Seismicity has continued a systematic increase along the S margin of the resurgent dome. Swarms occurred on 4 and 14 Febuary, the latter containing >100 located events, one reaching M 3. Another swarm on 27-28 February contained several of M >2, and on 3 March more than 200 shocks were recorded, the largest at M 2.8. In the two weeks following the 3 March swarm, seismic activity remained relatively quiet, with a few days having as many as ten recorded events. Depths remained between 8 and <3 km. The 2-color geodimeter system has continued to detect enhanced strain rates of ~5 microstrain/year since September. On the SW rim of the caldera, Mammoth Mountain was generally quiet, although a series of events occurred there on 6 March, the largest reaching M 1.7.

Geological Summary. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

Information Contacts: D. Hill, USGS Menlo Park.