REVIEW OF LONG VALLEY CALDERA ACTIVITY FOR 1998

U.S. Geological Survey

Volcano Hazards Program

345 Middlefield Rd., Menlo Park, CA

Three events dominated activity in Long Valley caldera and vicinity through 1998. First was the declining phase of accelerated uplift of the resurgent dome and persistent earthquake swarm activity in the south moat of the caldera that began in June-July 1997 and peaked in November-December, 1997. The second and third were the magnitude M=5.1 earthquakes on 8 June and 14 July 1998, which were centered just west of the Hilton Creek fault and 2 to 4 km south of the caldera, respectively, together with their respective aftershock sequences.

DECLINING PHASE OF THE 1997 CALDERA UNREST

The final phase of the strong earthquake swarm activity in the south moat of the caldera that dominated caldera unrest through the second half of 1997 extended into January, 1998. A M=4.8 earthquake on December 31^st , which was located beneath the west end of the airport, marked a shift in the center of the most intense earthquake activity from beneath the western part of the south moat (between the Highway 203 - 395 junction and the east side of Mammoth Lakes) to the east-central section of the south moat (beginning 2 km east of the Highway 203-395 junction and extending eastward to the vicinity of Doe Ridge and the east end of the airport). Activity in early January earthquake included a burst of strong swarm activity on January 1^st between the Fish Hatchery and the airport with more than a dozen M>3 earthquakes and well over 2,000 earthquakes detected and located through January 5^th. A M=4.1 earthquake at 6:11 AM (PST) on January 6 beneath the west end of the airport was the last of nine 4.0 <M <4.9 earthquakes that occurred in the south moat beginning with the initial M=4.1 earthquake on 13 November 1997.

The gradual decline in south-moat earthquake activity to background rates by mid-summer was interrupted by occasional M>3 earthquakes and brief bursts of elevated swarm activity, particulary through February and March. The latter include swarms on February 16 at the southern margin of the resurgent dome with a M=3.2 earthquake and over 80 smaller events; a swarm on March 2 near the 203-395 Highway junction with over 80 small events, eight of which had magnitudes M>2.5; a swarm on March 6-7 with over 180 small events, seven of which had magnitudes of M>2.5; and a swarm on March 15-16 near Hot Creek that included a M=3.0 event and over 90 smaller earthquakes. The last six months of the year included only five earthquakes in the caldera with magnitudes of M>3.0. The two largest had magnitudes of M=3.7. One occurred at 11:50 PM on July 14^th in association with a swarm near the 203-395 Highway junction on July 14 (this event followed the M=5.1 earthquake of July 14 located 13 km to the ESE by 2 hours). The other was associated with a swarm just 3 km east of Mammoth Lakes on December 7-9 (the M=3.7 event occurred at 8:16 AM on the 7th).

The two-color geodimeter measurements confirmed that inflation of the resurgent dome also gradually slowed from a peak rate that approached 30 cm/year during the most intense activity in mid November, 1997, to roughly 15 cm/year from December 1997 through March 1998. By mid-May, the inflation rate had dropped to between 1 and 2 cm/year, a rate that persisted through the end of the year.

LP EARTHQUAKES

Long-period (LP) earthquake activity at depths between 10 and 25 km (6 to 15 miles) beneath Devils Postpile and the southwest flank of Mammoth Mountain continued through 1998. We detected some 140 of these LP events in 1998, or just over half the number (250 events) detected during 1997. Although this represents a significant decrease in the LP activity rate with respect to 1997 rates, the 1998 rate is still distinctly higher than any time from the onset of LP activity in mid-1989 through 1996. During that entire 8-year period (1989-1996), we detected a total 165 LP events.

One difference in the character of the LP activity that emerged in 1998 with respect to earlier years was the tendency for the many of the earthquakes to be preceded by several tens of seconds of a tremor-like signal with a dominant frequency of around 1 Hz. Tremor-like signals occasionally preceded earlier LP earthquakes, but the duration of the signal was generally shorter and the dominant frequence somewhat higher (2-3 Hz). An LP earthquake with a magnitude of M=1.8 at a depth of 12 km on November 11 was the largest LP event recorded since the initial activity in 1989. This earthquake was followed by a week of elevated LP activity that included roughly 25 smaller events.

Occasional LP earthquakes also continue to occur at depths between 15 and 30 km centered beneath an area roughly 7 km west of the Mono Craters. One of the larger LP events to date in this area occurred at a depth of about 24 km on September 26^th with an estimated magnitude M=2.4.

CO₂ GAS EMISSIONS AT MAMMOTH MOUNTAIN.

Field studies of CO₂ flux around the flanks of Mammoth Mountain over the summer months suggest that the overall flux rate has been slowly decreasing over the past three years. High-resolution, spatial-temporal flux measurements in the Horseshoe Lake tree-kill area carried out by John Rogie from Penn State clearly document strong, spatially-coherent fluctuations in flux rates over intervals ranging from hours to months. Processes controlling the observed variations in flux rates are still under investigation. The continuous soil-gas concentration data collected by Ken McGee and Terry Gerlach continue to show a pronounced increase in CO₂ soil-gas concentrations during winter months associated with the blanketing effect of snow cover. Using airborne measurements of CO₂ concentrations carried out in a fixed-wing aircraft in September and November, Terry Gerlach detected a CO₂ plume down wind from Mammoth Mountain consistent with multiple CO₂ sources around the flanks of the mountain. This demonstrated ability to successfully detect a CO₂ plume from an aircraft, together with existing airborne COSPEC technology for measuring sulphur dioxide (SO2) concentrations in the atmosphere, provide an important additional tool for monitoring gas emissions associated with magmatic unrest in the area.

THE M=5.1 EARTHQUAKES OF 8 JUNE AND 14 JULY

The gradual slowing of activity within the caldera through the spring and early summer was countered by the occurrence of two M = 5.1 earthquakes just south of the caldera in the vicinity of the Hilton Creek fault. The first of these earthquakes, which was located just 1.5 km south of the caldera at a depth of 6.7 km beneath the surface trace of the east-dipping Hilton Creek fault, occurred at 10:42 PM (PDT) on June 8. The second at 9:53 PM (PDT) on July 14 was centered 3 km to the south-southeast at a depth of 6.2 km. Both earthquakes were located within the footwall of the east dipping Hilton Creek fault, a major range-front normal fault with over 20 m of pure dip-slip, post-glacial (Holocene) offset. Neither earthquake, however, appears to have involved slip on the Hilton Creek fault itself.

Both earthquakes were followed by rich aftershock sequences that gradually tailed off through the end of the year and, together, included 9 earthquakes with magnitudes in the range M=4.0 to 4.5. In combination, the aftershock epicenters define a nearly orthogonal pattern in map view with those to the June 8 earthquake confined to a west-northwest lineation through the mainshock epicenter and those to the July 14 earthquake confined to a slightly more diffuse south-southwest lineation through the mainshock epicenter. Both aftershock lineations cut across the Hilton Creek fault and intersect at a point just east of the June 8 epicenter (approximately 1 km east of the mapped trace of the Hilton Creek fault).

Preliminary focal mechanisms for the two mainshocks determined by Doug Dreger (U.C. Berkeley) using data from regional broad-band seismic stations indicate that the June 8 event involved dominantly strike-slip displacement and the July 14 event involved dominantly

normal, dip-slip displacement. The T-axes (directions of maximum extension) for both focal mechanisms show a common, ENE-WSW orientation. Based on the aftershock distributions, it appears that the June 8 mainshock involved right-lateral slip on a west-northwest striking plane. The dip-slip planes of the July 14 mainshock, however, strike to the NNW forming a high angle to the NNE trend of the aftershock distribution for that earthquake. This suggests that, although dip-slip displacements associated with the mainshock may dominate, the July 14 earthquake sequence involved a mix of dip-slip and strike-slip displacements along a complex fault geometry. A brief surge in the aftershock activity to the July 14 M=5.1 earthquake from December 13-17 included three M>4.0 earthquakes: a M=4.0 and 4.1 events at 8:14 and 9:54 PM (PST), respectively, on December 13, and a M=4.1 event at 2:32 AM on the 17^th. The focal mechanisms for all three events are dominantly strike-slip consistent with left-lateral displacement along NE-striking fault planes sub-parallel with the NNE trend of the aftershock sequence.

On July 30, Art Sylvester and his students from the University of California, Santa Barbara, re-surveyed a level line that crosses the Hilton Creek fault near the McGee Creek campground. They found no elevation changes across the fault with respect to a survey run in July of 1997. This null result provides further evidence that M=5.1 earthquakes of June 8 and July 14 did not involve slip on this major, range-front fault. (The results of this leveling survey are posted under the UCSB Geology Department web page at:

http://www.geol.ucsb.edu:8080/~geodesy/level_lines/X0036_McGEE_CREEK.html. )

Earthquake activity elsewhere in the region showed no significant variations from background activity over the past several years. In particular, small earthquakes (including a number of M>3 events) continued throughout the Sierra Nevada block south of the caldera and along the western margin of the Owens Valley.