Report on Long Valley (United States) — September 1984
Scientific Event Alert Network Bulletin, vol. 9, no. 8 (September 1984)
Managing Editor: Lindsay McClelland.
Long Valley (United States) Seismicity at relatively low level and inflation
Please cite this report as:
Global Volcanism Program, 1984. Report on Long Valley (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 9:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198409-323822.
37.7°N, 118.87°W; summit elev. 3390 m
All times are local (unless otherwise noted)
The following is from a USGS report.
"Following decay of the January 1983 earthquake swarm activity to background levels in the spring of 1983 (SEAN 07:02, 08:01, 08:02, and 08:03), seismic activity has persisted at a relatively low level. Within the caldera, this background level typically involves several earthquakes (magnitude greater than or equal to 1) per day and an occasional locally felt M 3 event. A M 4.2 earthquake on 28 April 1984 and a M 3.8 event in a swarm that began 16 July 1984 are the largest events to occur in the caldera since the January 1983 swarm. Deformation data show evidence for only modest inflation of the resurgent dome following the January 1983 swarm. Frequently repeated trilateration measurements, however, show continued extensional deformation within the caldera at rates of several microstrain units per year.
"With the reduced rates of seismic activity and ground deformation in the caldera during the last year, the possibility of an imminent eruption appears more remote than during the previous 2 years. In a letter dated 11 July 1984, the Director of the USGS advised California officials that a volcanic eruption does not pose an immediate threat to public safety in the Long Valley region and that the Long Valley region does not satisfy the imminent threat criteria for an official Hazard Warning for volcanic activity.
"The Long Valley region had been under a Notice of Potential Volcanic Hazard since May 1982. Prior to September 1983, this was the lowest of three levels used by the USGS to express the relative urgency of a potential geologic hazard when formally notifying the public and responsible officials. In September 1983, the 3-level terminology was changed to a single-level system termed a Hazard Warning. The USGS is continuing its intensive seismic, geophysical, and geodetic monitoring of Long Valley Caldera that was initiated in mid-1982."
Geologic Background. 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, CA.