Ground Magnetic Data from within the Long
Valley Caldera, California: A Website
for Data Distribution
The past two decades have been a period of unrest for
the Long Valley caldera of eastern California (Figure
1). 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 – 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 (Figure 2). 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..
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