REVIEW OF LONG VALLEY CALDERA ACTIVITY FOR 1999

U.S. Geological Survey

Long Valley Observatory

345 Middlefield Rd. Menlo Park, CA 94025

This final year of the millennium saw some of the lowest activity levels within Long Valley since the onset of unrest in 1979-80. The dominant event in the region involved a M=5.6 earthquake on 15 May located in the Sierra Nevada 7 km south of the caldera. This earthquake and its aftershocks served to extend the rupture zone of the 8 June and 14 July M=5.1 earthquakes of 1998 roughly 8 km further to the south-southwest into the Sierra Nevada block. Carbon dioxide emissions from the flanks of Mammoth Mountain showed little change from the relatively high flux rates of the previous several years.

CALDERA UNREST

SEISMICITY

Both earthquake activity and ground deformation were subdued within Long Valley caldera throughout 1999. The two largest earthquakes within the confines of the caldera were M=2.9 and M=3.1 events on January 1 beneath the southern margin of the caldera (5 km ESE of Mammoth Lakes) and March 27 beneath the southern margin of the resurgent dome (9 km east of Mammoth Lakes), respectively. On February 24-25, a swarm of some 42 small earthquakes located just outside the caldera were centered 1 to 2 km east of Lake Mary in the Lakes Basin (5 km WSW of Mammoth Lakes). The largest events in this sequence were M=3.2 and M=2.9 earthquakes at 3:10 PM (PST) on the 24^th and 3:03 AM on the 25^th respectively (Figure S7)

Aside from its relative quiescence, two aspects of 1999 caldera seismicity are noteworthy. One is the abrupt onset of a decrease in seismicity rate within the caldera that coincides with the occurrence of the May 15, M=5.6 earthquake, which was located in the Sierra Nevada south of the caldera (see below). The other is the brief swarm of small earthquakes beneath the north flank of Mammoth Mountain that occurred within the hour following the M=7.1 Hector Mine earthquake of October 16 (0946 UT), the epicenter of which was located in the Mojave Desert some 430 km southeast of the caldera. (Figure S8, S9) The former is further evidence of still poorly understood interactions between seismic activity within the caldera and that in the Sierra Nevada block south of the caldera. The latter appears to be a more subtle example of remote triggering similar the activity triggered in Long Valley caldera and elsewhere in the western U.S. by the M=7.3 Landers earthquake of June 28, 1992, the epicenter of which was also located in the Mojave Desert 420 km south of the caldera (Hill and others, 1992).

The putative triggered response of Mammoth Mountain to the Hector Mine earthquake of October 16 is interesting for several reasons:

• It represents the only recognized candidate for remote triggering north of the Hector Mine epicenter. The Hector Mine mainshock ruptured to the south, and the most obvious instances of triggered seismicity were in the direction of rupture propagation (principally in the Salton Trough, south of the mainshock epicenter). In contrast, the Landers mainshock ruptured to the north, and again all the recognized triggered response was again in the direction of rupture propagation (north of the mainshock epicenter). Note that the Hector Mine epicenter is located just 30 km north-northeast of the Landers epicenter.
• Mammoth Mountain did not respond to the Landers earthquake. The swarm activity triggered by the Landers earthquake was concentrated in the south moat of the caldera and adjacent sections of the Sierra Nevada block. In the case of the Hector Mine earthquake, the north flank of Mammoth Mountain was the only place in the vicinity of the caldera that showed evidence of a triggered response.
• As was true for the Landers earthquake, the caldera also showed a transient strain response to the Hector Mine earthquake as detected by the borehole dilatometers (see the report by Johnston and others). As with Landers, the transient strain response is much larger than can be attributed to the triggered seismicity alone. In the Hector Mine case, the strain transient is consistent with aseimic (silent) slip on a north-striking normal fault beneath the north flank of Mammoth Mountain (see Johnston and others’ report).

DEFORMATION

Aside from the transient response to the October 16 Hector Mine earthquake detected by the borehole dilatometers, deformation within the caldera remained stationary through 1999. The center of the resurgent dome currently stands nearly 80 cm higher than in the late 1970’s prior to the onset of the last two decades of caldera unrest. It’s noteworthy that, in contrast to Yellowstone and Campi Flegrei (Italy) calderas, which showed pronounced uplift through the early 1980’s followed by partial subsidence, Long Valley caldera has yet to show any significant subsidence. Rabaul caldera in Paupa New Guinea, the other large caldera with well-documented deformation over the last couple of decades, showed sustained uplift at varying rates through the 1980’s and early 1990’s with no evidence of subsidence until the onset of eruptive activity in September 1994.

DEEP LP EARTHQUAKES

The rate of deep LP “volcanic” earthquake activity beneath the west flank of Mammoth Mountain, which began in 1989-1990, tapered off following the elevated rate that persisted from early 1997 through the end of 1998 (Figure S10). Deep LP earthquakes activity detected in 1999 included roughly 30 events compared with an average of roughly 200 events per year during the 1997-98 interval (Figure S10). Initial results from the analysis of data collected during the 1997 seismic experiment (which involved the temporary deployment of some 60 3-component, digital seismic stations and provides much improved constraint on the hypocentral locations of the LP earthquakes over the routine locations obtained from the permanent seismic network) indicates that these LP events are occurring within a north-striking planar distribution that dips steeply (roughly 80 degrees) to the west at depths of 10 to 20 km beneath the surface. The up-dip extension of this planar distribution intersects the surface along a line that is essentially collinear with the southern extension of the Inyo volcanic chain through Mammoth Mountain.

CARBON DIOXIDE (CO₂ ) AND HELIUM (He) GAS EMISSIONS AT MAMMOTH MOUNTAIN

CO₂ soil-gas concentrations measured at fixed depths in the Horseshoe Lake tree-kill area continue to show an annual variation with snow depth and occasional temporary fluctuations during the snow-free months. The only notable fluctuation in CO₂ concentrations during 1999 involved a three-week concentration increase at the SKI monitoring site on the north flank of Mammoth Mountain that began four days after the May 16, M=5.6 earthquake, which was in the Sierra Nevada located 23 km to the southeast. Whether these two events are somehow related remains unclear.

Additional sampling and analyses of helium isotopic composition in the MMF steam vent on the north side of Mammoth Mountain show that the trend of decreasing ³He/ ⁴He at the MMF vent since 1997 was interrupted by a rise recorded in May 1999, following a period of increased LP activity in the fall of 1998. With respect to the cold CO₂ emissions from the soils on Mammoth Mountain, radioactive carbon measurements made in 1999 on cores from trees at the margin of the HSL tree kill area indicate that the CO₂ discharge in that area peaked in 1991, declined for several years, and then has been relatively constant since about 1995. Continuous measurements of diffuse gas flow rate at several locations, along with periodic measurements over grid points laid out at different tree-kill areas, show that short-term variations in gas flux as large as a factor of 2 can occur at individual discharge areas in response to variations in atmospheric conditions. Such variations, however, correlate well between different discharge areas, suggesting that similar mechanisms are involved at each area. Comparable variability has been observed in the rate of dissolved gas discharge in the groundwater system in response to variations in seasonal precipitation and recharge. Field comparisons of chamber-based equipment used by each of three groups making gas-flow measurements at Mammoth Mountain showed that results obtained by one group were significant higher than those obtained by the other two groups as a result of several factors, the most important being disturbance of the soil surface prior to measurement

REGIONAL EARTHQUAKE 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 two years (Figure S11). 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 12 km [7 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.