LONG VALLEY OBSERVATORY QUARTERLY REPORTS

COMBINED July – December 2007

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 REPORTS

July – December 2007

CONTENTS

EARTHQUAKES

CALDERA ACTIVITY AND MAMMOTH MOUNTAIN

SIERRA NEVADA ACTIVITY

REGIONAL ACTIVITY

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

CONTINUOUS BOREHOLE AND STRAIN MEASUREMENTS

TILT MEASUREMENTS

MAGNETIC MEASUREMENTS

BACKGROUND

HIGHLIGHTS

CO₂ STUDIES

HORSESHOE LAKE

MAMMOTH MOUNTAIN

MONO CRATERS

HYDROLOGIC MONITORING

ANNUAL SUMMARY 2007

SUMMARY FOR JULY – DECEMBER 2007

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the first half of 2007. 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. The deformation pattern since 2003 has been characterized by gradual subsidence that appears to have flattened out in early 2007. The center of the resurgent dome remains some 75 cm higher than prior to the onset of unrest in 1980. Seismic activity within the caldera included only 39 M ≥ 1 earthquakes during this six-month period, the largest of which was a M = 1.9 earthquake on August 6. The Sierra Nevada south of the caldera continued to show a higher activity rate with 8 M ≥2.8 earthquakes, the largest of which was a M = 3.5 earthquake at 9:34 AM (PST) on November 1 located 2 miles south of McGee Mountain. Sierra Nevada activity included a swarm of nearly 80 small earthquakes (M ≤ 2.3) on September 20-22 centered 0.5 mile south of Convict Lake. The carbon dioxide flux in the vicinity of Mammoth Mountain remains elevated but has shown evidence of a fluctuating decline since 1995. Measurements of the CO₂ degassing rate at Horseshoe Lake, which averaged 2005 averaged about 100 tons of CO₂ per day (t/d) through 2005, had decreased to ~ 39 t/d by mid-2006. The August 2007 measurements, however, showed a relative increase to ~75 t/d. Sporadic episodes of geysering in Hot Creek that began in May 2006 has continued but at a generally declining rate.

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)

Note: Seismic activity in this report uses the automatic computer-generated (Earthworm) solutions rather than the final hand- check (CUSP processing) solutions. The computer-generated epicentral locations and magnitude estimates have become increasingly reliable with time, and they do not suffer from backlogs that can develop in CUSP processing due to an abrupt increase in the rate of earthquake activity elsewhere in northern California.

LONG VALLEY CALDERA AND MAMMOTH MOUNTAIN ACTIVITY:

Low levels of earthquake activity beneath Long Valley caldera and Mammoth Mountain continued through the last six months of 2007. None of the earthquakes during this period exceeded magnitude M=2.0. The only activity of note was a cluster of small earthquakes beneath the Basalt Canyon area (0.5 mile northwest of the Hwy 395-203 Junction) between 9:00 and 10:00 AM (PST) on December 21. The largest earthquake in this sequence had a magnitude of M = 1.7 (Figure S6)

SIERRA NEVADA ACTIVITY

As has been true since 1999, earthquake activity in the Sierra Nevada block south of the caldera continues at a higher rate than within the caldera with most of the activity concentrated in a band extending from the southern margin of the caldera for some 20 km to the south-southwest (Figures S1-S7). Nine of these earthquakes had magnitudes between 2.9 and 3.0. The only event larger than M=3.0 was a M = 3.5 earthquake at 9:34 AM (PST) on November 1 located near McGee Creek 2 miles south of McGee Mountain (Figure S5). A swarm of nearly 80 small earthquakes on September 20-22 was centered 0.5 mile south of Convict Lake. The largest event in this sequence was a magnitude M = 2.3 earthquake at 1:05 PM (PST) on September 20 (Figure S3).

REGIONAL ACTIVITY

Elsewhere in the region, low level activity in the Adobe Hills area (10 to 15 miles east of Mono Lake) included three earthquakes with magnitudes between 2.9 and 3.0 on July 20, August 18, and December 31, respectively.

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

John Langbein, Stuart Wilkinson, Mike Lisowski, Eugene Iwatsubo, and Jerry

Svarc

Over the past 7 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 G1. The close correlation of variations in baseline lengths between the EDM measurements that began in 1984 and the GPS measurements, the first of which began in 1999 (Figures G1, G2, G3), has allowed us to discontinue the expensive, labor-intensive EDM measurement in October of 2006. In the future, we will rely entirely on the GPS measurements.

Recent results from the GPS data indicate that the gradual shortening of the baseline lengths by 1 to 3 cm from 2004 through 2006 (consistent with subsidence of the resurgent dome by a comparable amount) reversed trend through mid 2007 and has shown slow extension over the past six months (Figure G2). Over the long term, the center of the resurgent dome remains some 75 cm higher than prior to the onset of caldera unrest in 1980 (Figure G3). Also see; http://lvo.wr.usgs.gov/monitoring/index.html#deformation

Figure G-1 Map showing 2-color EDM baselines

Figure G2.Line-length changes for the EDM baselines (red crosses) measured from CASA for the period February 12, 1999 through October 3 2007 compared with continuous GPS data for the same lines (black circles). Note that the EDM measurements were discontinued in October 2006.

Figure G3. Line-length changes for the EDM baselines (red crosses) measured from CASA for the period June 1984 through October 3, 2007 compared with continuous GPS data for the same lines (black circles). Note that the EDM measurements were discontinued in October 2006.

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 a 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).

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.

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Figure D1. Locations of dilatometers and tiltmeters.

Highlights

The data during this quarter has been relatively quiet at all sites. Pressure corrected data are shown in Figures D2. Comparative pore pressure and strain data at the Postpile and Big Springs dilatometer sites is shown in Figure D3. The 1.5 microstrain drop in compressional strain for POPA from July-October reflects a decrease in ambient pore pressure (Figure D3) associated with the normal drop in summer runoff on the nearby San Joaquin River.

Figure D2. Dilatational strain for borehole dilatometers POPA, PLV1, MX, and BS f or June – December 2007.

Figure D3. Comparison of dilatational strain and pore pressure for borehole dilatometers POPA and BG (Big Springs) for June – December 2007.

TILT MEASUREMENTS (Mal Johnston, Roger Bilham, Doug Myren and Stuart Wilkensen)

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 D1. 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 in Figures T1-T3. 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

Fig T1 shows the long base data from June 1, 2007 to Jan 1, 2008. No changes of note are apparent. Data from the tiltmeters in the deep boreholes at Big Springs and Motorcross are shown in Figure T2.Data from the short base tiltmeters are shown in Figures T3. Very little of geophysical interest occurred this period.

Figure T1. East-west and north-south components of the long-base tiltmeter for 1 June through December 2007. Positive slopes indicate tilt down the south and east, respectively.

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Figure T2. East-west and north-south components for the borehole tiltmeters installed with the Big Springs (BS) and Motocross (MX) dilatometers for June – December 2007.

MAGNETIC MEASUREMENTS (M.J.S. Johnston, S. Wilkinson, Doug Myren, Y. Sassai, and Y. Tanaka)

Background

Local magnetic fields at 18 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 Figures M2.. As these instrument are getting old we are having great difficulty keeping them alive. Dedicated work by Stuart Wilkinson is greatly appreciated.

Highlights:

Nothing unusual during this quarter.

Figure M2. Differential magnetometer data for the stations shown in Figure M1.

CO₂ STUDIES

HORSESHOE LAKE TREE-KILL AREA (Cindy Werner, Mike Doukas and Ken McGee 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 until August 2007 when the telemetry transmitters were removed. Station HS1 is located near the central portion of the Horseshoe Lake tree kill in an area of high CO₂ ground flux and has both a 0-100% sensor and a 0-50% CO₂ sensor. Station HS2 is located in a lower flux area near the margin of the tree kill and HS3 is at the edge of the tree-kill zone in the group campground area. Stations located away from Horseshoe Lake include SKI, located near the former Chair 19 in the Mammoth Mountain Ski Area and SRC, installed as a background site, located at Shady Rest Campground adjacent to the USFS Visitor Center in the town of Mammoth Lakes. 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 were collected from the sensors every hour and telemetered every three hours via GOES satellite through July. The GOES transmitting antennas, mounted inside the USFS structures except for SRC, produce strong signals to the satellite even after significant snow buildup on the roofs of the structures. The antenna for SRC is located on top of the building and is vulnerable to damage by snow and wind. All monitoring sites have backup data loggers that also record ambient temperature. The data loggers are now the primary method of data recording and storage. Snow data are obtained from a U.S. Bureau of Reclamation monitoring station at Mammoth Pass

Data for 2007 for are shown in Figure C1 along with snow depth (SWE) at Mammoth Pass. [Note: all dates and times in UT. Data not corrected for pressure and temperature.] Station HS3 stopped transmitting in February and was not repaired.

The typical annual buildup of CO₂ at Horseshoe Lake seen most years on many of the stations was absent during 2007 due to the much smaller than average snow pack. The cause of the distinct CO₂ peak recorded at HS1A, HS2 and SKI during March is not clear at this time.

During the annual monitoring station servicing trip to Long Valley in August, CVO gas project personnel also conducted their thirteenth annual soil CO₂ efflux survey at Horseshoe Lake. The results are shown in Figure C2. The long term degassing rate average through 2005 had been about 100 tons of CO₂ per day at Horseshoe Lake. However, the 39 t/d measured in 2006 suggested a decline in the CO₂ degassing rate. The rate measured in August 2007 was 75 t/d, up from 2006 but still below the long term average.

Ken McGee retired from the USGS at the end of 2007.

Figure C1. Carbon dioxide soil gas concentrations for 2007

FigureC2 – Horseshoe Lake carbon dioxide flux: 1995-2007

Carbon Dioxide Measurements at Mammoth Mountain (Christopher Farrar, Deborah Bergfeld, William Evans, Cindy Werner)

MAMMOTH MOUNTAIN (Christopher Farrar, Deborah Bergfeld, William Evans, Cindy Werner)

Background

During August and September 2007 we carried out field investigations to assess changes in the quantities of CO₂ emissions and areas of emissions in four locations around Mammoth Mountain and at one location just southeast of the mountain (fig. cf-1). Known CO₂ emission sites on Mammoth Mountain form a discontinuous ring around the south, west, and north sides of the mountain at altitudes between 2,700 and 3,000 meters. The largest areas and most prominent, by virtue of associated tree-kill, emit CO₂ at ambient air temperatures diffusely from soils (sites HSL, RC, and CH-12 in fig cf-1). A few smaller areas of CO₂ emission, at or above tree-line on the north flank of the mountain, are associated with thermal ground and identifiable steam vents (MMF is the largest of these areas). All four of the areas reported on here have been known since 1994 and CO₂ measurements have been made, primarily by USGS personnel, several times over the past 12-14 years. In August 2007 CO₂ measurements and gas samples were collected for the first time in the Mill City Mine site (MC in fig cf-1) located near Mammoth Rock and downslope of an old mine adit.

CO₂ emissions were calculated from ground-based measurements of changes in CO₂ concentration in closed chambers (volume approximately 4 liters) over intervals from 1 to 3 minutes. CO₂ concentrations in the chambers were measured using infrared non-dispersive gas analyzers. Locations of measurement sites were determined with hand-held GPS receivers (generally precise to about 4-6 m).

Results

The results of the measurements made in 2007 are compared to similar measurements made in 2006 in table cf-1. At Mill City Mine, the total CO₂ emission was an estimated 1.4 t/d, from a relatively small area of about 2000 square meters. No measurements of CO₂ prior to 2007 are available for comparison. Here soil temperatures in localized areas were above ambient. The presence of H₂S was evident from the odor. The chemical composition of soil gas collected from 30 cm deep is given in table cf-2. The gas composition can be compared to that of Mammoth Mountain fumarole (MMF), a vent which discharges gas of magmatic origin. The gas is predominantly CO₂ at both sites but with atmospheric gases (N₂, O₂, and Ar) about ten times more concentrated in the Mill City sample. He and H₂ are remarkably similar in samples from both sites. The ¹³C of -4.0 is similar to the -4.7 in MMF and is consistent with a magmatic or mantle origin for the gas. When snow conditions permit in late spring, we plan to resample gas at the Mill City site and include analysis for He isotope ratio.

Estimates of total CO₂ emission from the four sites on Mammoth Mountain based on data collected in 2007 are similar to the estimates made for 2006. The total estimated for 2007, 76 t/d, is about 25 percent higher than the estimated total for 2006. The difference is not considered significant and could be related to differences in the snowpack and melting during the winter and summer of 2006 compared to 2007. Greater snowpack, melting, and subsequent recharge in 2006 than 2007 may have been more effective at scrubbing CO₂ from the unsaturated zone and transporting more CO₂ away in the ground-water flow system. Part of the differences in estimated emissions relate to slightly

different boundaries used in the surveys between years.

MONO CRATERS (Deborah Bergfeld, Jim Howle, and William Evans).

Background

Recent volcanism in the Long Valley region has mostly occurred 10 to 20 km north of the Long Valley caldera. In June 2007 we initiated a study of diffuse CO₂ emissions over a variety of young volcanic features in the region, including dacite flows and the ~ 660 year old cinder cone on Paoha Island, and seven locations along the Mono Craters volcanic chain, from Panum Crater near the shore of Mono Lake, to Devil’s Punch Bowl in the south (fig. 1). CO₂ flux measurements were made using a field-portable infrared analyzer. Gas samples were collected in evacuated bottles from bubbling springs on Paoha Island, and from thermal ground and cold tephra deposits on North Coulee.

Highlights

Flux measurements on Paoha Island showed no detectable flux of CO₂ around the young cinder cone on the north end the island. Measurements over dacite flows adjacent to Hot Springs Cove on the east side of the island showed only minor CO₂ emissions localized around steam vents. The average flux from 25 sites was < 8 gm^-2d^-1 with only 4 sites displaying an anomalous flux > 15 gm^-2d^-1. These data are in agreement with results from gas samples collected from bubbling springs along the shore of Hot Springs Cove, which contain predominantly C₁ through C₆ hydrocarbons and N₂ with less than 4% CO₂.

Sites along the Mono Craters volcanic chain were selected based on young emplacement ages, proximity to faults, areas of known temperature anomaly, and areas showing evidence of hydrothermal alteration. Of the seven selected sites, only North Coulee showed anomalous CO₂ efflux. North Coulee is one of the larger late-Holocene volcanic features in the Mono Craters chain and is composed primarily of a glassy rhyolite flow that is partially blanketed on the west by tephra deposits. Some areas contain discrete zones of discolored altered rock that provide evidence of former fumarolic activity; however we found only one small area that had temperatures greater than 40° C.

The field site on North Coulee covered a ~ 244,000 m² area that included many of the alteration zones. The CO₂ flux was measured at 224 locations and at most sites was anomalously high, with an average flux over the entire study area of 109 gm^-2d^-1. The average falls within the range of average fluxes from tree-kill sites on Mammoth Mountain. The estimated discharge of CO₂ is around 27 t d^-1, which is comparable to the magmatic CO₂ output from the Long Valley hydrothermal system. There is no soil on North Coulee, and vegetation is generally limited to a few scattered pine trees, therefore biogenic CO₂ emissions should essentially be zero. Given that 63 sites had fluxes below detection limits this supposition is well supported. Two gas samples collected from cold high-emissions areas contained mostly air with CO₂ concentrations around 4.7%. The d¹³C-CO₂ value of one sample was -3.7‰, similar to the carbon isotope composition of gas collected from Shady Rest fumarole and basalt fumarole on the Long Valley caldera resurgent dome.

The diffuse CO₂ efflux at North Coulee is large and is comparable to fluxes in the Long Valley caldera, but is highly unusual in that subsurface CO₂ concentrations are ≤ 5%. Diffuse degassing areas in Long Valley have soil CO₂ concentrations that are much higher, up to 90%. Given the lack of vegetation, the youthful nature of the Mono Craters volcanism, and the similarity in the carbon isotope composition of the cold gas collected in this study with that of CO₂ from areas around Long Valley, we assume that the CO₂ source at North Coulee is magmatic.

Deep long-period (LP) earthquakes identified during the 1989 seismic swarm at Mammoth Mountain have been associated with the release of CO₂-rich fluids from rising basaltic magma. Deep LP events have also been recorded west of the late-Holocene volcanic features in the Mono Craters chain, and are observed at other young volcanic fields in Northern California (Pitt et al., 2002). Two of these, Lassen Volcano and Clear Lake, are characterized by gas vents, degassing springs and areas with high diffuse emissions of CO₂. Tree kills have not been observed at these locations but like Mammoth Mountain, accidents involving high concentrations of CO₂ have resulted in human fatalities.

The large quantity of CO₂ emissions from North Coulee and the presence of deep LP earthquakes west of the Mono Craters chain provide an incentive for additional work in the area. Future work will include collection of additional gas samples for helium isotopes, and a second survey of CO₂ flux around the North Coulee.

Reference:

Pitt, A.M., Hill, D.P., Walter, S.W., Johnson, M.J.S., 2002, Midcrustal, long-period earthquakes beneath Northern California volcanic areas: Seismological Research Letters, 73, no.2, p.144-152.

Figure 1. Sketch map showing flux measurement sites (CC, DF and numbered black squares). Volcanic features of the Mono and Inyo volcanic chains are shown in light gray. CC = cinder cone, CD = Casa Diablo, DF = dacite flow, HSC = Hot Spring Cove, HTK = Horseshoe Lake Tree Kill, LP = Long Period Earthquake, ML = Mammoth Lakes, MM = Mammoth Mountain, NC = North Coulee, PI= Paoha Island, PB = Punch Bowl, PC = Panum Crater.

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 four wells, LKT, LVEW, SF, and CH-10B (locations in 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 and CH-10B are given in figures H2 and H3.

Figure H2. Hydrographs for well LKT, based on filtered daily mean values. The rise, beginning in mid-2006 is from a strong recharge pulse derived from the above average winter 2006 snow-pack. The level generally declined through 2007 without any substantial recharge in this part of the caldera.

Figure H3. Hydrographs for well CH10B, based on filtered mean daily fluid levels. The large fluid level rise in mid-2006 is due to high recharge from above average precipitation during the winter of 2006. The fluid level during 2007 was generally low and showed only a small rise of about 0.2 m during the summer from recharge.

Figure H4. Unfiltered fluid levels in well LVEW in meters above mean sea level.

The fluid level in well LVEW is controlled by the pressure in a fractured rock aquifer at a depth of about 3000 m below land surface and therefore is largely buffered against seasonal fluctuations.

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 H5) 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 H5. Discharge, water temperature, and specific conductance at Hot Creek Flume (HCF), based on daily mean data.

Water temperature and specific conductance generally increased through 2007 owing to the low winter 2007 snow-pack and resultant snow-melt runoff. Discharge in Hot Creek during 2007 was the lowest since 1992.

THERMAL WATER DISCHARGE ESTIMATE

Estimates of total thermal water discharge (figure H6) 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.

Figure H6. Estimated thermal water discharge for springs in Hot Creek Gorge.

Thermal springs in Hot Creek Gorge, throughout 2007, have continued to exhibit variability in discharge, temperatures, and vent locations. Unusual thermal spring activity, including geyser-like fountaining to heights of 2 m above the creek began in late May 2006. The fountaining diminished in frequency and vigor (fountain height) through the later part of 2006 and first half of 2007 but then showed periods of increased activity through the last half of 2007. Nevertheless, the estimated total thermal water discharge from springs in the gorge remained within 8 percent of the mean discharge (243 L/s) for the period 1988-2007. The shift in the locations of active spring vents along the banks of Hot Creek, from mostly along the left bank (northwest side) to mostly along the right bank is the most notable change in the springs. The U.S. Forest Service closure of the area for swimming remains in effect because of the unpredictable behavior of the springs and areas of soil instability along the banks of Hot Creek.

REVIEW OF 2007

Relative quiescence has prevailed in Long Valley caldera and adjacent sections of eastern California from early 2000 through 2007. Intra-caldera earthquake activity in 2007 included minor swarms beneath Mammoth Mountain on January 17-26 and March 13, a swarm beneath the southeast margin of the resurgent dome (2.5 km WSW of Hot Creek) on March 13-15, and a cluster of M ≤ 1.7 earthquakes in the Basalt Canyon area on December 21. The largest earthquake within the caldera was a M=2.1 event on March 15 located ~2.5 km WSW of Hot Creek. Earthquake activity in the Sierra Nevada south of the caldera continued at a higher rate than within the caldera. The largest earthquake in the region was a M=4.6 event on June 12 near Lake Dorothy (1.5 km SSW of Mount Morrison). Aftershocks to this earthquake persisted through the remainder of June and included some 27 M>2 earthquakes. A swarm on September 20-22 centered 0.5 miles south of Convict Lake included over 80 earthquakes, the largest of which had a magnitude M=2.3. A M=3.5 earthquake at 9:34 AM (PST) on November 1 was located near McGee Creek, 2 miles south of McGee Mountain. The deep long-period (LP) earthquakes beneath Mammoth Mountain continue but at a much reduced rate. Fewer than five were detected and located during 2007.

Seismic activity elsewhere in the region included a M=3.0 earthquake on January 10 beneath the Volcanic Tableland near the Pleasant Valley Reservoir in the Owens Gorge (8 km west of Rovana) that was followed by a sequence of small earthquakes that gradually decayed away through the 19^th. A second cluster of small earthquakes occurred in the same area from May 25 through 31 (Figures S1, S5). The largest event in this sequence was a M=2.4 earthquake on May 25. A swarm of earthquakes centered just 2 km northeast of downtown Bishop began on April 1 and persisted through April 4 (Figure S4). The largest, a M=3.0 earthquake at 10:09 PM on the 3^rd, was preceded by a M=2.7 earthquake at 12:26 PM on the 3^rd – both large enough to produce felt shaking in the Bishop area. These earthquakes were centered at depths of 13 to 15 km, respectively. Persistent, low-level activity in the Adobe Hills (10-15 miles east of Mono Lake) included three earthquakes with magnitudes between 2.9 and 3.0 on July 20, August 18, and December 31, respectively.

No significant deformation was detected within the caldera during 2007. The 70-cm inflation of the resurgent dome in the central section of the caldera accumulated between 1978 and 1999 remains in place with only minor fluctuations since 1998 (see Figure G3). This long-term deformation pattern is indirectly reflected in the dilatational strain recorded by the POPA borehole dilatometer located 16 km to the west of the center of inflation. The record shows increasing compressional strain through the mid-1990 followed by a gradual decrease beginning around 2000 (Figure A5).

Figure A5. Long-term record of dilatational strain at the Devils Postpile borehole strain meter (POPA) from August 1, 1984 to January 1, 2008. Increasing values with time indicate compressional strain. The annual spikes coincide with peak, mid-summer run-off in the adjacent San Joaquin River system.

Elevated levels of magmatic carbon dioxide (CO₂) efflux in isolated areas around the flanks of Mammoth Mountain continue to pose local hazards, although it appears that beginning sometime in 2002 the average flux rate began a gradual slowing trend (Figure C2). Annual measurements in the Horseshoe Lake, Reds Creek, Chair 12, and the Mammoth Mountain fumarole areas, which together averaged over100 tons/day through the late 1990’s, showed values fluctuating between 60 to 80 tons/day over the last couple of years. A new measurement at an area near the Mill City Mine found a CO₂ flux of ~ 1.4 tons/day from a 2,000 square meter area with slightly elevated soil temperatures. This CO₂ has a magmatic carbon isotopic signature similar to that of the Mammoth Mountain fumarole. This Mill City Mine site is also the apparent source of the H₂S odor that is frequently reported by residents and hikers through the area. New background measurement of diffuse CO₂ flux on Paoha Island in Mono Lake and the adjacent Mono Craters found only minor CO₂ emissions except from the summit of North Coulee. There the flux is ~ 27 tons/day, which is comparable to the CO₂ output from the Long Valley hydrothermal system.

The Long Valley caldera hydrothermal system showed no significant changes in 2007 with respect to earlier years. The total thermal water discharge from springs in Hot Creek gorge remained within 8% of the mean discharge for 1988 through 2006. The geyser-like fountaining in Hot Creek that began in May 2006, however, has continued on an irregular basis accompanied by changes in locations of many of the active spring vents from the left to right banks (looking downsteam). Hot Creek remains closed to swimming because of its unstable behavior.