LONG VALLEY OBSERVATORY QUARTERLY REPORT

APRIL-JUNE 2000

Long Valley Observatory

U.S. Geological Survey

Volcano Hazards Program, MS 910

345 Middlefield Rd., Menlo Park, CA 94025

http://quake.wr.usgs.gov/VOLCANOES/LongValley/

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.

CONTENTS

QUARTERLY REPORT: April-June 2000

EARTHQUAKES

CALDERA ACTIVITY

SIERRA NEVADA ACTIVITY

REGIONAL ACTIVITY

DEFORMATION

TWO-COLOR EDM SUMMARY

GPS – CONTINUOUS MEASUREMENTS

DILATATIONAL STRAIN AND TILT

Instrumentation

Highlights

MAGNETIC MEASUREMENTS

INSTRUMENTATION

HIGHLIGHTS

CO₂ STUDIES IN LONG VALLEY CALDERA

HYDROLOGIC MONITORING

LONG VALLEY OBSERVATORY QUARTERLY REPORT

April-June 2000

The onset of relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the first half of 2000. The resurgent dome, which essentially stopped inflating in early 1998, has shown minor subsidence in the past six to nine months. Seismic activity within the caldera has typically included fewer than five small earthquakes per day, most with magnitudes less than M=2.0. 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. Most of the earthquake activity in the region continues to occur as aftershocks to the three M > 5 earthquakes of 8 June 1998 (M=5.1), 14 July 1998 (M=5.1), and 15 May 1999 (M=5.6) in the Sierra Nevada south of the caldera.

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

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

CALDERA ACTIVITY:

Earthquake activity within Long Valley caldera remains low with only a few (typically fewer than five) events per day large enough to be detected and located by the real-time computer system (generally M > 1). The only earthquake within the caldera during this quarter with magnitude greater than M=2.0 was a M=2.3 event on April 27 at 7:36 AM (PDT) that occurred as part of a cluster of small events beneath Mammoth Mountain (Figure S1).

SIERRA NEVADA ACTIVITY:

Earthquake activity within the Sierra Nevada block south of the caldera during the second quarter of 2000 continued to be concentrated in the aftershock zone for the three M5 earthquakes of 8 June 1998 (M=5.1), 14 July 1998 (M=5.1), and 15 May 1999 (M=5.6), which defines a 15-km-long, linear zone of epicenters extending to the south-southwest into the Sierra Nevada from the southeastern margin of the caldera. Seven of the earthquakes along this zone had magnitudes in the range M=2.5 to 3.0. The largest were M=3.0 events; one at 11:44 PM (PDT) on April 22 located 5 km SE of Red Slate Mountain (16 km WSW of Tom’s Place) and the other at 2:44 PM on June 23 near Big McGee Lake (12 kmWSW of Tom’s Place).

REGIONAL ACTIVITY:

Minor activity elsewhere in the region included a M=2.9 earthquake beneath the north end of Round Valley at 2:30 AM on April 27. Sporadic activity in Fish Lake Valley (30 km east of Boundary Peak at the north end of the White Mountains) continued with three M=2.6 events (11:46 AM on June 6 and 11:20 AM and 10:32 PM on June 7, respectively) plus a cluster of over 40 small earthquakes on June 21 including a M=3.0 earthquake at 11:00 PM. The Fish Lake Valley area, which is near the north end of the Death Valley-Furnace Creek fault, has produced a number of small to moderate earthquakes over the past 20 years including a M=5.5 earthquake on September 24 1982 and half a dozen events with magnitudes of M=4.0 or greater. To the south, M=3.0 and M=2.9 earthquakes located 20 km south of Big Pine occurred at 2:44 and 2:57 PM on June 21^st. These events fall within the northern end of the rupture zone for the great 1872 Owens Valley earthquake.

DEFORMATION

TWO-COLOR EDM SUMMARY (John Langbein, Stuart Wilkinson, and Adam Heffernan)

A two-color Electronic Distance Meter (EDM) is used to monitor the lengths of approximately 10 baselines in and near the Long Valley Caldera shown in Figure EDM-1. The precision of each length measurement is between 0.5 and 1.0 mm. The 8 baselines shown with heavy lines that use CASA as a common end point are measured several times each week. Other baselines that have CASA in common are measured at less frequent intervals of 1 to 2 months. The remaining baselines are currently measured once per year. With the frequent measurements, we can monitor temporal changes in the deformation. With the annual measurements, we can monitor the spatial extent of deformation.

The measurements of length changes shown in Figures EDM-2 and EDM-3 for the frequently measured baselines show that the extension rates decreased to essentially zero in late-1998 followed by the onset of gradual contraction in early 1999. These two-color data indicate that the baselines spanning the resurgent dome have contracted by roughly 1 cm over the past year. This compares with over 35 cm of extension from mid-1983 when the measurements were started through mid-1998. The center of the resurgent dome currently stands some 80 cm higher than in the late 1970’s prior to the onset of caldera unrest.

Figure EDM-1 Map showing 2-color EDM baselines

GPS – CONTINUOUS MEASUREMENTS. (John Langbein, Elliot Endo, Frank Webb, Tim Dixon, Stuart Wilkinson, and Adam Heffernan USGS-Menlo Park, USGS-CVO, JPL, and U. Miami)

Over the past 6 years, 12 GPS (Global Position System) receivers have been installed within and near the Long Valley Caldera. Of these, eight were installed in the past 2 years by Elliot Endo of the Cascades Volcano Observatory. The locations of receivers within the caldera are shown in Figure GPS-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. The three component displacement data are shown in Figure GPS2-4 for all 12 receivers along with two other sites, CMBB and MUSB located on the western slope of the Sierra Nevada. The site at CASA now has two receivers; one operating since 1994 and the second one, CA99, installed this past summer.

The travel-time measurements from each receiver is processed daily to produce a position in a reference frame with North America fixed. Additional processing involves removing a temporal, common-mode signal from each time-series of displacements as well as the gross outliers. To re-adjust the data to a more local reference frame, a rate is removed from each time series. This rate is the average displacement rate from 1996 to the present of the 2 Sierra Nevada stations, CMBB and MUSB. In the plots, to show any deviation from a constant rate, the local rate is also removed and that rate is posted next to the trace of the residual displacements. These preliminary GPS data are consistent with no significant deformation within Long Valley caldera over the past year.

DILATIONAL STRAIN MEASUREMENTS (Malcolm Johnston, Doug Myren, Bob Mueller and Stan Silverman)

I. Instrumentation

Dilational strain measurements are being recorded continuously at the Devil's Postpile, POPS, and at a site, PLV1, just to the north of the town of Mammoth Lakes in Long Valley and at the two new sites, MCX and BSP (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).

Figure D1. Location map for borehole dilatometers (triangles) and tiltmeters (solid circles). LB is the Long Base tiltmeter.

Data from the strainmeters are transmitted using satellite telemetry every 10 minutes to a host computer in Menlo Park. The data are also recorded on site on 16-bit digital recorders together with 3-component seismic data and on backup analog recorders. A summary of the high-frequency seismic and strain data is also transmitted by satellite.

II. Dilatometer Highlights

We are finally able make corrections for the annual changes in the POPA dilatometer that apparently resulted from massive ground water changes in the region to the west of Mammoth Mt. during the annual snow melt. Last summer we drilled a well near the Postpile dilatometer and installed a pressure transducer to monitor the water table. Figure D2 shows POPA (top) and the water level POPP (bottom). There is a clear correspondence between the two that will allow us to determine addmittance and make corrections to the POPA record.

Otherwise, this period has been relatively quiet on the four LV strainmeters. Raw data are shown in Figure D3. The longer term changes on BG02 and MX02 in Fig. D3 are due to cement curing. All instruments are shown in real-time on the WWW

(see http://quake.wr.usgs.gov/QUAKE/crustaldef/longv.html).

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

I. Instrumentation

Instruments recording crustal tilt in the Long Valley caldera are of two types - 1) a long-base instrument in which fluid level is measured in fluid reservoirs separated by about 500 m and connected by pipes (this instrument (LB) 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.(All Others). Figure D1 shows the locations of the seven tiltmeters that are installed in Long Valley, California.

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.

II. Tiltmeter Highlights

The data from the long base tiltmeter is shown in Figure T2. Very little of geophysical interest occurred this year and the data are generally uneventful. We unfortunately lost data during the latter part of the year when power to the vaults was cut in July by buldozers extending the Mammoth Lakes airport. Power was only recently restored. Data for the shallow borehole tiltmeters Escape, Fossil, Little Antelope, Casa, Sherwin, Valentine, Motor Cross and Big Springs

are shown in Figures T3-T10. The only event of interest occurred on Valentine (VA) in mid-February. While hints of this event may have been detected on the closest tiltmeter Sherwin (SH), single recordings like this indicate some local origin and are generally discounted.

Data can be viewed in real time on

http://quake.wr.usgs.gov/QUAKE/longv.html

MAGNETIC MEASUREMENTS (R.J. Mueller and M.J..S. Johnston)

BACKGROUND

Local magnetic fields at Hot Creek (HCR) and Smokey Bear Flat (SBF) in the

Long Valley Caldera have transmitted data via satellite telemetry to Menlo Park since January 18, 1983. Satellite telemetry has been operating at station Sherwin Grade (MGS) since January, 1984. Between August 1998 and August 1999, eight additional magnetometers, together with a 3-component system and a magnetotelluric system (MT), were installed at existing telemetry locations inside and adjacent to the Long Valley Caldera in cooperation with Dr. Yosi Sasai (Univ. of Tokyo) and Dr. J. Zlotnicki (CNRS, France). 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 1. 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 the

caldera are shown in Figures 2-5. Each of these stations are referenced to a site on Sherwin Grade (MG) located to the south of the caldera.

HIGHLIGHTS

The differenced data for the 10 magnetic field stations, referenced with station MGS, are shown in figures 2, 3, and 4. Missing data are due to telemetry problems at site BSP and HCR. The long term rate changes for differences HCR-MGS (+1.0 nT/a) and SBF-MGS (-0.6 nT/a) are continuing from 1991 to 1999 (Figure 6). No significant changes in magnetic field are observed during this reporting period. Changes during the first part of April and mid-May are due to magnetic storm activity and are not due to tectonic sources.

During August, 1999, three new magnetometers (MXP, VAP, and LAP), from the

University of Tokyo, where installed at existing locations in the Long Valley Caldera (Locations-Figure 1, Data-Figure 4). All three are operational with data being recorded onsite and with USGS satellite telemetry (Figures 2-4). A 3-component magnetometer was co-located with the POP station and an additional total field magnetometer was installed over a borehole at the PLV station (double circles on Figure 1) and a second MT experiment was installed at LAP.

Figure M2. Magnetic field differences in nanoTesslas (nT) between stations SPF-MGS and HCR-MGS from 1984 through March 2000 (see Figure M1). Station MGS, which serves as a reference station, is located along Highway 395 20 km southeast of the caldera.

CO₂ STUDIES (Ken McGee, Terry Gerlach, Mike Doukas, and Rich Kessler; 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, EQF, located near Earthquake Fault, 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.

We lost our project technician, Rich Kessler, during the report period. Although he voluntarily left to take a position with the University of Oregon Physics Department, there were clear signals that his term appointment would not be renewed later this year. Thus, we are without a technician and without the ability to do as much as before. We are evaluating whether we can continue to support the number of CO₂ monitoring stations that we currently operate.

Raw data for the 2nd quarter of 2000 for many of the telemetered monitoring stations are shown in the attached figure along with snow depth (SWE) at Mammoth Pass and Long Valley precipitation events. [Note: all dates and times in UT. Data not corrected for pressure and temperature.] The snow depth (SWE) curve declined to zero on June 13. An unfortunate power failure at the HS1 monitoring site in February resulted in a loss of data from that site but HS1 is now back on line. Data from HS2 and HS3 show a slow decline in CO₂ levels from their winter high values. Besides a noticeable dip in CO₂ concentration at HS3 on May 11-12 lasting about 24 hours and a small peak at EQF on June 8-9, the CO₂ record from these monitoring stations was relatively quiet during the report period.

Figure C1 Map showing locations of the continuous CO₂ -monitoring stations.

***HIGHLIGHT*** In contrast to the relatively quiet record described above, we recorded a significant CO₂ peak at the Laurel Spring monitoring station (2^nd figure) on April 27 that pegged the 0-20% sensor offscale for about 16 hours. Later, on June 21, we recorded the first of a continuing series of sharp CO₂ peaks at LSP superimposed on a rising CO₂ baseline. We are watching the situation closely as that trend has continued into July. Earlier monitoring at this site several years ago suggested that gas events recorded here are possibly related to activity around Mammoth Mountain.

Figure C2. Carbon dioxide (CO₂) concentrations for the monitoring stations in Figure C1 for April-June 2000.

HYDROLOGIC MONITORING (Devin Galloway, Chris Farrar, Jim Howle, Michelle Sneed, Liz Colvard, and Mike Sorey: U.S. Geological Survey, Sacramento, Carnelian Bay, and Menlo Park, CA.

Hydrologic data collected for the USGS Volcanic Hazards Program in this report include ground-water level data from five wells and streamflow, water temperature, and specific conductance from one site on Hot Creek (figure H1). Additional data are available upon request – contact: Chris Farrar or Jim Howle at Carnelian Bay 530.546.0187.

BACKGROUND

Ground-water levels in wells and discharge from springs 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 a more complete description of 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 in 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. The influence of barometric pressure changes is then removed from the water-level records. The influence of earth tides is removed from the water-level records using frequency analysis and the calculated theoretical earth tide. The result yields the filtered water-level record. Filtered data for wells LKT, SF, CW-3, and CH-10B are given in Figures H2, H3, and H4. Analysis of the records from LVEW to provide filtered data is not yet complete; therefore raw data are presented for this site (Figure H3a).

SURFACE WATER MONITORING

Site HCF is located downstream of the thermal springs in Hot Creek Gorge (Figure H1). Stage, water temperature, and specific conductance (Figure H4b) are recorded every 15-minutes. The data are stored in 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 nonthermal water from the Mammoth Creek basin. Changes in specific conductance are related to changes in the mixing ratio of thermal and nonthermal components and when used in conjunction with data collected monthly at a site on Hot Creek above the thermal springs (data not shown here) can be used to estimate the total discharge of thermal water from springs in Hot Creek Gorge.

FIGURE CAPTIONS

Figures EDM-2 and EDM-3. Line-length changes along the frequently measured two-color geodimeter baselines for the past year (EDM-2) and since mid-1983 (EDM-3).

Figure GPS-1. Location map for continuous GPS stations operating within Long Valley caldera.

Figures GPS-2, -3, and -4. North, vertical, and east displacement components in mm for the continuous GPS stations over the past year with the average trend removed. The removed trend is reported to the right of the station name as mm/y ± a standard deviation. Thus the station KRAK has moved an average of 3.9 ± 2/3 mm south, 6.7 ± 2.4 mm east, and 7.0 ± 3.8 mm down over the past year.

Figures M3-M5. Magnetic field differences in nanoTessla (nT) for stations within the caldera (see Figure M1) with respect to the reference station, MGS, for January-March 2000.