The 1997 Los Angeles Basin Passive Seismic Experiment – a dense, urban seismic array to investigate basin lithospheric structures

Abstract

In 1997, 18 seismic stations were installed in the Los Angeles basin to record teleseismic, regional, and local earthquakes. The goals of the experiment were to quantify amplification of ground motion due to variations in sedimentary environments and subsurface structures, and to examine the tectonic extensional and compressional history of the Los Angeles basin and San Gabriel Mountains by tomograhic imaging. The linear array spanned the entire basin between Seal Beach to the south and Azusa to the north. The stations consisted of three-component, short-period seismometers and timing was controlled by GPS receivers. Most locations were homeowner backyards with continuous A/C power sources and battery backup. The stations operated between March and November, 1997. The long experiment time was needed to record as many large teleseisms as possible at the seismically and culturally noisy Los Angeles basin sites. Data were recorded continuously at 25 sps and triggered at 50 sps. The total array length was 50 km with an average station spacing of 3-4 km. This was a much denser seismic array than any other in the region, and the close spacing of seismometers is providing high-resolution information about the geometry of structures several kilometers below the surface. The high density of stations is making it possible to observe amplitude variations on length scales of a few kilometers and to obtain travel times in seismically and culturally noisy regions by cross-correlation with quiet stations using the highly coherent waveforms. This array location was chosen because it spans the entire Los Angeles basin, and it covers an area known for its high seismic hazard, unique geology and active plate motion. It was also the site of one leg of the 1994 Los Angeles Region Seismic Experiment in which onshore and offshore explosions were recorded along essentially the same line for very detailed structure in the upper 20 km of the Earth's crust.

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