Delayed Seismicity Rate Changes Controlled by Static Stress Transfer

• Published in: Journal of geophysical research. Solid earth 2017-10, Vol.122 (10), p.7951-7965
• Main Authors: Kroll, Kayla A., Richards‐Dinger, Keith B., Dieterich, James H., Cochran, Elizabeth S.
• Format: Article
• Language: English
• Subjects: Aftershocks; Constants; Coulomb failure; Delay; delayed shutdown; Deserts; Earthquakes; Geophysics; GEOSCIENCES; Mathematical models; Modelling; postseismic deformation; rate state; Seismic activity; Seismicity; seismicity rate increase/decrease; seismicity shutdown; Stress; Stress transfer; Stresses; Time dependence
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2017JB014227

On 15 June 2010, a Mw5.7 earthquake occurred near Ocotillo, California, in the Yuha Desert. This event was the largest aftershock of the 4 April 2010 Mw7.2 El Mayor‐Cucapah (EMC) earthquake in this region. The EMC mainshock and subsequent Ocotillo aftershock provide an opportunity to test the Coulomb failure hypothesis (CFS). We explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative CFS change imparted by the Ocotillo event. Based on simple CFS calculations we divide the Yuha Desert into three subregions, one triggering zone and two stress shadow zones. We find the nominal triggering zone displays immediate triggering, one stress shadowed region experiences immediate quiescence, and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown. We quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of CFS change with the rate‐state earthquake rate formulation of Dieterich (1994), assuming that each subregion contains a mixture of nucleation sources that experienced a CFS change of differing signs. Our modeling reproduces the observations, including the observed delay in the stress shadow effect in the third region following the Ocotillo aftershock. The delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations. We find that rate‐state models of time‐dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity. Key Points Seismic quiescence and triggering is observed related to coseismic static stress changes Decreased seismicity rates may be obscured by a small subset events with positive CFS Coupled Coulomb/rate‐state models can explain time‐dependent delay in seismicity rate decreases due to negative coseismic stress changes