Tomographic Image of a Seismically Active Volcano: Mammoth Mountain, California

Abstract:

High-resolution tomographic P wave, S wave, and VP /VS velocity structure models are derived for Mammoth Mountain, California using phase data from the Northern California Seismic Network and a temporary deployment of broadband seismometers. An anomalous volume (∼50 km³) of low P and low S wave velocities is imaged beneath Mammoth Mountain, extending from near the surface to a depth of ∼2 km below sea level. We infer that the reduction in seismic wave velocities is primarily due to the presence of CO₂ distributed in oblate-spheroid pores with mean aspect ratio α ∼8 x 10⁻⁴ (crack-like pores) and gas volume fraction φ ∼4 x 10⁻⁴. The pore density parameter κ = 3φ / (4πα) = na³ = 0.12, where n is the number of pores per cubic meter and a is the mean pore equatorial radius. The total mass of CO₂ is estimated to range up to ∼1.6 x 10¹⁰ kg if the pores exclusively contain CO2, although he presence of an aqueous phase may lower this estimate by up to one order of magnitude. The local geological structure indicates that the CO₂ contained in the pores is delivered to the surface through fractures controlled by faults and remnant foliation of the bedrock beneath Mammoth Mountain. The total volume of CO₂ contained in the reservoir suggests that given an emission rate of 5 x 10⁵ kg day−1, the reservoir could supply the emission of CO₂ for ∼8 to ∼90 years before depletion. Continued supply of CO₂ from an underlying magmatic system would significantly prolong the existence of the reservoir.