Hello San Andreas, Goodbye Cascadia

The western margin of the United States is comprised of a two-part plate boundary between the San Andreas transform fault and the Cascadia Subduction Zone (CSZ). The transition between these two plate boundaries is at the rugged Mendocino Triple Junction (MTJ). At this location the juvenile northern San Andreas Fault is being created in the wake of the northward trajectory of the MTJ while the CSZ is becoming consumed from the south. This dynamic interplay of migrating plate boundaries gives rise to multiple phenomena including anomalous crustal thickening and thinning, dynamic volcanism, fault creation and reorganization, altered configurations of local river watersheds, and the magnificent topography of northern California.

Leland O'Driscoll is studying the crustal and mantle structure beneath northern California (see map) using modern tomographic imaging techniques learned within Gene Humphreys' research group. During the summer of 2007, Leland deployed an 80-station broadband seismometer array, an effort requiring site permitting, instrument installation, and data collection. This array will be used to investigate variations amongst four major western U.S. geologic provinces: the southern Klamath Mountains, the Franciscan subduction complex, the southern Cascades volcanic arc, and the northern Sierra Nevada Batholith/Sierra Foothills. In addition to finite frequency tomography, other seismic imaging techniques will be applied to data collected from this array, including direct imaging receiver functions by Alan Levander’s research group at Rice University and further tomographic imaging from Richard Allen’s research group at UC Berkeley.