LONG VALLEY OBSERVATORY QUARTERLY REPORT

OCTOBER-DECEMBER 2004

AND

ANNUAL SUMMARY FOR 2004

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 2004

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 OF 2004 ACTIVITY

SUMMARY FOR OCTOBER-DECEMBER 2004

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. 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 this quarter was an isolated M=3.2 event near the south margin of the caldera on October 1 located 2 km west of Big McGee Lake (~13 km south of the caldera boundary). 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.

The earthquake swarm that began in the Adobe Hills 20 km east of Mono Lake on September 18 and included M=5.5 and 5.4 earthquakes on the afternoon of the 18^th continued through the end of the year but with gradually declining activity levels.

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: A backlog in hand checking (CUSP processing) earthquakes this quarter has resulted in incomplete representation of seismic activity for the caldera and Sierra Nevada block this quarter plotted in Figures S1-S3. The unchecked Earthworm (automatic computer-generated) locations and magnitudes for this period are plotted in Figure S4 and in the Annual Summary seismicity plots (Figures A2-A4). While the unchecked epicentral locations are generally reliable, some of the automatically determined magnitudes may be too large. The processing backlog is a result of the many earthquakes in the Adobe Hills earthquake sequence that began in mid-September (see Regional Activity below).

CALDERA ACTIVITY:

Earthquake activity within Long Valley caldera remained low through the third quarter of 2004 averaging fewer than 3 earthquakes per day large enough to be located by the realtime computer system (Figures S1-S4, S6). None of these earthquakes had magnitudes in excess of M=2.6.

REGIONAL 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. Most of the activity continues to be concentrated along the north-northeast trending zone defined by the sequence of three M>5 earthquakes in 1998-99 extending from the southwest margin of the caldera to the vicinity of Grinnell Lake in the Sierra Nevada (Figures S1-S5).

The largest earthquake in the Sierra Nevada block this quarter was a M=3.2 earthquake at 2:52 AM on October 1 located 2 km west of Big McGee Lake (~13 km south of the caldera boundary; Figures S1, S4).

Elsewhere in the region, the energetic earthquake swarm that began on September 18, 2004, in the Adobe Hill 20 km east of Mono Lake with M=5.5 and 5.4 earthquakes, continued through the final three months of 2004. The activity during this period included a M=4.7 earthquake on October 9, some 13 earthquakes with magnitudes M>3, and several hundred smaller events.

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.

Review of the previous year of a combination of GPS and EDM data indicate negligible deformation. This is best summarized in Figure G-2, which shows length changes in the two-color EDM baselines (Figure G-1) 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 G-2. Line-length changes for the EDM baselines (red circles) measured from CASA for the period January 11, 2004 through January 11, 2005 compared with continuous GPS data for the same lines (black 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

Strain data during this quarter has been relatively quiet at all sites. Raw data are shown in the top four frames of Figures D-2.  Pore pressure and strain data at the Postpile (POPA) and Big Springs dilatometer corrected for pore pressure are shown in the bottom four panels of Figure D-2.

The M=9.0 Sumatra earthquake of December 26, 2004, was clearly recorded by the strain meters in Long Valley caldera (see Figure D-3). Notably, however, this earthquake did not trigger deformation or seismicity in Long Valley. The peak surface waves were about 0.3 microstrain with the primary energy at about 20 seconds.These amplitudes are smaller than those produced by the Denali, Hector Mine and Landers earthquakes, which did produce triggered seismicity and deformation within the caldera, and they are comparable to the Hokkaido, Pertrolia, Loma Prieta events, which did not produce a triggered response. Free oscillations of the Earth were produced by the Sumatra event and these are clearly evident in the power spectra shown in Figure D-4.

Figure D-3. Straingram of the M=9.4 Sumatra earthquake of December 26 2004 as recorded on the Motocross dilatational strain meter.

Figure D-4. Spectrogram of the M=9.4 Sumatra earthquake recorded on the Motocross dilatational strain meter (Figure D-3).

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 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 were unavailable for most of this quarter until Roger Bilham repaired the instrument in early December 25 (see Fig T-1). Also shown in Figure T-1 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 T-2. Very little of geophysical interest occurred this period and the data are generally uneventful.

.

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 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.

Highlights

Not much to report for this quarter (see Figure M-2). Analysis of the data during triggered slip at the time of the Hector Mine is included in a paper now submitted for publication.

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 October through December from most of the telemetered monitoring stations are shown in the attached figure along with snow depth (as water equivalent) at Mammoth Pass. [Note: all dates and times in UT. Gas data not corrected for pressure and temperature.] The records from all of the stations show the typical fall perturbation of the record due to the beginning of the winter snowfall season. The fall snowpack is heavier than normal for this time period and suggests that we could be in for a very wet winter. Data for the full year are shown in the second figure.

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 2004.

Figure C-3. Carbon dioxide (CO₂) concentrations for the monitoring stations in Figure C1 for the year 2004.

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, H6, and H7. 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, H4, and H5).

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.

Data from wells LVEW and SF were not recorded between October 2003 and June 2004 due to construction of new equipment shelters and changes in the type of equipment used for measurements. A pressure transducer was installed in LVEW and fluid-level recording began in June 2004. Fluid-level recording began in SF during November 2004. Unfiltered fluid levels relative to an arbitrary datum are shown for 2004 in figure H5.

Figure H5. Unfiltered fluid levels in wells SF and LVEW and atmospheric pressure on the resurgent dome. Fluid level altitudes relative to mean sea level are approximately 2110 meters in LVEW and 2232 meters in SF.

Figure H6. Hydrographs for well CW3, based on unfiltered values from January 1988 through August 1993 and filtered daily mean values from September 1993 through September 2004.

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 9.4 Sumatra earthquake in December.

Figure H7. 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.

Fluid pressures in well CW3 during January 2004 reached the lowest level measured since 1995. Fluid pressures in well CH10B during April 2003 reached the lowest level measured since 1987. Fluid pressures in CW3 began rising in early 2004 and in CH10B began rising in mid-2003, however pressures in both wells are still low relative to long-term means. 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 H8) 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 H8. Discharge, water temperature, and specific conductance at Hot Creek Flume (HCF), based on unfiltered daily mean data.

THERMAL WATER DISCHARGE ESTIMATE

Estimates of total thermal water discharge (figure H9) 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 estimated thermal water discharge increased through most of 2004, reaching values near the long-term mean. The decline in discharge with subsequent rise is lagged about six months compared to fluid pressures measured in well CH10B (fig H7).

Figure H9. Estimated thermal water discharge for springs in Hot Creek Gorge.

SUMMARY OF 2004 ACTIVITY

The relative quiescence in Long Valley caldera that began in early 1999 has persisted through 2004. Earthquake activity in the adjacent Sierra Nevada block south of the caldera has gradually died away over the same period, although background levels remain somewhat higher than within the caldera.

Deformation

The resurgent dome continues to undergo minor fluctuations in deformation as reflected in changes the lengths of baselines spanning the resurgent dome (Figure A1). The change in distance between the CASA and KRAC monuments (top line in Figure A1), which closely tracks elevation changes at the center of the resurgent dome, has fluctuated at the ±1.5 cm level following the ~10-cm uplift associated with the strong 1997-98 activity. Thus over the past six years, the center of the resurgent dome has sustained the roughly 75-cm uplift that accumulated during the recurring unrest from 1979 through 1999.

Figure A1. Line-length changes across the resurgent dome with respect to the monument CASA for 1997-2004 based on the 2-color EDM measurements (red crosses) and continuous GPS data (black circles). See Figure G-1 for monument locations.

Seismicity

Earthquake activity both within the caldera and the Sierra Nevada block to the south remained low through 2004. The two most notable earthquake sequences within the caldera involved 1) a minor swarm at the end of January and the first few days of February in the south moat and 2) a M=3.0 earthquake on September 20 located at the south margin of the caldera just north of Convict Lake. The latter was the first M≥3.0 earthquake within the caldera since the cluster of earthquakes that included a M=3.0 event on November 4, 2002 (just 24 hours after the M=7.9 Denali Fault earthquake in Alaska) centered beneath the south moat just south of the Highway 395-203 junction. The swarm in early February this year was located in the same general area of the south moat but, interestingly, the epicenters fell along a southwest trend in contrast to the WNW trend shown by most earthquake sequences in that area.

Earthquake activity within the adjacent Sierra Nevada block continues to be somewhat elevated with respect to that in the caldera through 2004. The Sierra Nevada activity included ~7 earthquakes of M≥3, the largest of which was a M=3.7 earthquake on January 12 located 2 km east of Red Slate Mountain (19 km south of the caldera and 15 km WSW of Tom’s Place). Most of the activity remains concentrated in the NNE-trending aftershock zone associated with the three M>5 earthquakes of June and July 1998 and May 1999.

The most noteworthy seismic activity in the general vicinity of Long Valley caldera during the year was the prolonged earthquake swarm in the Adobe Hills centered roughly 20 km east of Mono Lake and 20 km NNE of Long Valley caldera (see Figure A2). Its onset was marked by a M=2.3 earthquake at 12:02 AM on September 18 followed by M=3.2 and 4.1 earthquakes at 12:07 and 12:08 AM, respectively. Activity continued to intensify through mid-afternoon of the 18^th with M=5.5 and M=5.4 earthquakes at 4:02 and 4:43 PM, respectively. These M>5 earthquakes produced widely felt shaking over the area from Bridgeport to Bishop. Activity gradually declined through the remainder of the year and into early 2005. By the end of December, this Adobe Hill swarm had produced well over 1,000 detectable earthquakes including ~48 with magnitudes M≥3 and 6 with M≥4.

Previous earthquake swarm activity in the Adobe Hills includes a prolonged swarm of comparable intensity in the summer and fall of 1980 with M=4.8 and M=4.9 earthquakes on September 7. At the time, however, this 1980 Adobe Hills swarm was eclipsed by the much more energetic activity in Long Valley caldera and the adjacent Sierra Nevada block associated with the onset of caldera unrest that began with the sequence of four M6 earthquakes in May of 1980.

Figure A4. History of long-period earthquake activity beneath the southwest flank of Mammoth Mountain from 1 June 1989 through 2004. The vertical bars indicated the number of events per week and the solid line tracks the cumulative number of LP earthquakes with time. The period from June 1989 through 1998 includes only those events sufficiently well recorded to be located. From 1999 through 2004, all detected LP events are included.

The mid-crustal long-period volcanic earthquakes, which began beneath the southwest flank of Mammoth Mountain during the 1989 Mammoth Mountain earthquake swarm, continued to occur through 2004 but at a much reduced rate (Figure A4) compared with the peak in LP activity from early 1997 through mid-1998.

Carbon Dioxide

Carbon dioxide (CO₂) concentrations measured in the Horseshoe Lake tree-kill area on the south flank of Mammoth Mountain show no significant changes for 2004 with respect to the past several years. If the total CO₂ flux from the flanks of Mammoth Mountain is decaying from peak values in the early 1990’s, it is doing so only very slowly.

The survey of scattered areas of vegetation die-off and diffuse CO₂ flux on the resurgent dome completed by Deb Bergfeld and colleagues (see the April-June 2004 report) indicates anomalous CO₂ emissions from the kill areas are around 9 tonnes/day (compared with ~300 tons/day from Mammoth Mountain). d¹³C-CO₂ values of the diffuse emissions are similar to values previously reported for CO₂ from hot springs and thermal wells around Long Valley indicating a common source. The areas of elevated CO₂ flux tend to be associated with locally elevated soil temperatures. Some of the older areas near the Casa Diablo power plant are likely related to geothermal power production but development of new areas may reflect a delayed response of the hydrothermal system to the 1997 unrest episode. That episode included an additional 10-cm uplift of the resurgent dome accompanied by intense earthquake swarm activity in the south moat.

Hydrology

Thermal spring discharge in Hot Creek Gorge, which had dropped precipitously by about 20 percent in the last half of 2003 followed by a recovery beginning in January 2004, reached normal discharge values by June 2004.  Fluid levels in key monitoring well continued to decline with levels in wells CW3 and LKT reaching their lowest values since records began in 1985. The fluid level in LKT showed a ~20 cm drop coincident with the onset of the Adobe Hills earthquake sequence in mid September.