SCEC Award Number 24204 View PDF
Proposal Category Individual Research Project (Single Investigator / Institution)
Proposal Title Modeling potential seismic rupture of the SAF creeping segment
Investigator(s)
Name Organization
Nadia Lapusta California Institute of Technology
SCEC Milestones B1-1, D1-1 SCEC Groups FARM, PBS, Seismology
Report Due Date 03/15/2025 Date Report Submitted 04/08/2025
Project Abstract
 The creeping section of the San Andreas fault is thought to provide a barrier between the southern and northern locked parts of the fault, given no historic or paleoseismic large earthquake ruptures in the area. At the same time, there is evidence for slip deficit and high temperature rises within the creeping section consistent with large seismic slip. Lab studies indicate that fault gouge from the creeping fault zone at SAFOD can significantly weaken at seismic slip rates in a manner consistent with thermal pressurization (TP) of pore fluids. We seek to understand, through numerical model-ing, whether the creeping section can be ruptured by large earthquakes (Mw 7.0 or larger) due to TP for realistic fault properties while still arresting smaller events such as the 2004 Mw 6 Parkfield earthquake. We find that there is indeed a realistic range of fault properties for that allows Mw 7 events to propagate through the creeping segment. As the rapid slip penetrates into the VS (creep-ing) region due to inertial effects, there is a competition between the increase in friction and de-crease in the effective normal stress due to shear heating. Our initial models had relatively high slip rates, with average values of 5-6 m/s, whereas a typical average observed value is ~1 m/s. To create more realistic models, we have reduced the slip rates by employing a maximum rate threshold, Vlim, as a proxy for off-fault inelasticity. For Vlim = 3 m/s, the slip rates are quite realistic, averaging to 1 m/s.
Intellectual Merit Our modeling indicates that dynamic weakening due to thermal pressurization (TP) of pore fluids can lead to rupture of creeping fault segments which are typically considered to be aseismic. Models with dynamic TP in otherwise stable segments can explain the absence of microseismicity at the bottom of the locked (seismogenic) zone of several SAF segments (e.g., Carrizo and Coachella). This project indicates that the same physical mechanism can allow large dynamic rupture to span the creeping segment, for temperature increases consistent with the field inferences from the creeping segment.
Broader Impacts Understanding whether the creeping segment can sustain seismic rupture is important for seis-mic hazard and emergency response planning, as the resulting event could potentially cause destruction in two large metropolitan areas – Los Angeles and San Francisco – preventing the emergency responders and supplies from one of the areas reaching the other. Furthermore, our study adds to the evidence that large mature faults are susceptible to co-seismic weakening due to thermal pressurization of pore fluids, with important implications for the state of stress and frequency-magnitude earthquake statistics on such faults.
Project Participants Caltech PhD students Victor Vescu (2D modeling), Mary Agajanian (3D modeling), and William Chen (learning the codes to join the project in the future).
Exemplary Figure Figure 2: Simulated fraction of the creeping section ruptured for a Mw 7-style event and varying TP parameters, with Vlim = 15 m/s (left) and Vlim = 3 m/s (right) that represent effects of off-fault inelas-ticity. The case with Vlim = 3 m/s has realistic average slip rates of ~1 m/s during the dynamic event. In both cases, there are realistic parameter combinations that allow for rupture of the entire creeping segment. From Vescu et al., SCEC poster, 2024.
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