Seismic investigation of magmatic unrest beneath Mammoth Mountain, California, USA

Abstract

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 CO 2 gas, resulting in tree die-off in 1990 and posing a signifi cant 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 crosscorrelation. 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 CO 2 fl ux is a shallow CO 2 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 fl uid in the upward-migration of the seismicity.

Fluid‐faulting interactions: Fracture‐mesh and fault‐valve behavior in the February 2014 Mammoth Mountain, California, earthquake swarm

“…This is near the deepest previously observed seismicity in this area (Figure 1), likely just above the brittle-ductile transition [Hill, 1992]. An arc of seismicity on the swarm's southeastern edge appears to form an eastward continuation of the ring structure identified by Prejean et al [2003], which was activated primarily during the swarm in late 1989 [Hill et al, 1990;Lin, 2013]. As in the 2014 swarm, slip in 1989 on this outwardly dipping structure accommodated normal faulting oriented such as to raise the summit of the mountain relative to its surroundings [Prejean et al, 2003], and its activation is consistent with uplift of the central edifice driven by pressure from below.…”

Seismically triggered anomalies in the isotope signatures of mantle‐derived gases detected at degassing sites along two neighboring faults in NW Bohemia, central Europe

“…Martens and White [] suggested that carbon dioxide exsolved at far greater depths than the dyke intrusion may have triggered the seismicity. Detailed investigations into seismic activity at Mammoth Mountains in eastern California found low Vp/Vs ratios within the upwardly migrating seismicity, suggesting the involvement of fluid (CO 2 ) in the seismic swarm [ Lin , ]. Progressively increasing Vp/Vs ratios were recorded before the April 2006 L'Aquila earthquake in Italy.…”

Tomographic image of a seismically active volcano: Mammoth Mountain, California

“…The NCSN uses a V P / V S ratio of 1.719 for routine event location in the Mammoth Mountain region. Previous tomographic studies in the region use a value of V P / V S =1.67 [ Julian et al , ; Foulger et al , ; Lin , ]. Poisson's ratio ν for granite and dense metamorphic rocks varies between 0.1 and 0.3 with a mean value of ∼0.2 [ Gercek , ].…”

“…Julian et al [] derived models of P wave and V P / V S structures using NCSN and temporary station phase data collected in 1989–1990, Foulger et al [] reproduced the models of Julian et al [] and derived P wave, S wave, and V P / V S models from NCSN and temporary station phase data collected in 1997, as well as differences between the two studies. Lin [] derived a P wave velocity model using phase data from 1989 to 1990.…”