Improving Back-Projection Imaging with a Novel Physics-Based Aftershock Calibration Approach

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Large earthquakes involve complex patterns of slip along fault surfaces that themselves are complex. These complexities have significant impacts on predicting expected ground motions from major earthquakes, and thus on the estimated hazard from such events. Understanding these complexities is key, but progress towards this goal has been slowed by the limited resolution of current imaging methods. This project is improving the resolution and accuracy of such images through a novel method that uses the relative motions recorded by dense clusters of seismometers to resolve the rupture process of large earthquakes. The technique is analogous to procedures used to locate and track moving sources with antennas. The enhanced resolution of the proposed method allows testing of improved physics-based earthquake rupture models, and is improving our capacity to model earthquake hazards. The project takes full advantage of the increasing availability of global dense seismic networks including the Earthscope USArray, and supports the PhD work of a female graduate student. The research results are being shared via conferences and journals, as well as via outreach to public schools and in undergraduate classes. Seismic array processing, also known as back-projection (BP) imaging, is an emerging technique that utilizes high-frequency (HF) seismic waveforms to provide detailed information about source processes of earthquakes. Even though the technique offers unique, high-resolution observations, it has been shown to have shortcomings. While BP images from each array located on various continents are capable of revealing fine details of the earthquake rupture processes, the exact locations of HF sources do not align, leading to discrepancies regarding such properties as rupture length and speed. The team has developed a novel aftershock calibration to mitigate the source location uncertainties of the arrays and tested it on the 2015 M7.8 Nepal-Gorkha earthquake. The team is now applying their method to data from the 2004 M9.2 Sumatra, 2010 M8.8 Chile, and 2011 M9.1 Japan earthquakes, and developing BP images that are mutually consistent across multiple arrays and reveal additional fine detail about the earthquake processes. This work is narrowing the gap between seismic observations and earthquake simulations, and providing key information to improve predictions of ground motions from large earthquakes and thus reduce seismic risk.