SUMMARY OF LONG VALLEY CALDERA ACTIVITY FOR 1992

U.S. Geological Survey
Office of Earthquakes, Volcanoes, and Engineering
345 Middlefield Rd., Menlo Park, CA 94025

Sporadic earthquake swarm activity continued to occur in the southern half of Long Valley caldera through 1992 but at a slightly lower rate than in 1991. Inflation of the resurgent dome, however, continued at essentially the same 3-cm/year uplift rate that prevailed through 1991. Most level lines in the caldera were resurveyed for the first time since 1988 during the summer of 1992. Results confirm an additional 10 cm of uplift over the central part of the resurgent dome over this four year period bringing the total uplift accumulated since 1975 to nearly 60 cm. Certainly the most remarkable and energetic event in the caldera during 1992 was the abrupt surge in local seismicity that began immediately following the June 28, M=7.3 Landers earthquake, which was located in southern California some 400 km south of the caldera. Local seismicity increased not only within Long Valley caldera following the Landers earthquake but along most of the eastern California shear zone as far north as the southern Cascade volcanoes (Lassen Peak and Mount Shasta), the Geysers geothermal area, California, Cedar City, Utah, and in Yellowstone National Park over 1200 km north of the Landers epicenter. This unambiguous example of seismicity remotely triggered by a large regional earthquake forces re-evaluation of some long-held ideas on mechanical interactions in the earth's crust (see Hill et al., 1993, Science, v. 260, pp. 1617-1623 for further details). The surge in seismicity triggered by the Landers earthquake was the strongest "swarm" in the caldera during 1992. It included two M>3 earthquakes and over 250 smaller events in the first six days after the Landers earthquake. A transient deformation pulse accompanied the triggered seismicity in Long Valley with peak strain amplitudes of 0.25 ppm recorded on the borehole dilatometer just west of the caldera boundary and a long-base tiltmeter in the east-central section of the caldera.

Careful scanning of seismograms recorded on 16 mm film from selected stations in the caldera has revealed a series of more than 60 earthquakes with the character of long-period (LP) volcanic earthquakes that began occurring beneath the southwest flank of Mammoth Mountain at depths of 10 to 28 km in mid-1989. These LP events are notable for their deficiency in energy a frequencies above about 5 Hz compared with the much more common volcano-tectonic (VT) earthquakes. Their onset coincides with the 1989 swarm of shallow (depths < 10 km) volcano-tectonic earthquakes beneath Mammoth Mountain that persisted from May through December, 1989. The focal depths of the LP events are systimatically deeper than the deepest VT earthquakes. Although most of these LP earthquakes are located between Mammoth Mountain and Red Cones (a pair of post-glacial basaltic cinder cones 4 km southwest of Mammoth Mountain), two were located 5-7 km west of Mono Craters and one beneath the west moat of the caldera. Since their onset in mid-1989, these LP earthquakes have occurred at intervals that vary from weeks to months with individual occurrences typically consisting of several events within a few minutes with the largest event last in the sequence. Their (amplitude) magnitudes have ranged from M= 0.7 to 2.2. Similar sporadic occurrences of LP earthquakes at mid-crustal depths have been recognized beneath the young volcanic areas in Japan as well as beneath Lassen Peak and Medicine Lake caldera in the southern Cascade Range of northern California. This growing body of evidence from Japan and the western United States suggests that occasional LP earthquakes at mid- to lower-crustal depths are common beneath areas of Holocene volcanism and not indicative of imminent volcanic activity (see Pitt and Hill, 1994, Geoph. Res. Lett, v. 21, pp. 1679-11682 for further details).