Aftershock forecasts following the M6.4 and M7.1 Ridgecrest, California, earthquakes of July 2019

Jeanne L. Hardebeck, Andrew J. Michael, Morgan T. Page, Nicholas J. van der Elst, Michael Barall, Andrea L. Llenos, Eric M. Martinez, & Sara K. McBride

Published August 12, 2019, SCEC Contribution #9467, 2019 SCEC Annual Meeting Poster #270

The M6.4 and M7.1 Ridgecrest earthquakes were the first significant California earthquakes since the USGS developed a new national capability for aftershock forecasting. The USGS has for decades issued aftershock forecasts in California, based on Reasenberg and Jones (Science, 1989). We updated this method to improve uncertainty estimates, the handling of early catalog incompleteness, and adaptation as aftershock sequences progress (Page et al., BSSA 2016). We also developed new regionalized aftershock parameters for California (Hardebeck et al., SRL 2019). The aftershock forecasts are integrated with other earthquake products on the USGS event webpages, and are communicated with a template that provides basic information as well as detailed numerical forecasts.

We issued the first aftershock forecast 35 minutes after the M6.4 Ridgecrest event, using generic parameters for hydrothermal regions, which have the highest aftershock productivity in California. We updated the forecast over the following day and a half, adapting to a slightly lower productivity, and reducing uncertainty, using data from the sequence. At the time of the M7.1 earthquake, the forecast showed a 1 in 200 chance of a M≥7 in the following week, ~50,000 times the background probability at that location (e.g. Field et al., BSSA 2014). The first forecast following the M7.1 was issued 47 minutes after that event, using generic parameters. We regularly updated the forecast using the observed sequence, adapting to its lower productivity and faster decay rate.

We will evaluate the success of the forecasts by comparing them with the observed numbers of aftershocks. The Reasenberg and Jones model assumes that all aftershocks are triggered by a single mainshock, which may or may not be a good approximation for this sequence with two larger events each triggering aftershocks. We plan to implement ETAS forecasting (Ogata, JASA 1988), which is better suited for sequences with multiple mainshocks.

Key Words
Ridgecrest, aftershock forecast

Citation
Hardebeck, J. L., Michael, A. J., Page, M. T., van der Elst, N. J., Barall, M., Llenos, A. L., Martinez, E. M., & McBride, S. K. (2019, 08). Aftershock forecasts following the M6.4 and M7.1 Ridgecrest, California, earthquakes of July 2019. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes