Understanding properties of active faults in Seal Beach by seismicity analysis

Abstract:

Strike-slip faults are capable of producing earthquakes on both their main fault and secondary or potentially unknown faults. The location of earthquakes, in turn, can help resolve the structure of the faults. Cross-sections produced by oil-industry data in Seal Beach show a complicated flower structure associated with the Newport-Inglewood Fault (NIF). The spatial and depth distribution of earthquakes along the NIF can testify the flower structure and find which strands are active. Current SCSN catalog using regional stations show earthquakes in Seal Beach are clustered from 6 to 11 km depth. However, there are only about 200 events during the past ten of years in the Southern California Seismic Network (SCSN) catalog. By taking advantage of the ultra-dense nodal array deployed in Seal Beach, we can detect many more small-magnitude local earthquakes to better understand the complexities of the branching fault structures.

In this study, we use three-week continuous seismic data from a 2‐D array of ~5200 industrial nodal seismometers with ~100 m spacing in Seal Beach, California, for earthquake location. To minimize the anthropogenic events, we only use data from local time 6 pm-5 am. During this period, the SCSN catalogue has no events in our study region but we detect and locate thousands of events. The events are clustered along the NIF, the Los Alamitos Fault, and the Seal Beach coastline. The spatial distribution of the events along NIF, although having the same trend as the surface trace of the fault, is offset ~4 km to the east and ~3 km to the west. Most of the located events are shallower than 1 km. The NIF has been thought to be aseismic in the basin due to the lack of shallow seismicity (< 5 km). It will be necessary to reevaluate this if the shallow events are seismic.