Seismic Velocity Change Patterns Along the San Jacinto Fault Zone Following the 2010 M7.2 El Mayor-Cucapah and M5.4 Collins Valley Earthquakes

Abstract

We study temporal changes of seismic velocity (dv/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 Pinon Flat Observatory. The imaged dv/v sequences are proxies for evolving material properties in the crust surrounding the SJFZ. The temporal and the spatial coincidence of the observed dv/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 M7.2 El Mayor-Cucapah earthquake and the M5.4 Collins Valley earthquake that occurred 94days 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 10km. The distribution of the obtained dv/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. Plain Language Summary Motion along geologic fault zones not only occurs during small and large potentially devastating earthquakes. Some faults creep aseismically, which means the slip motion is too slow to generate earthquake waves. To develop a more complete understanding of fault behavior, it is important to detect and locate such creep transients. A particularly interesting problem is how earthquakes and episodic creep events, which tend to occur in a zone below the earthquakes, interact with each other. Creep events are usually detected using satellite-based methods. In this study we applied the most modern ambient seismic wavefield analysis techniques to first detect and locate changes in the rock properties around a continental strike slip fault, the San Jacinto fault, Southern California, that are caused by two successive deep creep events along that fault. These results are supported by data from a colocated deformation meter that picked up signals from the same creep events that are thought to be triggered, initiated by the two earthquakes indicated in the title. The observed deformation patterns-they are not uniform along and across the fault-and their temporal evolution can help us better understand the processes that happen on and off faults in earthquake-prone regions.