LONG VALLEY OBSERVATORY QUARTERLY REPORT

JANUARY-MARCH 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: January-March 2000

EARTHQUAKES

CALDERA ACTIVITY

SIERRA NEVADA ACTIVITY

REGIONAL ACTIVITY

DEFORMATION

TWO-COLOR EDM SUMMARY

GPS – CONTINUOUS MEASUREMENTS

DILATATIONAL STRAIN

Instrumentation

Highlights

MAGNETIC MEASUREMENTS

INSTRUMENTATION

HIGHLIGHTS

CO₂ SOIL GAS CONCENTRATIONS AND AIRBORNE MEASUREMENTS

HYDROLOGIC MONITORING

LONG VALLEY OBSERVATORY QUARTERLY REPORT

January-March 2000

The relative quiescence that has persisted in Long Valley caldera since the spring of 1998 continued through the first quarter of 2000. The resurgent dome has shown no significant deformation for the past year, and 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 (M ³ 1) to be detected and located by the real-time computer system. The only earthquakes within the caldera during this quarter with magnitudes M ³ 2.0 were M=2.3 and M=2.1 earthquakes at 4:00 AM and 6:19 AM on January 7^th, both located near the southern margin of the resurgent dome (Figure S1).

SIERRA NEVADA ACTIVITY:

Earthquake activity within the Sierra Nevada block south of the caldera during the first 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. Half a dozen of the earthquakes along this zone had magnitudes in the range M=2.8 to 3.0. The largest earthquake in the area, however, was a M=3.8 earthquake at 8:04 PM on January 20^th located beneath the area between Mt Morrison and Convict lake just south of the caldera. This earthquake was accompanied by a number of smaller events, six of which had magnitudes of M=2.0 or greater (Figure S1).

REGIONAL ACTIVITY:

Activity elsewhere in the region included two M=3.2 earthquakes located 15-16 km east of Mono Lake on January 2 and January 19, respectively. A M~4 earthquake and a cluster of smaller events occurred on March 4 beneath the northwest side of Fish Lake Valley (roughly 15 km east of Boundary Peak at the north end of the White Mountains). This 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.

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 rates are nearly zero relative to the measurements made since 1984. In contrast, the rates during the later part of 1997 were has high as 20 cm/yr on the KRAKATAU baseline during the most recent unrest in the caldera. Several baselines over the past year show slight contraction which is probably the result of geothermal power production at Casa Diablo located 1 km west of the central EDM site at CASA.

In addition to the frequently measured baselines, the annual survey data spanning the mid-98 to mid-99 interval shows a correspondingly small or negligible deformation over the past year.

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. Highlights

The dilatational strain data show no unusual during the first quarter of 2000. Figures summarizing the dilatometer and tiltmeter data for the first half of 2000 will appear in the monitoring report for the second quarter (April-June). The dilatometer and tiltmeter 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.

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 20 km southeast of the caldera.

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.

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.

During August, 1999, three new magnetometers (MXP, VAP, and LAP -- not shown in Figure M1), 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 M2-4). 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 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 SOIL GAS CONCENTRATIONS AND AIRBORNE MEASUREMENTS (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 (see Figure C1). 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, 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. Raw data for the 1st quarter of 2000 for many of the telemetered monitoring stations are shown in the attached figure along with Long Valley seismicity. [Note: all dates and times in UT. Data not corrected for pressure and temperature.] An unfortunate power failure at the HS1 monitoring site in February has resulted in a loss of data from that site. It is our intention to put that station back on line once snow levels decline to the point where it is safe to again work in that building.

A gas flight was made at Mammoth Mountain on March 22. Sixteen orbits were made at 200-ft intervals from 12,500 ft to 9,500 ft. The CO₂ content of the background air was remarkably uniform – much more so than ever observed in previous flights; most of the observed variability was in a +/- 0.5 ppm range. There was no evidence of CO₂ pollution from potential anthropogenic sources on or around the mountain. CO₂ from the mountain was detected mainly in the lower three orbits. The anomalies are clear and sharp in the data with peaks rising to as much as 3 ppm above background. They suggest that more volcanic CO₂ resides below the 9,500-ft orbit, which for safety reasons is normally our lowest orbit. The indicated above-background CO₂ emission rate is about 50 tons/day, but we feel this is an underestimate because of the likelihood of more volcanic CO₂ below the 9,500-ft orbit. The flight was made at noon and lasted a little over an hour. We have found this to be a good time to fly in the summer and fall for taking advantage of plume lofting by orographic winds; in winter, however, it may be too early in the day for orographics to be effective. It is also possible that the results were effected by the absorption of CO₂ in snow melt, which started about four days before the flight – a few weeks earlier than in recent years. However, we think this is unlikely to be significant because the continuous CO₂ sensor records usually do not show significant CO₂ absorption until after melting becomes far more advanced.

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 January-March 2000.
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.