Shallow Seismicity in the Long Beach–Seal Beach, California Area

Abstract

Seismicity can help to locate fault zones that are often difficult to characterize in densely populated urban areas. In this study, we use three dense nodal arrays consisting of thousands of sensors to detect and locate seismic events in the Long Beach–Seal Beach area of California. Small events can be detected at sufficient signal-to-noise levels during the night, when urban noise is relatively low. We detect and locate >1000 events with ML below 2. Most of the located events are clustered at very shallow depth (0–2 km). The results support previous suggestions that the shallow Newport-Inglewood fault is a wide splayed fault in this area. The seismicity pattern also compares well with some newly identified faults from reflection seismic surveys. The shallow events, which elude detection by the regional seismic network, underscore the complex nature of the faults and their seismic hazard.