Application of Seismic Array Processing to Earthquake Early Warning

Earthquake early warning (EEW) systems that issue warnings prior to the arrival of strong shaking are essential in mitigating earthquake hazard. Currently operating EEW systems work on point-source assumptions and are of limited effectiveness for large events, for which ignoring finite-source effects result in magnitude underestimation. Here, we explore the concept of characterizing rupture dimensions in real time for EEW using small-aperture seismic arrays located near active faults. Back tracing array waveforms allow estimation of the extent of the rupture front (as a proxy of the rupture size) and directivity in real time, providing complementary EEWcapabilities forM >7 earthquakes to existing EEW systems. We implement it in a simulated realtime environment and analyze the 2004M 6 Parkfield, California, earthquake recordings by the U.S. Geological Survey Parkfield dense Seismograph ARray (UPSAR) array and the 2010M 7.2 El Mayor–Cucapah earthquake recordings by strong-motion sensors in San Diego, California. We find it important to correct for the bias in back azimuth induced by dipping structures beneath the UPSAR array, based on data from smaller events. Our estimated rupture length is 30% shorter than those inferred from other studies but still reasonable for EEW purposes. We attribute this difference to rupture directivity effects and the limited field of view of a single array. The accuracy of the approachmay be improvedwith a network of arrayswith overlapping fields of view.We demonstrate this by tracking the 2011 Tohoku earthquake rupture with two clusters of Hi-net stations in Kyushu and northern Hokkaido. The obtained results are consistent with teleseismic back-projection results and yield reasonable estimates of rupture length and directivity. Compared with other proposed finite-fault EEW approaches, the array method is less affected by the coarseness of a Global Positioning System or seismic network and provides a high-frequency characterization of the rupture that yields more suitable predictors of ground shaking for certain structures.