LONG VALLEY CALDERA MONITORING REPORT

July-September 1999

U.S. Geological Survey

Volcano Hazards Program

345 Middlefield Rd. Menlo Park, CA 94025

CALDERA ACTIVITY

SEISMICITY

Earthquake activity remained low within the caldera through the second quarter of 1999. A few scattered events within the south moat and beneath Mammoth Mountain all had magnitudes less than M=3.0, and only four earthquakes within the caldera had magnitudes of M=2.0 or greater.

DEFORMATION

The two-color geodimeter deformation data show that the resurgent dome has remained stable over the past year with the center of the resurgent dome standing roughly 80 cm higher than in the late 1970's. The continuous data from the borehole dilatometers and tiltmeters, as well as the continuous differential magnetometer data showed no changes significantly above background noise levels during this period.

MAMMOTH MOUNTAIN CO₂

CO₂ soil-gas emissions in the Horseshoe Lake tree-kill area and other sites around Mammoth Mountain continue with little change aside from the annual variations in shallow soil-gas concentrations associated with waxing and waning of snow cover. The only noticeable anomaly occurred with a small increase in CO₂ concentrations at the SKI monitoring site in late September (the SKI site is located on the north flank of Mammoth Mountain a few hundred meters up hill from the main ski lodge: see the report by McGee and others).

REGIONAL ACTIVITY

Earthquake activity in the Sierra Nevada south of the caldera continues to be dominated by the gradually slowing aftershock sequences to the M=5.1 earthquakes of 8 June and 14 July 1998 and the M=5.6 earthquake of 15 May 1999. The aftershocks to these events are largely confined to a linear band of epicenters that extends roughly 17 km SSW from the southern margin of the caldera boundary roughly coincident with the trend of lower McGee Creek. The largest of these aftershocks included M=3.7 and 3.8 earthquakes on July 16 and a M=2.7 earthquake on August 16.

RESPONSE

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