Longbase Strainmeter Recordings of Baja California Earthquake

By Frank Wyatt, UCSD/SIO

The recent Baja California earthquake (M 7.2) resulted in broad-scale deformation of the earth in the border regions of California and Mexico. Of particular concern immediately following this event was the possibility that the Baja earthquake might lead to the triggering of an even larger event on the faults extending into the highly populated regions of Mexico and California... most notable on the San Andreas Fault.

HPWREN connectivity was crucial to the monitoring of an important source of information on the state of the southern San Andreas and San Jacinto faults: recordings from the longbase strainmeters operated by the Institute of Geophysics and Planetary Physics, University of California, San Diego.

Within minutes of the strong shaking we were able to log into the HPWREN data-streams from the three strainmeter sites alongside the Salton Sea (DHL and SCS) and near the town of Anza, California (PFO), and begin to monitor the earth-strain. Our findings were reported to the agencies charged with assessing the hazards associated with the earthquake.

This figure presents the processed records from the north-south strainmeter at Durmid Hill, near the southernmost terminus of the San Andreas fault. These data have been edited to remove the permanent deformation (offset) coincident with the earthquake, filtered to suppress the transient seismic coda, and adjusted for the ever-present earth-tide deformation. What remains is a substantial post-seismic transient. Some degree of earth-strain relaxation is anticipated following such a large, nearby event. Reassuringly this deformation was clearly not growing in rate, instead following an exponential- decay.

A previous article about the longbase laser strainmeters can be found at /news/20081117/. For more information please check http://pfostrain.ucsd.edu/pfo/.

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