Long Valley Volcanic Region Moment Tensors
Summary
Most tectonic events can be characterized by a double-couple (DC) model
of faulting which describes shear along a linear fault plane. Earthquakes
with coseismic volume changes indicate that tectonic forces were not the
only factor contributing to failure. In geothermal or volcanic areas, such
events are thought to be influenced by fluid migration, in either liquid or
gas form, or a change in the state of matter of a fluid (Ross et al., 1999
; Dreger et. al., 2000). In November 1997, at the peak of an episode of
unrest in the Long Valley caldera, four M4.X earthquakes with significant
volumetric components occured in the south moat of the caldera (Dreger et.
al., 2000).
To ascertain the extent of fluid influenced faulting triggered by the hydrothermal
or magmatic system in the Long Valley volcanic region we modeled the waveforms
of 33 high quality events recorded in the NCSN catalog, which were greater
than M3.5, using BDSN data and four different source models: DC, deviatoric,
DC+isotropic, and the full moment tensor (FMT). For the DC and DC+isotropic
models, a grid search method iterating over strike, dip, rake, DC moment
and isotropic moment, which is equal to zero in the pure DC case, was used
to find the solution which best fit the observed three-component waveforms.
For the deviatoric and full moment tensor models, the second rank symmetric
seismic moment tensor was solved by linearly inverting complete three-component
broadband seismograms in the time domain using a weighted least squares approach.
We focused on a 100 km wide circular area centered at Long Valley caldera
which also encompassed the Mono-Inyo Craters to the north and the seismically
active Sierra Nevada mountain block to the south. Green's functions were
determined using the SoCal velocity model which is appropriate for the
eastern California and Sierra Nevada regions. Both the data and the Green's
functions were bandpass filtered between 0.02 to 0.05 Hz using a causal
Butterworth filter. The F-test was used to determine if the FMT solution
improvement of fit to the data was significant at or above the 95% confidence
level. We also performed several analyses to determine the stability
of the solution, the depth sensitivity of the isotropic components, and the
possibility of obtaining a spurious isotropic component.
Of the 33 events, 28 had statistically insignificant non-DC components.
The remaining five events had statistically significant positive volumetric
components. The majority of these fluid influenced faulting events
occured during the 1997 crisis in the Long Valley caldera. The remaining
event occurred in the Sierra Nevada block during the Red Slate Mountain
earthquake swarm. We were not able to analyze the source process of
earthquakes in or near the vicinity of the Mono-Inyo volcanic chain
because events greater than M3.5 were not recorded during the time interval
investigated by this study.
References
Dreger, D. S., H. Tkalcic, and M. Johnston, Dilational Processes Accompanying
Earthquakes in the Long Valley Caldera, Science, 288, 122-125,
2000.
Ross, A., G. R. Foulger, and B.R. Julian, Source processes of industrially-induced
earthquakes at The Geysers geothermal area, California, Geophysics,
64, 1,877 - 1,889, 1999.
Modeling Magmatic and Hydrothermal
Volcanic Sources
Helpful discussions with Michael Manga
Summary
I modeled the two-dimensional propagation of a magma filled crack
using the method of Spence and Turcotte (1985) who assumed laminar flow
and a constant rate of fluid injection. I chose to investigate a range of
1) magma viscosities from felsic to mafic, 2) medium elastic properties
from batholith to lava flow, and 3) magma supply rates. This experiment
showed that even the least viscous magma does not flow fast enough to satisfy
the seismological constraints imposed by the 1997 fluid influenced faulting
events.
References
Spence, D. and D. Turcotte, Magma-Driven Propagation of Cracks, J.
Geophys. Res., 90, 575-580, 1985
Abstracts
Templeton, D. C. and D. S. Dreger, Fluid Influenced Faulting in the Long
Valley Volcanic Region, AGU Fall Meeting 2003.
Templeton, D. C. and D. S. Dreger, Source Processes of Long
Valley Caldera Seismicity Determined by Moment Tensor Inversion, Seism.
Res. Lett., 73, 253, 2002.
Active Tectonics Research
Group at UC Berkeley
UC
Berkeley Seismological Laboratory
Department of Earth and Planetary
Sciences
University of California, Berkeley