Seismogeodetic Imaging of Active Crustal Faulting

Citation

Inbal, Asaf (2017) Seismogeodetic Imaging of Active Crustal Faulting. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z92B8W02. https://resolver.caltech.edu/CaltechTHESIS:10032016-142052951

Abstract

Monitoring microseismicity is important for illuminating active faults and for improving our understanding earthquake physics. These tasks are difficult in urban areas where the SNR is poor, and the level of background seismicity is low. One example is the Newport-Inglewood fault (NIFZ), an active fault that transverses the city of Long-Beach (LB). The catalog magnitude of completeness within this area is M=2, about one order of magnitude larger than along other, less instrumented faults in southern California. Since earthquakes obey a power-law distribution according to which for each unit drop in magnitude the number of events increases by a tenfold, reducing the magnitude of completeness along the NIFZ will significantly decrease the time needed for effective monitoring. The LB and Rosecrans experiments provides a unique opportunity for studying seismicity along the NIFZ. These two array contain thousands of vertical geophones deployed for several-months periods along the NIFZ for exploration purposes. The array recordings are dominated by noise sources such as the local airport, highways, and pumping in the nearby oil fields. We utilize array processing techniques to enhance the SNR.We downward continue the recorded wave field to a depth of a few kilometers, which allows us to detect signals whose amplitude is a few percent of the average surface noise. The migrated wave field is back-projected onto a volume beneath the arrays to search for seismic events. The new catalog illuminates the fault structure beneath LB, and allows us to study the depth-dependent transition in earthquake scaling properties.

Deep aseismic transients carry valuable information on the physical conditions that prevail at the roots of seismic faults. However, due the limited sensitivity of geodetic networks, details of the spatiotemporal evolution of such transients are not well resolved. To address this problem, we have developed a new technique to jointly infer the distribution of aseismic slip from seismicity and strain data. Our approach relies on Dieterich (1994)'s aftershock model to map observed changes in seismicity rates into stress changes. We apply this technique to study a three month long transient slip event on the Anza segment of the San Jacinto Fault (SJF), triggered by the remote M_w7.2, 2010 El Mayor-Cucapah (EMC) mainshock.

The EMC sequence in Anza initiated with ten days of rapid (≈100 times the longterm slip rate), deep (12-17 km) slip, which migrated along the SJF strike. During the following 80 days afterslip remained stationary, thus significantly stressing a segment hosting the impending M_w5.4 Collins Valley mainshock. Remarkably, the cumulative moment due to afterslip induced by the later mainshock is about 10 times larger than the moment corresponding to the mainshock and its aftershocks. Similar to sequences of large earthquakes rupturing fault gaps, afterslip generated by the two mainshocks is spatially complementary. One interpretation is that the stress field due to afterslip early in the sequence determined the spatial extent of the late slip episode. Alternatively, the spatial distribution is the result of strong heterogeneity of frictional properties within the transition zone. Our preferred model suggests that Anza seismicity is primarily induced due to stress transfer from an aseismically slipping principal fault to adjacent subsidiary faults, and that the importance of earthquake interactions for generating seismicity is negligible.

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