Including Three-dimensional Finite Fault and Distributed Slip Models in Ground Motion Forecasting for Earthquake Early Warning

Abstract:

Many Earthquake Early Warning (EEW) systems aim to warn users of impending shaking before it reaches them by using real-time source models derived from seismic and/or geodetic data as input to ground motion prediction equations. The ShakeAlert EEW system currently includes one algorithm that characterizes the earthquake as a point source and another that uses a line source representation, providing location, strike, and rupture extent. Source-to-site distance is a fundamental input for ground motion prediction (GMP), and including finite fault effects via a line source enables more accurate GMP than using a point source when rupture dimensions are comparable to or exceed source-site distances (Böse et al., 2017). GMP for EEW might be further improved by instead using distance measures derived from 3D source geometries and distributed slip models (DSMs). Here we systematically evaluate which finite fault information is most impactful when included in the GMP calculation.

We compare the true ground motion (calculated using the full 3D source information) to that obtained using simplified point and line source representations for a suite of synthetic source models with different magnitudes, geometries, and faulting styles. We then assess which parameters for the line and 3D representations of the true source model introduce the most error in GMP if incorrectly estimated by systematically varying these source parameters and comparing the calculated ground motion from the incorrect model to that from the true model. Finally, we assess the improvement DSMs can provide by comparing the predicted ground motion obtained assuming uniform slip to that obtained with a rupture distance derived from the slip distribution using the method of Thompson and Baltay (2018). These assessments will help delineate which finite fault information significantly improves GMP and whether this depends on the type of earthquake (i.e., magnitude and faulting style) that occurs.