An Automated Method for Developing a Catalog of Small Earthquakes Using Data of a Dense Seismic Array and Nearby Stations

Abstract

We propose a new automated procedure for using continuous seismic waveforms recorded by a dense array and its nearby regional stations for P‐wave arrival identification, location, and magnitude estimation of small earthquakes. The method is illustrated with a one‐day waveform dataset recorded by a dense array with 99 sensors near Anza, California, and 24 surrounding regional stations within 50 km of the dense array. We search a wide range of epicentral locations and apparent horizontal slowness values (⁠0–15  s/km⁠) in the 15–25 Hz range and time shift the dense array waveforms accordingly. For each location–slowness combination, the average neighboring station waveform similarity (avgCC) of station pairs <150  m apart is calculated for each nonoverlapping 0.5 s time window. Applying the local maximum detection algorithm gives 966 detections. Each detection has a best‐fitting location–slowness combination with the largest avgCC. Of 331 detections with slowness <0.4  s/km⁠, 324 (about six times the catalog events and 98% accuracy) are found to be earthquake P‐wave arrivals. By associating the dense array P‐wave arrivals and the P‐ and S‐wave arrivals from the surrounding stations using a 1D velocity model, 197 detections (⁠∼4 times of the catalog events) have well‐estimated locations and magnitudes. Combining the small spacing of the array and the large aperture of the regional stations, the method achieves automated earthquake detection and location with high sensitivity in time and high resolution in space. Because no preknowledge of seismic‐waveform features or local velocity model is required for the dense array, this automated algorithm can be robustly implemented in other locations.