LONG VALLEY OBSERVATORY QUARTERLY REPORT

JULY-SEPTEMBER 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

July-September 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 MEASUREMENTS

CONTINUOUS MEASUREMENTS: MAMMOTH MOUNTAIN

RESURGENT DOME

HYDROLOGIC STUDIES

SUMMARY FOR JULY-SEPTEMBER 2003

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the first half of 2003. The resurgent dome, which has shown minor fluctuations in uplift and subsidence since early 2000, showed essentially no change during the 3^rd quarter of 2003. 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. The largest earthquake in the region this quarter was a magnitude M=3.2 earthquake in the Sierra Nevada near Grinnell Lake (13 miles SE of Mammoth Lakes) at 12:35 AM (PDT) on August 18. 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. Recently established efforts to measure CO₂ within the caldera reveal isolated areas of slightly elevated emission rates on the resurgent dome and in the Basalt Canyon area (located north of Highway 203 roughly one mile west of the geothermal plant).

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 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=2.6 earthquake on at 7:51 AM (PDT) on September 19 associated with a cluster of smaller events located in the south moat beneath the east of Mammoth Lakes (Figure S3).

REGIONAL ACTIVITY

As has been true since 1999, earthquake activity in the Sierra Nevada block south of the caldera continues at a slightly higher rate than that within the caldera. Most of these earthquakes continue to be located within the aftershock zone of the M=5.6 earthquake of May 1999. The largest earthquake in the region this quarter was a M=3.2 earthquake at 12:35 AM on August 18 located just east of Grinnell Lakes in the Sierra Nevada south of the caldera (12 miles SE of Mammoth Lakes: see Figure S2).

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 May 2002 through mid-October 2003 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 May 2002 through October 17, 2003.

Five baselines from the frequently measured, two-color EDM network 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 was followed by an episode of gradual expansion that began in late 2001 and persisted through early 2003. This expansion has since slowed, and the resurgent dome has shown no significant deformation through mid-2003. 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 November 13, 2003 compared with continuous GPS data for the same lines (crosses).

Figure G-3. Line-length changes for the EDM baselines (circles) measured from CASA for the period November 2002 through November 13, 2003 compared with continuous GPS data for the same lines (crosses).

CONTINUOUS BOREHOLE STRAIN MEASUREMENTS (Malcolm Johnston, Doug Myren, 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 third quarter of 2003 show no significant changes. The steady decrease in dilatational strain at POPA through the third quarter reflects a recurring annual strain cycle (see Figure D-3).

Figure D-2. Dilatational strain for July-September 2003 from the POPA, Big Springs (BS), and Motor Cross (MX) borehole dilatometers. Also shown is the pore pressure in a well adjacent to the POPA dilatometer.

Figure D-3. Long-term plots of dilatational strain for the POPA and Big Springs dilatometers with associated pore-pressure records.

TILT MEASUREMENTS (Mal Johnston, Vince Keller, 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

Data for the tiltmeters show no significant changes for the period April-June 2003 (see Figures T-1 and T-2).

Figure T-2. East-west and north-south components for the shallow borehole tiltmeters for July-September 2003. Increasing values indicate tilt down to the east and north, respectively.

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 M-1. 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.

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 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.

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, 2003.

Diffuse CO₂ Studies (Deborah Bergfeld, Chris Farrar, , Jim Howle and William Evans: U. S. Geological Survey, Menlo Park, and Carnelian Bay, CA).

Background

In July 2003, we constructed and sampled two new CO₂ flux grids at tree kill locations on the resurgent dome. The new grids, Basalt Fumarole (BF) and Ridge Tree Kill (RTK) include 25 and 37 sample locations respectively, and encompass around 9,000 and 6,000 m² each (Table 1). The thermal ground at BF is long-lived and gas samples have previously been collected from vents on the west side of the grid. In contrast, tree kills at RTK are relatively recent. The RTK area is west of two older small areas of tree kill at FVN and FVS (Fig. 1). A brief investigation of soil temperatures and flux at RTK in November 2002 revealed soil temperatures as high as 49 and 68 °C at depths of 50 and 70 cm, respectively. When we returned in July 2003 it appeared that the extent of the tree kill had increased.

During the July 2003 field investigation we also measured fluxes at previously established grids at Casa Diablo (CD), Basalt Canyon (BC) and Dead Tree Hill (DTH). Grid locations are shown on figure 1 and grid details are presented in table 1. All of the grids studied thus far contain sections of thermal ground and areas of vegetation kill. Visible steam emissions are observed in most of the grids.

Figure 1. Generalized map showing the location of the CO₂ flux grids.

Flux and Soil Temperature at BF and RTK

Diffuse CO₂ emissions at BF are high, and there is a large range in flux values across the grid. (Fig. 2a). Soil temperatures collected at 10 cm depth were between 22 and 74 °C and are positively correlated with CO₂ flux (r = 0.75). The average flux at BF is 290 g m^-2d^-1, which is just slightly lower than averages for the nearby BC grid (Figs 2a-c). The larger flux at BC reflects the presence of several sites with focused CO₂ emissions. Tree kill at BF is concentrated along the west side of the grid and is likely related to elevated soil temperatures and / or CO₂ flux.

Table 1. Information about diffuse flux grids and CO₂ emissions.

At RTK there are no focused gas vents and the average flux is around 14 g m^-2d^-1 (Fig. 3), which is typical of background soil CO₂ fluxes. Soil temperatures collected at 10 cm depth were between 19 and 44 °C and are moderately correlated with flux (r = 0.6). CO₂ emissions at nearby grids FVN and FVS are similar to RTK (Table 1). The relation between tree kill and CO₂ flux and soil temperatures at RTK is not obvious. Measurement sites around dying trees commonly had fluxes and soil temperatures around background levels.

Figure 2. Histograms showing the distribution of CO₂ fluxes from the BF grid (2a) compared with fluxes from the nearby BC grid (2b, 2c).

Figure 3. Histogram showing the distribution of CO₂ fluxes at the RTK grid.

Temporal Variations in CO₂ Flux

Replicate measurements at the CD grid over 3 field investigations show relatively large variations in average CO₂ fluxes (Fig. 4a-c). These variations are expected, and detailed studies have shown CO₂ flux is commonly influenced by atmospheric conditions. At this time we assume the observed change in flux at CD is not related to power production. This assumption is supported by similar variations in average flux observed over 2 sets of measurements at BC and DTH (Table 1).

Figure 4. Histograms showing the variation in CO₂ fluxes at the CD grid.

CO₂ Emissions from the Resurgent Dome

Estimates of total emissions in tonnes of CO₂ per day (t d^-1) are calculated using the average grid flux multiplied by the grid area. As demonstrated by the grid at DTH, relatively low fluxes over a large area can yield substantial estimates of CO₂ emissions (Table 1). In such areas, a high percentage of sites have fluxes typical of biogenic activity making it necessary to discern the potential for biogenic CO₂ contributions. At this time we have not characterized the biogenic input at all of the low flux grids. Flux measurements and isotopic analyses of soil-CO₂ in the winter months when activity is negligible will allow us to evaluate biogenic contributions, and will aid us in quantifying anomalous diffuse CO₂ emissions over a larger portion of the resurgent dome.

At high flux locations such as CD, BC and BF characterization of background CO₂ flux emissions is much less important and can essentially be ignored, especially when most of the vegetation is dead. Summing average CO₂ emissions from the three high flux grids CD, BC and BF indicates around 6.4 t d^-1 of anomalous CO₂ is emitted from only 29,240 m² on the resurgent dome.

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.

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.