Seismic investigation of magmatic unrest beneath Mammoth Mountain, California, USA
Guoqing Lin
Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA
Guoqing Lin; Seismic investigation of magmatic unrest beneath Mammoth Mountain, California, USA. Geology 2013;; 41 (8): 847–850. doi: https://doi.org/10.1130/G34062.1
I present a reinvestigation of the seismic activity during an 11-mo-long seismic swarm between 1989 and 1990 beneath Mammoth Mountain in eastern California (United States). This swarm, believed to be the result of a shallow intrusion of magma beneath Mammoth Mountain, was followed by the emission of magmatic CO2 gas, resulting in tree die-off in 1990 and posing a significant human health risk around Mammoth Mountain. In this study, I apply a state-of-the-art approach to estimate the ratio of compressional- to shear-wave velocity (Vp/Vs) within similar event clusters using differential times from waveform cross-correlation. A high-resolution three-dimensional seismic velocity model and a precise earthquake location catalog are obtained to complement the analysis of the in situ Vp/Vs ratios. The majority of the relocated seismicity below sea level is distributed in an anomalous body with low Vp and high Vp/Vs, consistent with the seismic expression of the magma body that triggered the seismic swarm. Abrupt changes in Vp/Vs ratios, with lower values for the shallow depths, are observed near sea level, indicating the source of the 1989 CO2 flux is a shallow CO2 reservoir instead of the hypothesized basaltic magma at mid-crustal depths of 10-25 km. From sea level to the surface, low Vp/Vs ratios in a near-vertical zone suggest involvement of fluid in the upward-migration of the seismicity.
body waves
elastic waves
magmatism
P-waves
S-waves
seismicity
velocity structure
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