A small seismograph network of six stations now monitors earthquakes in and near Lassen Volcanic National Park. The first 14 months of recording has revealed a northwest-trending seismic zone passing through the park. This zone is the resolved equivalent of a diffuse zone of historical epicenters passing through Lassen Park and Truckee, California, and is parallel to nearby lineaments in California, Oregon, and Nevada recognized from surface geology. Three dense concentrations of earthquakes correlate very closely with three geothermal areas. One concentration also outlines the north and east sides of the 4-km-diameter Mt. Tehama caldera. The recent dacite plug domes of Lassen Peak and Chaos Crags are nearly aseismic, however. Several approximate focal mechanism solutions indicate primarily normal faulting with east-west extension. This implies the northwest-trending seismic zone is undergoing extension and right-lateral shear. Extension directions near the center of the network display a radial symmetry that could be caused by a broad updoming or magma injection centered near Lassen Peak or Chaos Crags.

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