Examining precursory stress changes during earthquake swarms in California and Nevada from seismicity rate observations

Yu Jiang, & Daniel T. Trugman

Submitted September 7, 2025, SCEC Contribution #14538, 2025 SCEC Annual Meeting Poster #TBD

Distinguishing between competing physical theories of earthquake nucleation requires advancing our resolution of fine-scale temporal variations in stressing rates driving fault slip. An improved characterization of fault stressing rates can be achieved by analyzing the mechanical response to variations in seismicity rates of a representative crustal volume. Two main end-member hypotheses prevail in current earthquake science literature: the pre-slip model which emphasizes slow slip in promoting large ruptures, and the cascade model which relies on stress transfer between earthquakes and does not require a slow slip phase. If pre-slip occurs before earthquakes, it is expected to generate a seismicity rate above background levels before the mainshock, providing a potential indicator for identifying pre-slip processes.

Here we introduce a novel methodology to infer the timing and evolution of precursory slow slip from seismicity rates based on rate-and-state friction law. The temporal evolution of stressing rates is modelled as an asymmetric ramp function, selected to capture various fault behaviors: acceleration, steady-state, and deceleration. From the rate-and-state model, we derive an integral form for R/r, the ratio of observed to background rate, and obtain its closed-form expression. Using a Bayesian framework, we estimate probability distributions for model parameters, such as the time and amplitude of shear stressing, frictional-stress parameter, and characteristic time. We can reliably reconstruct stress history from synthetic simulations.

Applying this framework to eight earthquake swarms in California and Nevada, we find that six exhibit elevated seismicity rates above background preceding the largest events. Modeling of these precursory phases indicates cumulative stress changes ranging from 0.01 to 2 MPa. Independent validations of precursory surface movement signals are provided by GPS data (e.g., 2008 Reno-Mogul and 2020 Westmorland swarms) and InSAR observations (2011 Hawthorne swarm). These results suggest that stress evolution before an energetic event can be inferred from seismicity rate observations, providing new insights into earthquake nucleation physics.

Citation
Jiang, Y., & Trugman, D. T. (2025, 09). Examining precursory stress changes during earthquake swarms in California and Nevada from seismicity rate observations. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)