LONG VALLEY OBSERVATORY QUARTERLY REPORT

October-December 2001

and

Annual Summary for 2001

 

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 to 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

October-December 2001

 
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: 4th Quarter – 2001         

 

RECENT HYDROLOGIC TESTING IN WELL LVEW

 

ANNUAL SUMMARY: 2001

 

 

SUMMARY: 4th quarter 2001

 

The quiescence in Long Valley caldera that began in the spring of 1998 continued through 2001. The resurgent dome, which essentially stopped inflating in early 1998, showed minor subsidence (of about 1 cm) through the fourth quarter of 2001. The center of the resurgent dome currently stands just under 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 remained 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). None of the earthquakes within caldera had magnitudes as large as M=2.0 this quarter. As usual, all were located within the south moat, along the southern margin of the resurgent dome, or beneath Mammoth Mountain.

 

 

 

 

 

 

 

SIERRA NEVADA ACTIVITY:

Most earthquakes occurring within the Sierra Nevada block south of the caldera during the fourth quarter of 2001 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. A M=3.1 earthquake at 12:21 PM followed by a M=2.5 earthquake at 3:11 PM, both on October 2nd, were located just east of this aftershock zone with epicenters directly beneath the surface trace of the Hilton Creek fault (Figure S1). The largest earthquake during this period was a M=3.4 event at 1:26 PM on December 2nd located 2 miles east of Mount Morgan and near the epicenter of the M=5.6 earthquake of May 15, 1999. This earthquake was followed by a small aftershock sequence (Figure S3, S4).

 

 

REGIONAL ACTIVITY:

A magnitude M=2.9 earthquake occurred in Chalfant Valley at 8:33 PM on October 24, 12 miles north of Bishop. This earthquake was the largest in a small cluster that included a M=2.5 earthquake at 1:01 PM on the 23rd.

 

 

 

 

 

 

 

 

 

DEFORMATION

 

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

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 G1 Map showing 2-color EDM (Electronic Distance Meter) 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 by roughly 2 cm since mid 2000. 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 just under 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 from the beginning of measurements in early 1984 through March 4, 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, 16 GPS (Global Position System) receivers have been installed within and near the Long Valley Caldera. The locations of the GPS receivers within the caldera are shown in Figure GPS-1. Data from these receivers and will take over the long-term monitoring as the two-color EDM is phased out in 2002. Horizontal displacement rates for the GPS stations in Figure G3 are plotted in Figure G4.

 

The travel-time measurements from each receiver are 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 G3 Locations of continuous GPS stations.

 

 

 

 

Figure G4. Displacement rates for the continuous GPS sites in mm/yr for 1999.0 to 2002.0

 

 

 

 

 

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 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.(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 tiltmeters showed no significant changes this quarter. Long-term trends reflect secular instrumental drift and not geologically significant changes.

 

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. Station locations for differential magnetometers and magneto-telluric (MT) arrays.

 

 

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.

 

 

 

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

 

 

 

HIGHLIGHTS

The differenced data for the 10 magnetic field stations, referenced with station MGS, are shown in Figures M3, 4, and 5. Missing data are due to telemetry problems. 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 2001 (Figure M2). No significant changes in magnetic field are observed during this reporting period.  Changes around Oct 22, Nov 7, and Nov 24 are due to magnetic storm activity and are not due to tectonic sources. The data from station DMC is noisy and believed to be an instrumental problem.

 

CO2 STUDIES IN LONG VALLEY CALDERA: 4th Quarter – 2001

Ken McGee, Terry Gerlach, and Mike Doukas, Cascades Volcano Observatory, Vancouver, WA

 

INSTRUMENTATION

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, and EQF, located near Earthquake 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, 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

 

HIGHLIGHTS

Data for 2001 for most of the telemetered monitoring stations are shown in Figure C1 along with snow depth (SWE) at Mammoth Pass.  [Note: all dates and times in UT.  Data not corrected for pressure and temperature.]  The obvious breaks in the data in August occur during annual servicing of the monitoring stations.  In addition, longer breaks in the data from the stations at HS1 and SKI were caused by power problems at each site.  The typical annual buildup of CO2 during the winter months in the core area of the anomaly at Horseshoe Lake can be seen in the plots for HS1A, HS1B, and HS2.  This year for the first time, there is a strong suggestion of a winter CO2 buildup under the snow at the SKI monitoring site as well.  There are few, if any, additional CO2 degassing events of any significance during 2001.

CVO gas project personnel traveled to Long Valley several times during 2001 for a variety of activities including: servicing the CO2 monitoring stations, making soil CO2 efflux measurements, and emergency repair trips.  One airborne mission to Mammoth Mountain for CO2 plume measurements was flown in November. 

 

 

Figure C1. Carbon dioxide concentrations

 

Recent Hydraulic Testing in Well LVEW (Chris Farrar, Jim Howle, Mike Sorey, Bill Evans, Evelyn Roeloffs:  USGS, Carnelian Bay, CA, Menlo Park, CA, and CVO,WA, Ron Jacobson, and Joe Henfling: Sandia National Lab, NM)

 

 

Three hydraulic tests have been run in LVEW during May 2000, July 2000, and September 2001; all after completion of phase III drilling which reached a total depth of 2997 m.   The May 2000 test was a shake-down run to test equipment, procedures, and the quality of water for surface discharge permit.  The pump discharge rate for the May and July 2000 tests was ~ 0.83 l/s.  During May the pump was run for three periods, totaling 24 hours and produced ~75,000 liters of fluid (about 3 well-bore volumes).  A 94-hour pumping test was run in July 2000, producing about 291,000 liters (~10.7 well bore volumes).  Early-time drawdown data (<10,000 seconds) from July closely match a type-curve solution for a well obtaining water from a single high-angle fracture.   Data obtained after 10,000 seconds from a pressure transducer set 390 m below the surface are more difficult to interpret because of the large density changes that took place in the fluid column after pumping began, resulting in the measured pressure rise at 390 m (fig. LP1).

 

To overcome the density variation problem during pumping, a repeat of the test was done in September 2001 with a high-range pressure transducer set at 2600 m below land surface.  At this setting the transducer accurately measures the pressure of the total fluid column regardless of density variations.   A plot of drawdown versus log of time (fig. LP2) shows a linear section between 100 and 1000 seconds that transitions between 1000 and 10,000 seconds to a curved section, with flattening slope between 10,000 seconds and the end of the pumping phase.  The linear part of the plot closely matches a solution for radial flow in porous media, with a transmissivity of 1 darcy-meter.  The flattened slope after 10,000 seconds suggests that drawdown reached a constant head/pressure boundary.

 

Analyses of water samples (Table 1) show a progressive change in chemical composition between May 2000 and September 2001, suggesting the latest samples are the least contaminated from drilling fluids and better represent the composition of the formation water.  Although the ratio of dissolved boron to dissolved chloride (fig LP3), varies only slightly from .054 in May 2000 to .046 in September 2001, the concentrations of both constituents increase by 41 and 67 percent respectively.  These elements and the ratio of total dissolved solids to chloride for the three periods progressively shift toward a mixing line between non-thermal and thermal waters in Long Valley.  The major cation composition is similar to other geothermal waters sampled around the resurgent dome in Long Valley Caldera in terms of concentrations and ionic ratios.  The major anion composition shows LVEW water contains less chloride than other geothermal waters.  The major ion chemistry most closely resembles water from two thermal springs, LHC on the east side of the resurgent dome and BAL in the east moat.  The low tritium value, 0.2 +/- 0.2 TU, collected July 23, 2000, indicates that neither recent (< 50 yr) recharge water nor modern water (~ 5 TU) injected during drilling make up a large proportion of the sample.  The oxygen 18/16 and D/H ratios of all the samples are consistent with a recharge source on the resurgent dome or from the east moat or south moat of the caldera.  The He-3/4 ratio (from Mack Kennedy, LBNL) in dissolved gas is lower than the ratios in most thermal waters in the caldera but still suggests a strong magmatic component, somewhat diluted by crustal helium.  The carbon 13/12 ratio closely matches that of other geothermal waters and magmatic CO2 in Long Valley-Mammoth Mtn area.

 

Table 1.

Water Chemistry and Isotopic Composition 

 

 

 

 

 

 

Date

Temp

 pH

 ALK

NH3

  Ca

   Mg

   Na

   K

  Cl

 SO4

   F

SiO2

B

TDS

   D/H

O18/16

 

    C

 

--------------------------------------- milligrams per liter  -------------------------------------------

 

5/5/2000

29.4

7.3

644

15.8

14

0.90

463

11.3

56.2

159

4.9

152

3.04

1490

    --

     --

7/22/2000

39.0

7.2

664

12.0

10

0.64

388

11.9

77.9

94.6

8.8

137

3.73

1250

-125.9

-16.33

7/23/2000

41.0

7.2

655

12.3

10

0.64

385

12.1

79.4

92.3

9.1

138

3.82

1240

-126.0

-16.34

9/11/2001

36.8

7.2

643

10.6

7.7

0.60

370

12.0

94.0

88.0

7.9

130

4.30

1200

-125.9

  -16.45

Table 2.

Gases dissolved in water

 

 

 

 

 

 

 

 

 

Date

Ar

O2

N2

CO2

CH4

H

He

He3/4

  C

 

 

 

 

 

 

 

 

     ------------- micromoles/kg ---------------

 R/Ra

delta 13/12

 

 

 

 

 

 

9/11/2001

20

0.1

1310

2510

9.6

0.02

0.22

3.66

-6.4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REVIEW OF 2001

 

ACTIVITY HIGHLIGHTS

Activity levels in Long Valley caldera and vicinity were incrementally lower in 2001 than in 2000 thus continuing the trend of extended quiescence that began toward the end of 1999. Low level seismic activity within the caldera typically included five or fewer earthquakes per day large enough to be located by the online computer system. Most were smaller than magnitude M=2.0, and none were as large as M=3.0 (the largest was a M=2.8 earthquake beneath the southern margin of the caldera 0.5 mile north of Convict Lake on May 21).

 

Seismic activity in the Sierra Nevada south of the caldera continued to be concentrated within the aftershock zone of the 1998-99 sequence of three M5 earthquakes. The 2001 activity included eight earthquakes of M=3.0 or larger. The largest was the M=3.4 earthquake of December 2 located near the epicenter of the M=5.6 earthquake of May 15, 1999 (Figures S7, S8, and S9).

 

Mid-crustal long-period (LP) volcanic earthquakes continue to occur at depths of 10 to 25 km beneath the west flank of Mammoth Mountain although at a much reduced rate compared with the peak in activity in 1997-98 (Figure S10). Altogether, some 60 LP earthquakes were detected during 2001 with over 15 of these occurring in a cluster on February 10th.

 

Figure S10. Temporal occurrence of deep (depth 10 to 25 km) long period (LP) earthquakes beneath Mammoth Mountain from 1989 through 2001. Vertical bars indicate number of LP earthquakes per week (left axis) and the continuous curve indicates the cumulative number of events (right axis).

 

Deformation within the caldera was limited to continuing slow subsidence of the resurgent dome at a rate of roughly 1 cm/year. All together, the center of the resurgent dome has lost some 2 cm in elevation since inflation stopped in late 1998 leaving the center of the resurgent dome roughly 75 cm or so higher at the end of 2001 than in the late 1970’s. The continuous strain and deformation monitoring networks detected no short-term deformation transients during the year. The same is true for the magnetometer networks.

 

The diffuse carbon dioxide (CO2) degassing at the Horseshoe Lake tree kill area and other sites around the flanks of Mammoth Mountain has shown no significant change over the past several years. The total CO2 flux continues to fluctuate about 200 tones per day with the Horseshoe Lake area contributing roughly 90 tones per day.

 

ISSUES AND FUTURE DIRECTIONS

The lull in caldera unrest over the past couple of years has provided an opportunity to look back over the wealth of data collected during the previous two decades of activity and to more thoroughly investigate the nature and significance of the processes driving the unrest. The better we understand what has happened, the better we will be able to assess the future unrest episodes and their significance in terms of potential volcanic hazards. As we look more carefully at data from the intense unrest during the 1997-98 episode in the south moat, for example, it is becoming increasing clear that fluids (magmatic brine or perhaps magma) played a central role in this activity. This underscores the value of a closely integrating the seismic, deformation, and hydrologic monitoring efforts – an effort that we will emphasize through 2002.

 

The lull has also provided the time to complete a number of pending items. One such item is a revision of the USGS Response Plan for Volcanic Unrest in the Long Valley Caldera – Mono Craters Region, California. The updated version of this plan was released in March 2002 as USGS Bulletin 2185. This bulletin is currently available in electronic form at http://geopubs.wr.usgs.gov/bulletin/b2185/. Printed versions will be available toward the end of April 2002.

 

A significant change in deformation monitoring will occur during 2002 as we phase out the two-color EDM monitoring network in favor of the continuous GPS network, which now consists of 18 stations in and around the caldera. The GPS data have been tracking the 2-color EDM data quite nicely over the past couple of years (check the GPS link under Deformation on the LVO web page: http://lvo.wr.usgs.gov ). Although we sacrifice a factor of two to four in resolution with this switch, we gain significant reliability and flexibility in deformation monitoring over the long run.

 

Plans for installing the down-hole instrument package in the Long Valley Exploratory Well (LVEW) in late summer 2002 are on track. Once completed, high-quality data on seismicity, strain, pore pressure, and other parameters from this 2.5 km-deep borehole observatory, which is centered directly over the inflation center for the resurgent dome, will be incorporated with the rest of the Long Valley caldera monitoring data stream. Also this summer, we plan to install two broadband, three-component seismic stations (CMG 40T’s) in the vicinity of Mammoth Mountain to enhance our ability to detect and interpret an future occurrences of long period (LP) or very-long-period (VLP) earthquake activity in the area.