A series of earthquakes occurred on August 3, 2009
in the Gulf of California, the largest of which was felt in the Tucson
and Phoenix area of southern Arizona. The largest event, a magnitude
6.9 occurred at 10:59 am Tucson time. It took the initial P wave a
little over one minute to travel from the source to Tucson. The earthquake
was a strike-slip event (horizontal motions on a vertical fault) associated
with the plate boundary between the North America and Pacific plates.
The lack of vertical faulting probably prevented the generation of
a tsunami from this large earthquake. At this location the plate boundary
is a series of short normal faults and longer strike slip faults causing
the Gulf of California to open up. The mainshock was followed by a
magnitude 6.2 event 50 km to the northwest as well as several aftershocks.
There was also a magnitude 5.8 foreshock about 4 minutes before the
large 6.9 event, but at the time there was no way to know that it
was a foreshock and a larger event was going to occur.
The earthquake was felt in Tucson on the 10th floor
of the Gould Simpson Building at the University of Arizona and in
several of the tall building downtown. Most of Tucson is built on
a sedimentary basin that amplifies the ground shaking more than structures
built on hard rock at a similar distance from the earthquake. It is
rare to feel earthquakes in Tucson but this event reminds us that
we are not very far from a major plate boundary.
