August 26, 2012

Earthquake sequence near Brawley

By Jamison H. Steidl, Ph.D., Research Seismologist, Earth Research Institute and Adjunct Professor, Earth Science, UC Santa Barbara

Very interesting earthquake sequence today, with vertical acceleration records twice the horizontal for the M5.3 at 19:33 UTC. This event was followed by two other events, M5.5 at 20:57 UTC and a M4.8 at 23:33 UTC. The M4.8, being the closest to the WLA site, had the largest pore pressure increase so far. Pore pressure response is very interesting as well. First screenshot (clickable for an expanded version) is all the pore pressure sensors from shallow to deepest at the WLA site. Second screenshot is the older re-instrumented USGS/NEES site (NP.5210) 80 meters from the WLA site, with both 3-component acceleration and pore pressure response shown on same plot.

This is the largest ground motion record we've recorded so far at the NEES facility. No liquefaction, but another great example of pore pressure ratcheting effect from multiple earthquakes, and a good look at the details of the initiation phase of liquefaction.

These records demonstrate why continuous recording is so important on these arrays. This high quality data is possible because of the operations and maintenance funds supported through the National Science Foundation's George E. Brown Jr., Network for Earthquake Engineering Simulation program [Award numbers CMS-0217421, CMS-0402490, and CMMI-0927178]. Real-time telemetry of this data back to UCSB and Caltech is provided by the ANR/HPWREN system.

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