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 Piñon 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 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 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.

Seismic Attenuation Monitoring of a Critically Stressed San Andreas Fault

“…Whereas the hypothesis of lateral homogeneity is easy to accept (see the study by Li et al, 2004 and consider the short distances traveled by seismic waves), the time independence one is more stringent, for slowness cannot be separated from attenuation in our analysis. However, previous work on the central section of the San Jacinto fault zone (Hillers et al, 2019) confirm our hypothesis that the variability is small, apart from the effects of large mainshocks. We discuss the issue further in section 4 and the supporting information.…”

The 2018 Geothermal Reservoir Stimulation in Espoo/Helsinki, Southern Finland: Seismic Network Anatomy and Data Features

“…The reference waveform is the stack of correlation functions from the analysis period, days 145 to 230. To improve the quality of the waveforms we stack correlations over ±1 day and apply an SVD-based Wiener filter [Moreau et al, 2017;Hillers et al, 2019b] with the dimensions five days and five samples. To assess if dv/v estimates are spurious or resolve genuine medium changes we consider spectral power on the vertical V and combined horizontal channels H, and the spectral partition ratio H 2 /V 2 as a proxy for the W S -to-W P energy density ratio in a diffusion regime [Shapiro et al, 2000], as markers of wavefield properties [Hillers et al, 2019b].…”

The 2020 coronavirus lockdown and seismic monitoring of anthropic activities in Northern Italy

“…In general, the value of H/V is related to how seismic energy in the ambient-noise field is distributed in the form of compressional, shear and surface waves 7,25,26 ; changes in H/V after vs. before the lockdown would reveal whether the reduction in anthropic noise affects one of these seismic phases/components more or less importantly than the others; in other words, whether traffic and industry-induced vibration can be associated to one particular constituent of the seismic field. Figure 6 shows that the lockdown measures have no effect on H/V, and we infer that, while anthropic noise is reduced significantly by the lockdown (Fig.…”