Applying True Source Depth to ShakeAlert Algorithms for Intraslab Earthquakes

Abstract:

ShakeAlert, the network-based earthquake early warning system in development for the West Coast of the United States, currently uses a fixed focal depth of 8 km to estimate earthquake location and magnitude. This is an appropriate way to reduce computational costs without compromising alert accuracy in California, where earthquakes typically occur on shallow crustal faults. In the Pacific Northwest, however, the most common moderate-magnitude events occur within the subducting Juan de Fuca slab at depths between ~35 - 65 km. We investigate whether assuming a fixed shallow depth is sufficient to produce accurate alerts for intraslab earthquakes or if applying true earthquake depth to ShakeAlert algorithms improves alert accuracy and warning time. Using a dataset of seismic recordings from 36 M4.5+ intraslab earthquakes from Chile and Cascadia, we replay events through the EPIC and eqInfo2GM algorithms to estimate source parameters and predict ground shaking intensity. Each event is replayed twice, once using a fixed 8 km depth and a second time using the true earthquake depth. For each depth scenario, we analyze MMI alert threshold contours using various performance metrics to determine the number of correctly alerted sites and measure warning times. Using a variance reduction calculation, we also measure the accuracy of ground motion intensity maps for each depth scenario by comparing them to actual ShakeMaps. By comparing the results of the fixed and true depth scenarios, we determine if applying true depth improves the accuracy of alert regions and provides longer warning times during intraslab earthquakes, i.e. whether this is a valuable addition to ShakeAlert in subduction zone settings.