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Published Online August 5, 2004
Science DOI: 10.1126/science.1101304

Reports

Submitted on June 9, 2004
Accepted on July 28, 2004

Evidence for Deep Magma Injection Beneath Lake Tahoe, Nevada-California

Kenneth D. Smith 1*, David von Seggern 1, Geoffrey Blewitt 2, Leiph Preston 1, John G. Anderson 1, Brian P. Wernicke 3, James L. Davis 4

1 Nevada Seismological Laboratory, University of Nevada Reno, Reno, NV 89557, USA.
2 Nevada Seismological Laboratory, University of Nevada Reno, Reno, NV 89557, USA; Nevada Bureau of Mines & Geology, University of Nevada, Reno, NV 89557.
3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 92215.
4 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA.

* To whom correspondence should be addressed.
Kenneth D. Smith , E-mail: ken{at}seismo.unr.edu

A deep earthquake swarm in late 2003 at Lake Tahoe, California (Richter Magnitude [ML] <2.2; depth 29-33 kilometer), was coeval with a transient displacement of 6 millimeters horizontally outward from the swarm and 8 millimeters upward measured at GPS station Slide Mountain (SLID) 18 km to the northeast. During the first 23 days of the swarm, hypocentral depths migrated at a rate of 2.4 millimeters/second up-dip along a 45 kilometer2 structure striking N30°W and dipping 50°E. SLID's 20 millimeter/year transient velocity implies a lower bound of 200 nanostrain/year (parts per billion per year) on local strain rates, an order of magnitude greater than the 1996-2003 regional rate. The geodetic displacement is too large to be explained by the elastic strain from the cumulative seismic moment of the sequence suggesting an aseismic forcing mechanism. Aspects of the swarm and SLID displacements are consistent with lower-crustal magma injection under Lake Tahoe.


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Science. ISSN 0036-8075 (print), 1095-9203 (online)