LONG
VALLEY OBSERVATORY QUARTERLY REPORT
OCTOBER-DECEMBER
2000
AND
ANNUAL SUMMARY FOR 2000
Long Valley Observatory
U.S. Geological Survey
Volcano Hazards Program, MS
910
345 Middlefield Rd., Menlo
Park, CA 94025
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.
CONTENTS
QUARTERLY REPORT:
OCTOBER-DECEMBER 2000
EARTHQUAKES
CALDERA ACTIVITY
SIERRA NEVADA ACTIVITY
REGIONAL ACTIVITY
DEFORMATION
TWO-COLOR EDM SUMMARY
GPS – CONTINUOUS MEASUREMENTS
DILATATIONAL STRAIN
Instrumentation
Highlights
TILT MEASUREMENTS
Instrumentation
Highlights
MAGNETIC
MEASUREMENTS
INSTRUMENTATION
HIGHLIGHTS
HYDROLOGIC
MONITORING
BACKGROUND
GROUND WATER MONITORING
SURFACE WATER MONITORING
CARBON
DIOXIDE (CO2) STUDIES IN LONG VALLEY CALDERA
REVIEW OF 2000
CALDERA UNREST
SEISMICITY
DEFORMATION
DEEP LP EARTHQUAKES
VERY-LONG-PERIOD VOLCANIC EARTHQUAKES
HYDROLOGY AND CARBON DIOXIDE MONITORING
REGIONAL EARTHQUAKE ACTIVITY
LONG VALLEY OBSERVATORY QUARTERLY REPORT
OCTOBER-DECEMBER 2000
The
relative quiescence that has persisted in Long Valley caldera since the spring
of 1998 continued through the fourth quarter of 2000. The resurgent dome has
shown no significant deformation this quarter, and 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. Most of the earthquake
activity in the region continues to occur as aftershocks to the three M > 5
earthquakes of 8 June 1998 (M=5.1), 14 July 1998 (M=5.1), and 15 May 1999
(M=5.6) in the Sierra Nevada south of the caldera.
EARTHQUAKES (D.P.
Hill and A.M. Pitt)
CALDERA
ACTIVITY:
Earthquake
activity within Long Valley caldera remains low with only a few (typically fewer
than five) events per day large enough (M ³ 1) to be detected and
located by the real-time computer system. None of the earthquakes occurring
within the caldera during this quarter had magnitudes as large as M=2.0.
SIERRA NEVADA ACTIVITY:
Most earthquake activity within the Sierra Nevada block south of the caldera during the fourth quarter of 2000 was 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. Three earthquakes within this aftershock zone had magnitudes as large as M=3.0: specifically, a M=3.0 event at 9:46 PM on October 12, a M=3.2 event at 1:25 PM on December 12, and a M=3.1 event at 8:57 PM on December 20. A cluster of earthquakes beneath the base of Wheeler Crest at the western margin of Round Valley on November 12 included M=2.6 and M=3.6 events at 10:51 and 11:05 PM, respectively.
REGIONAL
ACTIVITY:
The
largest earthquake in the greater Long Valley region this quarter was a M=3.9
event at 6:45 PM on December 28 located in Chalfant Valley some 12 km north of
Bishop. This earthquake was accompanied by a number of aftershocks, all of
which were smaller than M=3.0.
DEFORMATION
TWO-COLOR EDM
SUMMARY (John Langbein, Stuart Wilkinson, and Stefan Kerby)
A two-color Electronic Distance Meter (EDM) is used to monitor the lengths of approximately 70 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 Base-lines and monuments for the 2-color EDM network
The measurements of length changes shown in Figures EDM-2
and EDM-3 for the frequently measured baselines have shown a slight contraction
over the resurgent dome of 1 cm/yr over the two years following the 1997-1998
unrest. However, during the past year, these baselines show no significant
deformation. In contrast, the rates during the later part of 1997 were has high
as 20 cm/yr extension on the KRAKATAU baseline during the most recent unrest in
the caldera., 
Figures EDM 2 (top) and 3 (bottom).
In addition to the frequently measured baselines, the annual survey measurements spanning the mid-1999 to mid-2000 interval shows a correspondingly small or negligible deformation over the past year.
Modeling of all of the EDM data over the 1998.5 to 2000.5 epoch suggest a model of deflation at 8km in the southern part of the resurgent dome.
GPS CONTINUOUS MEASUREMENTS.
John Langbein, Elliot Endo, Eugene Iwatsubo, Jerry Svarc,
Stuart Wilkinson, Stefan Kirby, Frank Webb, and Tim Dixon
USGS-Menlo Park, USGS-CVO, JPL, and U. Miami
Over the past 7 years, there have been 14 GPS (Global Position System) receivers installed within and near the Long Valley Caldera. Of these, 10 were installed in the past 3 years by CVO. The locations of all the receivers are shown in Figure GPS-1. We 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 GPS-2, 3 for all 14 receivers along with two other sites, CMBB and MUSB located on the western slope of the Sierra Nevada.
Figure GPS-1: Locations of continuous in Long Valley caldera
The travel-time measurements from each receiver are processed daily to produce a position in a reference frame with North America fixed. Additional processing is done by removing a temporal, common-mode signal from each time-series of displacements. Gross outliers are also removed. To re-adjust the data to a more local reference frame, a rate is removed from each time series. That rate is the average displacement rate from 1995 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.
A comparison of the length change derived from the GPS data and the length changes measured by the two color EDM can be found at: http://quake.wr.usgs.gov/research/deformation/twocolor/compare_GPS_EDM.html
Figure GPS-4. Comparison of EDM (+)and GPS (o) line-length
changes from CASA99 to stations SAWC, TILC, KNOL, and KRAK for the period 7/99
– 3/01.
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
(POPA), Motor Cross (MX), Big Springs (BS), and at a site, PLV1, just to the
north of the town of Mammoth Lakes in Long Valley. The site locations are shown
in Figure D1. The instruments are Sacks-Evertson dilational strain meters and
consist of stainless steel cylinders filled with silicon oil that are cemented
in the ground at a depth of about 200m.
Changes
in volumetric strain in the ground are translated into displacement and voltage
by a 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).
Data
from the strainmeters are transmitted using satellite telemetry every 10
minutes to a host computer in Menlo Park. The data are also transmitted from
some sites with 24-bit digital seismic telemetry, together with 3-component
seismic data. At sites where this does not occur, data are
recorded
on site on 16-bit digital recorders and a summary of the high-frequency seismic
and train data is transmitted by satellite.
Figure D1. Locations of borehole
dilatometer and tilt stations in Long Valley caldera
II. HIGHLIGHTS
The most interesting events recorded this year are strain histories recorded on POPA during Long Period (LP) events under Mammoth Mt. on July 6, 2000 and August 13, 2000 (see Fig. D1 for locations of all strainmeters). The first event showing the strain record in "raw" form and that in filtered form to illustrate the short period and long period extensional component of the transient, is displayed in Figure D2. This event generated strain offsets on all three working strainmeters in the caldera region. These offset are seen in the expanded plot of longer-term data also shown in Figure D2. The upper plot on the left shows the raw data for 12 hours on July 6 with the offsets superimposed on Earth tides while the lower plot on the left show the data with the earthtides removed. Net extensions of 0.9 and 0.4 nanostrain were observed on POPA and MX respectively, while an extension of -0.3 nanostrain was observed on BG. Thus at POPA we see a transient of about 1.5 nanostrain superimposed on a net offset of 0.9 nanostrain. The seismically determined depth of the LP event was about 3 km. To explain the strain offset using a pressure source at this depth, we need a pressure increase of 1.4 MPa associated with the event.
The second event occurred on August 13 and the
"raw", low- and highpass filtered data are shown in Figure S10 in the
2000 Annual Summary. This event generated an almost identical transient at POPA
but no corresponding strain offsets. This event had a seismically determined depth of about 3.75 km
and requires a pressure transient of about 2 MPa to generate the observed
strain signal at POPA. We do not know if transient signals were
recorded at MX and BG at these times since these instruments were not yet
connected to high sample rate telemetry.
A
summary of data from the four borehole strainmeters in Long Valley for the
entire year is shown in Figure D3. The annual term in the POPA data is due
to snow load and snow melt in the San Joaquin headwaters. We are now monitoring
the water table near the strainmeter.
To view data see: 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.
II. HIGHLIGHTS
The data from the long base tiltmeter is shown in Figure T1. Very little of geophysical interest occurred this year and the data are generally uneventful. We unfortunately lost some data during the summer when some instrumentat failures occurred. Data for the shallow borehole tiltmeters Escape, Fossil, Little Antelope, Casa, Sherwin, Valentine, Motor Cross and Big Springs are shown in Figures T2.
Data can be viewed in real time on:
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 2000, ten
additional magnetometers, together with a 3-component magnetometer system and
three magnetotelluric systems (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 M1.
Temporal changes in local magnetic field are isolated using simple differencing
techniques.
Figure M1. Map showing locations of magnetometer and
magnetotelluric (MT) stations in Long Valley caldera
DATA
Plots of
daily averaged data from the telemetered magnetometer stations in the
Long Valley
region are shown in Figures M2-5. Each of these stations are referenced to a
site on Sherwin Grade (MG) located to the south of the caldera.
HIGHLIGHTS
The
differenced data for the 10 magnetic field stations, referenced with station
MGS, are shown in Figures M3-5. Missing data are due to telemetry problems. The
long-term rate changes for differences HCR-MGS (+1.0 nT/a) and SBF-MGS (-0.6
nT/a) are continuing from 1991 to 2001 (Figure M2). No significant changes in
magnetic field are observed during this reporting period. Changes during mid July, mid-August, and
November are due to magnetic storm activity and are not due to tectonic
sources. Station PLV was struck by
lightning during the end of August and will not be operational until May
2001. Missing data during the last part
of December are due to telemetry problems at site MGS.
Figure M2.
Differential magnetic data for stations SBF and HCR with respect to MGS since
1984.
During August
2000, one new magnetometer (DCM) and one new magnetotelluric system (LAP) were
installed in the Long Valley Caldera
(Locations-Figure M1, Data-Figure M4. All are operational with data
being recorded onsite and with USGS satellite telemetry.
During
August, three new magnetometers (MXP, VAP, and LAP), from the University of
Tokyo, were installed at existing locations in the Long Valley caldera
(locations-Figure M1, Data- Figure M4). All three are operational with data
being recorded onsite and with USGS satellite telemetry (Figures 2-4). A 3-component
magnetometer was co-located with the POP station and an additional total field
magnetometer installed over a borehole ant the PLV station (Figure M1). The
second MT experiment was installed at LAP.
Figures
2-5. Differential
magnetic data for the year 2000.
HYDROLOGIC MONITORING
(Chris Farrar, Jim Howle, Michelle Sneed, Devin Galloway,
and Mike Sorey: U.S. Geological
Survey, Carnelian Bay, Sacramento, and
Menlo Park, CA).
Hydrologic
data collected for the USGS Volcanic Hazards Program in this report include
ground-water level data from five wells and stream flow, water temperature, and
specific conductance from one site on Hot Creek (figure H1). Additional data are available upon request –
contact: Chris Farrar or Jim Howle
at Carnelian Bay 530.546.0187.
BACKGROUND
Ground-water
levels in wells and the discharge of springs can change in response to strain
in the Earth’s crust. The network of
five wells and one surface water station provides hydrologic data that
contributes to monitoring deformation and other changes caused from magmatic
intrusions and earthquakes in Long Valley Caldera.
GROUND-WATER LEVEL MONITORING
Ground-water
levels are measured continuously in five wells, LKT, LVEW, SF, CW-3, and CH-10B
(figure H1), using pressure transducers that are either submerged below the
water surface or placed above ground and sense back-pressure in a
nitrogen-filled tube extending below the water surface. Barometric pressure is also measured at each
site using pressure transducers. The
data are recorded by on-site data loggers and telemetered on a three-hour
transmit cycle using the GOES satellite and receivers at Menlo Park and
Sacramento. All sites are visited
monthly to collect data from on-site recorders and to check instrument calibrations.
Data
processing is done in the Sacramento Office.
Records of barometric pressure are used in combination with the
water-level records to determine aquifer properties from the observed
water-level response to atmospheric loading and earth tides. The influences of barometric pressure
changes and earth tides are removed from the water-level records. The result yields the filtered water-level
record that may contain other hydraulic and crustal deformation signals. Filtered data for wells LKT, CW-3, and
CH-10B are given in figures H2, H5, and H6.
Analysis of the records from LVEW and SF to provide filtered data is not
yet complete; therefore raw data are presented for these two sites (figure H3).
Periods of missing data are due to use of the well for testing or water supply, or because of instrumentation problems.
Periods of missing data are due to use of the well for testing or because of instrumentation problems.
SURFACE WATER MONITORING
Site
HCF is located downstream from the thermal springs in Hot Creek Gorge (figure
H1). Stage, water temperature, and
specific conductance (figure H7) are recorded every 15-minutes. The data are recorded by an on-site data
logger and telemetered every three hours.
Specific conductance is a measure of total dissolved ionized
constituents. Water at HCF is a mixture
of thermal water from springs along Hot Creek and non-thermal water from the
Mammoth Creek basin. Changes in
specific conductance are related to changes in the mixing ratio of thermal and
non-thermal components of stream flow.
Estimates
of total thermal water discharge (figure H8) are computed from monthly
measurements of discharge, and boron and chloride concentrations collected at a
non-recording site (HCA) located upstream of the Hot Creek gorge thermal area
and at site HCF downstream.
