LONG VALLEY CALDERA MONITORING REPORT
January-June 1998
(combined 1st and 2nd quarter reports for 1998)
U.S. Geological Survey
Volcano Hazards Program
345 Middlefield Rd. Menlo Park, CA 94025
CALDERA ACTIVITY
SEISMICITY
The surge in earthquake activity within the caldera that began on December 31st with a M=4.8
earthquake at 12:37 PM (PST) beneath the west end of the airport continued through mid-January, 1998. This turned out to be the final episode in the persistent sequence of energetic
earthquake swarms that visited the south moat of the caldera through the second half of 1997.
The waning phase of this swarm included well over 2,000 earthquakes detected and located by the
realtime computer system through January 5 (see note below). Most of the activity was
concentrated in the area of the Fish Hatchery and the airport, although swarm activity on January
1st included six M>2.5 earthquakes with a M=3.8 earthquake at 4:18 AM all located just 2 miles
SE of Mammoth Lakes. The more intense activity in the vicinity of the airport included over 60
M>2.5 earthquakes with more than a dozen of these having magnitudes of M=3.0 or greater (lots
of locally felt shaking at the Fish Hatchery and airport). A M=4.1 earthquake at 6:11 AM on
January 6th also located beneath the west end of the airport turned out to be the last of eight
M=4.0 or greater earthquakes to occur within the caldera since the M=4.1 of November 13, 1997,
which was located beneath the southwest section of the resurgent dome.
[Note on earthquake counts: The large number of earthquakes in Long Valley caldera and vicinity over the last year (over 14,000 events with magnitudes of M~1.2 or greater) has generated a large backlog in standard processing of the earthquake data for final archiving of event times, locations, and magnitudes. The number of earthquakes reported in the accompanying figures reflect the incomplete state of this processing. The initial, automatic processing, which is accomplished in near-real time by on-line computers, is quite reliable under normal circumstances. This real-time system does a good job of detecting earthquakes within and adjacent to the caldera with magnitudes as small as M=0.5, and it normally detects and locates all earthquakes of magnitude M=1.2 and larger. During periods of intense swarm activity, this real-time system, tends to overestimate magnitudes of small earthquakes that occur in rapid-fire sequences. Location errors may increase as well. The second (CUSP) processing stage is intended to catch and correct these and other errors that may show up in the on-line processing. CUSP processing, however, requires off-line, interactive analysis of seismograms by a small group of analysists. Major earthquake sequences such as the 1997-98 Long Valley swarms quickly build up a backlog for CUSP processing that may take many months to work through. The upshot is this:
The seismicity maps and time-magnitude plots in the Long Valley monitoring reports are
based on CUSP processing. The figures in this report reflect the current backlog in CUSP
processing, and thus they significantly under-represent the number of earthquakes that
actually occurred during the swarms of 1997-98. The discrepancy is illustrated in Figure
S13, which compares the numbers of earthquakes detected by the realtime computer
system and the current number of events for which CUSP processing has been completed]
Gradually declining swarm activity included another burst in the vicinity of the airport on January
9 to 11 with nine earthquakes of M=3.0 or greater. The largest in this sequence was a M=3.7
earthquake at 11:33 AM on the 9th. Activity in the vicinity of the Highway 203-395 junction from
January 17-20 included a M=3.3 event at 1:13 AM on the 17th and a M=3.0 event at 1:21 AM on
the 20th. A cluster of earthquakes near Whitmore Hot Springs on January 24th included a M=3.4
earthquake at 5:48 AM. Small earthquakes within the caldera continued to occur at an average
rate of 30 to 40 events per day as detected and located by the realtime computer system through
February. Magnitude M=3.0 and greater earthquakes in February included a M=3.0 event at 2:24
PM on the 5th located beneath Juniper Ridge in Mammoth Lakes, a M=3.5 event at 11:39 AM on
the 9th in the vicinity of the Highway 203-395 junction, a M=3.0 event at 3:46 PM on the 12th
located a mile south of the airport, a M=3.2 event at 4:56 AM on the 16th located near the mouth
of Fumarole Valley (5 miles E of Mammoth Lakes) associated with a swarm of over 80 smaller
events, a M=3.0 event at 3:54 PM on the 18th near the 203-395 junction.
Caldera activity in March included four minor earthquake swarms against a background activity
level that declined from an average of 20 events per day detected and located by the realtime
computer system to fewer than 10 events per day by the end of the month. A swarm on March 2nd
near the Highway 203-395 junction included over 80 events, eight of which had magnitudes of
M=2.5 or larger. The largest was a M=3.0 earthquake at 3:36 PM on the 2nd. Swarm activity on
the 6th and 7th included over 180 detected and located events with seven M>2.5 earthquakes. The
largest was a M=2.8 event at 5:37 PM on the 6th. On March 15-16, an earthquake swarm in the
vicinity of Hot Creek and the airport included over 90 detected and located events, six of which
had magnitudes of M=2.5 or greater with the largest a M=3.0 event at 6:45 AM on the 16th.
Declining earthquake activity within the caldera through the second quarter (April through June)
included occasional small earthquakes and minor swarms throughout the area of the south moat
from the vicinity of Mammoth Lakes east to airport and Whitmore Hot Springs. Only five
earthquakes during the three-month period had magnitudes as large as M=3.0. Two occurred
near the southern margin of the resurgent dome on April 26th. These were M=3.0 and 3.1 events
at 6:50 AM and 12:08 PM on the 26th, respectively, associated with a minor swarm that continued
into the 27th. Magnitude M=3.0 events at 11:35 PM on May 9th and 1:13 AM on May 15th ,
respectively, were located between the 203-395 Highway junction and Mammoth Lakes. The May
15th event was located beneath the eastern edge of Mammoth Lakes (near The Trails
development). At 8:10 PM on May 23rd, a 2.9 earthquake occurred beneath the airport (7 miles
ESE of Mammoth Lakes). At 4:27 AM on May 30, a M=2.5 earthquake located beneath Old
Mammoth produced locally felt shaking. Activity within the caldera continued to decline through
June, although as described under "Regional Activity" below, a M=5.1 earthquake on June 8th
located just south of the caldera and near the northern end Hilton Creek fault produced felt
shaking throughout the area.
The long-period (LP) earthquake activity centered at depths of 10 to 25 km beneath the
southwest flank of Mammoth Mountain, which sustained rates of 15 to 20 events per week from
November through the first week of April, declined abruptly in mid-April. We have detected
almost no LP activity beneath the Mammoth Mountain-Devils Postpile area since mid April.
DEFORMATION
Data from the two-color geodimeter network spanning the caldera show that the episode of
increased extensional deformation and uplift of the resurgent dome that began in mid-1997
gradually slowed from January through mid-May of this year. These data indicate essentially no
deformation since mid-May. The 8 km-long baseline spanning the resurgent dome from CASA to
KRAKATAU, which spans the resurgent dome, extended a total of 10 cm from mid-May 1997
through mid-May 1998. Most of this extension coincided with the intense earthquake swarm
activity in the caldera from November through January (see the July-December 1997 report). In
the past, the length change in the CASA-KRAKATAU line has closely matched the uplift of the
resurgent dome based on leveling data. Indeed, a 10-cm uplift of the central sections of the
resurgent dome from May 1997 through May 1998 is consistent with the continuous GPS data for
the CASA and KRAKATAU monuments. The lack of significant deformation of the resurgent
dome since mid-May is supported by the continuous GPS data and long-base tiltmeter (located
just north of the airport) data, which shows virtually no change in tilt since mid-May. (Note: only
the east component of the long base tiltmeter is operational. We hope to have the north
component back in operation by fall.).
REGIONAL ACTIVITY
Earthquake activity within the region of Long Valley caldera was dominated by a ML=5.1
earthquake and its aftershocks located just south of the caldera boundary near the northern end of
the Hilton Creek fault. The M=5.1 mainshock occurred at 10:24 PM (PDT) on June 8th at a depth
of 6.7 km (4 miles) directly beneath the surface trace of the Hilton Creek fault (epicenter 1 mile
south of the caldera boundary, 3 miles SE of the airport, 7 miles WNW of Tom's Place, and 11
miles ESE of Mammoth Lakes). The Hilton Creek fault dips to the east at between 45 to 60
degrees (from the horizontal), and thus the hypocenter of this earthquake falls below (ie. within
the footwall block) this major, range-front, normal fault. The aftershock sequence define a NW-SE lineation of epicenters that cuts across the NNW-striking surface trace of the Hilton Creek
fault, and the mainshock focal mechanism shows dominantly right-lateral, strike-slip displacement
along a plane coincident with the NW strike of the aftershock distribution. Taken together, it is
clear that this M=5.1 event involved slip on a subsidiary fault rather than the Hilton Creek fault
itself.
The M=5.1 mainshock was preceded by a foreshock sequence that included a M=4.0 earthquake
just over 26 hours earlier (at 3:55 AM on the 7th ) with virtually the same hypocentral location.
Foreshock activity also included a cluster of smaller earthquakes (all M=2.0 or smaller) located
within 2 km of the mainshock epicenter that occurred over the week prior to the mainshock.
Looking back earlier in the year, clusters of small earthquakes (magnitudes M~2 or smaller)
occurred in the immediate vicinity of the June 8 M=5.1 epicenter in both March and May.
The vigorous aftershock sequence that followed the June 8 mainshock included over 1,100
earthquakes with magnitudes of M=1.2 or greater by the end of June. Six of the aftershocks had
magnitudes of 4.0 or greater, the largest of which was a M=4.3 earthquake at 1:07 PM on June
26th. A second M=5.1 earthquake occurred 36 days later (at 9:53 PM on July 14th ) located just 3
km (1.8 miles) SSW of the June 8 event. More about this second M=5.1 earthquake and its
aftershocks in the July-September quarterly report.
Aside from the M=5.1 earthquake and its aftershocks, seismic activity within the Sierra Nevada
block south of the caldera showed little change through the first half of 1998 compared with the
last half of 1997. The largest two events over this six-month period were M=3.3 earthquakes: one
at 3:38 PM on April 22nd located in the vicinity of Mount Morgan (9 miles WSW of Tom's Place)
and the other at 1:33 AM on June 25 located 9 miles south of the caldera in the vicinity of Lake
Virginia. The latter occurred in a brief burst of activity that included two other earthquakes with
magnitudes close to M=3.0. Additional M~3 earthquakes in the Sierra Nevada block included a
M=3.0 event at 11:19 PM on May 6th located 1 mile west of Mount Morrison and a M=3.1 event
at 3:06 PM on May 24th located 0.5 mile west of Convict Lake. Of interest but unknown
significance is the relative lull in the level of activity in the Sierra Nevada block over the 11 day
period (May 25 to June 7) prior to the M=4.0 foreshock to the June 8 M=5.1 earthquake.
Elsewhere, a M=3.2 earthquake at 10:50 PM on March 20 located 2.4 miles NW of Bishop
produced locally felt shaking in the Bishop area.
RESPONSE
The condition remained GREEN (no immediate risk) throughout the first half of 1998. The M=5.1 earthquakes of June 8 and July 14 were both 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.