Long Period Seismicity at Mammoth Mountain, California

Abstract

AbstractLong period (LP) seismicity is a ubiquitous feature at Mammoth Mountain, California. Phase‐based locations exhibit source depths ranging from ∼8 to 25 km. Between September 2011 and October 2013, over 200 LP events were added to the Northern California Earthquake Data Center catalog. Three typical LP events were used as master events, and continuous waveform correlation was performed over the 2‐year period, with the results indicating that thousands of LP events were present in the continuous seismic data. Using phase‐weighted stacking, representative network stacks for the three events were produced. Full waveform inversion of the representative events using Green functions that adequately represent the topography of Mammoth Mountain and the high‐resolution velocity structure produced through local earthquake tomography suggest that all three event types are located within ∼100 m of the surface in the vicinity of the Chair 12 tree kill and CO2 emission area and within ∼1 km of the Mammoth Mountain Ski Area base lodge. The source mechanisms are purely volumetric and can be attributed to a subhorizontal hydrothermal crack excited into acoustic resonance by the rapid discharge of CO2 gases trapped in the crack. An accompanying vertical force that is opposite in phase to the volume change in the crack can be attributed to pressure and momentum changes in an adjacent fluid‐filled conduit linking the crack to the surface. The implication of this work is that LP seismicity at Mammoth Mountain occurs almost continuously and in direct contradiction to the traditional view—at very shallow depths.