LONG VALLEY OBSERVATORY QUARTERLY REPORT

JULY-SEPTEMBER 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

APRIL-JUNE 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 JULY-SEPTEMBER 2004

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the second quarter of 2004. The resurgent dome, which essentially stopped inflating in early 1998 and showed minor subsidence (of about 1 cm) through 2001, was 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. The largest earthquake this quarter was an isolated M=3.0 event near the south margin of the caldera on September 20. 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.

An earthquake swarm that began in the Adobe Hills 20 km east of Mono Lake on September 18 included M=5.4 and 5.4 earthquakes on the afternoon of the 18^th that produced felt shaking throughout the region.

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:

Earthquake activity within Long Valley caldera remained low through the third quarter of 2004 averaging fewer than 3 earthquakes per day large enough to be located by the realtime computer system (Figures S1-S4, S6). The two most noteworthy events included 1) a swarm on the 4^th of July that included some 14 small earthquakes (the largest a M=2.7 earthquake at 7:51 PM) located just north of Hot Creek near the southeast margin of the resurgent dome, and 2) a M=3.0 earthquake at 4:24 PM on September 20 located near the south rim of the caldera just north of Convict Lake (see Figs S1, S3, and S5).

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 Grinnell Lake in the Sierra Nevada (Figures S1-S5).

The largest earthquake in the Sierra Nevada block this quarter was a M=3.0 earthquake at 12:32 AM on July 19 located 2 km south of Grinnell Lake (~9 km south of the caldera boundary). The overall activity levels showed minor surges in earthquake numbers and magnitudes during the first three weeks of August and again in mid-September. The September “surge” included a M=2.8 event on 14th (see Figure S4).

Elsewhere in the region, the onset of a prolonged earthquake swarm in the Adobe Hills centered roughly 20 km east of Mono Lake was marked by a M=2.3 earthquake at 12:02 AM on September 18 followed by M=4.0 and 4.1 earthquakes at 12:07 and 12:08 AM, respectively. Activity continued to intensify through mid-afternoon of the 18^th with M=5.5 and M=5.4 earthquakes at 4:02 and 4:43 PM, respectively. These M>5 earthquakes produced widely felt shaking over the area from Bridgeport to Bishop. By the end of September, this swarm had produced over 700 detectable earthquakes including 33 with magnitudes M>3 and 6 with M>4.

Previous earthquake swarm activity in the Adobe Hills includes a prolonged swarm of comparable intensity in the summer and fall of 1980 with M=4.8 and M=4.9 earthquakes on September 7. At the time, however, this 1980 Adobe Hills swarm was eclipsed by the much more energetic activity in Long Valley caldera and the adjacent Sierra Nevada block associated with the onset of caldera unrest that began with the sequence of four M 6 earthquakes in May of 1980.

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-2, which shows length changes in the two-color EDM baselines (Figure G-1) together with line-length changes determined from the continuous GPS data.

Also see; http://lvo.wr.usgs.gov/monitoring/index.html#deformation

Figure G-1 Map showing 2-color EDM baselines

Figure G-2. Line-length changes for the EDM baselines (red circles) measured from CASA for the period May 1984 through October, 2004 compared with continuous GPS data for the same lines (black 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.

Highlights

The data during this quarter has been relatively quiet at all sites. Data plots for this period will be included in the 4^th quarter report.

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

Very little of geophysical interest occurred this period and the data are generally uneventful. Data plots for this period will be included in the 4^th quarter report.

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 M-1. 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.

Highlights

Not much to report for this quarter. Data plots for this period will be included in the 4^th quarter report.

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, 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. Precipitation data are collected by the USFS at the Mammoth Lakes Visitor Center.

Data for July through September from most of the telemetered monitoring stations are shown in the attached figure along with precipitation events as recorded at the USFS Ranger Station in Mammoth Lakes. [Note: all dates and times in UT. Gas data not corrected for pressure and temperature.] The records from all of the stations show the normal low baselines characteristic of this time of year. The slight offset in some of the records in August is due to on site annual servicing of the monitoring stations. During the August servicing trip, the annual soil CO₂ efflux survey was conducted at Horseshoe Lake. Results are still being evaluated and will be reported later.

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 April-June 2004.

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).

Figure H2. Hydrographs for well LKT, based on filtered daily mean values.

Data from wells LVEW and SF were not recorded between October 2003 and June 2004 due to construction of new equipment shelters and changes in the type of equipment used for measurements. A pressure transducer was installed in LVEW and fluid-level recording was begun in June 2004. Fluid-level recording is expected to begin in SF beginning September 2004.

Figure H5. Hydrographs for well CW3, based on unfiltered values from January 1988

through August 1993 and filtered daily mean values from September 1993 through September 2004. Periods of missing data are due to use of the well for testing or because of instrumentation problems. 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. Fluid pressures in well CH10B during April 2003 reached the lowest level measured since 1987. Fluid pressures in CW3 began rising in early 2004 and in CH10B began rising in mid-2003, however pressures in both wells are still low relative to long-term means. 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 through most of 2004, reaching values near the long-term mean. The decline in discharge with subsequent rise lags about six months behind the fluid pressures measured in well CH10B (fig H6).

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