SCEC Award Number 21091 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Can a Lack of Lake Loading Explain the Earthquake Drought on the Southern San Andreas Fault System?
Name Organization
Matthew Weingarten San Diego State University
Other Participants Ryley Hill, Ph.D. Candidate, SDSU-SIO Joint PhD Program
SCEC Priorities 3f, 5c, 2c SCEC Groups SAFS, EFP, Geology
Report Due Date 03/15/2022 Date Report Submitted 08/12/2022
Project Abstract
We use new geologic and paleoseismic data to demonstrate that the past 6 major earthquakes on the Southern San Andreas Fault (SSAF) correlate with high-stands of the ancient Lake Cahuilla, a ∼236 km^3 body of water adjacent to the SSAF. In order to investigate possible causal relationships, we computed time-dependent Coulomb stress changes due to variations in the lake level over the last ∼1100 years. Simulations were performed using a fully coupled 3-D finite element model incorporating a poroelastic crust overlying a viscoelastic mantle. We find that the Coulomb stress perturbations on the SSAF are positive (i.e., promoting failure) throughout the lake loading history. For a plausible range of lake ages and material properties of the Earth’s crust, the estimated stress perturbations are of the order of 0.5 MPa, likely sufficient for triggering. Stress perturbations are dominated by pore pressure changes, but are enhanced by the poroelastic “memory" effect whereby increases in pore pressure due to previous lake high stands do not completely vanish by diffusion and constructively interfere with the undrained response in subsequent high stands. Our preferred model suggests that the lake loading complemented the interseismic stress accumulation on average by as much as 16-44%. The destabilizing effects of lake inundation are enhanced by a non-vertical fault dip, presence of a fault damage zone, and lateral pore pressure diffusion. Our model may be applicable to other regions where hydrologic loading, either natural or anthropogenic, was associated with significant seismicity.
Intellectual Merit Our work is directly applicable to SCEC research priorities 3f, 5c, and 2c. We study the mechanical effects of fluid flows on earthquake occurrence throughout the earthquake cycle and combine patterns of stress accumulation with paleoseismic observations from the last millennium. The significance of our findings is enhanced by both the public and academic interest in the Southern San Andreas Fault. The SSAF continues to captivate readers from both communities because it poses the largest seismic hazard in California with the potential to produce M7+ earthquakes. In addition to its hazard potential, the SSAF’s abnormal quiescence is a long-pondered question: why hasn’t the SSAF produced a major earthquake event in the past 300 years when its average recurrence interval is documented to be ~180 years? Our results indicate that the SSAF event history is modulated by the presence of ancient Lake Cahuilla – the fault is promoted towards failure during lake highstands and stabilized during low lake levels. Without the presence of lake loading in the past 300 years, we discuss the implication that a decreased water level could have stabilized the SSAF in the modern era and delayed a major earthquake on the fault. Our work is of broad societal relevance, especially to the ~23 million Southern California residents, as well as relevant to other regions where active seismogenic faults are subject to hydrologic loading from natural or anthropogenic sources.
Broader Impacts This work was multi-disciplinary in nature: the combination of data on the SSAF from paleoseismic, geodetic, geologic and hydrogeologic sources was necessary for the project's success. The collaboration of researchers from all of these sub-disciplines within the SCEC community showed how the value of the communities’ cross-disciplinary nature. In addition, this award primarily supported the work of Ph.D. student Ryley Hill at SDSU. Results are to be presented at the upcoming AGU Fall Meeting, SCEC annual meeting and as part of lectures to the Joint Geophysics Program as well as the SDSU Computational Sciences Program.
Exemplary Figure Figure 3 | Stress Effects of Lake Loading on Earthquake Cycle. A) Maximum ΔCFS (mPa) on the SSAF as a function of time C.E. (years) for 7 km depth. Color lines correspond to models assuming different permeability of the fault zone, from highest (Model 1) to lowest (Model 5, no permeability contrast with the host rocks). B) Variations suggested by our preferred model (model 2), assuming an average tectonic stressing rate of 16 kPa/yr (dashed line; also see Methods). Dark and light blue solid lines denote the maximum and minimum stress perturbations, respectively, based on a grid search across all possible earthquake timings (±1σ). Circles represent the minimum and maximum modulation in the grid search results for a range of σ. Stress perturbations are with respect to the (unknown) background stress at the beginning of simulation. The averaged stress load contribution plotted here is 32.33% for the lowest mean total and 43.70% for the largest mean total.