LONG
VALLEY OBSERVATORY QUARTERLY REPORT
April-June
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
April-June 2001
CONTENTS
EARTHQUAKES
CALDERA ACTIVITY
SIERRA NEVADA ACTIVITY
REGIONAL ACTIVITY
DEFORMATION
TWO-COLOR EDM SUMMARY
GPS – CONTINUOUS MEASUREMENTS
DILATATIONAL STRAIN AND TILT
Instrumentation
Highlights
MAGNETIC
MEASUREMENTS
INSTRUMENTATION
HIGHLIGHTS
HYDROLOGIC
MEASUREMENTS
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 roughly 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).
EARTHQUAKES (D.P.
Hill and A.M. Pitt)
CALDERA
ACTIVITY:
The level of earthquake activity within Long Valley caldera increased slightly over last quarter but generally remains 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 most noteworthy seismic events within the caldera include: (1) a cluster of small earthquakes (all with magnitudes M<2) in the south moat of the caldera April 2 and 3; (2) a cluster of half a dozen small earthquakes beneath the east flank of Mammoth Mountain, all with magnitudes M<2, between 7:45 and 11:06 PM (PST) on April 27; (3) a magnitude M=2.5 earthquake at 6:40 AM (PST) on May 3 beneath the Convict Moraine (1 mile south of the airport and 7 miles east-southeast of Mammoth Lakes); and (4) a M=2.8 earthquake at 6:44 AM (PDT) on May 21 beneath the southern margin of the caldera 0.5 miles north of Convict Lake (8 miles east-southeast 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. Three of these earthquakes had magnitudes above M=3.0. A M=3.1 earthquake at 6:52 AM on April 20 was located one mile south of Red Mountain (5 miles southwest of Tom’s Place). A M=3.1 earthquake at 1:35 AM on April 28 was accompanied by eight smaller events all located beneath Big McGee Lake (11 miles west-southwest of Tom’s Place). A M=3.2 earthquake at 1:22 AM on May 4 located 2 miles south of McGee Mountain (8 miles west-southwest of Tom’s Place) was followed by some 12 smaller aftershocks. A tight cluster of a dozen small earthquakes on May 24 was located 1 mile northwest of Mount Morrison. The largest in this sequence was a M=2.8 earthquake at 4:46 AM on the 24th.
REGIONAL
ACTIVITY:
Notable
earthquake activity elsewhere in the region included a pair of magnitude M=2.5
earthquakes at 10:47 and 10:52 AM (PDT) on June 2 located 5-6 miles
west-northwest of Bishop. Between April 8 and April 29, a cluster of some 12
small earthquakes occurred beneath the surface expression of the White
Mountains fault zone at the base of the western escarpment of the White
Mountains (18 miles north of Bishop). Most of these occurred on April 22, and
the largest was a M=2.2 earthquake at 1:37 AM on the 22nd.
DEFORMATION
TWO-COLOR EDM
SUMMARY (John Langbein, Stuart Wilkinson, and Stefon Kirby)
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 1 cm over the past year. 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 80 cm higher than
in the late 1970’s 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 July 20, 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. An important technical achievement concerns our ability to now view the data at high sample rates using seismic data telemetry. Data from the Post Pile (POP) and Motocross (MCX) instruments now come back by 24-bit digital telemetry over phone lines. Data from the Big Springs instrument (BSP) will soon come back by 24-bit satellite telemetry.
The dilatometer data plots for the first quarter will be included in the second quarter monitoring report. 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.
II. Tiltmeter Highlights
The
long-base tiltmeter has had operational problems for much of the first quarter.
Data from the shallow borehole tiltmeters will be included in the second
quarter report. None of these data have shown geophysically significant changes
during 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.
Figure M1. Locations of
differential magnetic field stations within Long Valley caldera. The reference
station MGS (not shown) is located along Highway 395 approximately 20 km
southeast of the caldera.
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
The
differenced data for the 10 magnetic field stations, referenced with station
MGS, are shown in figures 3, 4, and 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 into 2001 (Figure 2). No
significant changes in magnetic field are observed during this reporting
period. Changes around April 12th
are due to magnetic storm activity and are not due to tectonic sources. Station PLV was struck by lightning at the
end of August 2000 and will not be operational until August 2001.
Figure M2. Magnetic field differences between the
station pairs SMF - MGS and HCR – MGS
in nano
Tessela (nT) for 1984 through mid-2001.
HYDROLOGIC MONITORING (Chris
Farrar, Jim Howle, and Michelle Sneed:
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. The steep pressure drops recorded during late 1997 in all three wells probably are mostly caused by the high rate of crustal extension in the central part of Long Valley Caldera during that same period. 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 (figures H3 and H4).
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.
Water levels in CW3 are known to be affected by pumping at
the Casa Diablo geothermal field.
Examples of these effects include the large pressure drop in 1991 and
the distinct peak in 2000. 
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. Water temperatures change in response to ambient temperatures and the mixing ratio.
