Coseismic Offsets on PBO Borehole Strainmeters: Real, or Spurious?

We have observed coseismic strain offsets during many significant earthquakes, at all locations in the 74-instrument PBO borehole strainmeter (BSM) network. The M7.2 El Mayor-Cucapah earthquake of April 4, 2010 induced the largest offsets thus far, on BSMs located within the San Jacinto fault zone - the "Anza cluster". Here we present analyses of trends in the observed offsets for the Anza cluster, as well as inspection of their inferred borehole lithology. We find that offsets rarely agree with elastic dislocation theory in magnitude and sign, and speculate that they are controlled more by localized geologic constraints than by triggered fault slip, as has been suggested in previous studies (e.g. Linde and Johnson, 1989).