LONG VALLEY OBSERVATORY QUARTERLY REPORT

APRIL-JUNE 2002

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

April-June 2002

CONTENTS

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

HELIUM ISOTOPE VARIATIONS IN MAMMOTH MOUNTAIN FUMAROLE

SUMMARY

The quiescence in Long Valley caldera that began in the spring of 1998 continued through the first quarter of 2002. The resurgent dome, which essentially stopped inflating in early 1998 and showed minor subsidence (of about 1 cm) through 2001, began renewed inflation earlier this year at a rate of 1 to 2 cm/year. The center of the resurgent dome still stands roughly 80 cm higher than prior to 1980. 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.

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)

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 most notable activity involved a cluster of small earthquakes beneath the west flank of Mammoth Mountain between 12:24 and 1:02 PM (PDT) on June 26. This cluster included four earthquakes with magnitudes of M~2.0 (Figure S3).

SIERRA NEVADA ACTIVITY:

Earthquake activity continued within 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. This activity was dominated by a cluster of earthquakes near the southern end of this aftershock zone centered just east of Grinnell Lake that began on June 6 and persisted through the end of June. The largest events in this sequence were earthquakes with magnitudes M=3.3 at 2:37 PM on June 2, M=3.0 at 8:36 AM on June 9, and M=3.2 at 5:26 PM on June 17. (Figures S3, S4).

DEFORMATION

TWO-COLOR EDM SUMMARY (John Langbein, Stuart Wilkinson, and Stefon Kirby)

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.

Figure EDM-1 Map showing 2-color EDM baselines

The measurements of length changes shown in Figure EDM-2 for the frequently measured baselines show that the gradual contraction that began in early 1999 appears to have stopped in mid-2000. These two-color data indicate that the baselines spanning the resurgent dome began another episode of extension in early 2002 (Figure G2). Based on the relation between leveling and 2-color data, the center of the resurgent dome remains about 80 cm higher than in the late 1970’s prior to the onset of caldera unrest.

Figure G2. Line-length changes for the EDM baselines measured from CASA for the period October 1, 2001 through September 25, 2002.

GPS – CONTINUOUS MEASUREMENTS. (John Langbein, Elliot Endo, Frank Webb, Tim Dixon, Stuart Wilkinson, and 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 GPS-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 indicate inflation of the resurgent dome by just over 1 cm since the beginning of the year.

Figure GPS-1. Map showing continuous GPS sites. Stations with triangles are part of the experimental Real Time Kinematic (RTK) sites intended to provide near real time GPS deformation data.

Figure GPS 2. Displacement rates for continuous GPS sites in mm/year for 2001.2 to 2002.7. Solid arrows are significantly above the noise level with the associated error ellipses indicating the 95% confidence interval. Light arrows are within uncertainly levels. The dark arrows indicate inflation of the resurgent dome by between 1 to 2 cm in the last 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, PLV, 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 th

e 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 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

The borehole dilatometers show no geophysically significant signals this quarter. Real-time plots for these instruments are available at

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. 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 north-south component of the long-base tiltmeter is not currently working. The east-west component has shown steady tilt down to the east at a rate averaging 0.1 to 0.2 microradians per month consistent with the two-color EDM and continuous GPS data.

Real time plots of the data from these instruments can be viewed at 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 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 the

caldera are shown in Figures M2-5. Each of these stations are referenced to a site on Sherwin Grade (MG) located to the south of the caldera.

Figures M2-M5. Variations in magnetic field (in nanoTessla) for stations in Figure M1 with respect to the reference station MGS located southeast of the caldera on the Volcanic Tableland.

HIGHLIGHTS

The differenced data for the 10 magnetic field stations, referenced with station MGS, are shown in figures M3, M4, and M5. Missing data are due to telemetry problems at station MGS and DMC. The long-term rate changes for differences HCR-MGS (+1.0 nT/a) and SBF-MGS (-0.6 nT/a) are continuing from 1991 through 2002 (Figure 2). No significant changes in magnetic field are observed during this reporting period. The data from station DMC is noisy and believed to be an instrumental problem. The telemetry outage at MGS is due to a faulty power connection to the satellite telemetry transmitter which was fixed in Sept., 2002.

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, 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. Precipitation data are collected by the USFS at the Mammoth Lakes Visitor Center.

Data for the months of April through June 2002 for most of the telemetered monitoring stations are shown in the attached figure along with snow depth (SWE) at Mammoth Pass and precipitation events. [Note: all dates and times in UT. Gas data not corrected for pressure and temperature.] The record from HS1A and HS1B reflects the usual effect of the winter snow pack. Note the decline in CO₂ concentration along with snow depth in the last half of May. The snowpack began to accumulate last fall with steep increases in late November and December. Snow accumulation in the first three months of 2002 fell off sharply and thus the CO₂ levels at the HS1 sensors did not achieve typical winter values until March. During April, CO₂ concentrations actually began to decline at the HS1 sensors until a significant snowfall on April 17 kicked values high again. A minimal snow effect at SKI was recorded last winter but was not observed this year, although a small event of unknown origin was recorded at SKI in early April. After months of no activity, we recorded three large CO₂ peaks at the Laurel Spring monitoring station in mid May (not shown). Early in the third quarter of 2002, two stations, HS3 and EQF, stopped transmitting but since we no longer have a technician on the project, repair of these stations will have to wait until August at the time of the annual maintenance trip.

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

Figure C2. Carbon dioxide (CO₂) concentrations for the monitoring stations in Figure C1 for April-June 2000. CAUTION: Raw Data - not corrected for pressure or temperature.