LONG VALLEY OBSERVATORY QUARTERLY REPORT

OCTOBER-DECEMBER 2003

AND

ANNUAL SUMMARY FOR 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

October-December 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

ANNUAL SUMMARY FOR 2003

SUMMARY FOR OCTOBER-DECEMBER 2003

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the last quarter of 2003. The resurgent dome, which has shown minor fluctuations in uplift and subsidence since early 2000, showed essentially no change during the 4^th 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=3.6 earthquake beneath the western edge of Round Valley at 6:07 PM (PST) on November 10. 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).

A four-day workshop, “Understanding a Large Silicic Volcanic System: An Interdisciplinary Workshop on Volcanic Processes in Long Valley Caldera-Mono Craters”, was held October 8-12, 2003, at the Mammoth Mountain Lodge on the north flank of Mammoth Mountain, Mammoth Lakes, CA. The workshop included over 65 participants from academia, government agencies, and the private sector, with participants from Italy, Japan, New Zealand, and Great Britain. Goals of the workshop were to 1) develop an interdisciplinary assessment of our current understanding of the Long Valley – Mono Craters volcanic system, and 2) identify outstanding questions that might be resolved with new observations and experiments as a framework for guiding future proposals to both NSF and the USGS Volcano Hazards Program. Wide-ranging discussions during the workshop emphasized that, although we have learned a great deal about this complex magmatic system over the past 25 years, a number of important questions have yet to be resolved. The National Science Foundation (NSF) and the Volcano Hazards Program of the U.S. Geological Survey supported the workshop with Paul Segall (Stanford University) and Dave Hill (USGS) as principal organizers.

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 fourth quarter of 2003 averaging fewer than 5 earthquakes per day large enough to be located by the realtime computer system (Figures S1-S5). The greatest activity was associated with a cluster of small earthquakes beneath the southeastern margin of the resurgent dome on November 8 that included a dozen events, three of which were above magnitude 2. The largest was a M=2.2 earthquake at 9:02 PM (PST). Altogether, the caldera produced only five M>2 earthquakes during this quarter with none exceeding M=2.2.

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=3.6 earthquake beneath the west margin of Round Valley (20 km southeast of the caldera) at 6:07 PM on November 10 (Figure S2).

Activity of note beyond the immediate vicinity of Long Valley caldera included a series of small earthquakes beneath the Bridgeport Reservoir (~2 km north of Bridgeport) in mid-December, the largest of which were M=3.5 and 4.0 events at 1:17 PM and 4:20 PM (PST), respectively, on December 10. These earthquakes produced felt shaking in the Bridgeport area. On December 20 at 8:35 AM, a M=4.1 earthquake occurred beneath the western margin of Eureka Valley on the east side of the Inyo Mountain that produced felt shaking in the Bishop-Big Pine area.

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. Grey arrows indicate marginally significant changes.

Figure G-1. Horizontal displacement vectors in mm/year for continuous GPS sites in and around Long Valley Caldera from August 2002 through March 16, 2004.

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-3a,b 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 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-3a. Line-length changes for the EDM baselines (circles) measured from CASA for the period January 1, 1999 through January 25, 2004 compared with continuous GPS data for the same lines (crosses).

Figure G-3b. Line-length changes for the EDM baselines (circles) measured from CASA for the period January 2003 through January 25, 2004 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 data during this quarter has been relatively quiet at all sites.Raw data are shown in Figures D2. Pore pressure data at the Postpile dilatometer sites are also shown in Figure D2. As usual these are out of phase with the strain records at Postpile.

Good records of strain were obtained as a result of the December 22 M6.5 San Simeon earthquake. These are shown in Figure D2.  This event, in contrast to other large events such as the M7.9 Denali earthquake, the M7.4 Landers and the 7.1 Hector Mines events did not trigger deformation and seismicity in Long Valley Caldera.

Figure D-2. Dilatational strain for September-December 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. Arrows indicate the M=6.5 San Simeon earthquake of 22 December 2003. Bottom three panels show the high-frequency strain records for the San Simeon earthquake as recorded on POPA, MX, and BS dilatometers.

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. 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 is shown in Figure D1. During this period, technical problems and difficulties with satellite data collection platforms plagued primarily

the north component of this tiltmeter. The "event" on about December 10 resulted from repairs done at that time by Roger Bilham. The increase in lbew.flt on about November 1 resulted from rainfall. Very little of geophysical interest occurred this period

and the data are generally uneventful. Data from the tiltmetes in the deep boreholes at Big Springs and Motorcross are also shown in Figures T2.

Figure T-2. Tilt components for the longbase tiltmeter together with the tiltmeters installed with the Big Springs (BS) and Motocross (MX) borehole dilatometers.

A major effort to upgrade all the tiltmeters installed at about 7 m deep was initiated in mid-September and all were replaced with automatic self-leveling tiltmeters. Data from these borehole tiltmeters (Escape, Fossil, Little Antelope, Casa, Sherwin, and Valentine) are shown in Figures T3. The "events" in  mid-September are not of geophysical origin. They relate to the installation of replacement tiltmeters.

Figure T3. Tilt components for the shallow borehole tiltmeters for September-December 2003.

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.

HIGHLIGHTS

 While there are some unusual changes at individual stations in November these most probably relate to induction effects from solar flare activity at this time that is incompletely removed by the simple differencing technique we use to isolate local magnetic field effects.

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 (Fig C1). 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 October through December from most of the telemetered monitoring stations are shown in Figure C2 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 for October and most of November. There is a slight unexplained dip in the baselines of several of the stations near the end of October. In December, the baselines from HS1A and HS1B begin to rise and transition into the typical pattern of spikes and dips in response to the beginning of significant snowfall accumulation at Horseshoe Lake.

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 October-December, 2003.

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

BACKGROUND

The coincidence of areas of tree and brush kills, elevated soil temperatures and CO₂ discharge on and around portions of Long Valley caldera provide evidence of changes in surficial conditions. Many of the areas of vegetation kill are located near the Casa Diablo power plant, and increases in soil temperatures and CO₂ emissions can be linked to the onset and / or increases in geothermal fluid production in the late 1980’s and early 1990’s. At other locations such as the Teapot (Tpt), Ridge Tree Kill (RTK) and Isha Tree Kill (ITK) development of dead zones became apparent only in the last few years (Fig E1). These new areas are located on opposite sides of regional faults and it is unclear if the onset of elevated soil temperatures is associated with a single underlying source. A third type of thermal area in the caldera is represented by Hot Bubbling Pool (HPB) and Shady Rest Fumarole (SRF). Thermal ground at these sites is long established and much of the ground is bare or supports only sparse grasses or low lying brush. Patches of dead brush at HBP however, show that thermal conditions there too have changed.

In November 2003 we completed the initial phase of our investigation of diffuse CO₂ flux at discrete areas of vegetation kill across the caldera. At the end of the survey CO₂ fluxes had been measured at ten primary and five secondary grids (Fig. E1). The grids range in size from approximately 800 to 36,000 m². Results for all grids to date are summarized in Table E1. In this report we also present findings from an initial examination of CO₂ flux in the south moat and soil-gas chemistry from locations at four grids.

Figure E1. Generalized map showing the location of the primary (black squares) and secondary (gray squares) grids, gas sample locations from this and previous studies (white circles) and flux traverses (dotted lines). Basalt group includes BCE, BC, and BF.

Table E1. Results from primary and secondary grids.

Results from new locations

Four of the five new grids constructed in November contain areas of vegetation kill but only one grid, Basalt Canyon Extension (BCE), contributes significantly to anomalous CO₂ emissions (Table E1). The average flux calculated for BCE is a minimum estimate since emissions at four sites were too high to measure accurately and were excluded from the data set. Total emissions from the new grids at HBP, SRF, ITK and Tpt are less than 1 t d^-1, however the presence of recent tree kills at ITK and Tpt emphasizes a need for continued observation at these grids.

In this quarter we also began an investigation of CO₂ emissions away from plant-kill areas. Because of the association between the 1997-1998 south moat earthquake swarms and magmatic fluid intrusion, some study of south moat CO₂ emissions is needed. Our initial survey of 40 locations over portions of the south moat did not identify any high flux sites. Although much more work would be required to fully characterize the south moat flux, early findings indicate the average flux is about 3 g m^-2 d^-1, similar to other background values observed during the investigation.

CO₂ emissions

The majority of excess geothermal CO₂ emissions at the caldera are derived from the Basalt Canyon (BC), Basalt Fumarole (BF) and BCE grids located west of the power plant (Table E1). These areas emit roughly 8 tonnes of CO₂ per day from about 32,000 m² of thermal ground. The nearby Casa Diablo (CD) grid adds additional CO₂ to the overall budget. Replicate measurements at the CD grid indicate that CO₂ emissions are around 1.9 t d^-1 from a 14,400 m² area.

Several lines of evidence suggest that the high flux area represented by the Basalt group (BC, BF and BCE) may be expanding. Fluxes at several BC locations in the November survey were too high to measure accurately, stressed and dying trees continue to appear south of BC (Fig. E2) and background fluxes at the BFW grid were high compared to background fluxes at the FIG grid. Fluxes have not been measured around the dying trees but a limited survey of soil temperatures showed temperatures at five locations were 5 to 10 degrees higher than background values.

Figure E2. Image map showing high flux areas near the Casa Diablo power plant. Black circles are grid points, Fault locations are approximate.

Soil gas chemistry

Initial results from soil gas samples reconfirm the presence of isobutane (the working fluid used by the power plant) at high flux locations at the CD, BC and BF grids (Table E2 These results are not surprising since earlier studies have previously identified isobutane in gas samples collected from fumaroles within or near the grids. The soil gas sample at RTK in contrast, was predominately air and did not contain measurable concentrations of isobutane. Additional soil-gas samples have been collected at SRF, ITK and HBP. Results from these samples will be presented in a future report.

Table E2. Results from analyses of soil-gas samples.

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 December 2003 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.

Fluid pressures in wells CW3 and CH10B dropped to the lowest levels measured since 1995 and 1987 respectively. 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.

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 decline in discharge is consistent with the decline in fluid pressure measured in well CH10B (fig H6).

REVIEW OF 2003

With the close of 2003, Long Valley Caldera has sustained nearly five years of relative quiescence – the longest such interval since the onset of unrest in 1978.

Deformation

The slow inflation of the resurgent dome at a rate of ~ 1 cm/year that persisted through most of 2002 leveled off in early 2003 with essentially no change through the end of the year (Figure A1). With the gradual subsidence from early 1999 through 2001 and the uplift through 2002, the center of the resurgent dome currently stands only about 0.5 cm higher than in early 1999 and remains roughly 80 cm higher than in the late 1970s. The continuous deformation monitoring instruments (tiltmeters and dilatometers) as well as the magnetometers showed no significant changes through the year consistent with the GPS and EDM data.

Figure A1. Line length changes across the resurgent dome with respect to the monument CASA for 1997 through 2003 based on the 2-color EDM measurements (red crosses) and continuous GPS data (black circles). See Figure G1 for monument locations.

Seismicity

Seismic activity within the caldera remained low through 2003 as it has for the previous four years averaging fewer than five earthquakes per day large enough to be located by the realtime computer system (M ~ 0.5 and above). As in the past, most of these earthquakes were confined to the south moat and the southern margin of the resurgent dome (Figure A2). The largest intra-caldera earthquake during the year was a M=2.4 event on September 19 at 7:51 AM (PDT) associated with a cluster of smaller events in the south moat beneath the east margin of Mammoth Lakes. An earthquake sequence of comparable intensity was centered beneath the southeastern margin of the resurgent dome on November 8. This sequence included three M>2 earthquakes, the largest of which was a M=2.2 earthquake at 9:02 PM (PST).

Most of the earthquake activity in the Sierra Nevada block south of the caldera continued to be concentrated in the north-northeast lineation of epicenters that represents the aftershock zone of the three M>5 earthquakes of June and July 1998 and May 1999 (Figure A2). A notable exception was the M=4.0 earthquake of March 8 (7:35 AM, PST) that was located 1 km south of Laurel Mountain (~5 km south of the caldera boundary and 11 km east-southeast of Mammoth Lakes). This earthquake produced felt shaking in the Mammoth Lakes area and was accompanied by over 50 smaller earthquakes, the largest of which was a M=3.2 event. The Grinnell Lake area near the southern end of the seismicity lineation in the Sierra Nevada (Figure A2) was one of the more persistently active areas through the year. It produced M=3.2 earthquakes on June 15 and August 18 as well as a host of smaller earthquakes.

Occasional M~3 earthquakes elsewhere in the region included: a M=3.2 earthquake on January 23 3 km east of Red Slate Mountain (midway along the seismicity lineation in Figure A2), a M=3.0 earthquake on March 18 located beneath the Volcanic Tableland 10 km east of Crowley Lake, a M=3.1 earthquake on August 31 located 2 km east of Lake Dorothy in the Sierra Nevada, a M=3.0 earthquake on October 26 located 20 km west of Bishop, and a M=3.5 earthquake on November 10 in Round Valley. Altogether, ten earthquakes of M=3 or greater occurred in the area during 2003, the largest being the M=4.0 event on March 8 near Laurel Mountain.

The mid-crustal (10- to 25-km-deep) long period (LP) volcanic earthquakes, which began during the 1989 Mammoth Mountain earthquake swarm, have continued beneath the southwest margin of Mammoth Mountain but at a much-reduced rate with respect to the activity levels during the first half of 1997 (Figure A5). LP activity for 2003 was limited to the first and last quarters of the year with no LP earthquakes detected from April through September.

Figure A5. History of long-period (LP) volcanic earthquake activity beneath the southwest flank of Mammoth Mountain from 1 June 1989 through 2003. The vertical bars indicate the number of events per week and the solid line tracks the cumulative number of earthquakes with time.

Carbon dioxide

The carbon dioxide (CO₂ ) emissions from the tree-kill areas around the flanks of Mammoth Mountain have shown no evidence of significant change over the last several years. In particular, data from the CO₂ sensors at Horseshoe Lake are relatively flat and uneventful for 2003 except for the normal winter excursions due to snow accumulation.  A soil CO₂ efflux survey of Horseshoe Lake in August gave an emission rate of 135 tons/day, which is slightly higher than the rate for 2002.  However, the emission rate trend from 1995 through 2003 based on linear regression is relatively flat at about 100 tons/day and suggests a termination of the elevated CO₂ condition at Horseshoe Lake is unlikely anytime soon. The Horseshoe Lake tree-kill area produces roughly one third of the total CO₂ flux from the flanks of Mammoth Mountain.

Deborah Bergfeld and her colleagues (see the "Diffuse CO₂ Studies" section) describe several sites within the caldera showing a combination of vegetation-kill, and elevated CO₂ concentrations and soil temperatures that appear to be associated with the geothermal system within the caldera. The areas that produce the greatest CO₂ emissions are in the vicinity of the geothermal plant and have been known for some time. Initially the formation of these areas likely occurred as a result of superficial changes linked to increases in geothermal fluid production in the late 1980s and early 1990s. Bergfeld also describes some recently identified sites with elevated soil temperatures on the resurgent dome above Fumarole Canyon that may reflect a delayed response to the 1997 earthquake swarm activity in the area. Total CO₂ emissions at these sites are marginally above background levels.

Hydrology

Hydrologic monitoring data show that declining fluid pressures in key monitoring wells over the past several years continued through 2003. Fluid pressures in four of five key monitoring wells during 2003 were at the lowest values since 1995 and for three of these wells the pressures were the lowest since the late 1980s. The data also show a sharp decline in thermal water discharge from springs in Hot Creek Gorge began in August 2003 and persists to the end of 2003.  The decline in discharge is approximately 18 percent of the long-term mean discharge.

The decline in thermal water discharge from Hot Creek Gorge springs is consistent with the low fluid pressures recorded in wells CW3 and CH10B, both of which tap the south moat hydrothermal system. The reason for this decline is not clear. Geothermal production from the Casa Diablo power plant has not changed significantly over the past year and the caldera has shown no significant unrest.

Long Valley Exploratory Well (LVEW) – Installation of a deep borehole observatory.

During the week of August 2, a team of scientists and drilling experts from the oil industry successfully installed a 30-meter-long geophysical instrument string at a depth ~2.4 km (7,500 feet) in the Long Valley Exploratory Well (LVEW). The instrument string includes two three-component seismometers (4 Hz natural frequency, one at 2592 m and the other at 2264 m), a dilatometer (2254 m), a 48-m-long vertical-axis optical fiber strainmeter centered at 2150 m, and pass-through tubes designed to track pore pressure in the open hole beneath the instrument package. The instrument components are all specially designed to operate in the 100^o C environment at the installation depth. Those involved in the design and installation of the instrument string include Peter Malin (Duke University), who designed the borehole seismometers, Selwyn Sacks and Alan Linde (Carnegie Institution of Washington) who designed the high-temperature version of the borehole dilatometer, Mark Zumberg (U.C. San Diego) who designed the optical fiber strain meter, and Evelyn Roloffs (USGS) who designed the pass-through configuration for hydrological monitoring. The down-hole seismometers are producing beautifully clean seismograms and an impressively noise-free signal from the bottom-hole seismometer as telemetered to Menlo Park. As signals from the remaining components of the LVEW deep borehole observatory come on line over the next few months, they will greatly enhance the power of the LVO network as both a monitoring and research tool enhance the power of the LVO network as both a monitoring and research tool.

Instrumentation of LVEW as a deep borehole observatory represents the final stage (Phase IV) of a major drilling project that began in the mid-1980 with support variously from the Department of Energy, the California Energy Commission, the International Scientific Drilling Project (ICDP), DOSECC (the Drilling, Observation, and Sampling of the Earth’s Crust consortium funded by NSF), and the USGS (for a summary of Phases I-III, see Sorey, Hill, and McConnell in California Geology, v. 53, pp. 4-11, 2000).

Long Valley Caldera Workshop: October 8-12, Mammoth Mountain Inn

A four-day workshop on Long Valley Caldera “Understanding a Large Silicic Volcanic System: An Interdisciplinary Workshop on Volcanic Process in Long Valley Caldera-Mono Craters” was held from the evening of October 8 through October 12, 2003, in the Mammoth Mountain Ski area facilities perched on the southwest rim of the caldera. The National Science Foundation and the U.S. Geological Survey jointly supported the workshop, which included over 65 participants from academia, government science agencies, and the private sector, with participants from Italy, Japan, New Zealand, and Great Britain. The workshop was convened by Paul Segall (Stanford University) and Dave Hill (USGS) with the able assistance from Marcus Bursik (SUNY, Buffalo) and Gillian Foulger (Durham University, UK). A summary of the workshop will appear in a coming issue of EOS.