How low should we alert? Exploring earthquake early warning thresholds for a range of target intensities
- Date & Time
- Online-only seminar via Microsoft Teams
We investigate how the choice of alerting threshold for a given shaking intensity target affects the quality of ShakeAlert earthquake early warning (EEW) alerts using a suite of earthquakes in the west coast of the United States. The ShakeAlert system takes a source-based approach to EEW, where the alert region for a specified modified Mercalli intensity (MMI) is computed from ground-motion models using magnitude and location estimates as the main inputs. Currently, median-expected ground motions are used in the alert region calculation, which is unable to reliably alert all of the regions that experience shaking at the MMI level used to generate the alert. To ameliorate this, EEW systems can expand the size of the alerting region for a target MMI level (alert target) by choosing a lower MMI for the alert region computation. However, this comes with the tradeoff of increasing the number of inaccurate alerts issued to people who experience shaking below the alert target. We aim to find optimal MMI thresholds for alert targets ranging between MMI 4.0-6.0 using a ShakeMap catalog of 140+ M5.0-7.3 earthquakes in California, Oregon, and Washington. We compute the quality of each alerting strategy relative to the alert target in terms of the area and population alerted. Additionally, we evaluate the alert quality of the current ShakeAlert alerting thresholds for this range of MMI alert targets. We further discuss how this MMI threshold alerting strategy is equivalent to incorporating ground-motion-model uncertainty into the alert region computation, and we compare the optimal MMI thresholds found by this analysis with the combined standard deviation of the ground-motion models used by ShakeAlert.