Ground Magnetic Data from within the Long Valley Caldera, California

The unrest began in 1978 and continued through late 1999 and included recurring swarms of moderate earthquakes, as well as uplifting of the Resurgent Dome, which has totaled approximately 80 cm. It is believed that the seismicity is accompanied by magmatic intrusion beneath both the Resurgent Dome at a depth of about 7 km; 10 km and the South Moat Seismic Zone (SMSZ) at a depth of about 15 km (Sorey and others, 2003).

Seismic surveys within the caldera's topographic boundary have indicated the seismicity beneath the northwest section of the caldera is associated with fluid injection into narrow conduits and fractures (Stroujkova and Malin, 2000). Like the dominant regional structural trend, these conduits run in a northwest-southeast direction and are only expressed at the surface by a slight topographic relief of about 3 m. Merged aeromagnetic data (Roberts and Jachens, 1999) over the caldera show a magnetic low in the west and a high in the east (Figure 3).