LONG VALLEY OBSERVATORY QUARTERLY REPORT

JANUARY-MARCH 2003

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 2003

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 2003

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the last half of 2002. The resurgent dome, which essentially stopped inflating in early 1998 and showed minor subsidence (of about 1 cm) through 2001, began renewed inflation early last year at a rate of 1 to 2 cm/year. The center of the resurgent dome still stands roughly 80 cm higher than prior to 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=4.0 earthquake and its aftershocks centered in the Sierra Nevada 5 km south of the caldera boundary (1 km south of Laurel Mountain) on the morning of March 8. 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:

Earthquake activity within Long Valley caldera remained low through the first quarter of 2003 averaging fewer than 5 earthquakes per day large enough to be located by the realtime computer system (Figures S1-S5). The largest earthquake within the caldera during this period was a M=2.4 earthquake on at 4:27 PM (PST) on March 1 located just inside the southern margin of the caldera 4 km west of Lake Crowley (Figure S3).

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. The most energetic activity in the region this quarter involved a M=4.0 earthquake at 7:35 AM on March 8 located 1 km south of Laurel Mountain (5 km south of the caldera boundary: see Figure S3). This earthquake produced felt shaking in the Mammoth Lakes area and was followed by an aftershock sequence that included over 50 smaller earthquakes, the largest of which was a M=3.0 earthquake at 9:37 PM on the 8^th. On March 18, a M=3.0 earthquake at 11:27 AM occurred beneath the Volcanic Tableland 10 km east of Tom’s Place (Figure S3).

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.

The vectors representing horizontal displacements from continuous GPS since January 2002 are shown in Figure G-1. The vectors pointing radially away from the center of the resurgent dome indicate expansion across the resurgent dome of about 3 cm since the most recent episode of inflation began in the spring of 2002.

Figure G-1. Horizontal displacement vectors in mm/year for continuous GPS sites in and around Long Valley Caldera from January 2002 through May 2003.

There are now 5 baselines from the frequently measured, two-color EDM network that are also measured by continuous GPS. The location of the EDM and GPS networks is shown in Figures G-1 and G-2. A comparison of the length-changes derived from GPS and those measured by the EDM are shown in Figure G-3 for the last three 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). Since the USGS installations use more modern receivers, they have better day-to-day repeatability than the JPL operated receivers. 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.

More plots of both the GPS and EDM data can be found at:

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

Figure G-2 Map showing 2-color EDM baselines

The measurements of length changes shown in Figure G-3 for the frequently measured EDM baselines (together with the associated GPS data) show that the gradual contraction that began in early 1999 stopped in late 2001. These two-color data indicate that the baselines spanning the resurgent dome began another episode of extension in early 2002. 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 G-3. Line-length changes for the EDM baselines (circles) measured from CASA for the period January 1, 1999 through June 15, 2003 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.

Highlights

The strain data for the first quarter of 2003 has shown no significant changes. The data for this quarter will be included in the April-June 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

The data from the long base tiltmeter will be included in the April-June report. Very little of geophysical interest occurred this year and the data are generally uneventful.

Data for the shallow (1-m deep) borehole tiltmeters Escape, Fossil, Little Antelope, Casa, Sherwin, and Valentine, which are plotted in Figures T-2a, show no significant changes.

The deep (100-m deep) borehole tiltmeters, Motor Cross and Big Springs, are shown in Figure T-2b. These instruments were cemented in the fall and the transients shown at this time reflect the thermal curing of the expansive grout. Since being grouted in the data quality has improved and the drift rate has decreased.

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.

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

Data for the first three months of 2003 from most of the telemetered monitoring stations are shown in the attached figure along with snow depth (SWE) at Mammoth Pass and precipitation events. [Note: all dates and times in UT. Gas data not corrected for pressure and temperature.] The record from HS1A and HS1B reflects the effect of the winter snow pack, although through the first quarter of this year the snow accumulation has only been about 75% of normal (26.28 inches SWE at Mammoth Pass by March 31). Thus the CO₂ levels at the HS1 sensors have not achieved typical sustained winter values through the end of March. A minimal snow effect at SKI recorded in the winter of 2001 was not observed in 2002 and has not been observed yet this year. Carbon dioxide levels at the remaining monitoring stations were relatively flat during this quarter.

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).

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.

Water levels in all five monitor-wells continued to decline steadily or with only minor periods of rise during the first quarter of 2003. This pattern is similar to that recorded for 2001-2002. By the end of March 2003, water levels in LKT reached the lowest level since 1996, the lowest level in SF since the beginning of record (1996), the lowest level in CW3 since 1995, and the lowest level in CH10B since records began in 1983. The low water levels are due to in part to below average annual precipitation in 1999, 2000, and 2003.

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.

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. Thermal water discharge in Hot Creek gorge was lower in total during 2002 than in any year since 1994.