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Geology topics

C. D. Farrar

Publications and source records attributed to C. D. Farrar.

At least 19 recordsLinked to original sources
USGS2012

Helium isotope and gas discharge variations associated with crustal unrest in Long Valley Caldera, California, 1989-1992

The onset of anomalous seismic activity in 1989 beneath Mammoth Mountain on the southwestern rim of the Long Valley caldera, California, was followed within ∼4 months by a large increase in 3 He/ 4 He in vapor discharged from a fumarole on the north side of the mountain. The helium isotopic ratio at this vent rose to a maximum of 6.7 RA in July 1990 and subsequently declined to values near 5 R A . Potential sources of the 3 He‐rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous rocks, and volatile release from a 3 He‐rich gas chamber situated above previously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7 R A ), the persistence of relatively high values (>5 R A ) over a period of 3 years, the increase in the flux of total He relative to gases in air‐saturated water, and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion may be as shallow as 2 km, consistent with the prompt appearance of increased 3 He/ 4 He ratios in the fumarolic gas, and that the intrusive process may have persisted for ∼1 year. In contrast, a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome‐south moat region during the 1989–1991 period resulted in at most relatively small changes in 3 He/ 4 He in fumarolic discharge at the southern edge of the resurgent dome. The more subdued response may result from a combination of greater intrusive depths and greater dilution of 3 He‐rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with Mammoth Mountain.

California↗
USGS2012

Effects of geothermal development on deformation in the Long Valley Caldera, eastern California, 1985-1994

Long Valley caldera in east central California has been the site of crustal unrest in the form of seismicity and ground deformation in response to magmatic inflation since 1980. Uplift of the resurgent dome has totaled ∼0.6 m over the 1975–1992 period. Within this region of uplift, and near the southwestern edge of the resurgent dome, is the Casa Diablo area, which experienced relative subsidence of 0.17 m between 1985 and 1992. Geothermal fluid production began at Casa Diablo in 1985 to supply a binary electric power plant; currently, three such plants generate a total of about 40 MW of electricity. The plants are supplied by 170°C water pumped from depths near 150m; all the produced water is reinjected at cooler temperatures at depths near 600 m. Analyses of data from (1) regional leveling lines and a network of bench marks in the geothermal well field area, (2) tilt observations from an L‐shaped array of bench marks, and (3) calculations relating pressure and temperature changes to subsidence indicate that relative subsidence at Casa Diablo results mainly from reductions in pressure in the shallow production reservoir and overlying formations and reductions in temperature in the underlying injection reservoir. Secondary effects on deformation are caused by offsets along the Taylor‐Bryant fault bounding the west side of the well field.

California↗
USGS2003

Inferences on the hydrothermal system beneath the resurgent dome in Long Valley Caldera, east-central California, USA, from recent pumping tests and geochemical sampling

Quaternary volcanic unrest has provided heat for episodic hydrothermal circulation in the Long Valley caldera, including the present-day hydrothermal system, which has been active over the past 40 kyr. The most recent period of crustal unrest in this region of east-central California began around 1980 and has included periods of intense seismicity and ground deformation. Uplift totaling more than 0.7 m has been centered on the caldera's resurgent dome, and is best modeled by a near-vertical ellipsoidal source centered at depths of 6-7 km. Modeling of both deformation and microgravity data now suggests that (1) there are two inflation sources beneath the caldera, a shallower source 7-10 km beneath the resurgent dome and a deeper source ???15 km beneath the caldera's south moat and (2) the shallower source may contain components of magmatic brine and gas. The Long Valley Exploration Well (LVEW), completed in 1998 on the resurgent dome, penetrates to a depth of 3 km directly above this shallower source, but bottoms in a zone of 100??C fluid with zero vertical thermal gradient. Although these results preclude extrapolations of temperatures at depths below 3 km, other information obtained from flow tests and fluid sampling at this well indicates the presence of magmatic volatiles and fault-related permeability within the metamorphic basement rocks underlying the volcanic fill. In this paper, we present recently acquired data from LVEW and compare them with information from other drill holes and thermal springs in Long Valley to delineate the likely flow paths and fluid system properties under the resurgent dome. Additional information from mineralogical assemblages in core obtained from fracture zones in LVEW documents a previous period of more vigorous and energetic fluid circulation beneath the resurgent dome. Although this system apparently died off as a result of mineral deposition and cooling (and/or deepening) of magmatic heat sources, flow testing and tidal analyses of LVEW water level data show that relatively high permeability and strain sensitivity still exist in the steeply dipping principal fracture zone penetrated at a depth of 2.6 km. The hydraulic properties of this zone would allow a pressure change induced at distances of several kilometers below the well to be observable within a matter of days. This indicates that continuous fluid pressure monitoring in the well could provide direct evidence of future intrusions of magma or high-temperature fluids at depths of 5-7 km. ?? 2003 Elsevier B.V. All rights reserved.

California↗
USGS2003

Deformation near the Casa Diablo geothermal well field and related processes Long Valley caldera, Eastern California, 1993-2000

Regional first-order leveling lines, which extend from Lee Vining, CA, to Tom's Place, CA, have been surveyed periodically since 1957 by the U.S. Geological Survey (USGS), the National Geodetic Survey (NGS), and Caltrans. Two of the regional survey lines, or leveling networks, intersect at the Casa Diablo geothermal well field. These leveling networks, referenced to a distant bench mark (C916) near Lee Vining, provide time-series vertical control data of land-surface deformation that began around 1980. These data are also useful for delineating localized subsidence at Casa Diablo related to reservoir pressure and temperature changes owing to geothermal development that began in 1985. A comparison of differences in bench-mark elevations for five time periods between 1983 and 1997 shows the development and expansion of a subsidence bowl at Casa Diablo. The subsidence coincides spatially with the geothermal well field and temporally with the increased production rates and the deepening of injection wells in 1991, which resulted in an increase in the rate of pressure decline. The subsidence, superimposed on a broad area of uplift, totaled about 310 mm by 1997. The USGS established orthogonal tilt arrays in 1983 to better monitor deformation across the caldera. One tilt array (DBR) was established near what would later become the Casa Diablo geothermal well field. This array responded to magmatic intrusions prior to geothermal development, tilting away from the well field. With the start of geothermal fluid extraction in 1985, tilt at the DBR array reversed direction and began tilting into the well field. In 1991, geothermal power production was increased by a factor of four, and reservoir pressures began a period of steep decline. These changes caused a temporary three-fold increase in the tilt rate. The tilt rate became stable in 1993 and was about 40% lower than that measured in 1991-1992, but still greater than the rates measured during 1985-1990. Data from the local leveling networks spanning the well field and the bounding graben were analyzed for several 2-year periods (1993-1995, 1995-1997, and 1997-1999). Annual rates of change across the normal faults bounding the graben have steadily decreased for each 2-year period between 1993 and 1999, reflecting the slowing decline in geothermal reservoir pressure. Horizontal control data from a two-color electronic distance meter (EDM) defined the lateral extent of subsidence at Casa Diablo. The EDM and leveling data elucidate the localized effect of the shallow source of subsidence and the broader effect of the deeper magmatic inflation source. Data from bench marks common to both the vertical and the horizontal control networks were used to assess the effect of subsidence on the EDM base station (CASA). Modeling of geodetic data collected during periods of little or no magmatic inflation indicated that the CASA two-color EDM station is being drawn toward the well field at a rate of 3-5 mm/yr. ?? 2003 Elsevier B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗
USGS1996

Hydrologic data for Long Valley Caldera, Mono County, California, 1987-93

Hydrologic data were collected during 1987-93 as part of the U.S. Geological Survey's long-term Volcanic Hazards Monitoring Program of the Long Valley Caldera, Mono County, California. The data are presented in graphs or tables. Data collected for the Long Valley Hydrologic Advisory Committee monitoring program also are presented. Hydrologic data collected include continuous record of ground-water levels in 6 wells, instantaneous measurement of ground-water levels in 55 wells, continuous record of discharge at 2 streams and 5 springs, instantaneous discharge measurements at 4 streams and 2 springs, ground-water temperature profiles of 3 thermal wells, continuous record of water temperature in 1 stream and 5 springs, vent gas temperatures at 2 fumaroles, and chemical and isotopic analyses of water samples collected at 13 ground-water and 6 surface-water sites. Precipitation amounts at three sites also are included.

Open-File Report↗
USGS1995

Forest-killing diffuse CO2 emission at Mammoth Mountain as a sign of magmatic unrest

Mammoth Mountain, in the western United States, is a large dacitic volcano with a long history of volcanism that began 200 kyr ago and produced phreatic eruptions as recently as 500 ± 200 yr BP. Seismicity, ground deformation and changes in fumarole gas composition suggested an episode of shallow dyke intrusion in 1989–90. Areas of dying forest and incidents of near asphyxia in confined spaces, first reported in 1990, prompted us to search for diffuse flank emissions of magmatic CO 2 , as have been described at Mount Etna and Vulcano . Here we report the results of a soil-gas survey, begun in 1994, that revealed CO 2 concentrations of 30–96% in a 30-hectare region of killed trees, from which we estimate a total CO 2 flux of ≥1,200 tonnes per day. The forest die-off is the most conspicuous surface manifestation of magmatic processes at Mammoth Mountain, which hosts only weak fumarolic vents and no summit activity. Although the onset of tree kill coincided with the episode of shallow dyke intrusion, the magnitude and duration of the CO 2 flux indicates that a larger, deeper magma source and/or a large reservoir of high-pressure gas is being tapped.

California↗
USGS1995

Hydrologic effects associated with the January 17, 1994 Northridge, California, earthquake

This report compiles hydrologic observations in southern California associated with the 1994 Mw = 6.7 Northridge, California earthquake. In southern California, the largest ground water level change was a drop of 52 cm at Crystalaire. Most of the steplike water-level changes recorded following the Northridge earthquake agreed in direction with the sign of the calculated coseismic volume strain field. A spring discharge increase at Southern Pacific Springs was reported to have preceded the earthquake by several days. Outside of southern California, water-level changes were also observed, but are not consistent in sign or size with the static strain field of the earthquake sequence.

California, Nevada, Oregon↗
USGS1990

Monitoring the hydrologic system for potential effects of geothermal and ground-water development in the Long Valley caldera, Mono County, California, U.S.A.

In the early 1980's, renewed interest in the geothermal potential of the Long Valley caldera, California, highlighted the need to balance the benefits of energy development with the established recreational activities of the area. The Long Valley Hydrologic Advisory Committee, formed in 1987, instituted a monitoring program to collect data during the early stages of resource utilization to evaluate potential effects on the hydrologic system. Early data show declines in streamflow, spring flow, and ground-water levels caused by 6 years of below-average precipitation. Springs in the Hot Creek State Fish Hatchery area discharge water that is a mixture of nonthermal and hydrothermal components. Possible sources of nonthermal water have been identified by comparing deuterium concentrations in streams and springs. The equivalent amount of undiluted thermal water discharged from the springs was calculated on the basis of boron and chloride concentrations. Quantifying the thermal and nonthermal fractions of the total flow may allow researchers to assess changes in flow volume or temperature of the springs caused by groundwater or geothermal development.

California↗
USGS1987

Hydrologic and geochemical monitoring in Long Valley Caldera, Mono County, California, 1985

Hydrologic and geochemical monitoring, to detect changes caused by magmatic and tectonic processes in the Long Valley caldera has continued through 1985. The monitoring included the collection of the following types of data: chemical and isotopic composition of water and gases from springs, wells, and steam vents; temperatures in wells, springs, and steam vents; flow rates of springs and streams; water levels in wells; and barometric pressure and precipitation at several sites. In addition, reservoir temperatures for the geothermal system were estimated from computations based on chemical geothermometers applied to fluid samples from wells and springs. Estimates of thermal water discharged from springs were made on the basis of boron and chloride fluxes in surface waters for selected sites in the Casa Diablo area and along the Mammoth-Hot Creek drainage. These data are presented in tables and graphs. The Long Valley area was relatively quiescent throughout 1985 in terms of geodetic changes and seismic activity. As a consequence , the hydrologic system varied mainly in response to seasonal influences of temperature, atmospheric pressure, and precipitation. However, spring flows near Casa Diablo were influenced by pumping at the geothermal production well field nearby. (Author 's abstract)

Water-Resources Investigations Report↗
USGS1986

Ground-water resources in Mendocino County, California

Mendocino County includes about 3,500 sq mi of coastal northern California. Groundwater is the main source for municipal and individual domestic water systems and contributes significantly to irrigation. Consolidated rocks of the Franciscan Complex are exposed over most of the county. The consolidated rocks are commonly dry and generally supply < 5 gal/min of water to wells. Unconsolidated fill in the inland valleys consists of gravel, sand, silt, and clay. Low permeability in the fill caused by fine grain size and poor sorting limits well yields to less than 50 gal/min in most areas; where the fill is better sorted, yields of 1,000 gal/min can be obtained. Storage capacity estimates for the three largest basins are Ukiah Valley, 90,000 acre-ft; Little lake Valley, 35,000 acre-ft; and Laytonville Valley, 14,000 acre-ft. Abundant rainfall (35 to 56 in/yr) generally recharges these basins to capacity. Seasonal water level fluctuations since the 1950 's have been nearly constant, except during the 1976-77 drought. Chemical quality of water in basement rocks and valley fill is generally acceptable for most uses. Some areas along fault zones yield water with high boron concentrations ( <2 mg/L). Sodium chloride water with dissolved solids concentrations exceeding 1,000 mg/L is found in deeper parts of Little Lake Valley. (Author 's abstract)

Water-Resources Investigations Report↗
USGS1985

Hydrologic and geochemical monitoring in Long Valley caldera, Mono County, California, 1982-1984

The Long Valley caldera is a potentially active volcanic area on the eastern side of the Sierra Nevada in east-central California. Hydrologic and geochemical monitoring of surface and subsurface features began in July 1982 to determine if changes were occurring in response to processes causing earthquakes and crustal deformation. Differences since 1982 in fluid chemistry of springs has been minor except at Casa Diablo, where rapid fluctuations in chemistry result from near surface boiling and mixing. Ratios of 3 He/ 4 He and 13 C/ 12 C in hot springs and fumaroles are consistent with a magnetic source for some of the carbon and helium discharged in thermal areas, and observed changes in 3 He/ 4 He between 1978 and 1984 suggest changes in the magmatic component. Significant fluctuations in hot spring discharge recorded at several sites since 1982 closely followed earthquake activity. Water levels in wells have been used as strain meters to detect rock deformation associated with magmatic and tectonic activity and to construct a water table contour map. Coseismic water level fluctuations of as much as 0.6 ft have been observed but no clear evidence of deformation caused by magmatic intrusions can be seen in the well records through 1984. Temperature profiles in wells, which can be used to delineate regionally continuous zones of lateral flow of hot water across parts of the caldera, have remained constant at all but two sites.

California↗
USGS1985

Monitoring the hydrothermal system in Long Valley caldera, California

An ongoing program to monitor the hydrothermal system in Long Valley for changes caused by volcanic or tectonic processes has produced considerable data on the water chemistry and discharge of springs and fluid temperatures and pressures in wells. Chemical and isotopic data collected under this program have greatly expanded the knowledge of chemical variability both in space and time. Although no chemical or isotopic changes in hot spring waters can be attributed directly to volcanic or tectonic processes, changes in hot spring chemistry that have been recorded probably relate to interactions between and variations in the quantity of liquid and gas discharged. Stable carbon isotope data are consistent with a carbon source either perform the mantle or from metamorphosed carbonate rocks. Continuous and periodic measurements of hot spring discharge at several sites show significant co seismic and a seismic changes since 1980.

California↗
USGS1981

Ground-water-level monitoring network, Hollister and San Juan Valleys, San Benito County, California

The addition of 17 wells to the existing 86-well network is proposed to improve the ground-water monitoring in the Hollister and San Juan Valleys in California. The new wells were selected on the basis of well-construction data, availability, location, accessibility, use, and condition, either to replace wells that are no longer accessible or to furnish needed additional data for planning artificial recharge, preparing water-level-contour maps, and digital ground-water modeling.

California↗
USGS1980

A water-quality monitoring network for Vallecitos Valley, Alameda County, California

A water-quality monitoring network is proposed to detect the presence of and trace the movement of radioisotopes in the hydrologic system in the vicinity of the Vallecitos Nuclear Center. The source of the radioisotopes is treated industrial wastewater from the Vallecitos Nuclear Center that is discharged into an unnamed tributary of Vallecitos Creek. The effluent infiltrates the alluvium along the stream course, percolates downward to the water table, and mixes with the native ground water in the subsurface. The average daily discharge of effluent to the hydrologic system in 1978 was about 100,000 gallons. In Vallecitos Valley, the Livermore Gravel and the overlying alluvium constitute the ground-water reservoir. There is no subsurface inflow from adjacent ground-water basins. Ground-water flow in the Vallecitos subbasin is toward the southwest. The proposed network consists of four surface-water sampling sites and six wells to sample the ground-water system. Samples collected monthly at each site and analyzed for tritium and for alpha, beta, and gamma radiation would provide adequate data for monitoring.

California↗