Magnitude
7.8 near Tecomán, Colima, Mexico
Wednesday, January 22, 2003 at 2:06:35 (UTC)
Clearly the Earth is still active. Your other instructor, Prof. Chuck
DeMets, was working on a Global Satellite Positioning (GPS) survey of ground
deformation in the region when the quake hit, and he has moved his instruments
to monitor ground deformation following the quake. Therefore, he will be
delayed in returning to Madison.
The earthquake was located in a subduction zone that lies along the western
margin of Mexico and the rest of Central America, where the oceanic Rivera plate
is being subducted eastward beneath Mexico.
This earthquake was fairly large in magnitude, but the loss of life appears
to be relatively small because it occurred in a rural area. The depth was
33.0 km (20.5 miles), which is relatively shallow for subduction-zone
earthquakes (which can be as deep as 700 km).
The above map shows the seismicity in the region since 1990, color-coded for
depth. Note how the depth of the quakes increases eastward, due to the
increasing depth of the subducted plate beneath Mexico.
The map above shows the historical seismicity since 1990 for "large" quakes,
that is above magnitude 7.0. Clearly the quake in January 2003 was not
unusual for the area.
The map above shows the seismic hazard, cast in terms of peak ground
acceleration.
Photos:
Below are several photos of the damage I collected from various News web
sites. You can see that the major issue in a populated area is collapsing
buildings - this is the most common way people are hurt or killed in
earthquakes. If you were standing out in the middle of a field, you would
fall down, but probably be unharmed.
