REVIEW OF LONG VALLEY CALDERA ACTIVITY FOR 1997
U.S. Geological Survey
Volcano Hazards Program
345 Middlefield Rd., Menlo Park, CA 94025
The first half of 1997 was relatively quiet within the caldera. Earthquake activity remained low
with the only M>3 event during the first six months occurring at 12:39 PM (PDT) on April 1st.
Deformation of the resurgent dome continued but with a gradual slowing trend that persisted
through most of May. The most noteworthy events during the first half of the year involved two
M>4 earthquakes located just south of Convict Lake (4 km south of the caldera boundary). The
first, a M=4.2 event, occurred at 3:26 PM (PST) on February 10th, just three days after the
opening of movie "Dante's Peak" (featuring the eruption of a fictitious "north-Cascades"
volcano). The second, a M=4.1 event, occurred at 10:13 AM on February 24th, the day following
the prime-time airing of the ABC movie for TV, "Volcano: Fire on Mountain" (featuring the
eruption of a fictitious volcano at an eastern California ski resort). Not surprisingly, these two
"volcano" earthquakes attracted considerable media attention. Earthquake activity in the shallow
crust beneath Mammoth Mountain (depths less than 10 km) at the southwest margin of the
caldera remained low through 1997. The rate of deep (depths greater than 6 miles or 10 km),
long-period (LP) earthquakes centered beneath the southwest flank of Mammoth Mountain and
the Devils Postpile area, however, increased in early 1997 and remained elevated through the end
of the year. Altogether, over 200 of these events were detected during 1997 (magnitudes all less
than M=2.0). This exceeds the total number of deep, LP events detected from their onset in l989
through 1996.
A gradual increase in the inflation rate of the resurgent dome beginning in late May 1997 marked
the onset of an episode of unrest in Long Valley caldera that intensified at an accelerating rate
through the summer and fall and culminated in a series of strong earthquake swarms from mid
November 1997 through early January 1998. Frequent baseline measurements with the two-color
geodimeter and continuous GPS data document an accelerating inflation of the resurgent dome
through mid-November. The extension rate of an 8-km baseline spanning the resurgent dome
peaked at over 20 cm/y during the 2nd week of November. Following a strong earthquake swarm
on November 22 that included M=4.5, 4.8, and 4.7 earthquakes, the deformation pattern briefly
changed to one dominated by right-oblique slip along the WNW striking "south-moat fault zone"
defined by the hypocenters of the M>4 earthquakes. Then, in early December, the deformation
resumed the pattern of dome inflation with a relatively steady 12- to 15-cm/y-extension rate that
persisted through the end of the year. During the period most rapid deformation from August
through December, the water levels in three wells around the margin of the resurgent dome
dropped by 1.5 to 2 feet, apparently in response to shallow dilatational strain associated with
upwarping of the resurgent dome. By year's end, the 8-km baseline spanning the resurgent dome
was 7 cm longer than in late May. The associated earthquake swarm activity, which began in
early July, developed over a broad, 9-mile (15-km)-long zone spanning the south moat and
southern margin of the resurgent dome. Commonly, several areas within this zone were active at
the same time. This earthquake swarm activity included more than 12,000 M>1.2 and 120 M>3.0
earthquakes over the seven-month period through mid-January with a cumulative seismic moment
of 3.3x1024 dyne-cm (the equivalent of a single M=5.4 earthquake). The peak in seismic activity
from mid-November through early January included eight M>4.0 earthquakes, three of which had
magnitudes of M=4.8 to 4.9. Focal mechanism data for the larger earthquakes indicate dominantly
right-lateral slip along a WNW-trending fault zone within the south moat, although broad band
seismograms admit the possibility of a dilatational component (volume increase) in the source
mechanism for some of the M>4 earthquakes. The great majority of earthquakes had the broad-band character of brittle, double-couple events (tectonic or volcano-tectonic earthquakes). A few
shallow (depths less than 1.8 miles or 3 km) events beneath the southern half of the resurgent
dome, however, had energy concentrated in the 1- to 3-Hz band typical of shallow, long-period
(LP) volcanic earthquakes. Whether the unusual appearance of these seismograms can be
attributed to the earthquake source or to wave propagation effects remains to be determined.
Monitoring data for gases around Mammoth Mountain showed two noteworthy changes in 1997,
both of which are more likely related to the increased deep LP earthquake activity beneath the
southwest flank of Mammoth Mountain than to the much stronger activity within the caldera 3 to
15 km to the east. The helium isotope ratio, 3He/4He, in samples collected from May through
October from the MMF fumarole on the northeast flank of Mammoth Mountain showed an
increase with respect to the gradually declining values in samples collected from late 1995 through
early 1997. Continuous CO2 monitors in tree-kill areas on opposite sides of Mammoth Mountain
(Horseshoe Lake on the south and Chair 19 on the north) detected an abrupt increase in CO2 soil-gas concentrations that began in late September and ended in early December (prior to significant
snow accumulation).
This episode of persistent caldera unrest during the second half of 1997 is the third most energetic activity in the caldera since the intense earthquake swarms of May 1980 (including four M=6 earthquakes) and January 1983 (including two M=5.3 earthquakes). This 1997 activity differs from those earlier episodes in the gradual acceleration in activity over nearly four months prior to the strongest seismic activity, which itself was spread out over nearly three months (from mid-November through early January). The strongest seismic activity in the earlier two episodes occurred within the first few days of the swarm sequences. This 1997 activity attracted considerable attention from the media and provided the first test of the recently adopted color-code notification system for ranking activity levels within the caldera. Although we remained in "condition green" (no immediate risk) throughout, peaks in activity on November 22 and November 30 came close to meeting the guidelines for a "condition yellow" (watch).