LONG VALLEY OBSERVATORY QUARTERLY REPORT

APRIL-JUNE 2003

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 2003

CONTENTS

EARTHQUAKES

CALDERA ACTIVITY

SIERRA NEVADA ACTIVITY

REGIONAL ACTIVITY

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

CONTINUOUS BOREHOLE AND STRAIN MEASUREMENTS

Instrumentation

Highlights

TILT MEASUREMENTS

Instrumentation

Data

MAGNETIC MEASUREMENTS

BACKGROUND

DATA

CO₂ STUDIES

SUMMARY FOR APRIL-JUNE 2003

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the first half of 2003. The resurgent dome, which has shown minor fluctuations in uplift and subsidence since early 2000, showed essentially no change during the 2^nd quarter of 2003. The center of the resurgent dome still stands roughly 80 cm higher than prior to 1980. Seismic activity within the caldera, which has typically included fewer than five small earthquakes per day since 1999, continues at this relatively low level. The largest earthquake in the region this quarter was a magnitude M=3.2 earthquake in the Sierra Nevada near Grinnell Lake (13 miles SE of Mammoth Lakes) at 10:25 AM (PDT) on June 15. 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 through the second quarter of 2003 averaging fewer than 5 earthquakes per day large enough to be located by the realtime computer system (Figures S1-S5). The largest earthquake within the caldera during this period was a M=1.8 earthquake on at 3:03 AM (PST) on May 28 associated with a cluster of half a dozen smaller events all located in the south moat just east of the airport (Figure S3).

REGIONAL ACTIVITY

As has been true since 1999, earthquake activity in the Sierra Nevada block south of the caldera continues at a higher rate than that within the caldera. The most energetic activity in the region this quarter involved a M=3.2 earthquake at 10:25 AM on June 15 located just east of Grinnell Lakes in the Sierra Nevada south of the caldera (12 miles SE of Mammoth Lakes: see Figure S3). This earthquake was followed by a number of smaller aftershocks, including a M=2.2 earthquake at 4:09 AM on the 16^th.

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

John Langbein, Stuart Wilkinson, Elliot Endo, Eugene Iwatsubo, and Jerry

Svarc

Over the past 6 years, 18 GPS (Global Position System) receivers have been installed within and near the Long Valley Caldera. Of these, 14 were installed by Elliot Endo of the Cascades Volcano Observatory. The locations of the 12 receivers within the caldera are shown in Figure G-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 (Figure G-2). The site at CASA now has two receivers; one operating since 1994 and the second one, CA99, installed this past summer.

The vectors representing horizontal displacements from continuous GPS since January 2002 are shown in Figure G-1. The vectors pointing radially away from the center of the resurgent dome indicate expansion across the resurgent dome of about 3 cm since the most recent episode of inflation began in the spring of 2002.

Figure G-1. Horizontal displacement vectors in mm/year for continuous GPS sites in and around Long Valley Caldera from May 2002 through October 17, 2003.

There are now 5 baselines from the frequently measured, two-color EDM network that are also measured by continuous GPS. The location of the EDM and GPS networks is shown in Figures G-1 and G-2. A comparison of the length-changes derived from GPS and those measured by the EDM are shown in Figure G-3 for the last three years. Although for the Casa-Krak and Casa-Knolls baseline the comparison could be extend back in time, this comparison includes only the GPS data from the sites installed by USGS. There are now two GPS stations at CASA (Casa and Ca99), and two stations at KRAK (Krak and Krac). Since the USGS installations use more modern receivers, they have better day-to-day repeatability than the JPL operated receivers. Finally, it should be noted that the EDM measurements on the Casa-Hot baseline have more scatter than desirable; this is because the optics have deteriorated on the Hot

reflector.

More plots of both the GPS and EDM data can be found at:

http://lvo.wr.usgs.gov/monitoring/index.html#deformation

Figure G-2 Map showing 2-color EDM baselines

The measurements of length changes shown in Figure G-3 for the frequently measured EDM baselines (together with the associated GPS data) show that the gradual contraction that began in early 1999 was followed by an episode of gradual expansion that began in late 2001 and persisted through early 2003. This expansion has since slowed, and the resurgent dome has shown no significant deformation through mid-2003. 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 G-3. Line-length changes for the EDM baselines (circles) measured from CASA for the period January 1, 1999 through Oct 17, 2003 compared with continuous GPS data for the same lines (crosses).

CONTINUOUS BOREHOLE STRAIN MEASUREMENTS (Malcolm Johnston, Doug Myren, and Stan Silverman)

Instrumentation

Dilational strain measurements are being recorded continuously at the Devil's Postpile (POP), Motorcross (MX) near the western moat boundary in the south moat, Big Springs (BS) just outside the norhtern caldera boundary, and at Phillips (PLV1), just to the north of the town of Mammoth Lakes. The site locations are shown in 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 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 D-1. Locations of dilatometers and tiltmeters.

Data from the strainmeters are transmitted using satellite telemetry every 10 minutes to a host computer in Menlo Park. The data are also transmitted with 24-bit seismic telemetry together with 3-component seismic data to Menlo Park.

Highlights

The strain data for the second quarter of 2003 show no significant changes. The strain increase on the POP dilatometer beginning on about May 12 reflects a pore pressure increase associated with shallowing of the water table during spring runoff in the San Joaquin drainage.

Figure D-1. Dilatational strain for April-June 2003 from the POPA, Big Springs (BS), and Motor Cross (MX) borehole dilatometers. Also shown is the pore pressure in a well adjacent to the POPA dilatometer.

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

Instrumentation

Instruments recording crustal tilt in the Long Valley caldera are of two types - 1) a long-base (LB) instrument in which fluid level is measured in fluid reservoirs separated by about 500 m and connected by pipes, which 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). For tiltmeter locations, see Figure D-1. Real time plots of the data from these instruments can be viewed at http://quake.wr.usgs.gov/QUAKE/longv.html.

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.

Highlights

Data for the tiltmeters show no significant changes for the period April-June 2003 (see Figures T-1 and T-2).

Figure T-1. East-west and north-south components of the long base tiltmeter for April-June 2003.

Figure T-2. East-west and north-south components for the borehole tiltmeters for April-June 2003. Increasing values indicate tilt down to the east and north, respectively.

MAGNETIC MEASUREMENTS (M.J.S. Johnston)

BACKGROUND

Local magnetic fields at 12 sites in the Long Valley Caldera are transmitted via satellite telemetry to Menlo Park every 10 minutes. 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 M-1. Temporal changes in local magnetic field are isolated using

simple differencing techniques.

Figure M-1. 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 and near the caldera are shown in Figure M-2.

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 April through June from 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 effect of the winter snow pack, although this year the snow accumulation was only about 75% of normal on April 1. Due to several late season snow events, maximum snow accumulation did not occur until early May. Thus the CO₂ levels at the HS1 sensors did not achieve typical sustained winter values until mid-April. A minimal snow effect at SKI recorded in the winter of 2001 was not observed in 2002 or 2003. A slight snow effect on the carbon dioxide levels can be seen during April and May at HS3. Except for a slight rise in the baseline at SKI, carbon dioxide levels at the remaining monitoring stations were relatively flat during this quarter. Levels at both HS1 sensors returned to their normal low baselines by June.

Figure C-1 Map showing locations of the continuous CO₂ -monitoring stations.

Figure C-2. Carbon dioxide (CO₂) concentrations for the monitoring stations in Figure C1 for April-June, 2003.

CO₂ Studies (Deborah Bergfeld, Chris Farrar, and William Evans: U. S. Geological Survey, Menlo Park, and Carnelian Bay, CA).

BACKGROUND

During May 2003, we completed construction of four semi-permanent grids for monitoring CO₂ flux from thermal areas on portions of the resurgent dome. The new grids range in size from around 3,700 to 6,000 m² and are located north and west of the Casa Diablo power plant and in a small canyon near Fumarole Valley (Fig. 1). An earlier grid established southwest of the entrance to the power plant covers over 10,000 m². All of the grids have at least one section of ground that is characterized by dead plants, high soil temperatures, and sites where steam vents along former root zones. The overall goals of this project are to use flux data and soil-gas chemistry to better delineate geothermal fluid pathways across the caldera and to produce heat flux estimates for portions of the resurgent dome. If and when the proposed expansion of the Casa Diablo well field takes place another objective will be to track changes in CO₂ emissions from the thermal areas. Changes resulting from increased production may only be observed at the grids located near the power plant but it is also possible that new thermal areas may develop. Until the new well goes on line, periodic monitoring of the established grids will provide a better understanding of natural variations in CO₂ emissions at these sites.

CO₂ FLUX

Locations for the five grids Casa Diablo (CD), Basalt Canyon (BC), dead tree hill (DTH), and Fumarole Valley north and south (FVN, FVS) are shown in figure 1. Our initial survey of the grids confirmed that all five have some locations with elevated CO₂ flux. The grids can be split into low flux and high flux groups. FVN, FVS, and DTH are the low flux grids that have extensive areas of tree kill but only a few high flux spots (Fig. 2). Our observations at these grids indicate that thermal activity may be in decline.

Figure 1. Generalized map showing the location of the five flux grids.

Figure 2. Image maps of CO₂ flux at DTH, FVN and FVS show only a few areas of high flux. CO₂ flux is reported as g m^-2d^-1.

In contrast, fluxes at BC and CD are high across most of the grid (Fig. 3). At CD high fluxes are focused around steaming ground and then drop off outside of these areas. Flux at BC is generally high across the entire site and the grid does not encompass the entire CO₂ anomaly and needs to be enlarged. Flux and soil temperature are positively correlated at these sites and, unlike the low flux grids, there is no appearance of waning thermal activity.

Figure 3. Image maps showing high CO₂ fluxes at the CD and BC grids. CO₂ flux is reported as g m^-2d^-1.

Estimates of the average flux for the grids are made using a minimum variance estimator on the ln flux data. Total CO₂ emissions are estimated by applying the average flux across the entire grid (Table 1). The largest CO₂ emissions are from BC and CD but due to the lack of coverage at BC it is possible that the estimate is low.

Location	# sites	area (m^2)	avg. flux (g m^-2d^-1)	emissions (t/d)	+/-
Low Flux Grids
DTH	43	46250	16	0.7	0.10
FVN	26	3700	21	0.1	0.01
FVS	26	4300	18	0.1	0.01
High Flux Grids
BC	36	6000	440	2.6	0.70
CD	35	10625	122	1.3	0.37

During future field excursions additional grids will be constructed west of the power plant and along a ridge of dead trees south of the Fumarole Valley grids. Wide spaced flux measurements will also be collected between the grids to establish values for normal background flux.