LONG
VALLEY OBSERVATORY QUARTERLY REPORT
July-September
2001
Long Valley Observatory
U.S. Geological Survey
Volcano Hazards Program, MS
910
345 Middlefield Rd., Menlo
Park, CA 94025
http://lvo.wr.usgs.gov
This report is a preliminary description of unrest in Long
Valley caldera and Mono-Inyo Craters region of eastern California. Information
contained in this report should be regarded as preliminary and is not be cited
for publication without approval by the Scientist in Charge of the Long Valley
Observatory. The views and conclusions contained in this document do not
necessarily represent the official policies, either express or implied, of the
U.S. Government.
LONG VALLEY OBSERVATORY QUARTERLY REPORT
July-September 2001
EARTHQUAKES
SIERRA NEVADA ACTIVITY
REGIONAL ACTIVITY
TWO-COLOR EDM SUMMARY
GPS CONTINUOUS MEASUREMENTS
DILATATIONAL STRAIN AND TILT
Instrumentation
Highlights
INSTRUMENTATION
HIGHLIGHTS
CO2 STUDIES IN LONG VALLEY CALDERA: 2nd
& 3rd Quarter 2001
SUMMARY
The
quiescence in Long Valley caldera that began in the spring of 1998 continued
through the first quarter of 2001. The resurgent dome, which essentially
stopped inflating in early 1998 and showed minor subsidence (of about 1 cm)
through the first half of this year, has shown no further changes this quarter.
The center of the resurgent dome currently stands just under 80 cm higher than
prior to 1980. Seismic activity within the caldera has typically included fewer
than five small earthquakes per day, most with magnitudes less than M=2.0.
Diffuse emission of carbon dioxide (CO2) in the tree-kill areas
around the flanks of Mammoth Mountain continue at the relatively high levels
that have persisted since 1996.
Up-to-date
plots for most of the data summarized here are available on the Long Valley
Observatory web pages (http://lvo.wr.usgs.gov).
CALDERA
ACTIVITY:
Earthquake
activity within Long Valley caldera remained low with only a few (typically
fewer than five) events per day large enough to be detected and located by the
real-time computer system (generally M > 1). The largest earthquake to occur
within the caldera this quarter was a M~2.0 event at 12:47 PM on September 12
located just 0.5 mile northwest of the geothermal plant (3 miles east of
Mammoth Lakes).
SIERRA
NEVADA ACTIVITY:
Most earthquakes occurring within the Sierra Nevada block south of the caldera during the first quarter of 2001 were again concentrated in the aftershock zone for the three M5 earthquakes of 8 June 1998 (M=5.1), 14 July 1998 (M=5.1), and 15 May 1999 (M=5.6), which defines a 15-km-long, linear zone of epicenters extending to the south-southwest into the Sierra Nevada from the southeastern margin of the caldera. The largest of these earthquakes was a M=3 event at 5:33 PM on September 12 located 1 mile south of Grinnell Lake (12 miles southwest of Toms Place).
REGIONAL
ACTIVITY:
A magnitude
M=2.9 earthquake at 11:55 PM on August 29, which was located 15 miles west
southwest of Bishop, produced felt shaking in the Bishop area.
Further
a field, an earthquake swarm that began on July 15 in the Coso volcanic field
(in the southern Owens Valley 120 miles south southeast of Long Valley caldera)
continued through early August. This swarm included over 30 earthquakes with
magnitudes of M=3.0 or greater including M=4.9 and 4.7 earthquakes on July 17th.
DEFORMATION
TWO-COLOR EDM
SUMMARY (John Langbein, and Stuart Wilkinson)
A two-color
Electronic Distance Meter (EDM) is used to monitor the lengths of approximately
10 baselines in and near the Long Valley Caldera shown in Figure EDM-1. The precision of each length measurement is
between 0.5 and 1.0 mm. The 8 baselines
shown with heavy lines that use CASA as a common end point are measured several
times each week. Other baselines that have CASA in common are measured at less
frequent intervals of 1 to 2 months. The remaining baselines are currently
measured once per year. With the frequent measurements, we can monitor temporal
changes in the deformation. With the annual measurements, we can monitor the
spatial extent of deformation.
Figure
EDM-1 Map
showing 2-color EDM baselines
The measurements of
length changes shown in Figure EDM-2 for the frequently measured baselines show
that the gradual contraction that began in early 1999 appears to have stopped
in mid-2000. These two-color data indicate that the baselines spanning the
resurgent dome contracted by roughly 2 cm since mid 2000. This compares with
over 35 cm of extension from the beginning of the 2-color EDM measurements in
mid-1983 through mid-1998. Based on the relation between leveling and 2-color
data, the center of the resurgent dome remains about just under 80 cm higher
than in the late 1970s prior to the onset of caldera unrest.
Figure
G2.
Line-length changes for the EDM baselines measured from CASA from the beginning
of measurements in early 1984 through December 19, 2001.
GPS CONTINUOUS
MEASUREMENTS. (John Langbein, Elliot Endo, Frank Webb, Tim Dixon, Stuart
Wilkinson, and USGS-Menlo Park, USGS-CVO, JPL, and U. Miami)
Over the past 6 years, 12 GPS (Global Position System) receivers
have been installed within and near the Long Valley Caldera. Of these, eight
were installed in the past 2 years by Elliot Endo of the Cascades Volcano
Observatory. The locations of receivers
within the caldera are shown in Figure GPS-1. It is intended that data from
these receivers and a few more additional installations will take over the
long-term monitoring supplied by the two-color EDM. The three component
displacement data are shown in Figure GPS2-4 for all 12 receivers along with
two other sites, CMBB and MUSB located on the western slope of the Sierra
Nevada. The site at CASA now has two receivers; one operating since 1994 and
the second one, CA99, installed this past summer.
The travel-time measurements from each receiver is processed
daily to produce a position in a reference frame with North America fixed.
Additional processing involves removing a temporal, common-mode signal from
each time-series of displacements as well as the gross outliers. To re-adjust
the data to a more local reference frame, a rate is removed from each time
series. This rate is the average displacement rate from 1996 to the present of
the 2 Sierra Nevada stations, CMBB and MUSB. In the plots, to show any
deviation from a constant rate, the local rate is also removed and that rate is
posted next to the trace of the residual displacements. These preliminary GPS data are
consistent with no significant deformation within Long Valley caldera over the
past year.
DILATIONAL STRAIN MEASUREMENTS (Malcolm
Johnston, Doug Myren, Bob Mueller and Stan Silverman)
I. Instrumentation
Dilational strain measurements are being recorded continuously
at the Devil's Postpile, POPS, and at a site, PLV1, just to the north of the
town of Mammoth Lakes in Long Valley and at the two new sites, MCX and BSP
(Figure D1). The instruments are Sacks-Evertson dilational strain meters and
consist of stainless steel cylinders filled with silicon oil that are cemented
in th
e ground at a depth of about 200m. Changes in volumetric strain
in the ground are translated into displacement and voltage by an expansion
bellows attached to a linear voltage displacement transducer. This instrument
is described in detail by Sacks et al.(Papers Meteol. Geophys.,22,195,1971).
Figure D1.
Location map for borehole dilatometers (triangles) and tiltmeters (solid
circles). LB is the Long Base tiltmeter.
Data from the strainmeters are transmitted using satellite
telemetry every 10 minutes to a host computer in Menlo Park. The data are also
recorded on site on 16-bit digital recorders together with 3-component seismic
data and on backup analog recorders. A summary of the high-frequency seismic
and strain data is also transmitted by satellite.
II. Dilatometer Highlights
The borehole dilatometers show no geophysically significant signals this quarter.
Real-time plots for these instruments are available at
http://quake.wr.usgs.gov/QUAKE/crustaldef/longv.html.
TILT MEASUREMENTS (Mal Johnston, Vince
Keller, Bob Mueller and Doug Myren)
I. Instrumentation
Instruments recording crustal tilt in the Long Valley caldera
are of two types - 1) a long-base instrument in which fluid level is measured
in fluid reservoirs separated by about 500 m and connected by pipes (this
instrument (LB) was constructed by Roger Bilham of the University of Colorado),
and 2) borehole tiltmeters that measure the position of a bubble trapped under
a concave lens.(All Others). Figure D1 shows the locations of the seven
tiltmeters that are installed in Long
Valley, California.
All data are transmitted by satellite to the USGS headquarters
in Menlo Park, Ca. Data samples are taken every 10 minutes. Plots of the
changes in tilt as recorded on each of these tiltmeters are shown. Removal of
re-zeros, offsets, problems with telemetry and identification of instrument
failures is difficult, tedious and time-consuming task. In order to have a
relatively up-to-date file of data computer algorithms have been written that
accomplish most of these tasks most of the time. Detailed discussion or
detailed analysis usually requires hand checking of the data. Flat sections in the data usually denote a
failure in the telemetry Gaps denote missing data. All instruments are scaled
using tidally generated scale factors.
The
tiltmeters showed no significant changes this quarter.
Real
time plots of the data from these instruments can be viewed at
http://quake.wr.usgs.gov/QUAKE/longv.html.
MAGNETIC MEASUREMENTS (R.J. Mueller and M.J..S. Johnston)
BACKGROUND
Local magnetic fields at
Hot Creek (HCR) and Smokey Bear Flat (SBF) in the
Long Valley Caldera have transmitted data via satellite
telemetry to Menlo Park since January 18, 1983. Satellite telemetry has been
operating at station Sherwin Grade (MGS) since January, 1984. Between August
1998 and August 1999, eight additional magnetometers, together with a
3-component system and a magnetotelluric system (MT), were installed at
existing telemetry locations inside and adjacent to the Long Valley Caldera in
cooperation with Dr. Yosi Sasai (Univ. of Tokyo) and Dr. J. Zlotnicki (CNRS, France).
These and other data provide continuous 'real-time' monitoring in this region
through the low frequency data system. The location of these sites is shown on
Figure 1. Temporal changes in local magnetic field are isolated using simple
differencing techniques.
DATA
Plots of daily averaged
data from the telemetered magnetometer stations in the
caldera are shown in Figures 2-5. Each of these stations are
referenced to a site on Sherwin Grade (MG) located to the south of the caldera.
HIGHLIGHTS
No
significant changes in magnetic field were observed during this reporting
period.
CO2
STUDIES IN LONG VALLEY CALDERA: 2nd & 3rd Quarter
2001
Ken
McGee, Terry Gerlach, and Mike Doukas, Cascades Volcano Observatory,
Vancouver, WA
The GOES-telemetered carbon dioxide monitoring network in the Mammoth Lakes area continued to transmit data on soil gas carbon dioxide concentrations throughout the report period. Station HS1 is located near the central portion of the Horseshoe Lake tree kill in an area of high CO2 ground flux while HS2 is located in a lower flux area near the margin of the tree kill and HS3 is outside the tree-kill zone in the group campground area. Stations located away from Horseshoe Lake include SKI, located near Chair 19 in the Mammoth Mountain Ski Area, SRC, located at Shady Rest Campground adjacent to the USFS Visitor Center in Mammoth Lakes, EQF, located near Earthquake Fault, and LSP, located near Laurel Spring in the inferred Long Valley caldera rim fault. At all sites, CO2 collection chambers are buried in the soil. Air from these collection chambers is pumped to nearby carbon dioxide sensors housed in USFS structures or culverts. Local barometric pressure is also measured at HS1 using a Vaisala Pressure Transducer. Data are collected from the sensors every hour and are telemetered every three hours via GOES satellite. The GOES transmitting antennas, typically mounted inside adjacent USFS structures, continue to produce strong signals to the satellite even after significant snow buildup on the roofs of the structures. All monitoring sites have backup data loggers that also record ambient temperature. Snow data are obtained from a U.S. Bureau of Reclamation monitoring station at Mammoth Pass.
Data for the middle six months of 2001 from most of the telemetered monitoring stations are shown in the attached figure along with snow depth (SWE) at Mammoth Pass. [Note: all dates and times in UT. Gas data not corrected for pressure and temperature.] The early portion of the record from these monitoring stations reflects the usual decline of the winter snow pack. Curiously, the distinct increase in CO2 concentration in April and early May at SKI (and noted in the first quarter of 2001 as well) also suggests a snow effect. The snow effect at SKI has been minimal to nonexistent in the past. By mid-summer, many of the monitoring stations had telemetry or other problems as shown by the gaps in the record. Since we no longer have a technician on the project, repair of these stations had to wait until August at the time of the annual maintenance trip. A scheduled airborne gas flight at Mammoth Mountain was not completed due to the lack of availability of an aircraft. It is hoped the flight can be rescheduled during the fourth quarter of this year.
Mike Sorey, Bill Evans, John Rogie, and Mack Kennedy
Samples collected in 2000 and 2001 from the Mammoth Mountain Fumarole (MMF), on the north side of Mammoth Mountain, continue the time line of variations in helium isotopic composition began in 1989. Beginning in March 2001, samples have once again been collected at relatively frequent intervals (1-2 months) to allow a possible relation between 3He/4He at MMF and the occurrence of LP events beneath the Mammoth Mountain region to be better delineated.
Values of 3He/4He plotted in Figure 1 show a general trend of declining magmatic component following the 1989-1990 period of significant increase in this component. However, short periods of increasing 3He/4He occurred in 1992, 1997 and 2001. The increase in 1997 may be related to increased magmatic degassing associated with increased LP activity beginning in the spring of 1997. Similarly, the significant increase in 3He/4He in samples collected from March-August 2001 may also be related to increased LP and VLP activity beginning in the summer of 2000.
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Figure 1. Helium isotope ratios in fumarole MMF on the north side of Mammoth Mountain and occurrences and magnitudes of long-period (LP) earthquakes and large regional earthquakes for the period 1989-2001. Lines depicting LP events are length-sdaled to their magnitude (data provided by Mitch Pitt), except for 2 very long-period (VLP) events at depths as shallow as 4 km that occurred in the summer of 2000.