SCEC Award Number 18160 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Searching for Transient Aseismic Slip Events on the San Jacinto Fault
Investigator(s)
Name Organization
Noel Bartlow University of California, Santa Cruz Susan Schwartz University of California, Santa Cruz
Other Participants 1 UCSC graduate student
SCEC Priorities 3a, 2a, 3b SCEC Groups FARM, Geodesy, Seismology
Report Due Date 04/30/2020 Date Report Submitted 07/09/2020
Project Abstract
 Understanding the interplay of seismic and aseismic slip is key in seismic hazard evaluation. It is particularly important to know if the same fault can host different slip modes and understand the transition between modes of slip. We investigate this in the trifurcation area of the Anza segment of the San Jacinto fault where deep creep driving seismicity below the geodetic locking depth has been proposed. We focus on local moderate-sized earthquakes triggering slow slip transients by combining the occurrence of burst-type repeating earthquakes, seismicity, and borehole strainmeter data. We find that all Mw > 4.5 earthquakes from 2010-2020 triggered slow slip transients with coplanar burst-type repeating earthquakes and off-fault microseismicity. These observations include newly detected triggered slow slip transients following the June 10, 2016 Mw 5.2 Borrego Springs earthquake and the April 4, 2020 Mw 4.9 earthquake. Slow slip geometries defined by burst-type repeating earthquakes are consistent with strain change observations and reveal multiple active planes both on and off of the triggering earthquake fault. We conclude that burst-type repeating earthquakes, similar to low-frequency earthquakes, are useful indicators of slow slip transients and can reveal faulting complexities during slow slip. Further, we identify the first evidence of spontaneous slow slip in this region from burst-type repeating families on two minor faults in 2015. Taken together, our observations support a model where deep microseismicity is located in a transitional region at the bottom of the seismogenic zone with spatially heterogeneous frictional properties that produces frequent slow slip transients.
Intellectual Merit The San Jacinto Fault Zone (SJFZ) is the most seismically active in southern California, with multiple recognized segments with different properties. The Anza segment contains a ~20 km long seismic gap, which represents a potential future earthquake hazard. It also exhibits episodes of deep creep and abundant seismicity below the 10 km geodetic locking depth. Understanding the spatial and temporal behavior of this deep creep is important for providing constraints on deep fault behavior in this region and quantifying the stress added to the shallow locked region and the potential for triggering future large earthquakes. We show that the combined analysis of burst-type repeating earthquakes and strainmeter data allows for the identification and analysis of slow slip transients that could not be identified using tremor and GPS time series in previous studies.
Broader Impacts This project was a collaboration between three women geophysicists at very different stages in their careers. It involved a senior scientist, an early career researcher and a graduate student. This project provided some support to the two junior members of the research team and constitutes a chapter in Ph.D student Heather Shaddox’s Ph.D dissertation.
Exemplary Figure Figure 2. (a) Seismicity map with focal mechanisms of the Mw 5.2 June 10, 2016 local triggering earthquake and repeating earthquake families (colored according to Julian day) and observed (red) and modeled (blue) strain changes. (b) Fault parallel cross-section along line A-A’. The majority of burst-type repeating earthquakes are outlined by the dashed magenta line and the rectangular dislocation plane is included as a black rectangle. (c) Fault perpendicular cross-section along B-B’ similar to 2b