LONG VALLEY CALDERA MONITORING REPORT

October-December 1998

U.S. Geological Survey

Volcano Hazards Program

345 Middlefield Rd. Menlo Park, CA 94025

CALDERA ACTIVITY

SEISMICITY

Activity levels within Long Valley caldera remained low through the final quarter of 1998. The only earthquake activity of note within the caldera during this period involved a minor swarm from December 7-9 located in the south moat just south of Highway 203 (roughly 2 miles east-southeast of the Mammoth Lakes Post Office and 0.5 mile east of the water treatment plant). The swarm included over 40 earthquakes detected and located by the realtime computer system. The largest of these, which had magnitudes of M=3.7, M=2.5, and M=2.9, occurred at 8:16 and 8:51 AM (PST) on December 7 and 4:44 PM on December 9, respectively.

DEFORMATION

Deformation monitoring data continued to show only minimal changes within the caldera this quarter. The two-color geodimeter measurements indicate that the average extension rate for this three-month period was less than 1 cm/y. With the possible exception of two minor transient tilt changes noted below, continuous deformation data from the borehole dilatometers and tiltmeters showed no significant changes.

The two minor tilt transients include: 1) A 0.5-microradian, down-to-the-northwest tilt that developed from October 13-15 on the long-base tiltmeter, which is located in the south moat just north of the airport, and 2) a 1-microradian, down-to-the-south tilt that developed from December 4-8 on the Little Antelope borehole tiltmeter, which is located at the south end of Little Antelope valley in the eastern section of the resurgent dome. Given that each of these tilt transients was recorded on but a single instrument leaves their significance in doubt. For what it's worth, however, we note that M>3.5 earthquakes followed the onset of each of these tilt transients by 2-3 days at distances of 6 to 7 km from the respective tiltmeters. In particular, the onset of the October 13-15 tilt transient on the long-base tiltmeter was followed roughly 2 days later by a M=3.5 event (6:12 UT on October15) located within the aftershock zone of the M=5.1 earthquake of 8 June 1998 (7 km southeast of the tiltmeter). Other M>3.5 aftershocks to this 8-June earthquake were not associated with tilt transients on the long-base tiltmeter. The onset of the December 4-8 tilt transient on the Little Antelope tiltmeter was followed roughly 3.5 days later by a M=3.7 earthquake (at 1516 UT on the December 7) that was part of the December 7-9 swarm beneath the southwestern margin of the resurgent dome (6 km southwest of the tiltmeter).

MAMMOTH MOUNTAIN CO₂

Airborne surveys using a LI-COR carbon dioxide analyzer mounted in a fixed-wing twin-engine aircraft conducted on September 22 and November 13 detected a distinct CO₂ plume downwind from Mammoth Mountain (see the report by Ken McGee and others). Data from both flights show clear evidence for multiple sources of CO₂ around the mountain. This demonstrated airborne capability for defining CO₂ -plume geometry coupled with the well-established co-spec technology for detecting SO₂ , will be an important monitoring tool should activity in the area escalate to the point of an impending eruption.

REGIONAL ACTIVITY

Earthquake activity in the region around Long Valley caldera was dominated by the gradually decaying aftershock sequences to the M=5.1 earthquakes of 8 June and 14 July, 1998, both of which were located in the footwall block of the Hilton Creek fault 2 to 4 km south of the caldera boundary (see the monitoring reports for the first 3 quarters of 1998 and the annual summary below). This aftershock activity included the M=3.5 earthquake of 11:12 PM (PDT) on October 14 (0612 UT on the 15^th) that followed the transient tilt signal on the long-base tiltmeter mentioned above. The most intense aftershock activity occurred in the aftershock zone of the 14 July earthquake from December 13-17. This temporary surge in aftershock activity included three M>4.0 earthquakes: M=4.0 and M=4.1 events at 8:14 PM and 9:54 PM, respectively, on the 13^th and a M=4.1 event at 2:32 AM on the 17^th. Elsewhere in the Sierra Nevada block south of the caldera, a cluster of small earthquakes centered roughly 3 km south of Red Slate Mountain (8 km southwest of the 14 July M=5.1 earthquake epicenter) included four events with magnitudes of M=2.0 or greater on November 8. The largest of these was a M=2.9 earthquake at 7:00 AM on the 8^th. This was followed by a M=3.7 earthquake at 7:38 PM on November 11 located 5 km to the east beneath the Pioneer Basin.

RESPONSE

The condition remained GREEN (no immediate risk) throughout the fourth quarter of 1998.