Prospective and Pseudo-prospective Aftershock Forecasts After the July 2019 M6.4 Searles Valley and M7.1 Ridgecrest Earthquakes

Simone Mancini, Margarita Segou, Tom Parsons, & Maximilian J. Werner

Published August 15, 2019, SCEC Contribution #9689, 2019 SCEC Annual Meeting Poster #268

On 4 July 2019, a M6.4 earthquake hit southern California west of Searles Valley, activating a system of NE-SW oriented left-lateral and NW-SE trending right-lateral strike slip faults. 34 hours later, a M7.1 earthquake struck near Ridgecrest, triggering thousands of aftershocks along a ~55 km long right-lateral rupture. Given the regional tectonic setting, bounded by the Garlock fault to the south and extending towards the Owens Valley fault to the north, where notable earthquakes occurred in the last ~150 years, concerns for future damaging events on nearby faults were immediately raised. The stress patterns from the combined effect of the M6.4 and M7.1 events show that the central segment of the Garlock fault receives a maximum 0.006-0.338 MPa increase and that positive stress changes are estimated on the Southern Sierra Nevada, Owens Valley, and Panamint Valley faults. We then combined the Coulomb stress results with the rate-and-state framework. We present (1) a set of early stress-based (CRS) and statistical (ETAS) aftershock forecasts that we developed shortly after the two mainshocks, and (2) later forecasts that use the aftershocks recorded through the first 2 weeks of the sequence. Our preliminary CRS models include the static stress changes following the M6.4 and the M7.1 events and feature synthetic source models with empirically determined fault length and fault constitutive parameters from past research. Increasingly complex CRS models incorporate optimized rate-state parameters and the first USGS spatially variable slip model for the M7.1 event. We introduce spatially heterogeneous receiver planes by combining the UCERF3 fault traces, available stress inversions and past focal mechanisms (1981-2019). The most complex CRS forecasts also incorporate secondary triggering effects from M2.5+ triggered events. We test the performance of the models in a pseudo-prospective mode for the first month of the sequence using the metrics of the Collaboratory for the Study of Earthquake Predictability, and we put forward a 1-year forecast.

The prospective 1-year forecasts of the ETAS and the enhanced CRS models support heightened rates in the Coso volcanic area. The complex CRS forecasts expect increased seismicity also along the central Garlock fault and the northern section of the South Sierra Nevada fault, where the cumulative probabilities of at least one M6+ event within the next year are ~10 orders of magnitudes higher than the background values.

Key Words
Aftershock forecasts, Coulomb rate-state modeling, ETAS models

Citation
Mancini, S., Segou, M., Parsons, T., & Werner, M. J. (2019, 08). Prospective and Pseudo-prospective Aftershock Forecasts After the July 2019 M6.4 Searles Valley and M7.1 Ridgecrest Earthquakes. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes