U.S. Geological Survey
Office of Earthquakes, Volcanoes, and Engineering
345 Middlefield Rd., Menlo Park, CA 94025
Following the buildup in activity through the end of 1993, earthquake activity within the caldera gradually decreased through the winter of 1994. The spring were particularly quiet with only one earthquake exceeding magnitude 2 during the 10-week period from April 18 through June 28. Activity increased again in late June to a moderate level that persisted through the end of the year. Just 10 to 12 M~3 earthquakes occurred in the caldera during 1994 compared with more than 30 during 1993. As in previous years, the earthquakes continue to be concentrated in the south moat of the caldera with a tendency for gradual northward encroachment into the central part of the resurgent dome in recent years.
Occasional small M<2 earthquakes continue to occur in the shallow crust beneath Mammoth Mountain (focal depths 4 to 10 km). In addition, we continue to record occasional deep, long-period (LP) earthquakes in the crustal volume beneath the southwest flank of Mammoth Mountain (depths 15 to 20 km). We recorded 11 such events during 1994, all with magnitudes less than 2.
Earthquake activity in the Sierra Nevada block south of the caldera persisted at a moderate level throughout 1994 with over 20 M~3 events. Most of this activity continues to be concentrated in a broad band extending south from Mount Morrison to Red Slate Mountain with more diffuse activity extending eastward to Round Valley, the Owens River gorge, and Bishop. Scattered small (M<2) earthquakes in Chalfant Valley are late aftershocks M=6.4, 1986 Chalfant Valley earthquake.
Swelling of the resurgent dome in the central part of the caldera continued through 1994 at a fairly steady strain rate of 2-3 ppm/y (corresponding uplift rate of 2-3 cm/y at the center of the resurgent dome). Aside from minor (possibly seasonal) fluctuations, baselines in the two-color geodimeter network extending north and east of CASA in particular have maintained relatively steady extension rates since mid-1991. Baselines extending west and southwest of CASA, however, show evidence of a gradually slowing extension rate beginning in mid-to late-l993.
Perhaps the most notable development in the caldera during 1994 was recognition that the areas of dead trees in the vicinity of Horseshoe Lake just south of Mammoth Mountain and Chair 12 and Reds Lake on the north side of Mammoth Mountain correlate with high concentrations of CO2 in the soil. The trees in these areas, some of which are 200 to 300 years old, began to die in mid-1990, and it seems likely that the high CO2 flux represents degassing of a magma body emplaced in the shallow crust during the April-December 1989 earthquake swarm beneath Mammoth Mountain. Analysis of the isotopic signature of the CO2 confirms that it has a magmatic source.