R. M. Russo
223 Williamson Hall
or by appointment
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
the 5.2 and 6.0 have occurred. All of the seisms occurred at very
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).
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
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.
. 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.
4. Schematic cross-section showing one possible mechanism for
September 10, 2006 Gulf of Mexico earthquake. Top view:
Gulf of Mexico crust and overlying sediments prior to redistribution by
hurricane Katrina. During the hurricane, Mississippi River
sediments deposited in shallow water near the Gulf Coast were
redeposited to deeper Gulf waters. Bottom view: Added delta
sediments in the deep Gulf increase the load on the underlying Mesozoic
oceanic crust of the Gulf, causing it to flex down. Shallow
portions of the crust undergo compression during flexure, producing the
earthquake on a steeply-dipping fault plane. The fault may have
occurred by reactivation of an older Rift sequence fault.
Want to know more? Please check the website links
mentioned in the captions, above! Or contact me via email.
gratefully acknowledge data and images from the IRIS Consortium and the