LONG VALLEY OBSERVATORY QUARTERLY REPORTS

COMBINED JULY-DECEMBER 2005

AND

ANNUAL SUMMARY FOR 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

July-December 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

ANNUAL SUMMARY FOR 2005

SUMMARY FOR JULY-DECEMBER 2005

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the second half of 2005. 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 75 to 80 cm higher than prior to the onset of unrest in 1980. Low-level earthquake activity continues within the caldera and the Sierra Nevada to the south with infrequent swarms of small (M ≤ 3) earthquakes. The largest earthquake in the region during the second half of 2005 was a M=4.0 event on 13 November in the Adobe Hills area east of Mono Lake. Emissions of carbon dioxide from the flanks of Mammoth Mountain show little evidence of change from previous years during this six-month period.

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: As noted in the January-June 2005 report, we are now reporting 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:

Earthquake activity within or immediately adjacent to the caldera for the second half of 2005 was dominated by a swarm of small (M<2) earthquakes beneath Mammoth Mountain that began on August 6 and persisted through August 14. This swarm included over 28 earthquakes, the largest of which was a M=1.8 event on the 6^th at 8:47 AM (PDT). All were shallow, most with focal depths less that 4 km. Two were as deep as 5 km. Activity remained low through the remainder of the year with only three earthquakes of magnitudes M > 2.0 beneath the caldera or Mammoth Mountain. The largest included M= 2.4 and 2.1 earthquake on August 30 located beneath the north flank of Mammoth Mountain.

MONO-INYO CRATRES ACTIVITY

Three small earthquakes with epicenters ~1.5 km west of Wilson Butte (midway along the west side of the Mono-Inyo volcanic chain) occurred on August 11-12. These earthquakes had magnitudes of M=1.1 to 1.4 and focal depths of roughly 5 km. Few earthquakes have been detected along the Mono-Inyo chain since the local seismic network was installed in the area in mid 1982.

SIERRA NEVADA ACTIVITY

Earthquake activity in the Sierra Nevada block south of the caldera continues to exceed that within the caldera. An increase in the level of activity that began in June continued through the remainder of year with a slowing trend from October through December (see Figure S7). The Sierra Nevada activity continues to be largely concentrated within the linear trend that extends SSE from the intersection of the Hilton Creek fault with the southern margin of the caldera. The most energetic activity involved a sequence of more than 50 earthquakes in the vicinity of Grinnell Lake on October 3-4 that included M=3.4 and M=3.7 earthquakes at 2:16 and 2:19 AM (PDT) on October 3 both located 1.5 mile SE of Grinnell Lake (Figures S4 and S7).

REGIONAL ACTIVITY

Elsewhere, sporadic earthquake activity persisted east of Mono Lake in the Adobe Hills area associated with the energetic earthquake swarm that began in September 2004. During the last half of 2005, this activity included M=3.1 and M=4.0 earthquakes on November 13 and 21, 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. 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 since September 1999. 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 September 1999 through February 2, 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 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).

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 and D5. The variations in dilatation at POPA correlate inversely with pore pressure (D3) reflecting seasonal variations in the water table. The large variations in strain at Motocross (MX in Figure D4a) are local and remain unexplained. None of the strain changes through the second half of 2005 reflect significant changes in the Long Valley caldera magmatic system.

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

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 float data from the long-base tiltmeter for 1 June 2005 through 14 March 2006.

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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.

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. The tilt data show little of geophysical interest for this period.

MAGNETIC MEASUREMENTS (M.J.S. Johnston)

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 technique

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.

No signals of geophysical significance to report for the last half of 2005 (see Figure M2).

Figure M2. Magnetic data for the twelve differential magnetometers operated in Long Valley caldera and vicinity with respect to the station MGS for 1 June 2005 through 14 March 2006. The reference station MGS is located in the Volcanic Tableland southeast of the caldera.

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 2005. 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 are collected from the sensors every hour and are telemetered every three hours via GOES satellite. The GOES transmitting antennas, mounted inside the USFS structures except for SRC, continue to 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. Snow data are obtained from a U.S. Bureau of Reclamation monitoring station at Mammoth Pass

Data for 2005 for are shown in Figure C2 along with snow depth (SWE) at Mammoth Pass. [Note: all dates and times in UT. Data not corrected for pressure and temperature.] There are major gaps in data in 2005 caused mainly by power and sensor problems. In September, the sensor at SRC failed and will not be replaced until summer due to budget constraints. The obvious break in the data for both sensors at HS1 beginning in January results from a power failure in the Lakes Basin. Although the station has a standby battery backup, the power was off long enough to drain the batteries and cause a loss of programming instruction to the GOES data collection platform and transmitter. Repair was not attempted until June due to deep snow when Stuart Wilkinson was able to get to HS1 and restart the station. Unfortunately, a more serious malfunction occurred after the June repair and was not fixed, due to budget constraints, until August. Power problems at HS1 have plagued us for some time so during our annual servicing trip in August, we made several modifications to the station to improve power reliability. So far we have not experienced any power problems at HS1 in the current winter.

The typical annual buildup of CO₂ at Horseshoe Lake can be seen during the first few months of 2005 in the plots for HS2 and HS3. Because of the unusually large snowpack during the early months of 2005, stations HS2 and HS3 recorded larger than normal CO₂ signals. It is possible some of the recorded anomaly at these stations was due to lateral transport of CO₂ through the snow from the core area of the Horseshoe Lake anomaly. The peaks at HS1 in December are a response to the early snowfall events this season. The cause of the distinct CO₂ peak at HS3 in November is not clear at this point. Beyond that unusual event, the network recorded few abnormal CO₂ degassing events during 2005.

During the annual monitoring station servicing trip to Long Valley in August, CVO gas project personnel also conducted another soil CO₂ efflux survey at Horseshoe Lake. The results, shown in the attached figure, do not differ significantly from earlier surveys. The long term degassing rate average continues to be about 100 tons of CO₂ per day at Horseshoe Lake.

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 of 26 December 2004.

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.

SUMMARY OF 2005 ACTIVITY

The relative quiescence in Long Valley caldera and vicinity that began in early 1999 has persisted through 2005. Sporadic, low-level earthquake activity continues within the caldera and, at a somewhat higher level, in the Sierra Nevada immediately south of the caldera. The resurgent dome remains 75 to 80 cm higher than before the onset of unrest in 1980. Carbon dioxide emissions continue at Horseshoe Lake and other sites around the flanks of Mammoth Mountain.

Deformation

The resurgent dome continues to show minor fluctuations in deformation as reflected in baseline length changes measured from the benchamark CASA at the southern margin of the resurgent dome (Figure A1). As reflected in length changes along the baseline between CASA and KRAK spanning the resurgent dome (top line in Figure A1), the elevation of the resurgent dome has fluctuated by ± 1.5 cm following the 10-cm uplift associated with the strong that persisted from mid-1997 through mid-1998.

Figure A1. Line-length changes across the resurgent dome with respect to the monument CASA for 1984 through 2006 based on the 2-color EDM measurements (red) and continuous GPS data (black). See Figure G1 for monument locations.

Seismicity

Low-level seismic activity within the caldera during 2005 included a swarm of some 25 small earthquakes on March 5 located in the south moat near fish hatchery, the largest of which was a M=3.0 earthquake at 7:09 PM (PDT). Activity returned briefly to Mammoth Mountain with a swarm of small (M<2) earthquakes that began on August 6 and persisted through August 14. This swarm included over 28 earthquakes, the largest of which was a M=1.8 event on the 6^th at 8:47 AM (PDT). All were shallow brittle-failure earthquakes, most with focal depths less that 4 km. Two were as deep as 5 km. Activity within the caldera remained low through the remainder of the year with only three earthquakes having magnitudes M > 2.0. The largest included M= 2.4 and 2.1 earthquake on August 30 located beneath the north flank of Mammoth Mountain.

The long-standing seismic quiescence along the Mono-Inyo volcanic chain was interrupted by three small earthquakes with epicenters located ~1.5 km west of Wilson Butte on August 11-12. These earthquakes had magnitudes of 1.1 to 1.4 and focal depths of roughly 5 km.

The rate of earthquake activity in the Sierra Nevada south of the caldera remained higher than within the caldera and continues to be largely concentrated within the linear trend that extends SSE from the intersection of the Hilton Creek fault with the southern margin of the caldera. This activity included a swarm on March 4 just east of Grinnell Lake (just 38 hours before the March 5 south-moat swarm) followed by a M=4.2 event 2 km south of Grinnell Lake on March 13 (this was the largest earthquake in the region during 2005). The latter was the largest earthquake in the area during the first half of 2005.

Elsewhere, sporadic earthquake activity continued east of Mono Lake in the Adobe Hills area associated with the energetic earthquake swarm that began in September 2004 (Figure A2). This this activity included M=3.1 and M=4.0 earthquakes on November 13 and 21, respectively.

Mammoth Mountain carbon dioxide emissions

Diffuse emission of carbon dioxide (CO₂) in the tree-kill areas around the flanks of Mammoth Mountain continue at a rate of roughly 300 tons/day, which has persisted since 1996. The CO₂ flux at the Horseshoe Lake tree-kill area has averaged about 100 tons/day over this period with individual semi-annual to annual measurements showing considerable scatter about this average (Figure A4).

Figure A4 Carbon dioxide (CO₂) emission rate at Horseshoe Lake for the years 1995 to 2005.

Hydrology

Hydrology monitoring showed that fluid levels in wells CW3 and LKT reached all-time lows in 2005 (Figures H2 and H3). Precipitation in the area for 2005 was close to the long-term mean, so it seems that these low levels reflect something besides simply variations in precipitation. Just what factors are contributing to the low fluid levels in these two wells, however, remains to be determined. Otherwise, the hydrology data show nothing out of the ordinary for 2005.