LONG VALLEY OBSERVATORY QUARTERLY REPORT

JULY-SEPTEMBER 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.

LONG VALLEY OBSERVATORY QUARTERLY REPORT

July-September 2000

 
CONTENTS

 

 

EARTHQUAKES

CALDERA ACTIVITY

SIERRA NEVADA ACTIVITY

MAMMOTH MOUNTAIN LONG-PERIOD 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

           

 

SUMMARY

 

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 and showed minor subsidence (of about 1 cm) through the first half of this year, has shown no further changes this quarter. The center of the resurgent dome currently 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. We detected just six deep, long-period earthquakes beneath Mammoth Mountain this quarter and two very-long-period (VLP) earthquakes at relatively shallow depths (< 4 km) beneath the mountain. Most of the earthquake activity outside the caldera 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=2.1 event on July 12 at 4:51 AM (PDT) beneath the west flank of Mammoth Mountain (Figure S1).

 

 

 

 

 

 

 

SIERRA NEVADA ACTIVITY:

Most earthquake occurring within the Sierra Nevada block south of the caldera during the third quarter of 2000 were again 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. The four largest earthquakes this quarter, however, were located outside this aftershock zone. These included a pair of M=3.0 earthquakes at 2:05 PM on August 15 and 6:27 PM on August 27, respectively, both located near the base of Wheeler Crest near Paradise Camp and a M=3.1 earthquake at 10:39 AM on September 5 located 2 miles southeast of Red Slate Mountain. The latter was accompanied by a sequence of 7 to 8 smaller events including a M=2.9 earthquake at 11:07 AM on the 5th. A M=2.9 earthquake at 5:56 AM on September 29 was centered 1.6 km west of Convict Lake (see Figures S1, S2, and S3).

 

 

MAMMOTH MOUNTAIN LONG-PERIOD EARTHQUAE ACTIVITY

Deep, long-period (LP) earthquake activity beneath Mammoth Mountain remained low this quarter. We detected just six deep LP events all with magnitudes of M=1.5 or smaller. Four of these occurred on 25-26 July.

 

 

Figure S6. Seismograms of the 06 July 2000 VLP earthquake. Top: Seismic band (100 – 0.1 s) from the POPA dilatometer. Bottom: Vertical component seismogram from Devils Postpile seismic station (DMP), which is located adjacent to the POPA dilatometer and has the standard Northern California Seismic Network frequency response in the band 1 – 25 Hz. This seismogram shows the spasmodic burst accompanying the VLP earthquake.

 

In addition, we detected two very-long-period (VLP) volcanic earthquakes with hypocenters beneath Mammoth Mountain: one on 6 July (0356 UT) and other on13 August (0007 UT). Both were recorded on the seismic band (100 – 0.1sec) of the POPA borehole dilatometer, which is located 4 km due west of the summit of Mammoth Mountain, and both are characterized by a single-sided, dilatational wavelet that smoothly develops and recovers over a periods of 30 to 120 sec with peak strain amplitudes of  ~ 1 nanostrain. The 6 July event (illustrated in Figure S6) resulted in a small (0.1-0.3 nanostrain) compressional offset. Both events were accompanied by spasmodic bursts of broad-band (brittle-failure) earthquakes. These VLP events are similar to the 13 October 1996 VLP earthquake, which was accompanied by a deep, long-period (LP) event midway through its dilatational excursion. Based on hypocentral locations determined for the associated impulsive events, we infer that the 13 October 1996 VLP may have been as deep as 15 km beneath the southwest flank of Mammoth Mountain (within the volume of post-1989, deep LP activity beneath Mammoth Mountain) and that both the 6 July and 13 August 2000 events occurred at shallow depths (<4 km below the surface) beneath Mammoth Mountain. These Mammoth Mountain VLP earthquakes are similar to VLP events recorded beneath the summit of Kilauea volcano, Hawaii, which Bernard Chouet and colleagues (1966) interpret as the result of small slugs of magma or magmatic brine moving through a crack-like restriction. The source for the 13 August 2000 event, for example, which was recorded by a 100 sec CMG3 seismometer located 4 km southeast of Mammoth Mountain (operated by the University of Nevada, Reno) and the Caltech TERRAscope station MLAC located 20 km east of Mammoth Mountain in addition to the POPA dilatometer, can be modeled as a north-striking opening crack. Spasmodic bursts were common during the 1989 Mammoth Mountain earthquake swarm, and the coincidence of spasmodic bursts with the 6 July and 13 August VLP earthquakes suggests that VLP activity may have been an integral part of the 1989 swarm activity (we had no instruments recording the 100-1 sec band at the time).

 

REGIONAL ACTIVITY:

A small (M=2.7) earthquake occurred beneath Merced Peak in Yosemite National Park (13 miles south of Yosemite Village) at 11:51 PM on August 2. Sporadic earthquake activity continued in the Fish Lake Valley area (30 km east of Boundary Peak at the north end of the White Mountains) with M=3.2 and M=3.0 earthquakes at 6:00 AM on August 6 and a pair of M=2.3 events at 10:38 and 10:51 PM, respectively, the same day. 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. A couple of small earthquakes (M=2.0 and 2.3) occurred beneath Black Point on the north Shore of Mono Lake at 11:00 AM on August 13 and 5:11 PM on August 14, respectively. On September 5, a M=3.0 earthquake occurred beneath the Sierra Nevada 10 miles west of Bridgeport.

 

 

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.

 

 

 

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 contracted slightly over 1 cm over the past year. This compares with over 35 cm of extension from the beginning of the 2-color EDM measurements in mid-1983 through mid-1998. 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 EDM-2. ( http://quake.wr.usgs.gov/QUAKES/geodetic/twocolor/lv_freq.gif ). Line-length changes along frequently measured EDM baselines since mid-1983.

 

 

 

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.

 

 

 


Figure GPS-1. Map showing locations of continuous GPS stations in Long Valley caldera

 

Figure GPS-2, 3, and 4. North, east, and vertical displacement components in mm for the continuous GPS stations since 1994.7 with the average trend removed. The removed trend is reported to the right of the station name as mm/y ± a standard deviation.

GPS-2 ( http://quake.wr.usgs.gov/QUAKES/geodetic/twocolor/pl_north_all.gif )

GPS-3 ( http://quake.wr.usgs.gov/QUAKES/geodetic/twocolor/pl_east_all.gif )

GPS-4 ( http://quake.wr.usgs.gov/QUAKES/geodetic/twocolor/pl_up_all.gif )

 

 

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 th

e 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

The borehole dilatometers show no geophysically significant signals this quarter. An important technical achievement concerns our ability to now view the data at high sample rates using seismic data telemetry. Data from the Post Pile (POP) and Motocross (MCX) instruments now come back by 24-bit digital telemetry over phone lines. Data from the Big Springs instrument (BSP) will soon come back by 24-bit satellite telemetry.

The dilatometer data plotted in Figure D2 have all been corrected for variations in atmospheric pressure. 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.(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

 

Data from the long-base tiltmeter are shown in Figure T2. The power line to this instrument was repaired in August, and data transmission resumed at that time. Some critical components on this instrument are still awaiting repair by Roger Bilham from the University of Colorado. Data from the shallow borehole tiltmeters are plotted in Figure T1, T2, and T3. None of these data show geophysically significant changes during this quarter.

 

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 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 3, 4, and 5. Missing data are due to telemetry problems at site PLV 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 2000 (Figure 2). No significant changes in magnetic field are observed during this reporting period.  Changes during mid July and mid-August are due to magnetic storm activity and are not due to tectonic sources. 

 

 During August, 2000, one new magnetometer (DCM) and one new magnetotelluric system (LAP) was installed in the Long Valley Caldera  (Locations-Figure 1, Data-Figure 4. All are operational with data being recorded onsite and with USGS satellite telemetry.

 

 

 

Figure M2. Magnetic field differences in nanoTesslas (nT) between stations SPF-MGS and HCR-MGS from 1984 through September 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. 

 

Raw data for the 3rd quarter of 2000 for many of the telemetered monitoring stations are shown in the attached figure along with Long Valley precipitation events. [Note: all dates and times in UT.  Data not corrected for pressure and temperature.]  Besides a couple of small peaks at HS1A and HS1B, the data from the monitoring stations reflect a relatively quiet period in the record.  The series of peaks at the Laurel Spring monitoring station reported last quarter diminished considerably this quarter.  All of the monitoring stations were serviced in August.  The precipitation data show a relatively dry summer with only one significant precipitation event.

 

The project had scheduled a gas flight at Mammoth Mountain in September to measure carbon dioxide emissions but was unable to conduct the flight due to the lack of availability of an aircraft.

 

 

 

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.