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New Techniques Using High-Density Seismic Array Data Analysis to Determine 3-D Fault and Crustal Structures in the Long Beach Basin

Abstract

Determining the depth orientation of the Newport-Inglewood Fault and other crustal

structures beneath large cities can improve the hazard analysis of large magnitude earthquakes.

Details of fault depth structure can also evaluate potential fault and crustal layer interactions

beneath populated areas or regions of interest. We propose a new method to find crustal layer

discontinuities utilizing point cloud methods derived from passive seismic data recorded in Long

Beach, California, from 2011-2012. Using a high-density array of 7,781 geophones within a

98-square-km area allows for excellent resolution of detailed structures at crustal depths. The

methodology provided here may prove to be a cost-effective way for seismologists and

companies to check hypotheses using passive seismic data before fully deploying or processing

data from an active source experiment. We first create a 3D model of the autocorrelated passive

data. Clear planes or surfaces within the 3D seismic data set are then located, and points

determined to not be in the layer are removed. We isolate each structural plane separately and

connect each point in the point cloud to another point forming small triangles. Each of the

triangular surfaces are filled in using Delaunay triangulation. We find that the Moho is dipping

inland at a 25° angle from horizontal with a slight curve in the upper 6 km. Crustal boundaries,

such as the upper crust, lower crust, and several minor layers in between, are also identified. The

three-dimensional structure of the northwest-southeast trending Newport-Inglewood Fault and

Los Alamitos Fault are also identified. This three-dimensional analysis of crustal structure will

aid in hazard analysis in the immediate Long Beach area and can potentially be replicated in

other seismically active areas where there is little knowledge of the underlying crustal structure.