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3 August 2009, Gulf of California Earthquake

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.

The seismic station near Tucson (Code name TUC) is part of the Global Seismic Network operated by IRIS and the USGS. It is also part of the Arizona Integrated Seismic Network that consists of 12 broadband seismic stations and 6 short period stations across the state. Eight of these stations were recently purchased by the Arizona Geological Survey as part of a collaborative project with the University of Arizona, Arizona State University and Northern Arizona University and funded by FEMA.


Seismograms Station Tucson (TUC)

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For more information on the 3 August 2009 Gulf of California earthquake go to: