LONG VALLEY CALDERA MONITORING REPORT

April-June 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. The already low seismic activity within the caldera apparently slowed further following the M=5.6 earthquake of May 15 located in the Sierra Nevada 8 km (5 miles) south of the caldera (see Regional Activity below).

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. None of the continuous deformation data from the borehole dilatometers or tiltmeters showed changes significantly above background noise levels during this period.

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. As described in the report by Ken McGee and others, the elevated concentrations gradually declined through May and early June to typical summer-fall values. The one anomaly showed up as an increase in CO₂ soil gas concentration at the Chair 19 site (just uphill from the main ski lodge) that persisted from May 19 through June 11. It=s not clear what may have produced this temporary increase in CO₂ concentration at this one site. McGee and others point out the onset of increased concentration began four days after the May 15 M=5.6 earthquake in the Sierra Nevada block, which was locate some 20 km (12 miles) east-southeast of Mammoth Mountain (see below). Whether this is simply a coincidence or involves some causal link remains unresolved.

REGIONAL ACTIVITY

Seismic activity in the region was dominated by the third in a sequence of M>5 earthquakes that have struck the area of the Sierra Nevada block south of the caldera in the last year. This latest M>5 earthquake was the largest of the three with a magnitude of M=5.6. It was located at the southern end of the aftershock zone to the M=5.1 earthquake of July 14, 1998 (roughly 8 km [5 miles] west-southwest of Tom=s Place). Its hypocenter was at a depth of 7 km (4 miles) beneath the stretch of McGee Creek that lies roughly 3 km (2 miles) east of Mount Baldwin. As with the M=5.1 earthquakes of June 8 and July 14, 1998, this May 15 earthquake was followed by a rich aftershock sequence, which in this case included over 30 M>3 earthquakes and 6 M>4 earthquakes. The epicenters for the aftershocks define a narrow zone elongated to the south-southwest that essentially extends the same trend defined the aftershocks to the July 14, 1998 earthquake some 12-13 km (8 miles) further into the Sierra Nevada. One of the nodal planes of the dominantly strike-slip focal mechanism for the mainshock is essentially parallel with the aftershock distribution consistent with left-lateral strike-slip displacement. As was true for the two M=5.1 earthquake of June and July 1998, this M=5.6 event was not accompanied or followed by any signs of increased earthquake activity or ground deformation within in the caldera (again the condition remained GREEN). Neither was it accompanied by any evidence for slip or increased seismicity along the nearby Hilton Creek fault. Aftershock activity to this May 15 earthquake gradually slowed through the end of June. Altogether, these three M>5 earthquakes and their aftershocks have included over 400 M>1.5 earthquakes, 15 of which had magnitudes of M=4.0 or greater.

RESPONSE

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