Los Angeles Region Seismic Experience (LARSE)

The LARSE project uses powerful computers to analyze transmitted and reflected underground sound waves to produce images of the Earth's crust, similar to the way in which medical CAT-scan images (transmitted X-rays) and ultrasound images (reflected sound waves) are created. Transmitted sound waves pass through the area being imaged, revealing the geologic structure by the ways they are bent or slowed. Reflected sound waves bounce off faults and rock layers, showing the shapes and depths of those features. Data collected along Line 1 of LARSE were used to create this composite image of the geologic structure beneath the Los Angeles region. Color bands in the upper part of the image, derived from computer analysis of transmitted sound waves, show the speed at which waves pass through different rock types—slower speeds (cooler colors) are in less dense rock. The deeper rectangular part of the image shows results from reflected sound waves—strongly reflective areas are in bright red. The intensely reflective zone deep beneath the San Gabriel Mountains is interpreted by LARSE scientists as a "master" blind thrust fault—a thrust fault that transfers stress and strain from near the San Andreas Fault to the network of faults in the San Gabriel Valley and Los Angeles Basin.