What Do Borehole Strainmeter Signals Tell Us about Earthquake-Related Stress Change?

Abstract:

We attempt to understand borehole tensor strainmeter signals in terms of stress changes associated with earthquake occurrence. Several borehole strainmeters installed in Anza, southern California were measuring borehole strains when two earthquakes (>M5) occurred nearby in 2010: M7.2 Baja California (BC) earthquake and M5.4 Southern California (SC) earthquake. We first calculate the principal strain tensor from noise-filtered strains in three azimuths, which may represent crustal deformation around the strainmeters. Then the strains are transformed into stress tensor based on a simple assumption of elastic crust, which gives changes in stress orientations and magnitudes as a function of time. We observed that changes in the orientations and the magnitudes of stresses were captured in the strainmeter signals when the two earthquakes occurred. The pre-earthquake stresses can be characterized by subtle increases in stress magnitudes predominantly in N-S direction, which is consistent with the focal mechanisms of the two earthquakes. When the earthquakes hit, sudden stress drops took place in approximately N-S direction, after which stresses were stabilized within 1 hour. The evaluated stress drops at the strainmeter site were at an order of 10^-2 MPa for the BC earthquake and 10^-3 MPa for the SC earthquake, which are lower by two order and three order of magnitude, respectively, than the maximum stress drops associated with the two earthquakes. Coulomb stress transfer models show that the modelled stresses transferred to the strainmeter locations are similar to, or an order lower than the strainmeter data. Our study demonstrates that the strainmeter data can be utilized for stress analysis, if strainmeters are installed in a way to capture representative crustal deformation.