LONG VALLEY OBSERVATORY QUARTERLY REPORTS

COMBINED JULY-DECEMBER 2006

 

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 2006

 

CONTENTS

 

 

EARTHQUAKES

CALDERA ACTIVITY AND MAMMOTH MOUNTAIN

                        Resurgent dome LP earthquake

                        Deep Mammoth Mountain swarm

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

            HIGHLIGHTS

CO2 STUDIES

HYDROLOGIC MONITORING

REVIEW OF 2006

           

 

SUMMARY FOR JULY-DECEMBER 2006

 

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the second half of 2006. 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 minor fluctuations about gradual subsidence. The center of the resurgent dome remains 75 to80 cm higher than prior to the onset of unrest in 1980. Seismic activity within the caldera remains low with no earthquakes as large as M=2.0. The largest earthquake in the region was a M=4.3 event near Grinnell Lake in the Sierra Nevada 16 km south of the caldera. Brief sequences of small (M<1.7), rapid-fire earthquakes (spasmodic bursts) beneath Mammoth Mountain occurred September 19 and November 23-34. The carbon dioxide flux in the vicinity of Mammoth Mountain remains high but shows evidence of a gradual decline since 1995. Sporadic episodes of geysering in Hot Creek that began June continued through December but at a 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:

The level of earthquake activity within Long Valley caldera remained low through the first six months of 2006 with nothing as large as magnitude M=2.0 (Figures S1- S8). A series of small earthquakes swarms occurred beneath Mammoth Mountain on September 19 and November 23-24 (Figures S3, S4, S5). Each was characterized by rapid fire sequences of small earthquakes (most with magnitudes M<1.5) with durations typically ranging from several minutes to a few hours. The most intense of these began on November 23 and included a M=2.1 earthquake at 6:14 PM on the 23rd. This sequence gradually died out over the next 24 hours.

 

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 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-S9). As was the case for the first half of 2006, the most intense of this activity was concentrated toward the southern end of this distribution in the vicinity of Grinnell Lake. Activity there included a M=3.8 earthquake at 2:54 PM on September 29 and a M=4.3 at 6:11 AM November 26. These were the largest earthquakes recorded in the Long Valley caldera region during the second half of 2006.

 

REGIONAL ACTIVITY

Occasional earthquake activity in the Adobe Hills area east of Mono Lake included a M=2.9 earthquake at 6:11 AM on September 1. A M=3.1 earthquake at 4:23 AM on August 2 was located beneath the southern end of the Volcanic Tableland some 10 km north-northwest of Bishop.

 

 

 

 

 

 

 

 

 

 

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.

 

The combination of GPS and EDM data indicate minor contraction across the resurgent dome since mid-2004.  This is best summarized in Figures G2 and G3, 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 February 12 1999 through February 12 2007 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 June 1984 through February 12, 2007 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 and D3. Comparative pore pressure and strain data at the Postpile dilatometer site is shown in Figure D4 and comparative pore pressure and strain data at the Big Springs dilatometer site is shown in Figure D5. Spectacular teleseismic records were obtained from the 15 November, M8.2 earthquake in the Kurile Islands although the peak strains applied to Long Valley caldera were 50-100 times less than those for which triggering of deformation and seismicity occurred. See Fig D6.

 

 

Figure D2. Dilational strain for POPA and PLV1 plus pore-pressure at the POPA site (bottom).

 

 

Figure D3. Dilatational strain at Motocross (MX) and Big Springs (BG) plus pore pressure at the Big Springs site (bottom)

Figure D4. Strain records from the POPA and MX dilatometers for the M=8.3 Kurile Islands earthquake of 11:14 (UTC), 15 November 2006

 

 

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.

 

 

 

Figure T1. East-west and north-south components of the long-base tiltmeter for 1 January through June 2006.



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Figure T2. East-west and north-south components for the shallow borehole tilt stations from 1 July through 31 December 2006.

 

Figure T3. East-west and north-south components for the borehole tiltmeters installed with the Big Springs and Motocross dilatometers for July-December 2006..

 

Figure T4. East-west and north-south tilt components for the shallow borehole tiltmeters for July-December 2006. See Figure D1 for locations.

 

Highlights

Fig T2 shows the data for the last two quarters of 2006. In late June, Roger Bilham of the University of Colorado replaced the sensors and repaired various parts that were failing. The primary problem with the NS component seems still to be a leak that allows atmospheric pressure to perturb the signal. The East-West component is now working well with little net tilt during the past 6 months. Data from the short base tiltmeters are shown in Figures T3-T8. Very little of geophysical interest occurred this period. The Long base tiltmeter show a slight tilt to the north. Most of the westerly tilt is probably seasonal. The data from the short base tiltmeters are generally uneventful. Data from the tiltmeters in the deep boreholes at Big Springs and Motorcross are shown in Figures

T10-T11. The only data of interest are some variations on MX in late June. These are instrumental problems associated with upgrading the power supply.

 

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 Figure M2.

 

Highlights:

Not much to report for this period other than it is a struggle to keep things running as the equipment gets older and older.

 

 

CO2 STUDIES  (Ken McGee, Mike Doukas and Cindy Werner, 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 2006.  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 2006 for are shown in the attached figure along with snow depth (SWE) at Mammoth Pass.  [Note: all dates and times in UT.  Data not corrected for pressure and temperature.]  There are no major gaps in the data in 2006 except for sensor HS1B which appears to have failed in December.  It is unlikely a repair will be attempted before summer.  Although power problems at HS1 have plagued us over the years, we made several modifications to the station to improve power reliability in 2005 and since then we have not experienced any power problems at HS1. 

The typical annual buildup of CO2 at Horseshoe Lake can be seen during the first few months of 2006 in the plots for HS1A, HS1B, HS2 and HS3.  Because of the unusually large snowpack during the early months of 2006, stations HS2 and HS3 recorded larger than normal CO2 signals much like the previous winter.  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 cause of the distinct CO2 peak at HS2 in December is not clear at this point although it appears remarkably similar to a peak at the same station in late November 2005.  Beyond that unusual event, the network recorded few abnormal CO2 degassing events during 2006.

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, differ from earlier surveys. 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 is out of the normal range and suggests a decline in the CO2 degassing rate.

 

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 July-Decmeber 2006.

 

Figure C-3. History of CO2 emission rates at Horsehoe Lake from 1995 through 2006

 

DIFFUSE CO2 STUDIES (Deborah Bergfeld, Christopher Farrar, William Evans and James Howle).

 

Background

During September and October 2006 we carried out field investigations to assess changes in the extent and quantity of CO2 degassing at four locations on and around Mammoth Mountain and at two locations east of the town of Mammoth Lakes, near the Casa Diablo geothermal power plant. The four sites around Mammoth Mountain are at elevations above 2700 m and include Chair 12 (C12), Reds Creek, (RC), Horseshoe Lake - Borrow Pit (HSL-BP), and Mammoth Mountain Fumarole (MMF) (Fig. 1). The two other sites at elevations below 2400 m include Shady Rest (SR) and Basalt Canyon (BC). CO2 fluxes at SR and BC were also measured in June 2006 and results are included herein for comparison.

 

C12, RC and HSL-BP are situated below tree line and are characterized by extensive areas of tree kill associated with large emissions of cold CO2. Emissions at the Horseshoe Lake tree kill area have been measured annually since 1995 by Gerlach, McGee, Doukas, and Werner of the Cascades Volcano Observatory, while the most recent report of emissions at C12 and RC was in 1998 by Farrar et al., (1998). The study at MMF was motivated by an accident during April 2006 that caused the deaths of three members of the Mammoth Mountain ski patrol, presumably of CO2 poisoning. MMF is located above tree line and as such, there is no kill zone to indicate areas of high CO2 emissions.

 

Increases in geothermal fluid production at the Casa Diablo power plant in the mid-1990s produced steaming ground and associated vegetation kills at BC, about 1.2 km west of the plant, and several other locations closer to the power plant. Our investigations of CO2 emissions at BC and SR were performed in response to recent changes in the geothermal well field that include two new production wells that went on line in July 2006. The BC site is located in a NE-SW trending canyon containing several gas vents and areas of steaming ground that are predominantly located along the canyon floor. CO2 fluxes at sites within the BC grid have been measured four times during 2003 and 2004 and averaged around 300 to 400 g m-2 d-1. The early studies at BC provide some background information to evaluate potential changes related to the new geothermal development.

 

The new production wells are located near the Shady Rest fumarole, a weak gas vent that is the westernmost surface expression of the Long Valley hydrothermal system. A second gas vent with sub-boiling surface temperatures is located north of the fumarole. A survey of diffuse CO2 degassing at Shady Rest fumarole in 2003 indicated anomalous CO2 emissions occur at locations away from the fumarole. In June 2006 we constructed an initial grid at SR, covering about 27,000 m2 focusing on patches of bare ground around the fumarole. Through that work we discovered the second gas vent and in September 2006 we expanded the study area to roughly 72,000 m2. The coverage at SR now includes both gas vents and extends into the edges of the healthy forest. In the future it may be necessary to increase the size of the grid along the north east perimeter as a group of mature pines appears to have died just prior to our investigation. It remains to be seen if the deaths are related to increasing soil temperatures, high CO2 fluxes or some biologic cause.

 

Highlights

Our results show the locations of sites with anomalous CO2 flux are generally restricted to an area of about 8700 m2 around Mammoth Mountain fumarole. Total CO2 emissions from the four areas on Mammoth Mountain during this study were around 80 tonnes per day. Specific information and selected statistics for each area are given in table 1. Because there are differences in the number of measurement sites, the size of the study areas, and the amount of background data included in the sample sets, additional work is needed before we can make quantitative comparisons of emissions data from 1996 and 2006. However, it is clear from table 1 that the maximum fluxes at all of the sites around Mammoth Mountain are significantly lower than they were in 1996 and it is arguable that overall emissions are declining. That said, it must be emphasized that in spite of this evidence, CO2 emissions at all 4 sites around Mammoth Mountain remain high and the asphyxiation hazard is an ongoing concern.

 

Results from the 2006 measurements at the geothermal sites indicate around 5 to 6 tonnes of CO2 per day is emitted at BC (table 2). These results are in line with data from the 2003-2004 studies and suggest that as of September there was no change in emissions related to the new production wells. The average flux at SR is somewhat lower than at BC and CO2 flux and soil temperature are positively correlated. Data collected thus far delineates a fairly large zone of anomalous CO2 emissions that extends north of the Shady Rest fumarole. These results provide the background data to monitor changes in the geothermal field over time.

 

 

Figure 1. Sketch map showing the measurement sites from this study. ML = Mammoth Lakes.

 

1 HSL measurements were made following a snow fall and fluxes may be suppressed. 1996 data in bold are from Farrar et al., 1998.

 

 

 

REFERENCES

Farrar, C.D., Neil, J.M. and Howle, J.F., 1999.  Magmatic Carbon Dioxide Emissions at Mammoth Mountain California. U.S. Geol. Surv. Water Resour. Invest. Rpt. 98-4217, 34 pp.

 

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, H3, 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 H4.

 

Figure H2.  Hydrographs for well LKT, based on filtered daily mean values.  The rise beginning in mid-2006 is from a strong recharge pulse, similar to the late-1990s.

 

 

 

 

 

 

Figure H3. Hydrographs for well CW3, based on unfiltered values from January 1988 through August 1993 and filtered daily mean values from September 1993 through Sep 2006.  Water levels in CW3 are affected by pumping at the Casa Diablo geothermal field.  The large water level rise beginning in late spring 2006 is due partly to strong recharge from above average precipitation in 2006 and partly owning to changes in the locations of wells used to supply geothermal fluid to the power plant at Casa Diablo.

Figure H4.  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 H5.  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.  Changes in density caused by increases in fluid temperature (see fig. H6) may also contribute to the rise.

 

 

Figure H6.  Maximum temperatures in well CH10B from logging done 1988 to 2006.  Temperatures have increased from < 94o C in 1991 to over 100o C in 2006.  The reasons for the temperature change are uncertain but may be in response to deformation of the resurgent dome, opening of fractures caused by local and distant earthquakes, changes in operations of geothermal wells at Casa Diablo, or a combination of these factors.

 

 

 

 

 

 

SURFACE WATER MONITORING

Site HCF is located downstream from the thermal springs in Hot Creek Gorge (figure H1).  Stage, water temperature, and specific conductance (figure H7) are recorded every 15-minutes.  The data are recorded by an on-site data logger and telemetered every three hours.  Specific conductance is a measure of total dissolved ionized constituents.  Water at HCF is a mixture of thermal water from springs along Hot Creek and non-thermal water from the Mammoth Creek basin.  Changes in specific conductance are related to changes in the mixing ratio of thermal and non-thermal components of stream flow.   Water temperatures change in response to ambient temperatures and the mixing ratio.

 

 

Figure H7.  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 H8) are computed from monthly measurements of discharge, and boron and chloride concentrations collected at a non-recording site (HCA) located upstream of the Hot Creek gorge thermal area and at site HCF downstream.    The quantity of thermal water discharged to Hot Creek is known to vary in response to seasonal variations in precipitation, snow-melt, earthquakes, and other processes.  It is believed that spring discharge may change in response to crustal strain. 

 

           

 

 

Figure H8.  Estimated thermal water discharge for springs in Hot Creek Gorge.

Unusual thermal spring activity, including geyser-like fountaining to heights of 2 m above the creek began in late May 2006.  The relatively high (10% higher than the mean) thermal water discharge in the later half of 2006 is consistent with the high water temperature measured in well CH10B (fig H.6).

 

REVIEW OF 2006

 

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through 2006. Earthquake activity within the caldera remains low. The largest intra-caldera earthquake during the year was a magnitude M=2.4 event on June 15 at 6:58 AM that occurred as part of a minor swarm beneath the southern margin of the resurgent dome. Activity levels in the Sierra Nevada south of the caldera continue to exceed those within the caldera, and the largest earthquake in the region in 2006 was a M=4.2 earthquakes near Grinnell Lake in the Sierra Nevada 16 km south of the caldera (Figures A2, A3, A4).

 

 

A series of small earthquakes swarms occurred beneath Mammoth Mountain on September 19 and November 23-24 (Figures S3, S4, S5). Each was characterized by rapid fire sequences of small earthquakes (most with magnitudes M<1.5) with durations typically ranging from several minutes to a few hours. The most intense of these began on November 23 and included a M=2.1 earthquake at 6:14 PM on the 23rd. This sequence gradually died out over the next 24 hours. The level of deep long-period earthquake activity beneath Mammoth Mountain declined through 2006 with only a dozen or so events large enough to locate.

 

 

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.

 

Ongoing measurements of the carbon dioxide (CO2) flux around Mammoth Mountain indicate that the flux rate is gradually declining. The hazard posed by high CO2 concentrations in isolated high-flux areas around Mammoth Mountain and Horseshoe Lake, however, remains a serious concern – particularly during the winter and spring months when CO2 - filled snow wells may develop around trees and adjacent to structures. CO2 flux measurements within the caldera reveal smaller areas of elevated flux (4 to 10 tons/day compared with a few hundred tons/day in the high flux areas around Mammoth Mountain) in the vicinity of the Basalt Canyon fumaroles and Shady Rest area in the west moat.

Noteworthy events in 2006 include: 1) unambiguous identification of a shallow long-period (LP) earthquake beneath the southern section of the resurgent dome on January 10 confirming that similar events observed during the 1997 cannot be dismissed as due to path effects on wave propagation from shallow brittle-failure earthquakes; 2) a swarm of deep, brittle-failure earthquakes in the lower crust (depths 25- to 35 km) southwest of Mammoth Mountain on June 16 with hypocenters distinctly below the deep LP earthquakes beneath the southwest flank of Mammoth Mountain (depth from 10 to 25 km); 3) An exceptionally heavy spring snowfall trapping gas emissions from the Mammoth Mountain fumarole (MMF) forming a snow cave, the collapse of which resulted in the tragic deaths of three ski patrol members presumably due to carbon dioxide poisoning on April 6, and 4) the onset of episodic geysering in Hot Creek in early June that forced temporary closure of Hot Creek to swimming as episodic geysering has continued through the remainder of the year but with waning intensity. The geysering appears to be a result of local changes in the shallow hydrothermal system in the vicinity of Hot Creek and not the result of significant new heat into the added to the system from depth.

 

A major field effort from July through September involved re-surveying the principal level lines through the caldera in conjunction with Global Positioning System (GPS) and microgravity measurements as a basis for linking the geoid and ellipsoid geodetic reference figures for future monitoring of deformation in the caldera. Having established this link will allow us to dispense with expensive, labor intensive leveling surveys in the future as a means of tracking caldera deformation as well as to relate results from future GPS survey and continuous GPS data to past leveling surveys. We expect the analysis of the data gathered under this field effort will be completed by mid-2007.