LONG VALLEY CALDERA MONITORING REPORT

January-March 1999

U.S. Geological Survey

Volcano Hazards Program

345 Middlefield Rd. Menlo Park, CA 94025

CALDERA ACTIVITY

SEISMICITY

Earthquake activity remained low throughout Long Valley caldera and vicinity through the first three months of 1999. Only three earthquakes occurred within or immediately adjacent to the caldera with magnitudes as large as M=2.9 to 3.3. The first was a M=2.9 earthquake at 3:45 AM (PST) on January 1 located beneath the southern margin of the caldera (5 km ESW of Mammoth Lakes). On February 24-25, a swarm of some 42 small earthquakes were centered 1-2 km east of Lake Mary (5 km WSW of Mammoth Lakes). The largest events within this sequence were M=3.2 and M=2.9 earthquakes at 3:10 PM (PST) on the 24^th and 3:03 AM on the 25^th, respectively. On May 27^th , a M=3.1 earthquake at 12:48 PM accompanied by half a dozen smaller events was located beneath the southern margin of the resurgent dome (1 km WSW of the Fish Hatchery and 9 km east of Mammoth Lakes).

DEFORMATION

The two-color geodimeter deformation data show that the resurgent dome has remained stable over the past six months with the center of the resurgent dome standing roughly 80 cm higher than in the late 1970's. None of the continuous deformation data from the borehole dilatometers or tiltmeters showed changes significantly above background noise levels.

MAMMOTH MOUNTAIN CO₂

The continuous measurements of CO₂ soil-gas concentrations around the base of Mammoth Mountain showed their usual seasonal increase associated with the blanketing effect of the winter snow cover.

The summary of gas discharges for the Mammoth Mountain area for the last half of 1998 by M.L. Sorey and colleagues, which was inadvertently omitted from the October-December 1998 monitoring report, summarizes the evidence indicating that the total CO₂ flux from the Mammoth Mountain area has been slowly decreasing since 1995. This report also indicates that the helium isotopic ratio (³He/⁴He) in gas samples collected from the Mammoth Mountain fumarole showed little change associated with the sustained earthquake swarm activity through the last half of 1997.

REGIONAL ACTIVITY

Earthquake activity within the Sierra Nevada block south of the caldera was concentrated within or in the vicinity of the aftershock zones to the pair of M=5.1 earthquakes on 8 June and 14 July last year located just south of the caldera and just west of the Hilton Creek fault (west to west-southwest of Tom=s Place). Interestingly, most of the aftershocks were concentrated near the southern end of the aftershock zone to the M=5.1 earthquake of 14 July 1998. As it turns out, these aftershocks are within 2 km of the epicenter of the M=5.6 earthquake of 15 May 1999 (37^o 31.7' N, 118^o 49.0' W, or 11 km WSW of Tom=s Place), which will be described in the next (April-June 1999) monitoring report. The largest earthquake in the area this quarter was a M=3.8 earthquake beneath Red Slat Mountain that occurred at 12:01 PM on 12 March some 4 km to the west-southwest of the dominant cluster of aftershock activity.

RELATED WORK

Additional field activities last summer that will contribute to our efforts to understand the processes driving the unrest in Long Valley caldera include 1) the installation of five additional magnetometers by Dr. Y. Sasai of the Earthquake Research Institute at the University of Tokyo, 2) a resurvey of self potential lines across the caldera by a group of French scientists headed by Dr. J. Zlotnicki, (see the section on magnetic measurements by Mueller and Johnston) and 3) a resurvey of microgravity stations in the caldera by M. Battaglia and Prof. Paul Segal of Stanford University working with Carter Roberts of the U.S. Geological Survey. Early results from the microgravity survey reported at the Fall 1998 American Geophyiscal Union meeting in San Francisco indicate that uplift of the resurgent dome is accompanied by a mass increase at depths of 10 to 12 km in support of evidence that the uplift is due to the addition of magma beneath the resurgent dome.

RESPONSE

The condition remained GREEN (no immediate risk) throughout the first quarter of 1999.