Using Dense Seismic Arrays to Determine Structure in the Los Angeles Basin

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With this study we plan to combine seismic data from oil-company surveys with the earthquake seismic data from the Southern California Seismic Network to determine the velocity in the top 1-3 km of the surface in the Los Angeles Basin. This information is important because it directly bears on the level of shaking that can be expected from earthquakes due to the variations soils and subsurface structures. It will be used along with actual measurements of ground shaking to determine maps of seismic hazard for the central part of the Los Angeles Basin. In addition, we will use the dense coverage provided by the oil-company surveys to determine the micro-earthquake activity in the basin. This is important for understanding the physical process that generates the earthquakes in Los Angeles.The subsurface velocity structure will be determined by surface waves and refracted body waves. These waves will be extracted from the ambient noise field by cross-correlation of the oil-company and earthquake-monitoring data. The source that generates these waves is ocean microseisms, and hence is always present. The correlations will be done over many days and will use small-aperture sub-arrays to minimize the background noise. The result will be a map of the near-surface shear and compressional wave velocities for the central basin, where the ray coverage is the densest. The near-surface velocities will then be added to the reference model of the Los Angeles region (SCEC velocity model) to make a complete velocity model that can be used to simulated earthquake-generated wave in the basin.