Crustal structure and seismicity distribution adjacent to the Pacific and North America plate boundary in southern California


  • Egill Hauksson


New three-dimensional (3-D) VP and VP/VS models are determined for southern California using P and S-P travel times from local earthquakes and controlled sources. These models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the Pacific-North America plate boundary. The models extend from the U.S.-Mexico border in the south to the southernmost Coast Ranges and Sierra Nevada in the north and have a 15-km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 22 km depth. The heterogeneity of the crustal structure as imaged by VP and VP/VS models is larger within the Pacific plate than the North American plate. Similarly, the relocated seismicity deepens and shows more complex 3-D distribution in areas of the Pacific plate exhibiting compressional tectonics. The models reflect mapped changes in the lithology across major geological terranes such as the Mojave Desert, the Peninsular Ranges, and the Transverse Ranges. The interface between the shallow Mono of the Continental Borderland and the deep Moho of onshore California forms a broad zone to the north beneath the western Transverse Ranges, Ventura basin, and the Los Angeles basin and a narrow zone to the south, along the Peninsular Ranges. The near-surface increase in velocity, from the surface to up to 8 km depth, is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins. At midcrustal depths a rapid increase in VP is imaged beneath the sediments of the large sedimentary basins, while beneath the adjacent mountain ranges the increase is small or absent.