LONG VALLEY OBSERVATORY QUARTERLY REPORTS

COMBINED JANUARY-JUNE 2005

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

January-June 2005

CONTENTS

EARTHQUAKES

CALDERA ACTIVITY

SIERRA NEVADA ACTIVITY

REGIONAL ACTIVITY

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

CONTINUOUS BOREHOLE AND STRAIN MEASUREMENTS

Instrumentation

Highlights

TILT MEASUREMENTS

Instrumentation

Data

MAGNETIC MEASUREMENTS

BACKGROUND

DATA

CO₂ STUDIES

HYDROLOGIC MONITORING

SUMMARY FOR JANUARY-JUNE 2005

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the second quarter of 2004. The resurgent dome, which essentially stopped inflating in early 1998 and showed minor subsidence (of about 1 cm) through 2001, was followed by gradual inflation through 2002. It has since held relatively steady showing only minor fluctuations about an average elevation roughly 80 cm higher than prior to the onset of unrest in 1980. The only notable seismic activity within the caldera was a swarm of some 25 small earthquakes on March 5 located in the south moat near fish hatchery that included a M=3.0 earthquake. The rate of earthquake activity in the Sierra Nevada south of the caldera remains higher than within the caldera. This activity included a swarm on March 4 just east of Grinnell Lake (just 38 hours before the south-moat swarm) followed by a M=4.2 event 2 km south of Grinnell Lake on March 13. The latter was the largest earthquake in the area during the first half of 2005. Diffuse emission of carbon dioxide (CO₂) in the tree-kill areas around the flanks of Mammoth Mountain continue at the relatively high levels that have persisted since 1996.

Up-to-date plots for most of the data summarized here are available on the Long Valley Observatory web pages (http://lvo.wr.usgs.gov).

EARTHQUAKES (D.P. Hill and A.M. Pitt)

Note: Beginning with this report, we will report seismic activity based on 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.

CALDERA ACTIVITY:

The most notable earthquake activity within Long Valley caldera during the first six months of 2005 was a swarm of more than 25 small earthquakes that began at 8:40 AM (PST) on March 5 located in the south moat near the southeast margin of the resurgent dome (in the vicinity of the fish hatchery). This earthquake swarm, which included a magnitude M=3.0 event at 7:09 PM on the 5^th , followed a the onset of a swarm in the vicinity of Grinnell Lake 18 km to the south by 38 hours (Figures S3, S8). Aside from this south-moat swarm, caldera activity remained low averaging fewer than 3 earthquakes per day large enough to be located by the realtime computer system (Figures S1-S4, 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 that within the caldera. Notable aspects of this activity through the first six months of 2005 include a swarm of over 30 small (M<2.3) earthquakes just east of Grinnell Lake that began at 7:18 PM (PDT) on March 3 and persisted through March 7. This swarm was followed 38 hours later by the south-moat swarm 18 km to the north (described above) and 9 days later by a M=4.2 earthquake at 2:09 PM on the 12^th located 2 km south of Grinnell Lake. The latter was the largest earthquake in the region during the first half of 2005. Sierra Nevada earthquake activity increased slightly through June. Most of this activity was concentrated in diffuse, north-west trending band between the bend in McGee Creek and Mount Morrison rather than along the north-northeast trending zone defined by the sequence of three M>5 earthquakes in 1998-99 where most of the Sierra Nevada activity has been concentrated over the past 5 years. (Figures S1-S5).

REGIONAL ACTIVITY

Elsewhere, the activity in the Adobe Hills area associated with the energetic earthquake swarm that began in September 2004 continued to decline through the first few months of 2005.

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

John Langbein, Stuart Wilkinson, Mike Lisowski, 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 G1. It is intended that data from these receivers and a few more additional installations will take over the long-term monitoring supplied by the two-color EDM (Figure G-2). The site at CASA now has two receivers; one operating since 1994 and the second one, CA99, installed this past summer.

Review of the previous year of a combination of GPS and EDM data indicate negligible deformation. This is best summarized in Figure G2, which shows length changes in the two-color EDM baselines (Figure G1) together with line-length changes determined from the continuous GPS data. Also see; http://lvo.wr.usgs.gov/monitoring/index.html#deformation

Figure G-1 Map showing 2-color EDM baselines

Figure G2.Line-length changes for the EDM baselines (red crosses) measured from CASA for the period July 2004 through July 2005 compared with continuous GPS data for the same lines (black circles).

Figure G3. Line-length changes for the EDM baselines (red crosses) measured from CASA for the period September 1999 through May 2005 compared with continuous GPS data for the same lines (black circles).

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. Raw data are shown in Figures D2.  Comparative pore pressure and strain data at the Postpile and Big Sprigs dilatometer site are shown in Figure D3. Large pore pressure changes beginning in April were produced by the large snow melt that is just finishing. These pore pressure changes are reflected in the dilatational strain signals as well as the tiltmeter signals as noted below. No strain changes of geophysical significance have occurred through the first half of 2005.

Highlights

Figure D2. Dilatometer sites for POPA, PLV1, Motocross (MX02), and Big Springs (BG02) for 1 January through 31 July 2005.

Figure D-3. Comparing dilatational strain and hydraulic pore pressure for the POP and Big Springs dilatometer installations for 1 January through 31 July 2005.

TILT MEASUREMENTS (Mal Johnston, Vince Keller, Bob Mueller and Doug Myren)

Instrumentation

Instruments recording crustal tilt in the Long Valley caldera are of two types - 1) a long-base (LB) instrument in which fluid level is measured in fluid reservoirs separated by about 500 m and connected by pipes, which was constructed by Roger Bilham of the University of Colorado, and 2) borehole tiltmeters that measure the position of a bubble trapped under a concave lens. 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. 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.

Figure T1. East-west and north-south components of the long-base tiltmeter for 1 January through 20 July 2005.

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Figure T2. East-west and north-south components for the borehole tiltmeters installed with the Big Springs and Motocross dilatometers.

Figure T3. East-west and north-south components for the shallow borehole tilt stations from 1 January through 30 June 2005.

Highlights

The data from the long base tiltmeter were unavailable for April 10 – May 10 because of instrument problems (Fig T1). Shown in Figure T2 are the data from the tiltmeters in the deep boreholes at Big Springs and Motorcross. Data from the short base tiltmeters are shown in Figure T3. As with the borehole dilatometers, the tilt changes beginning in April primarily reflect the effects of the heavy spring runoff associated with the exceptionally heavy snow load for the 2004-05 winter. The tilt data show little of geophysical interest for this period.

MAGNETIC MEASUREMENTS (M.J.S. Johnston)

Background

Local magnetic fields at 12 sites in the Long Valley Caldera are transmitted via satellite telemetry to Menlo Park every 10 minutes. These and other data provide continuous 'real-time'

monitoring in this region through the low-frequency data system. The location of these sites is shown on Figure M1. Temporal changes in local magnetic field are isolated using

simple differencing techniques.

Figure M1. Locations of differential magnetic field stations within Long Valley caldera. The reference station MGS (not shown) is located along Highway 395 approximately 20 km southeast of the caldera.

Highlights

No signals of geophysical significance to report for the first six months of 2005 (see Figure M2).

CO₂ STUDIES (Ken McGee, Terry Gerlach, and Mike Doukas, Cascades Volcano Observatory Vancouver, WA)

The GOES-telemetered carbon dioxide monitoring network in the Mammoth Lakes area continued to transmit data on soil gas carbon dioxide concentrations throughout the report period. Station HS1 is located near the central portion of the Horseshoe Lake tree kill in an area of high CO₂ ground flux while HS2 is located in a lower flux area near the margin of the tree kill and HS3 is outside the tree-kill zone in the group campground area. Stations located away from Horseshoe Lake include SKI, located near Chair 19 in the Mammoth Mountain Ski Area, SRC, located at Shady Rest Campground adjacent to the USFS Visitor Center in Mammoth Lakes, and LSP, located near Laurel Spring in the inferred Long Valley caldera rim fault. At all sites, CO₂ collection chambers are buried in the soil. Air from these collection chambers is pumped to nearby carbon dioxide sensors housed in USFS structures or culverts. Local barometric pressure is also measured at HS1 using a Vaisala Pressure Transducer. Data are collected from the sensors every hour and are telemetered every three hours via GOES satellite. The GOES transmitting antennas, typically mounted inside adjacent USFS structures, continue to produce strong signals to the satellite even after significant snow buildup on the roofs of the structures. All monitoring sites have backup data loggers that also record ambient temperature. Snow data are obtained from a U.S. Bureau of Reclamation monitoring station at Mammoth Pass. Precipitation data are collected by the USFS at the Mammoth Lakes Visitor Center.

Data for the first six months of 2005 from most of the telemetered monitoring stations are shown in the attached figure along with snow depth (SWE) at Mammoth Pass. [Note: all dates and times in UT. Gas data not corrected for pressure and temperature.] The record from these monitoring stations reflects the usual effect of the winter snow pack. The snow pack this year is very high and the CO₂ concentration at HS2 exceeded the range of the sensor (50%). A power outage in the lakes basin took the HS1 monitoring station off line in early January and, because of the deep snow, we have been unable to reach and restart the station. By end of the report period, however, the soil CO₂ concentration at HS3 is starting to decline. Shady Rest (SRC) and SKI continue to be unaffected to any large extent by the snow pack. The HS1 monitoring station remains off line.

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 January through June 2005.

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, CW-3, 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, CW-3, and CH-10B are given in figures H2, H4, and H5. 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. Unfiltered water-level data for well LVEW is shown in figure H3.

Figure H2. Hydrographs for well LKT, based on filtered daily mean values. A large drop in water level occurred in September 2004 in response to the Adobe Hills earthquake swarm. The rise in mid-2005 is from a strong recharge pulse.

Figure H3. Unfiltered fluid levels in well LVEW and atmospheric pressure on the resurgent dome. Fluid level altitude relative to mean sea level is approximately 2110 meters.

Figure H4. Hydrographs for well CW3, based on unfiltered values from January 1988 through August 1993 and filtered daily mean values from September 1993 through June 2005.

Periods of missing data are due to use of the well for testing or because of instrumentation problems. Water levels in CW3 are affected by pumping at the Casa Diablo geothermal field. Examples of these effects include the large pressure drop in 1991 and the distinct peak in 2000. During the Abobe Hills earthquake swarm, September 2004, the water level showed a coseismic drop, followed by a rise over a period of a few weeks. Similar but smaller amplitude changes were recorded following the 9.1 Sumatra earthquake in December 2005.

Figure H5. Hydrographs for well CH10B, based on filtered mean daily fluid levels. Fluid levels in this well showed coseismic pressure drops during the Adobe Hills swarm in September 2004.

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 H6) 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 H6. Discharge, water temperature, and specific conductance at Hot Creek Flume (HCF), based on daily mean data.

THERMAL WATER DISCHARGE ESTIMATE

Estimates of total thermal water discharge (figure H7) 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 H7. Estimated thermal water discharge for springs in Hot Creek Gorge.