Seismic Velocity Change Patterns Along the San Jacinto Fault Zone Following the 2010 M 7.2 El Mayor‐Cucapah and M 5.4 Collins Valley Earthquakes

We study temporal changes of seismic velocity ( d v / v ) in the crust around the central section of the San Jacinto fault zone (SJFZ), Southern California. Focusing on a 200‐day‐long period around April 2010, our analysis resolves two tens‐of‐days‐long successive episodes of reduced velocities that are compatible with signals from the long base strainmeter at the Piñon Flat Observatory. The imaged d v / v sequences are proxies for evolving material properties in the crust surrounding the SJFZ. The temporal and the spatial coincidence of the observed d v / v patterns with the occurrence of two proposed creep episodes suggest that the relative velocity changes reflect the response to deep creep events that follow the M 7.2 El Mayor‐Cucapah earthquake and the M 5.4 Collins Valley earthquake that occurred 94 days later on the San Jacinto fault. The main slip during the creep events was proposed to occur below 10‐km depth. Wavefield properties suggest sensitivity to medium changes above this source zone, in the top 10 km. The distribution of the obtained d v / v reductions shows a strong difference between large values to the west of the SJFZ and significantly smaller amplitudes to the east. The similarity to the seasonal velocity change pattern implies that the results are likely controlled by the contrast of mechanical properties across the fault, such as fault‐perpendicular shear modulus variations. Our analysis extends the spectrum of methods that can be used to study earthquake interaction, fault zone rheology and dynamics, triggering, and the interplay between creep episodes and earthquakes.