Article,
A magma mass beneath Casa Diablo? Further evidence from reflected seismic waves
Bulletin of the Seismological Society of America, 90 (2): 500--511 (Apr 1, 2000)
DOI: 10.1785/0119990071
Abstract
A forty-station seismographic network operating in Long Valley caldera
in California during the summer of 1997 recorded a number of earthquakes
with unusual secondary phases. The phases can be seen on several
stations throughout the network, allowing the use of particle motions
and differential travel times to examine their nature and points
of origin. Based on amplitude, frequency, and travel-time considerations,
it appears that some of the phases are mode-converted S-to-P reflections
from a zone below the Casa Diablo area near Mammoth Lakes, CA. The
locus of reflection points was imaged using time-domain signal migration
and stacking under the assumptions of first-order scattering theory.
The results indicate that the reflecting zone is a curved, 1.5 km
by 2 km surface 7.6 km below and 1.4 km west of the Casa Diablo geothermal
plant. Previous studies show low P-wave velocities and high S-wave
attenuations in the same location as the reflecting zone. An amplitude
analysis of the suspected reflections suggests they are produced
by large contrasts in material properties, with the possible range
of transitions including solid to partially molten or dry to fluid-saturated
rock. Given these observations plus the volcanic history of the caldera
and its recent tectonic activity, the most straightforward interpretation
of the reflections is that they come from either the top of a sill
or intrusion, or the overlying hydrothermal system of such structures.
10.1785/0119990071
