Implications of uncertainty in ground motion estimates on ShakeAlert alerting strategies: Considering the 2019 Ridgecrest earthquake sequence

Jessie K. Saunders, Brad T. Aagaard, Annemarie S. Baltay, & Sarah E. Minson

Published August 14, 2019, SCEC Contribution #9611, 2019 SCEC Annual Meeting Poster #273

The July 2019 Mw6.4 and Mw7.1 Ridgecrest earthquakes were the largest earthquakes to occur in Southern California in 20 years and were widely felt throughout the region. The lack of earthquake early warning (EEW) alerts issued to residents of Los Angeles County via the ShakeAlert L.A. app prompted calls for lowering the shaking intensity alert threshold from Modified Mercalli Intensity (MMI) IV to MMI III. In the ShakeAlert EEW algorithm, magnitude and location are input into a ground motion prediction equation, the output of which is fed into a ground motion to intensity conversion equation (GMICE) to determine which regions are likely to feel a certain MMI level. However, GMICEs are less accurate and less precise for low MMI levels (II and III) at distances greater than about 200 kilometers for large magnitude earthquakes, the parameters that would control alerts in L.A. for the Ridgecrest earthquakes.

We estimate the uncertainty in alert boundary locations based on the uncertainty in MMI from the GMICEs. We evaluate the implications of changing the alert threshold MMI in terms of this uncertainty as well as in terms of the area and population affected by correct and incorrect alerts. We also evaluate alternative alerting regions based on county and weather forecasting zone boundaries. We determine the quality of each alert strategy for the 2019 Mw6.4 and Mw7.1 Ridgecrest earthquakes by computing four quantities: (1) the area within the alert region that experiences shaking above the specified MMI threshold; (2) the area outside the alert region that experiences shaking above the threshold which should have received an alert; (3) the area within the alert region below the threshold that would still feel shaking (MMI > III); and (4) the area within the alert region that feels little to no shaking (MMI < III). We use ShakeMap MMI products as the ground truth for computing these alert quality metrics, and discuss how the alert quality changes when considering the Southern California Seismic Network ShakeMap produced using only instrumental data or the National Earthquake Information Center ShakeMap produced using both instrumental data and Did You Feel It? survey responses.

Key Words
earthquake early warning

Citation
Saunders, J. K., Aagaard, B. T., Baltay, A. S., & Minson, S. E. (2019, 08). Implications of uncertainty in ground motion estimates on ShakeAlert alerting strategies: Considering the 2019 Ridgecrest earthquake sequence. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes