U.S. Geological Survey
Volcano Hazards Program
345 Middlefield Rd. Menlo Park, CA 94025
The year began with M=3.2 and M=2.5 earthquakes (January 2 and January 4, respectively) in the area just west of the Hy 203-395 junction. Following these earthquakes, however, this source area for frequent earthquake swarms turned relatively quiet for the remainder of 1995. With a M=3.3 earthquake on January 14 located in the south moat just south of the airport, activity in the caldera made an eastward shift. Subsequent earthquake activity through the rest of 1995 in the caldera and adjacent areas was largely confined to a north-south corridor extending from the southeast margin of the resurgent dome to the south wall of the caldera and beyond into the Sierra Nevada block beneath Mount Morrison, Mount Baldwin, and as far south as the Pioneer Basin and Mount Huntington.
On March 4, M=4.4 and 4.3 earthquakes occurred near the southern caldera boundary just east of Convict Lake (near the epicenter of the first of the four M=6 earthquakes of May 25-27, 1980). These were the largest earthquakes to occur in the region during 1995. A swarm on March 19-20 in the south moat just south of the airport included more than 150 M > 1 earthquakes and three M > 3 events (the largest was a M=3.5 earthquake on the 19th; see Figures S9-9). The level of activity slowed both within the caldera and the Sierra Nevada block through mid-June. Activity within the caldera pickup up briefly with swarms on June 23 and 27 located near the mouth of Fumarole at the southern margin of the resurgent dome. Each of these swarms included a M~3 earthquake accompanied by more than 20 smaller events.
Beginning with the last two days of June, activity shifted from the caldera south to the Sierra Nevada block. A brief pause near the end of July interrupted the initial surge in activity only to be followed by a stronger surge in the number of earthquakes through August and September that gradually slowed through the end of the year (Figure S8). This swarm-like surge in Sierra Nevada seismicity included more than 20 M>3 earthquakes with individual clusters commonly producing from 20 to 30 events per day through much of August and September. The largest cluster, which occurred on September 17 and was located just south of Mount Morgan, included a M=3.7 earthquake and over 50 smaller events. This Sierra Nevada activity occurred in irregular clusters throughout the area from Mount Morrison south to Mount Huntington and Rock Creek Lake and west to Red Slate Mountain. Two areas that have produced few earthquakes in the past also showed an increase in activity during this period: one is the band of M<3 earthquakes with epicenters aligned subparallel with the southwestern stretch of the caldera between Mammoth Rock and Sherwin Creek; the other is the tight cluster earthquakes at a depth of 10 km directly beneath the outlet at the southern end of Crowley Lake. The latter included a M=3.5 earthquake at 8:20 PM (PDT) on July 20.
Mammoth Mountain continued to produce occasional small (M<2) earthquakes in the upper 10 km of the crust during 1995. In addition, the series of long-period "volcanic" earthquakes located at depths between 10 and 30 km beneath the southwest flank of Mammoth Mountain continue at a fairly steady rate of 20 to 25 M<2 events per year. Also noteworthy is the apparent lineation of earthquakes extending south of Mammoth Mountain well into the Sierra Nevada block. These epicenters are essentially in line with the Inyo Craters to the north and parallel with the series of north-striking faults south of Mammoth Mountain. Because of the sparse seismic network in this part of the Sierra, however, the locations of these earthquakes are less certain than those occurring within the caldera or the Sierra block to the east.
As documented by the two-color geodimeter data, the resurgent dome continued to inflate at a strain rate of 2 to 3 ppm/year (this corresponds to an uplift rate of 2 to 3 cm/year based on past comparisons with results from leveling data). A number of the geodimeter baselines, however, suggest that the extension rates may be gradually slowing with time. In mid-1995, most baselines showed a brief pause in extension followed by a period of increased extension rate (the timing of this pause with respect to the onset of the seismicity surge in the Sierra Nevada block to the south is intriguing). Similar, but less pronounced, variations in extension rate appear to have occurred at fairly regular intervals since 1991, however, with no systematic relation to seismicity variations either within the caldera or the Sierra Nevada block.
Based on field measurements of soil gas CO2 concentrations and associated tree-kill areas completed during the summer and fall, the areas with anomalously high CO2 concentrations on the flanks of Mammoth Mountain have expanded over last year. The expansion is reflected in the growth of 1994 tree-kill areas as well as newly developed tree-kill areas in the vicinity of Reds Creek on the west flank and on the north flank above the main ski lodge. Carbon dioxide (CO2) accompanied by small amounts of helium (He) continue to be the only gasses with high concentrations in the tree-kill areas, and with few exception, the soil-gas He/CO2 ratio is similar to that in the fumarole just south of the chair 3 lift on the east flank of the mountain.