vertical direction (bottom) as the seismic waves arrive at the station.