HR: 13:40h
AN: G43C-01 INVITED     [Abstracts]
TI: Moving Mountains and Deep Crustal Earthquakes: Evidence for Deep Magma Injection Beneath Lake Tahoe, Nevada-California
AU: * Blewitt, G
EM: gblewitt@unr.edu
AF: Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557 United States
AU: * Blewitt, G
EM: gblewitt@unr.edu
AF: Nevada Seismological Laboratory, University of Nevada, Reno, NV 89557 United States
AU: Smith, K D
EM: ken@quake.seismo.unr.edu
AF: Nevada Seismological Laboratory, University of Nevada, Reno, NV 89557 United States
AU: von Seggern, D
EM: vonseg@quake.seismo.unr.edu
AF: Nevada Seismological Laboratory, University of Nevada, Reno, NV 89557 United States
AB: We recently reported [Smith et al., Science 305, 2004] an unusually deep swarm of 1611 earthquakes that occurred in late 2003 at Lake Tahoe, California (depth 29-33 kilometer; Richter Magnitude [ML] $< $2.2; sum of the moment magnitude of all events is Mw 3.1). This swarm was coeval with a GPS transient displacement of 6 $\pm$ 0.3 mm horizontally outward from the swarm and 7.9 $\pm$ 1.0 mm upward measured at the GPS station on Slide Mountain, Nevada (SLID) 18 km to the northeast. Station SLID is a part of the 53-station Basin and Range Geodetic Network (BARGEN) network, continuously operating since 1996 and one of the PBO Nucleus stations. Here we focus on the results of the geodetic analysis for SLID, and other BARGEN stations within 200 km of SLID, starting on 1 January 2000. The SLID transient displacement is 9.8 mm in a direction normal to the planar structure defined by the deep earthquake swarm, spanning the same time period of the swarm. The geodetic displacement here 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. During the first 23 days of the swarm, hypocentral depths migrated at a rate of 2.4 millimeters/second up-dip along a 40-km$^2$ structure striking N30$\deg$W and dipping 50$\deg$ to the northeast. Assuming a stress drop of 10 MPa (reasonable for upper crustal earthquakes), this event has an equivalent seismic moment to a magnitude (Mw) 6.0 earthquake with a displacement of ~1 meter at the source. Applying Okada's model for a tensile crack at 28 km depth in the source region, a potency equivalent to a volume of 3.7 $\times$ 10$^7$ m$^3$, or volumetric moment equivalent of Mw 6.4, fits the SLID observations. As there is no established evidence of recent volcanism ($<$1 Ma) in the Tahoe region, this discovery suggests the hypothesis that such deep magmatic events in extensional regimes may be of a "normal" tectonic process of lower crustal evolution, perhaps providing a mechanism to sustain crustal thickness and crustal strength in zones of extension. If indeed this represents one episode of a longer-duration steady-state process, similar geodetic transients might be commonplace in the Basin and Range Province, though the single event reported here does little to constrain the characteristic time scale. EarthScope is well-posed to provide further evidence. The geodetic and seismological data until June 2004 indicated that this event stopped around January 2004. Here we present an updated analysis using a GPS and microseismicity data through November 2004.
DE: 8434 Magma migration
DE: 8494 Instruments and techniques
DE: 7280 Volcano seismology (8419)
DE: 1208 Crustal movements--intraplate (8110)
DE: 1236 Rheology of the lithosphere and mantle (8160)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting