The Effect of Earthquake Depth on Ground Motion Predictions for Earthquake Early Warning Alerting

Grace A. Parker, & Annemarie S. Baltay

Submitted August 16, 2021, SCEC Contribution #11521, 2021 SCEC Annual Meeting Poster #043

We investigate the influence of earthquake source depth on subduction zone ground motion model (GMM) predictions in the context of the U.S. Geological Survey (USGS) ShakeAlert Earthquake Early Warning System. ShakeAlert forecasts alert regions using a combination of earthquake source information and GMMs. Currently, the system assumes a constant source depth of 8 km, which is used to constrain other source parameters and in the estimation of ground motion. This value works well in California, where the majority of earthquakes occur in the shallow crust. However, the resulting alert quality is relatively untested for subduction zone slab earthquakes in Cascadia. We consider the contour product alert radius sensitivity to varying hypocentral depths from 20-200 km for a given magnitude event, using the Parker et al. (2021; Pea21) NGA-Subduction GMM and the Worden et al. (2012) ground motion to intensity conversion equation. We only consider the sensitivity of the output alert radii, not how algorithms might interpret tradeoffs between magnitude and source depth. We find that depth can have a strong influence on the resulting alert radii, because the increase in short-period ground motion with increasing source depth is larger than the decay due to the added distance to the rupture. For example, for an M7.5 earthquake, the MMI V alert radius can vary from 110 km for 20km depth to 210 km for 70km depth. Additionally, the same alert radius can be generated by multiple scenarios (e.g., an M7.5 earthquake at 50 km depth and an M8.5 at 25 km depth have the same MMI V alert radius of 150 km). For the 2001 M6.8 Nisqually and the 2018 M7.1 Anchorage earthquakes, using the true source depth in the Pea21 GMM, rather than 8 km in Pea21 or the operational ShakeAlert crustal GMMs, dramatically improves the ShakeAlert MMI V contour product prediction relative to the USGS ShakeMap intensity contour. These results provide motivation for ShakeAlert algorithms to focus on estimates of the true source depth in subduction zone settings. In the meantime, we propose spatially-varying depth probability density functions (pdfs) for input into ShakeAlert GMMs. These pdfs incorporate the USGS National Seismic Hazard Map source model, and follow ShakeMap convention for using location-based pdfs to determine event type and GMM combinations in near-real-time.

Key Words
Earthquake Early Warning, Source Depth, Subduction Zone, Ground Motion Prediction

Parker, G. A., & Baltay, A. S. (2021, 08). The Effect of Earthquake Depth on Ground Motion Predictions for Earthquake Early Warning Alerting . Poster Presentation at 2021 SCEC Annual Meeting.

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