LONG VALLEY OBSERVATORY
QUARTERLY REPORTS
COMBINED JANUARY-JUNE 2006
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 REPORTS
January-June 2006
CONTENTS
EARTHQUAKES
CALDERA ACTIVITY AND MAMMOTH MOUNTAIN
Resurgent
dome LP earthquake
Deep
Mammoth Mountain swarm
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
HIGHLIGHTS
CO2 STUDIES
MAMMOTH MOUNTAIN FUMAROLE
UPDATE
UPDATE ON GEYSERING IN HOT
CREEK
SUMMARY FOR JANUARY-JUNE 2006
The relative quiescence in
Long Valley caldera that began in the spring of 1998 continued through the
second quarter of 2006. The resurgent dome, which essentially stopped inflating
in early 1998 and showed minor subsidence (of about 1 cm) through 2001, was
followed by gradual inflation through 2002. It has since held relatively steady
showing only minor fluctuations about an average elevation roughly 80 cm higher
than prior to the onset of unrest in 1980. Noteworthy events through the first
half of 2006 include: 1) unambiguous identification of a shallow long-period
(LP) earthquake beneath the southern section of the resurgent dome confirming
that similar events observed during the 1997 cannot be dismissed as due to path
effects on wave propagation from shallow brittle-failure earthquakes; 2) a swarm
of deep, brittle-failure earthquakes in the lower crust (depths 25- to 35 km)
southwest of Mammoth Mountain with hypocenters distinctly below the deep LP
earthquakes beneath the southwest flank of Mammoth Mountain; 3) An
exceptionally heavy spring snowfall trapping gas emissions from the Mammoth
Mountain fumarole (MMF) forming a snow cave, the collapse of which resulted in
the tragic deaths of three ski patrol members presumably due to carbon dioxide
poisoning, and 4) the onset of episodic geysering in Hot Creek in early June
that forced temporary closure of Hot Creek to swimming as episodic geysering
has continued through the summer.
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)
Note: Seismic
activity in this report uses the automatic computer-generated (Earthworm)
solutions rather than the final hand- check (CUSP processing) solutions. The computer-generated epicentral
locations and magnitude estimates have become increasingly reliable with time,
and they do not suffer from backlogs that can develop in CUSP processing due to
an abrupt increase in the rate of earthquake activity elsewhere in northern
California.
LONG VALLEY CALDERA AND
MAMMOTH MOUNTAIN ACTIVITY:
The level of earthquake activity within Long Valley caldera remained low through the first six months of 2006 with nothing larger than magnitude M=2.4 (Figures S1- S8).
Resurgent dome LP earthquake: The most noteworthy seismic occurrence within the caldera was a shallow, long-period (LP) earthquake at a depth of less than 1 km beneath Fumarole Valley near southeast side of the resurgent dome on 10 January 2006. The waveform and spectra for this well-recorded (M~2) LP earthquake stands in clear contrast to those for a series ordinary (shear-failure) earthquakes with essentially the same location and focal depth that occurred in a swarm on 12-March swarm. The largest earthquake in this swarm was a M~1.8 event (Figure S11). We recorded a number of what appeared to be shallow LP earthquakes in the same area during the intense earthquake activity associated with the 1997 unrest episode. Until now, however, we could not preclude the possibility that the LP-like appearance of these earthquakes was a result of wave propagation from a shallow source rather than a property of the earthquake source itself. The contrast between the 10 January LP earthquake and the co-located shear-failure earthquakes of the 12-March swarm make clear that peaked spectrum at ~2 Hz of the LP earthquake (Figure S10) is indeed a source property. These shallow LP earthquakes beneath Fumarole Valley likely result from pressure oscillations in steam-filled cracks in the shallow hydrothermal system beneath the southern margin of the resurgent dome.
Figure S9. Seismograms for the shallow (< 1 km) LP earthquake beneath the resurgent dome on 10 January 2006 (top) compared with a M=1.8 typical shear-failure earthquake at essentially the same location and depth on 12 March 2006.
Figure S10. Spectrum of the 0348 UT M~2, 10 January 2006 resurgent dome LP earthquake (left) compared with that for the 1054 UT M=1.8 brittle-failure earthquake on 12 March 2006 (right) with the same location and focal depth.
Deep Mammoth Mountain swarm: Mammoth Mountain activity included an earthquake swarm on 16-June that appear to be typical shear-failure earthquakes (volcano-tectonic earthquakes) beneath the southwest flank of Mammoth Mountain at depths between 25 and 35 km (Figure S11). The largest event in this sequence was a M=2.5 earthquake at 0800 (PDT) on the 16th. This earthquake sequence is remarkable on two counts: 1) these are the deepest earthquakes yet recorded in the vicinity of Long Valley caldera, and 2) they are distinctly deeper than the 10- to 25-km deep LP earthquakes that developed during the 1989 Mammoth Mountain earthquake swarm. We interpret the zone at 6 to 10 km separating the shallow brittle-failure earthquakes and the deep-LP earthquakes beneath Mammoth Mountain as representing the brittle-plastic transition (and the ~350 to 400o C isotherm) in the upper, silicic crust. It seems likely that these deeper, brittle-failure earthquakes are occurring within a more mafic volume of the mid- to lower crust (which can remain in the brittle domain to temperatures as high as ~700o C). It’s noteworthy that a swarm of deep, brittle-failure earthquakes spanning the same depth range occurred beneath the Sierra Nevada crest in the vicinity of Lake Tahoe in late 2003. Smith et al., (Sciencexpress, 5 August 2004) conclude that this Lake Tahoe swarm was associated with a magmatic intrusion in the lower crust. We see no evidence for a significant intrusion associated with this deep Mammoth Mountain swarm.
SIERRA NEVADA 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. Through the first half of 2006,
this activity included three earthquakes of magnitude M=3.0 or greater, the
largest of which was a M=3.5 earthquake at 9:09 AM (PDT) on 11 June located
near Grinnell Lake 16 km south of the caldera (Figures S1-S8). Overall, the
rate of Sierra Nevada earthquake activity was slightly higher in June than
earlier in the year (Figures S5 and S7).
REGIONAL ACTIVITY
The largest earthquake in the
region through June was a M = 4.2 on 16 March located beneath the north end of
Cowtrack Mountain roughly 13 km east of Mono Lake. This is in the general area
of the Adobe Hills earthquake swarm of September 2004 that included two M~5
earthquakes.
DEFORMATION
SUMMARY OF EDM AND GPS MEASUREMENTS
John Langbein, Stuart Wilkinson, Mike Lisowski, 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 G1. 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.
Review of the previous year
of a combination of GPS and EDM data indicate negligible deformation. This is best summarized in Figures G2
and G3, which shows length changes in the two-color EDM baselines (Figure G1)
together with line-length changes determined from the continuous GPS data. Also
see; http://lvo.wr.usgs.gov/monitoring/index.html#deformation
Figure G-1 Map showing 2-color EDM baselines
Figure G2.Line-length changes for the EDM baselines (red
crosses) measured from CASA for the period July 2004 through July 2005 compared
with continuous GPS data for the same lines (black circles).
Figure G3. Line-length changes for the EDM baselines (red
crosses) measured from CASA for the period September 1999 through May 2005
compared with continuous GPS data for the same lines (black circles).
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 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).
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.
.
Figure D1. Locations of dilatometers and tiltmeters.
Highlights.
The borehole strain data during this quarter has been relatively quiet at all sites. Raw dilatational strain data are shown in Figure D2. Comparative pore pressure and strain data at the Postpile and Big Springs dilatometer sites are shown in Figure D3. The only event of note is a strain and tilt transient at the Motocross (MX) site early June (Figures D2, T3). This is an episodically recurring transient, and because corresponding transients do not appear on strainmeters and tiltmeters at other sites, it reflects some process local to the Motocross site.
Figure D2. Dilational strain for POPA, PLV1, Motocross (MX02), and Big Springs (BG02) for 1 January through 31 July 2006.
Figure D-3. Comparing dilatational strain and hydraulic pore pressure for the POPA and Big Springs dilatometer installations for 1 January through 31 July 2006.
TILT MEASUREMENTS (Mal Johnston, Vince Keller, Bob Mueller 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. For
tiltmeter locations, see Figure D1. 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 in Figures T1-T3. 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.
Figure T1. East-west and north-south components of the long-base tiltmeter for 1 January through June 2006.
.
Figure T2. East-west
and north-south components for the shallow borehole tilt stations from 1
January through 30 June 2006.
Figure T3. East-west and north-south components for the
borehole tiltmeters installed with the Big Springs and Motocross dilatometers
for January-June 2006..
Highlights
The data from the long base tiltmeter has been degrading for some time. The North-South component in particular has had problems. Fig T1 shows the data for the first two quarters of 2006. In late June, Roger Bilham of the University of Colorado replaced the sensors and repaired various parts that were failing. The East-West component is now working well but there are still some problems with the North-South leg. While we suspect the tube, new tests will soon reveal the exact nature of the problem. Data from the short base tiltmeters are shown in Figures T2. Very little of geophysical interest occurred this period. The Long base tiltmeter show a slight tilt to the south west. Most of the westerly tilt is probably seasonal. The data from the short base tiltmeters are generally uneventful.
Data from the tiltmeters in the deep boreholes at Big
Springs and Motocross are shown in Figures T3. The only data of interest are
some variations on MX in late June. These are most likely real since a
corresponding event is observed simultaneously on the MX strainmeter. However,
events at this time are not observed on POPA or Big Springs and apparently some
localized event occurred on or near the south caldera boundary. Isolation of
the tiltmeter preamplifiers that is expected reduce thermal problems is planned
for late August.
MAGNETIC MEASUREMENTS
(M.J.S. Johnston)
Background
Local magnetic fields at 18 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 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.
Highlights
No signals of geophysical significance to report for the first six months of 2006 (see Figure M2).
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 first
half of 2006. Station HS1 is located near the central portion of the Horseshoe
Lake tree kill in an area of high CO2 ground
flux and has both a 0-100% sensor and a 0-50% CO2
sensor. Station HS2 is located in a lower flux area near the margin of
the tree kill and HS3 is at the edge of the tree-kill zone in the group
campground area. Stations located away from Horseshoe Lake include SKI, located
near the former Chair 19 in the Mammoth Mountain Ski Area and SRC, installed as
a background site, located at Shady Rest Campground adjacent to the USFS
Visitor Center in the town of Mammoth Lakes. 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 except for SRC, continue to produce strong signals to the
satellite even after significant snow buildup on the roofs of the structures.
The antenna for SRC is located on top of the building and is vulnerable to damage
by snow and wind. 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 Data for the first six months of
2006 are shown in the attached figure along with snow depth (SWE) at Mammoth
Pass. [Note: all dates and times in UT. Data not corrected for pressure and
temperature.] Except for a malfunction at SRC, there are no major gaps in the
2006 data despite at least one power failure in the Lakes Basin. During that
power failure, the backup system, modified last summer, powered the stations
until normal power was restored a few hours later.
The
attached figure shows the typical winter buildup and decline of CO2 in the soil at
Horseshoe
Lake during the first six months of 2006. Because of the unusually large
snowpack during the early months of 2006, stations HS2 and HS3 recorded larger
than normal CO2 signals this year. This is
similar to 2005 which was also an above average snow year. It is possible some
of the recorded anomaly at these stations was due to lateral transport of CO2 through the snow from the core area of the
Horseshoe Lake anomaly near HS1. In April, the CO2
levels at HS3 on the margin of the Horseshoe Lake anomaly were the highest
recorded there since the station was established in 1997. This is in direct
correlation with the exceedingly large snowpack this winter which also peaked
in April. The cause of the slight rise in the CO2
baseline at SKI in late June is not clear at this point. In summary, the
network recorded few abnormal CO2 degassing
events during the first half of 2006 and responded normally to the winter
snowpack.
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 January through June 2006.
MAMMOTH MOUNTAIN FUMAROLE UPDATE
(W.C. Evans, C.D. Farrar)
On 6 April 2006, three members of the Mammoth Mountain Ski Patrol were killed at the Mammoth Mountain Fumarole (MMF), presumably due to carbon dioxide (CO2) poisoning. Heavy snowfalls had completely buried the warning fences encircling the fumarole and allowed a snow cave to form over the top of the vent and surrounding thermal area. Patrol members broke through the top of this cave and fell about 6 meters down through the cave, where gases apparently had accumulated to lethal concentrations. Although no gas measurements were made in the cave at this time, the fumarole is known to emit nearly pure CO2, a gas that is rapidly toxic at concentrations above ~15%.
USGS has monitored MMF for many years, and changes in the composition of the trace gas constituents (H2S, CH4, H2), carbon isotopes, and especially helium isotopes have been shown to correlate with deformation and seismicity. The changes in gas chemistry apparently reflect changes in magmatic degassing or in gas upflow, which might occur even in the absence of seismicity. Many magmatic gas species, such as H2S and CO, are much more toxic than CO2.
To investigate whether the deaths might reflect changes in gas composition and toxicity, MMF was sampled on 21 April after snow removal allowed for safe approach. The analysis of this gas is shown in Table 1, along with concentration ranges observed in previous years. For the 21 April sample, all of the gas constituents and the gas/steam ratio at within or near the range exhibited by previous samples from MMF spanning a 10-year period. No new toxic species were detected. Although the H2S concentration is near the upper end of the range observed previously, it is still at a level thought to be toxic only after fairly long exposure (~1 hour). The rapid loss of consciousness by the Ski Patrol members, as reported by eye-witnesses, strongly implicates CO2 as main culprit.
Mammoth Mountain is a strong emitter of CO2, and not surprisingly, MMF has a high gas/steam ratio compared to a normal geothermal fumarole like CDF (see Table 1), located near the power plant at Casa Diablo. The high gas/steam ratio at MMF likely contributes to the hazard, as there is insufficient latent heat to prevent rapidly accumulating snow from forming a cover over a concealed snow cave, and there is proportionally much more gas to fill up the cave.
Most of the CO2 at Mammoth Mountain is released as diffuse emissions on the flanks (tree-kill areas). However, small areas of diffuse efflux have been identified near MMF and at other areas near the summit and saddle. It remains to be answered whether an increase in gas efflux at MMF has occurred, but a new round of flux measurements at these higher elevation sites is planned for September after all snow has cleared and soils have dried.
Table 1. Composition of gas from Mammoth Mountain Fumarole in
April 2006 compared to the range seen in previous samples and to the
composition of gas from a fumarole near the Casa Diablo power plant.
|
Site |
|
MMF |
MMF |
CDF |
|
sample
date |
1993-2003 |
4/21/2006 |
9/1/1999 |
|
|
|
|
|
|
|
|
vol-% |
He |
0.0008-0.0023 |
0.0019 |
0.0008 |
|
|
H2 |
0.0255-0.0295 |
0.0242 |
0.0672 |
|
|
Ar |
0.0008-0.0081 |
0.0029 |
0.0466 |
|
|
O2 |
0.0178-0.118 |
0.0201 |
0.0353 |
|
|
N2 |
0.936-1.24 |
0.988 |
2.83 |
|
|
CH4 |
0.0016-0.0027 |
0.0025 |
0.0383 |
|
|
CO2 |
98.6-99.0 |
98.9 |
96.4 |
|
|
H2S |
0.0045-0.0689 |
0.0618 |
0.425 |
|
|
CO |
<0.001 |
<0.001 |
<0.001 |
|
|
SO2 |
<0.01 |
<0.01 |
<0.01 |
|
molar
gas/steam |
0.11-0.72 |
0.24 |
0.0012 |
|
UPDATE ON GEYSERING IN HOT CREEK GORGE.
(C.D. Farrar, S. Hurwitz, D. Bergfeld, W.C. Evans)
In early June, geysering was reported from some of the submerged hot springs on the bottom of Hot Creek, at locations both upstream and downstream from the paved access ramp. Geysering was subsequently observed by a team of USGS researchers (D. Hill, M. Mangan, D. Venezky) and has been monitored visually by USFS personnel. Activity can be described as intermittent splashing of muddy water to heights of a meter or more (5-m maximum observed) from the areas of the creek that normally show mild upwelling of bubbly hot water.
Although deformation and seismicity were at low levels, concern over the cause of the geysering prompted sampling efforts. The geysering began at a time of very high streamflow in Hot Creek, due to melting of the larger than normal snowpack, and many of the hot springs that are normally accessible at the edge of the creek were under water. Samples of gas bubbles were collected at three sites, and water samples were collected from two hot springs and from the creek at points upstream, within, and downstream from the area of hot spring discharge. The gas samples were compositionally similar to previous gas samples from the Gorge. The hot spring waters, as shown in the table, were also compositionally similar to previous samples (example shown from 1999).
The difference between upstream and downstream chloride concentrations in creek water was combined with the streamflow estimate to calculate total discharge of hot spring water in the Gorge, according to the usual practice. The result, 243 L/s, is close to the average obtained over many years of monitoring, as seen in Figure 1. Although streamflow estimates at such high flow rates are subject to a slightly greater uncertainty, there does not seem to be any indication of increased discharge of hot water associated with the geysering.
At about the same time the geysering began, high-frequency fluctuations appeared in the water level record in Core Hole 10B, a well that taps into the thermal aquifer a few hundred meters south of the gorge. The temperature profile measured in this well in July, showed that the well has warmed up over time and is now at or near boiling at the water surface (Figure 2). Boiling or bubbling sounds can actually be heard at the top of the well pipe.
The onset of geysering during high streamflow suggests that the geysering activity is related mainly to hydraulic pressure changes resulting from increased stream depth or increased recharge of cold water, or to some other aspect of the cold water-hot water interface in the gorge. Increased temperature of the hot water feeding the springs may be a factor, as suggested by the temperature record in Core Hole 10B, but the nature of the connection between the well and the hot springs is not known well enough to evaluate this possibility; plus the temperature changes in the well have occurred gradually over a period of years, in contrast to the sudden appearance of geysers. It will be interesting to see if geysering subsides with the streamflow.
The cause of the rising temperatures in Core Hole 10B, and its possible relation to caldera unrest, especially the shallow LP events recently identified by M. Pitt, needs additional study. To pursue this goal, the thermal profile will now be continuously monitored by a string of temperature sensors deployed in the well in July. Efforts to construct a time history of temperature profiles for other wells in Long Valley are also underway.
Table 1. Anion concentrations in Hot Creek and two hot springs within the gorge on 8 June 2006. 1999 hot spring data for comparison.
|
sample |
TempoC |
F mg/L |
Cl mg/L |
SO4 mg/L |
Br mg/L |
NO3mg/L |
PO4mg/L |
|
swimming hole |
12.1 |
0.53 |
9.62 |
15.8 |
0.0148 |
0.042 |
0.034 |
|
hot spring |
74.4 |
9.1 |
198 |
111 |
0.31 |
0.011 |
0.035 |
|
downstream |
13.3 |
0.45 |
8.60 |
6.83 |
0.014 |
0.052 |
0.033 |
|
upstream |
11.2 |
0.11 |
1.38 |
3.69 |
0.016 |
0.052 |
0.033 |
|
hot spring |
93.7 |
9.5 |
204 |
87 |
0.32 |
0.016 |
0.035 |
|
hot spring (1999) |
93 |
9.2 |
208 |
94 |
0.54 |
0.16 |
<0.1 |
Figure 1. Thermal water discharge from Hot Creek Gorge
calculated from dissolved chloride and sulfate concentrations in Hot Creek
water samples collected below USFS overlook. Most recent data point (June 8, 2006) is well within the
long-term average.
