SCEC Award Number 25053 View PDF
Proposal Category Collaborative Research Project (Multiple Investigators / Institutions)
Proposal Title The Interplay of Seismic and Aseismic Fault Slip During Earthquake Swarms in California and Nevada
Investigator(s)
Name Organization
Daniel Trugman University of Nevada, Reno Yu Jiang University of Nevada, Reno
SCEC Milestones D1-1, D2-2, D3-1 SCEC Groups Geodesy, Seismology, FARM
Report Due Date 03/15/2026 Date Report Submitted 02/10/2026
Project Abstract
 Earthquake swarms represent a ubiquitous mode of fault slip in which seismic and aseismic processes interact over weeks to years, yet the mechanics of strain release and the timing of aseismic slip in swarms remains to date poorly constrained. This project aimed to integrate seismicity observations with surface deformation measurements to improve understanding of fault behavior during earthquake swarms in California and Nevada and to assess the role of aseismic stressing in swarm evolution.
We develop T-rate, a physics-informed Bayesian inversion framework that reconstructs the temporal evolution of stress changes directly from seismicity rate observations using a modified rate-and-state friction law. The method parameterizes stressing rate using an asymmetric trapezoidal function, allowing acceleration, steady-state, and deceleration phases of transient loading to be resolved. To isolate stress-driven seismicity from earthquake-earthquake triggering, we apply declustering techniques to then estimate the ratio of transient to background seismicity rates. The approach is validated through comprehensive synthetic experiments.
Application to the 2008 Reno-Mogul swarm reveals three distinct stress-loading phases preceding the mainshock, with the final phase characterized by a rapid stress increase immediately before rupture. Independent GPS observations show 1-3 cm of surface displacement 3-4 days prior to the mainshock, providing strong evidence for aseismic slip. Extension of the analysis to seven additional swarms shows that five exhibit elevated non-earthquake-triggered seismicity consistent with transient aseismic stressing. These results demonstrate that seismicity rates can serve as a sensitive stress meter for quantifying aseismic fault processes, advancing SCEC’s goal of physics-informed characterization and forecasting of earthquake systems.
Intellectual Merit This project advances SCEC’s Earthquake System Science objectives by developing a physics-informed framework to quantify aseismic fault processes during earthquake swarms using seismicity data alone. By treating seismicity rates as a stress-sensitive observable within a Bayesian rate-and-state friction formulation, the project introduces T-rate, a creative and original inversion approach that reconstructs time-dependent stressing histories and associated uncertainties. This work improves fundamental understanding of strain partitioning and transient fault slip, particularly in regions with limited geodetic coverage, and directly contributing to physics-informed predictive analyses of seismicity and improved seismic hazard models for California and Nevada.
Broader Impacts This project supported the professional development of a postdoctoral researcher, Co-PI Dr. Jiang, in his first SCEC-supported role. It strengthened inter-institutional partnerships between UNR and international collaborators. By releasing the T-rate tool on open-access platforms like GitHub and Zenodo, the project enhances community research infrastructure. Benefitting society, these results improve physics-based seismic hazard models, especially in regions with sparse geodetic monitoring. This work directly supports SCEC’s mission to develop predictive analyses of seismicity, contributing to long-term earthquake preparedness and risk mitigation.
Project Participants The project was conducted primarily by Yu Jiang (postdoctoral researcher and Co-PI) and Daniel Trugman (PI) at the Nevada Seismological Laboratory, University of Nevada, Reno. The work benefited from scientific collaboration with Pablo González (CSIC, Spain), particularly through discussions on fault mechanics and stress modeling. Additional input was received through interactions within the broader SCEC community on earthquake declustering and aseismic fault processes. The analysis relied on open-access earthquake catalog from the U.S. Geological Survey (ANSS ComCat), continuous GPS data from the Nevada Geodetic Laboratory, and Sentinel-1 SAR data from the European Space Agency.
Exemplary Figure Figure 2. Stress and seismicity rate analysis of the 2008 Reno-Mogul, Nevada, swarm. (a) Modeled stress rate and stress are shown in blue lines and red lines; (c) is the same as (a) but stress rate and stress in the logarithm scale. (b) Observed and modeled seismicity rates are shown in red circles and blue lines respectively; (d) is the same as (b) but the seismicity rate is in the logarithm scale. The x-axis represents the number of days since 1/1/2008. The last day on the x-axis is 4/25/2008, one day before when the M 5.1 occurred.
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