Abstract

In 2003, a magma intrusion event occurred at 25–30 km in the lower crust under the northwestern corner of Lake Tahoe, as evidenced by both an earthquake swarm and a geodetic displacement. This study examines the seismicity associated with that event and subsequent seismicity in the upper crust. HYPODD relocations showed that the deep swarm of approximately 1600 microearthquakes at the intrusion site was concentrated on a planar area with a strike of N42°W, dipping at 39° to the northeast. The largest microearthquake in this swarm was M 2.2, and an anomalously high b-value of 2.0 is seen in the recurrence-versus-magnitude plot. The swarm progressed over this plane in a somewhat irregular pattern for a period of roughly 5 months. Focal mechanisms of the deep-swarm events are highly variable and do not reflect the known regional stress field. Two months after the deep-swarm activity started, a shallow swarm of approximately 1100 microearthquakes began at 10–12-km depths in the shallow crust almost immediately above the deep swarm and continued through 2005. This swarm had a maximum M of 2.4 and a relatively high b-value of 1.5. Based on HYPODD relocations, hypocenters in this swarm are concentrated in a narrow pipelike volume, and event depths progressed steadily upward over the more than 2 yr of observation. Focal mechanisms in this shallow swarm are more consistent with the regional stress field than those of the deep swarm. Within one focal depth horizontally of the deep swarm, postintrusion seismic activity increased significantly compared to prior years. Stress triggering from the deep magma intrusion, although based on sub-bar stress changes in the shallow crust, is a feasible explanation of the observed increase.

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