The project successfully installed a dense line of seismometers located every 4 miles between the coast of central California and the western Sierra Nevada mountains near Three Rivers. For about two years the seismometers recorded nearby and distant earthquakes as well as the background noise of the Earth in this region. The data collected were used to study the three dimensional (3D) structure beneath central California from the surface to about 150 miles depth by measuring how fast seismic waves propagate through this volume of rock. The data will be publicly available for future studies of the region.

For this project the 3D seismic images of the subsurface were used to determine how the present day plate boundary, which is the San Andreas fault, and the high elevations of the southern Sierra Nevada (such as Mt. Whitney) are related to the history of the plate boundary when it was a subduction zone where an oceanic plate sunk beneath the continent and fueled a chain of volcanic activity in what is now the Sierra Nevada. The main target of the study was a large structure, called the Isabella Anomaly, which propagates seismic waves at unusually high speeds beneath the Great Valley. The goal of this project was to determine whether the Isabella Anomaly was connected to the tectonic plate east or west of the Great Valley. An eastward origin would support the idea that the anomaly resulted from sinking of a dense root to the Sierra Nevada mountains, while a westward origin would support the idea that the anomaly is a fragment of an oceanic plate stuck beneath coastal California and being dragged northward beneath the San Andreas fault. We found that the Isabella Anomaly is continuous with structure west of the Great Valley supporting the hypothesis that it is a fossil slab from a former subduction zone. Distinguishing between these two hypotheses is important for understanding the deep conditions that affect motion on the San Andreas fault, which behaves differently above the Isabella Anomaly by sliding smoothly all the time rather than primarily sliding in earthquakes. Our results suggest that this change in dynamics of the San Andreas may related to presence of the fossil slab, which could release fluids that reduce friction in the overlying fault. Support for a westward origin of the Isabella Anomaly also indicates that the former dense root to the southern Sierra Nevada was smaller than previously inferred, which is a valuable insight for future studies of the formation of continental crust beneath volcanic chains.

The research activities above contributed to the training student scientists in modern geophysical data collection, seismic analysis methods, and scientific communication. The project supported undergraduate and graduate researchers. The primary student supported by the grant graduated with a master’s degree and is employed in geophysics research at a national laboratory. The project also provided research opportunities to broaden the training of 2 undergraduates, 2 other graduate students, and 2 postdoctoral scholars at the University of New Mexico. 

Last Modified: 05/01/2017
Modified by: Brandon Schmandt