Ground Magnetic Data from within the Long Valley Caldera, California

The past two decades have been a period of unrest for the Long Valley caldera
of eastern 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). The
western part has been modeled to relate to altered, low-magnetization (about
2.5 km thick) Bishop Tuff beneath the Resurgent Dome, indicating hydrothermal
alteration in the west, whereas the high in the east represents the unaltered
Bishop Tuff (Williams and others, 1977). The ground magnetic survey was
conducted to locate magnetic lows that might indicate altered zones reflecting
conduits for hydrothermal fluid flow in the northwest portion of the caldera.

[Summary provided by the USGS.]