LONG VALLEY OBSERVATORY QUARTERLY REPORT

JANUARY-MARCH 2004

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

January-March 2004

CONTENTS

EARTHQUAKES

CALDERA ACTIVITY

SIERRA NEVADA ACTIVITY

REGIONAL ACTIVITY

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

CONTINUOUS BOREHOLE AND STRAIN MEASUREMENTS

Instrumentation

Highlights

TILT MEASUREMENTS

Instrumentation

Data

MAGNETIC MEASUREMENTS

BACKGROUND

DATA

CO₂ STUDIES

HYDROLOGIC MONITORING

GROUND WATER LEVEL MONITORING

SURFACE WATER MONITORING

THERMAL WATER DISCHARGE ESTIMATES

SUMMARY FOR JANUARY-MARCH 2004

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the first quarter of 2004. The resurgent dome, which essentially stopped inflating in early 1998 and showed minor subsidence (of about 1 cm) through 2001 followed by gradual inflation through 2002. It has since held relatively steady showing only minor fluctuations about an average elevation roughly 80 cm higher than prior to the onset of unrest in 1980. Seismic activity within the caldera, which has typically included fewer than five small earthquakes per day since 1999, continues at this relatively low level. Earthquake activity in the region was dominated by a magnitude M=3.8 earthquake and a M=3.0 aftershock centered in the Sierra Nevada 2 km east of Red Slate Mountain on the evening of January 12. An isolated M=3 earthquake shook the southern Chafant Valley area on the evening of January 30. 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)

Processing note: Due to the large numbers of aftershocks following the M= 6.5 San Simion earthquake of 22 December 2003, routine checking (CUSP processing) of earthquake locations for the Northern California Seismic Network (of which the Long Valley network is a component) has a backlog. This is reflected in a data gap from February 7 – March 1 (Figure S6). The difference between the number of earthquakes with computer-only locations and those with CUSP locations for the first quarter of 2004 is illustrated in Figure S4.

CALDERA ACTIVITY:

Earthquake activity within Long Valley caldera remained low through the first quarter of 2004 averaging fewer than 5 earthquakes per day large enough to be located by the realtime computer system (Figures S1-S4, S6). None of the earthquakes had magnitudes as large as M=2.0. The most notable caldera activity this quarter was a swarm of a dozen or so small earthquakes (M<2) in a north-northeast lineation in the south moat between the Highway 395-203 interchange and the south wall of the caldera (Figure S2). This trend is interesting because it is nearly perpendicular to the east-west trend common to most swarm sequences in this part of the south moat.

REGIONAL ACTIVITY

As has been true since 1999, earthquake activity in the Sierra Nevada block south of the caldera continues at a higher rate than that within the caldera. Most of the activity continues to be concentrated along the north-northeast trending zone defined by the sequence of three M>5 earthquakes in 1998-99 extending from the southwest margin of the caldera to the vicinity of Grennell Lake in the Sierra Nevada (Figures S1-S5).

The Sierra Nevada produced three earthquakes this quarter with magnitudes greater than M=3.0. The largest was a M=3.7 earthquake at 5:14 PM (PST) on January 12 located 2 km east of Red Slate Mountain (15 km west-southwest of Tom’s Place). It was followed by a M=3.0 aftershock on the same day. A M=3.4 earthquake at 6:13 AM (PST) on January 28 was located beneath McGee Creek 2.4 km east-southeast of Mount Baldwin.

A M=3.2 earthquake at 10:57 PM (PST) on January 30 was located beneath the Chalfant Valley 12 km north of Bishop.

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

John Langbein, Stuart Wilkinson, Elliot Endo, Eugene Iwatsubo, and Jerry

Svarc

Over the past 6 years, 18 GPS (Global Position System) receivers have been installed within and near the Long Valley Caldera. Of these, 14 were installed by Elliot Endo of the Cascades Volcano Observatory. The locations of the 12 receivers within the caldera are shown in Figure G-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 (Figure G-2). The site at CASA now has two receivers; one operating since 1994 and the second one, CA99, installed this past summer.

Review of the previous year of a combination of GPS and EDM data indicate negligible deformation. This is best summarized in Figure G-1, which shows the displacement vectors from continuous GPS for the past year. (Note; the error ellipses for the vectors are greater than the vectors themselves)

There are now 4 baselines from the frequently measured, two-color EDM network that are also measured by continuous GPS. (A fifth baseline, knolls, has been discontinued since the EDM reflector was vandalized last year). The location of the EDM and GPS networks is shown in Figure G-2. Also, the common baselines are highlighted. A comparison of the length-changes derived from GPS and those measured by the EDM are shown in Figure G-3 for the last 4.5

years. Although for the Casa-Krak and Casa-Knolls baseline the comparison could be extend back in time, this comparison includes only the GPS data from the sites installed by USGS. There are now two GPS stations at CASA (Casa and Ca99), and two stations at KRAK (Krak and Krac). (Note that KRAK is now off-line; JPL has lost funding to support this station.) Since the USGS installations use more modern receivers, they have better day-to-day repeatability than the JPL operated receivers. Due to vandalism at the EDM site at KNOLLS, we have abandoned this reflector station. Finally, it should be noted that the EDM measurements on the Casa-Hot

baseline have more scatter than desirable; this is because the optics have deteriorated on the HOT reflector; we have replaced the old reflector with a new one and the repeatability has improved.

More plots of both the GPS and EDM data can be found at; http://lvo.wr.usgs.gov/monitoring/index.html#deformation

Figure G-1. Horizontal displacement vectors in mm/year for continuous GPS sites in and around Long Valley Caldera from January 2004 through June 2004.

Figure G-2 Map showing 2-color EDM baselines

Figure G-3. Line-length changes for the EDM baselines (circles) measured from CASA for the period January 1, 1999 through June 16, 2004 compared with continuous GPS data for the same lines (crosses).

CONTINUOUS BOREHOLE STRAIN MEASUREMENTS (Malcolm Johnston, Doug Myren, Bob Mueller and Stan Silverman)

Instrumentation

Dilational strain measurements are being recorded continuously at the Devil's Postpile (POP), Motorcross (MX) near the western moat boundary in the south moat, Big Springs (BS) just outside the norhtern caldera boundary, and at Phillips (PLV1), just to the north of the town of Mammoth Lakes. 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 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 D-1. Locations of dilatometers and tiltmeters.

Data from the strainmeters are transmitted using satellite telemetry every 10 minutes to a host computer in Menlo Park. The data are also transmitted with 24-bit seismic telemetry together with 3-component seismic data to Menlo park.

Figure D-2. Dilatational strain for POPA, PLV1, MX, and BG borehole dilatometers. Top frame shows pore pressure for POPA

Highlights

The data during this quarter has been relatively quiet at all sites. Raw dilatational data are shown in Figures D-2.  Pore pressure data at the Postpile dilatometer site are shown at the top, and as usual pore pressure is out of phase with dilatational strain at Postpile.

TILT MEASUREMENTS (Mal Johnston, Vince Keller, Bob Mueller and Doug Myren)

Instrumentation

Instruments recording crustal tilt in the Long Valley caldera are of two types - 1) a long-base (LB) instrument in which fluid level is measured in fluid reservoirs separated by about 500 m and connected by pipes, which 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). For tiltmeter locations, see Figure D-1. Real time plots of the data from these instruments can be viewed at http://quake.wr.usgs.gov/QUAKE/longv.html.

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.

Highlights

The east-west component of the longbase tiltmeter (Figure T-2) shows a reversal in trend from gradual down-to-the-east tilt to down-to-the west tilt in mid-March. This roughly coincides with the change from extension to contraction across the resurgent dome shown

Figure T-2. Data for the north-south and east-west components of the long-base tiltmeter (LB).

by the EDM and GPS data (Figure G-3) and is consistent with a change from gradual

inflation to gradual deflation of the resurgent dome. Because of instrument problems, data for the north-south component of the longbase tiltmeter are not reliable. The borehole tilt data (Figures T-3 and T-4) show no significant changes through the first quarter of 2004.

MAGNETIC MEASUREMENTS (M.J.S. Johnston)

BACKGROUND

Local magnetic fields at 12 sites in the Long Valley Caldera are transmitted via satellite telemetry to Menlo Park every 10 minutes. 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. 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 and near the caldera are shown in Figure M-2.

Highlights

 No unusual activity appears to have occurred during this period

CO₂ STUDIES (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 first three months of 2004 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 record from these monitoring stations reflects the usual effect of the winter snow pack. For the first time in several years, the CO₂ concentration at HS2 exceeded the range of the sensor (50%). This is probably due to the higher than normal build-up of snow during the early part of the winter.

Figure C-1 Map showing locations of the continuous CO₂ -monitoring stations.

Figure C-2. Carbon dioxide (CO₂) concentrations for the monitoring stations in Figure C1 for 2002. CAUTION: Raw Data - not corrected for pressure or temperature.

HYDROLOGIC MONITORING (Chris Farrar, Jim Howle, and Michelle Sneed: U.S. Geological Survey, Carnelian Bay and Sacramento, CA).

Hydrologic data collected for the USGS Volcanic Hazards Program in this report include ground-water level data from five wells; stream flow, water temperature, and specific conductance from one site on Hot Creek; and estimated thermal water discharge in Hot Creek Gorge (figure H1). Additional data are available on the web at -- http://lvo.wr.usgs.gov/HydroStudies.html

or 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).

Data from wells LVEW and SF were not recorded between October and March 2004 due to construction of new equipment shelters and changes in the type of equipment used for measurements.

Water levels in CW3 are 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.

Figure H6. Hydrographs for well CH10B, based on filtered mean daily fluid levels.

Fluid pressures in well CW3 during January 2004 reached the lowest level measured since 1995 but show a small rise between February and April 2004. Fluid pressures in well CH10B during April 2003 reached the lowest level measured since 1987 but have risen about 0.3 m through March 2004. These two wells tap the south moat hydrothermal system.

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.

Figure H7. Discharge, water temperature, and specific conductance at Hot Creek Flume (HCF), based on unfiltered daily mean data.

THERMAL WATER DISCHARGE ESTIMATE

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. The quantity of thermal water discharged to Hot Creek is known to vary in response to seasonal variations in precipitation, snow-melt, earthquakes, and other processes. It is believed that spring discharge may change in response to crustal strain.

The calculated discharge of thermal water from springs in Hot Creek Gorge shows a steep decline beginning with the measurement made on August 21, 2003. Between August and December, five measurements were made and all result in calculated discharge of thermal water approximately 18 percent lower than the long-term mean discharge. The estimated thermal water discharge increased between January and April 2004 by about 6 percent of the long-term mean. The decline in discharge with subsequent rise is lagged about six months compared to fluid pressures measured in well CH10B (fig H6).

Figure H8. Estimated thermal water discharge for springs in Hot Creek Gorge.