Imaging the Isabella Anomaly in Southern California: Surface Wave Tomography, Receiver Function Analysis, and Basin Analysis

Abstract:

Past seismic imaging studies of the Isabella anomaly in Southern California lacked resolution to distinguish between the two dominating hypotheses for its origin: a fossil slab origin or a foundering lithospheric root of the Sierra Nevada volcanic arc. To definitively distinguish the origin of the anomaly as one, or neither, of these possibilities is important in the understanding of the evolution of continental arcs and the process of subduction termination. To do this, we deployed an array of 44 broadband seismometers from the Isabella anomaly to the coast at latitude 36°N, starting in December 2013. The array has a station spacing of ~7 km spacing filling a gap between the southern and northern California permanent seismic networks (NCSN and SCSN). The array will continue to collect data until summer 2015. We will present preliminary surface wave tomography results using empirical interstation green's functions derived from ambient seismic noise recorded by more than 500 stations that currently or previously surround our array. Initial Ps receiver function images from common conversion point stacking will also be presented. We also use local earthquakes to model the shape and velocity of the Central Valley, as this will be important in correcting the tomography and receiver function images.We also deployed a dense array of short period stations in Peachtree Valley, where the broadband array crossed the San Andreas Fault (SAF). This network will be used to determine the location and level of seismicity on a segment of the SAF where it is creeping.This first batch of results from our broadband seismic experiment should provide new insight into how far the basaltic crust of the Monterey microplate extends inland beneath California and whether the Isabella anomaly is connected to North America or Pacific lithosphere.