U.S. Geological Survey
Office of Earthquakes, Volcanoes, and Engineering
345 Middlefield Rd., Menlo Park, CA 94025
1993 saw a gradual increase in the level of earthquake activity within Long Valley caldera through the year and an average level of activity significant above that for 1992. Over 30 M>3 earthquakes occurred within the caldera during 1993 with six of these associated with the November 27 swarm alone. This compares with a total of four M>3 earthquakes in the caldera for all of 1992. Indeed, 1992 stands as the least active year through the first four years of the decade. The level of earthquake activity in the Sierra Nevada block south of the caldera was also higher in 1993 than in 1992 (over 30 M>3 earthquake in 1993 compared with 14 in 1992). The Red Slate Mountain swarm of August 10-15 was the most intense earthquake sequence in the region during 1993. It included both the largest earthquake (a M=4.5 event ) during the year and the largest number of events for any one sequence or swarm (nearly 400 M>1 earthquakes) throughout the region.
Occasional long-period (LP) earthquakes continue to occur beneath the southwest flank of Mammoth Mountain at depths of 10 to 25 km. We detected eleven episodes of LP activity from March through December this year. Roughly half of these episodes involve solitary events; the remainder included bursts of several events occurring over time intervals of one to two minutes. All of these LP events were small, the largest having an amplitude-magnitude of M~1.
Although the earthquake activity in the caldera increased through 1993, the deformation rates across the caldera defined by the two-color geodimeter measurements remained steady at 2-3 ppm/year. This increased seismicity rate, however, is still not adequate to erase the growing deficit in the cumulative seismic moment in the caldera with respect to the moment represented by the geodetic deformation that has accumulated since 1990.
Both the two-color geodimeter data and the annual GPS surveys of the regional Long Valley caldera-Mono Craters geodetic network confirm that deformation within the caldera is dominated by inflation of the resurgent dome driven by a pressure (Mogi) source at a depth between 5 to 8 km. This single pressure source, however, cannot fully account for either the regional deformation pattern documented by the GPS data or the combined two-color geodimeter and 1988-1992 leveling data. In combination, these three deformation data sets seem to require a second, relatively deep (>15 km) inflation source somewhere beneath the southwest margin of the caldera and Mammoth Mountain, although the location of this source is poorly constrained. Such a pressure source may well be related to the deep LP earthquake activity beneath Mammoth Mountain. The leverage provided by analysis of the combined leveling and two-color data begins to bring the structure of the deformations sources in the caldera into sharper focus. This joint analysis, for example, indicates that the pressure source beneath the resurgent dome may have a flattened ellipsoidal shape and that normal slip on the medial graben faults may be accompanying inflation of the resurgent dome.