LONG VALLEY CALDERA MONITORING REPORT

July-September 1998

U.S. Geological Survey

Volcano Hazards Program

345 Middlefield Rd. Menlo Park, CA 94025

CALDERA ACTIVITY

SEISMICITY

The decline in earthquake activity within the caldera following the July 1997-January 1998 earthquake swarms continued through the third quarter of 1998. Only four earthquakes within the caldera during this three-month period had magnitudes of M=3.0 or greater. A M=3.7 earthquake at 11:50 PM on July 14^th accompanied a swarm of small earthquakes located beneath the southern margin of the resurgent dome near the 203-395 Highway junction. (This M=3.7 earthquake occurred just two hours after the M=5.1 earthquake located 8 miles to the southeast in the vicinity of the Hilton Creek fault -- see Regional Activity below.) An earthquake swarm beneath the eastern margin of the resurgent dome (about a mile northwest of the Hot Creek geological site) on August 8 included M=3.3 and M=3.2 earthquakes at 3:21 and 3:32 PM, respectively. A M=3.0 earthquake at 11:26 PM on August 21 was located near the southern margin of the caldera (1 mile south of Whitmore Hot Springs). This earthquake and a number of smaller ones in the same area were probably associated with the aftershock activity of the June 8 and July 14 M=5.1 earthquakes located just a few miles to the south in the vicinity of the Hilton Creek fault (again, see "Regional Activity" below).

The deep, long-period (LP) earthquake activity beneath the southwest flank of Mammoth Mountain averaged a few events per month this quarter. This relatively low-level activity continues the trend established in mid-April when the elevated LP earthquake activity that developed from 1997 through early 1998 abruptly slowed.

DEFORMATION

Deformation monitoring data showed minimal changes within the caldera this quarter. The two-color geodimeter measurements indicate resumption of very gradual extension across the resurgent dome. The average extension rate for this three-month period was roughly 1 cm/y, or about half the average extension rate that persisted from 1991 through early-1997. Continuous deformation data from the borehole dilatometers and tiltmeters showed no significant changes (the strain transient on the POPA dilatometer from September 2-7 does not seem to correlate with any other activity in the caldera or significant events in the aftershock activity to the June 8 and July 14 earthquakes) . It is noteworthy that the M=5.1 earthquakes of June 8 and July 14 (see "Regional Activity" below) were not accompanied by a significant changes in either deformation or seismicity within the caldera.

MAMMOTH MOUNTAIN CO₂

The included reports by M. Sorey, B. Evans, and J. Rogie and by K. McGee, T. Gerlach, M. Doukas, and R. Kessler summarizing results of their CO₂ studies this summer suggest that the overall flux may be slowly declining in the Horseshoe Lake tree-kill area. The seasonal drop in CO₂ soil-gas concentrations associated with snow melt was delayed to mid-June this year, presumably because of the heavy snow accumulations over the winter. Results from a series of high-resolution, spatial-temporal CO₂ flux measurements this summer emphasize that the flux undergoes strong fluctuations in rate over intervals ranging from hours to months. Ground water in wells on the flanks of Mammoth Mountain continue to show elevated concentrations of dissolved magmatic CO₂ and He (helium). These elevated concentrations in the ground water are presumably associated with the overall increase in soil-gas CO₂ emissions that began in 1990.

REGIONAL ACTIVITY

Aftershocks to the M=5.1 earthquake of June 8, which was located beneath the surface trace of the Hilton Creek fault 1 mile south of the caldera (see the January-June 1998 report), gradually declined through the first two weeks of July. Only two of these aftershocks had magnitudes of M=3.0 or greater during the interval from July 1 through July 13: a M=3.1 event at 5:14 AM on July 2 and a M=3.9 event at 9:54 AM on July 13.

Then at 8:53 PM on July 14, a second M=5.1 earthquake centered 3 km (1.8 miles) SSW of the epicenter of the June 8, M=5.1 event shook the area. The hypocenter of this July 14 event was 4.4 km (2.6 miles) south of the caldera boundary, 1.5 km (0.9 miles) west of the surface trace of the Hilton Creek fault, and at a depth of 6.2 km (3.7 miles). It's focal mechanism based on the University of California, Berkeley, broad-band seismic data shows dominantly normal slip on a NNW-striking fault plane with the T-axis plunging 14 degrees at an azimuth of 69 degrees. Thus, this earthquake appears to have involved normal slip on a fault subparallel with but to the west of the Hilton Creek fault (both the June 8 event and this earthquake occurred within the footwall block of the east-dipping Hilton Creek fault). As with the June 8 earthquake, this July 14 M=5.1 event was followed by a rich aftershock sequence that continued to slowly decay through the end of September. By the end of September, this aftershock sequence had included five events with magnitudes of M=4.0 or larger. The largest was a M=4.3 earthquake at 11:01 PM on July 31. (The other earthquakes of M=4 or greater included a M=4.1 event at 11:45 PM on July 14 and M=4.2, 4.0, and 4.1 earthquakes at 7:45 AM on August 2^nd, 6:44 AM on August 5^th, and 7:38 AM on September 11^th, respectively.) The aftershocks to the July 14 earthquake define a 10-km-long, NNE trending lineation through the mainshock epicenter. This aftershock lineation abuts and is nearly perpendicular to the ESE trend of the aftershocks of the June 8 mainshock, and it diverges southward from the Hilton Creek fault at an angel of roughly 40 degrees terminating beneath the upper reaches of McGee Creek midway between Mt. Morgan and Red Slate Mountain. The divergence between the NNE trend of the aftershock lineation and the NNW strike of the fault plane for the mainshock indicate that this earthquake sequence involves slip on a complex fault geometry. Many of the larger aftershock have fault plane solutions showing dominantly strike-slip motion consistent with left-lateral slip along the NNE trend of the aftershock sequence.

RESPONSE

The condition remained GREEN (no immediate risk) throughout the third quarter of 1998. The M=5.1 earthquakes of June 8 and July 14 were both close to but outside the caldera. Both showed the mainshock-aftershock behavior characteristic of earthquakes responding dominantly to tectonic rather than magmatic (or volcanic) processes, and neither was accompanied by changes in activity (earthquake swarms or increased ground deformation) within the caldera.