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At least 793 records · Page 44Linked to original sources

USGS2016

Origin of the pulse-like signature of shallow long-period volcano seismicity

Short-duration, pulse-like long-period (LP) events are a characteristic type of seismicity accompanying eruptive activity at Mount Etna in Italy in 2004 and 2008 and at Turrialba Volcano in Costa Rica and Ubinas Volcano in Peru in 2009. We use the discrete wave number method to compute the free surface response in the near field of a rectangular tensile crack embedded in a homogeneous elastic half space and to gain insights into the origin of the LP pulses. Two source models are considered, including (1) a vertical fluid-driven crack and (2) a unilateral tensile rupture growing at a fixed sub-Rayleigh velocity with constant opening on a vertical crack. We apply cross correlation to the synthetics and data to demonstrate that a fluid-driven crack provides a natural explanation for these data with realistic source sizes and fluid properties. Our modeling points to shallow sources (<1 km depth), whose signatures are representative of the Rayleigh pulse sampled at epicentral distances >∼1 km. While a slow-rupture failure provides another potential model for these events, the synthetics and resulting fits to the data are not optimal in this model compared to a fluid-driven source. We infer that pulse-like LP signatures are parts of the continuum of responses produced by shallow fluid-driven sources in volcanoes.

Journal of Geophysical Research B: Solid Earth↗

USGS2015

Seismic-sequence stratigraphy and geologic structure of the Floridan aquifer system near "Boulder Zone" deep wells in Miami-Dade County, Florida

The U.S. Geological Survey, in cooperation with the Miami-Dade Water and Sewer Department, acquired, processed, and interpreted seismic-reflection data near the North and South District “Boulder Zone” Well Fields to determine if geologic factors may contribute to the upward migration of injected effluent into that upper part of the Floridan aquifer system designated by the U.S. Environmental Protection Agency as an underground source of drinking water. The depth of the Boulder Zone at the North and South District “Boulder Zone” Well Fields ranges from about 2,750 to 3,300 feet below land surface (ft bls), whereas overlying permeable zones used as alternative drinking water supply range in depth from about 825 to 1,580 ft bls at the North and South District “Boulder Zone” Well Fields. Seismic-sequence stratigraphy and geologic structures imaged on seismic-reflection profiles created for the study describe the part of the Floridan aquifer system overlying and within the Boulder Zone. Features of the Floridan aquifer system underlying the Boulder Zone were not studied because seismic-reflection profiles acquired near the North and South District “Boulder Zone” Well Fields lacked adequate resolution at such depths. Stratigraphic analysis of seismic-reflection data collected from the study area was mainly applied to the Floridan aquifer system and used to identify four provisional seismic sequences, which extend vertically from near the base of the Floridan aquifer system upward to the lower part of the intermediate confining unit. These four seismic sequences compose a framework in which each sequence includes a major permeable unit of the Floridan aquifer system; from shallowest to deepest, these units are the Upper Floridan aquifer, Avon Park permeable zone, uppermost major permeable zone of the Lower Floridan aquifer, and Boulder Zone. The relations between seismic-sequence stratigraphy and hydrostratigraphy allow for detailed mapping of permeable zones and semiconfining units of the Floridan aquifer system at a level of resolution never before accomplished using well data alone. In addition to the preceding seismic-reflection analysis, interpretation of geophysical well log data from four effluent injection wells at the North District “Boulder Zone” Well Field delineated a narrow karst collapse structure beneath the injection facility that extends upward about 900 ft from the top of the Boulder Zone to about 125 ft above the top of the uppermost major permeable zone of the Lower Floridan aquifer. No karst collapse structures were identified in the seismic-reflection profiles acquired near the North District “Boulder Zone” Well Field. However, karst collapse structures at the level of the lowermost major permeable zone of the Lower Floridan aquifer at the South District “Boulder Zone” Well Field are present at three locations, as indicated by seismic-reflection data acquired in the C–1 Canal bordering the south side of the injection facility. Results from the North District “Boulder Zone” Well Field well data indicate that a plausible hydraulic connection between faults and stratiform permeability zones may contribute to the upward transport of effluent, terminating above the base of the deepest U.S. Environmental Protection Agency designated underground source of drinking water at the North District “Boulder Zone” Well Field.

Florida↗

USGS2012

Multicycle slip distribution along a laboratory fault

Slip distribution along a laboratory fault, which consists of eight spring-connected blocks that are elastically driven to slide on a frictional surface, has been examined for a “long” sequence of slip events to test the applicability of some conceptual models proposed recently in the literature. The distributions of large slip events are found to be quite variable and do not fit the uniform slip or characteristic earthquake models. The rupture initiation points are usually not near the corresponding maximum slip points, in contrast to observations by Thatcher (1990) and by Fukao and Kikuchi (1987) that earthquake hypocenters are commonly near corresponding regions of maximum slip in the fault planes. This contrast may suggest that either the present observations or theirs are not representative or the teleseismically determined hypocenters may not always be true rupture initiation points as usually assumed. Large slip events are also found to be a stress-roughening process. They are triggered by some small events after the stresses have been adjusted by some earlier small-to-moderate events to be near the critical levels at most locations along the fault. This suggests that earthquake prediction monitoring efforts should not be limited to a small region near an asperity but should be spread out to cover the entire fault segment in a seismic gap in order to detect the condition of simultaneous strain buildup.

Journal of Geophysical Research↗

USGS2012

Experimental deformation of topaz crystals: Possible embrittlement by intracrystalline water

Crystallographically oriented single‐crystal prisms of gem quality topaz (composition AlSiO (OH− F) where x = 0.04 ± 0.01) were deformed at a confining pressure of 1.50 GPa, a temperature of 800°C, and a strain rate of 2×10 s. Under nearly identical conditions, all crystals of anhydrous rock‐forming minerals that have been tested to date, such as olivine, quartz, feldspars, pyroxenes, and refractory oxides, deform plastically; in contrast, our topaz crystals failed by brittle fracture regardless of the orientation of the compression direction. No optical evidence for plastic deformation was detected. Another suite of experiments with compression perpendicular to the (001) cleavage at = 100°–950°C and a strain rate of 2×10 s displayed two regimes of behavior: (1) at >400°C, fracture strength was independent of temperature, and fracture occurred on one or two surfaces parallel to {103}; (2) at <400°C, the fracture strength increased rapidly with decreasing temperature, no macroscopic stress drop was observed, and many closely spaced conjugate fractures formed on (103) and (103). The anomalous brittleness of topaz compared to anhydrous silicate and oxide crystals indicates that intracrystalline “water” plays a role in the embrittlement. We suggest that water within the topaz crystals promotes fracture in ways similar to the mechanisms of slow crack growth aided by environmental moisture.

Journal of Geophysical Research B: Solid Earth↗

USGS2005

Candidate-penetrative-fracture mapping of the Grand Canyon area, Arizona, from spatial correlation of deep geophysical features and surficial lineaments

Some aquifers of the southwestern Colorado Plateaus Province are deeply buried and overlain by several impermeable shale layers, and so recharge to the aquifer probably is mainly by seepage down penetrative-fracture systems. The purpose of this 2-year study, sponsored by the U.S. National Park Service, was to map candidate deep penetrative fractures over a 120,000-km2 area, using gravity and aeromagnetic-anomaly data together with surficial-fracture data. The study area was on the Colorado Plateau south of the Grand Canyon and west of Black Mesa; mapping was carried out at a scale of 1:250,000. The resulting database constitutes a spatially registered estimate of deep-fracture locations. Candidate penetrative fractures were located by spatial correlation of horizontal- gradient and analytic-signal maximums of gravity and magnetic anomalies with major surficial lineaments obtained from geologic, topographic, side-looking-airborne-radar, and satellite imagery. The maps define a subset of candidate penetrative fractures because of limitations in the data coverage and the analytical technique. In particular, the data and analytical technique used cannot predict whether the fractures are open or closed. Correlations were carried out by using image-processing software, such that every pixel on the resulting images was coded to uniquely identify which datasets are correlated. The technique correctly identified known and many new deep fracture systems. The resulting penetrative-fracture-distribution maps constitute an objectively obtained, repeatable dataset and a benchmark from which additional studies can begin. The maps also define in detail the tectonic fabrics of the southwestern Colorado Plateaus Province. Overlaying the correlated lineaments on the normalized-density-of-vegetation-index image reveals that many of these lineaments correlate with the boundaries of vegetation zones in drainages and canyons and so may be controlling near-surface water availability in some places. Many derivative products can be produced from the database, such as fracture-density-estimate maps, and maps with the number of correlations color-coded to estimate the possible quality of correlation. The database contained in this report is designed to be used in a geographic information system and image-processing systems, and most data layers are in georeferenced tagged image format (Geotiff) or ARC grids. The report includes 163 map plates and various metadata, supporting, and statistical diagram files.

Data Series↗

USGS2010

Hydrologic conditions, stream-water quality, and selected groundwater studies conducted in the Lawrenceville area, Georgia, 2003-2008

Hydrologic studies conducted during 2003-2008 as part of the U.S. Geological Survey Cooperative Water Program with the City of Lawrenceville, Georgia, provide important data for the management of water resources. The Cooperative Water Program includes (1) hydrologic monitoring (precipitation, streamflow, and groundwater levels) to quantify baseline conditions in anticipation of expanded groundwater development, (2) surface-water-quality monitoring to provide an understanding of how stream quality is affected by natural (such as precipitation) and anthropogenic factors (such as impervious area), and (3) geologic studies to better understand groundwater flow and hydrologic processes in a crystalline rock setting. The hydrologic monitoring network includes each of the two watersheds projected for groundwater development?the Redland-Pew Creek and upper Alcovy River watersheds?and the upper Apalachee River watershed, which serves as a background or control watershed because of its similar hydrologic and geologic characteristics to the other two watersheds. In each watershed, precipitation was generally greater during 2003-2005 than during 2006-2008, and correspondingly streamflow and groundwater levels decreased. In the upper Alcovy River and Redland-Pew Creek watersheds, groundwater level declines during 2003-2008 were mostly between 2 and 7 feet, with maximum observed declines of as much as 28.5 feet in the upper Alcovy River watershed, and 49.1 feet in the Redland-Pew Creek watershed. Synoptic base-flow measurements were used to locate and quantify gains or losses to streamflow resulting from groundwater interaction (groundwater seepage). In September 2006, seepage gains were measured at five of nine reaches evaluated in the upper Alcovy River watershed, with losses in the other four. The four losing reaches were near the confluence of the Alcovy River and Cedar Creek where the stream gradient is low and bedrock is at or near the land surface. In the Redland-Pew Creek watershed, groundwater seepage gains were observed at each of the 10 reaches measured during September 2008. Continuous specific conductance, temperature, and turbidity data were collected at gage sites located on Pew and Shoal Creeks, which drain about 32 percent of the city area, and at a background site on the Apalachee River located outside the city boundary. Continuous surface-water monitoring data indicate that reduced precipitation during 2006-2008 resulted in lower turbidity and higher stream temperature and specific conductance than in 2003-2005. In comparison to the other two stream sites, water at the Apalachee River site had the lowest mean and median values for specific conductance, and the greatest mean and median values for turbidity during October 2005-December 2008. In addition to continuous water-quality monitoring, samples were collected periodically to determine fecal-coliform bacteria concentrations. None of the individual samples at the three sites exceeded the Georgia Environmental Protection Division (GaEPD) limit of 4,000 most probable number of colonies per 100 milliliters (MPN col/100 mL) for November through April. In the Redland-Pew Creek and Shoal Creek watersheds, the GaEPD 30-day geometric mean standard of 200 MPN col/100 mL for May-October was exceeded twice during two sampling periods in May-October 2007 and twice during two sampling periods in May-October 2008. Groundwater studies conducted during 2003-2007 include the collection of borehole geophysical logs from four test wells drilled in the upper Alcovy River watershed to provide insight into subsurface geologic characteristics. A flowmeter survey was conducted in a well south of Rhodes Jordan Park to help assess the interconnection of the well with surface water and the effectiveness of a liner-packer assembly installed to eliminate that interconnection. At that same well, hydraulic packer tests were conducted in the open-hole section of the well, and water samp

Georgia↗

USGS2020

Use of time domain electromagnetic soundings and borehole electromagnetic induction logs to delineate the freshwater/saltwater interface on southwestern Long Island, New York, 2015–17

The U.S. Geological Survey, in cooperation with the New York State Department of Environmental Conservation, used surface and borehole geophysical methods to delineate the freshwater/saltwater interface in coastal plain aquifers along the southwestern part of Long Island, New York. Over pumping of groundwater in the early 20th century combined with freshwater/saltwater interfaces at the coastline created saltwater intrusion in the upper glacial, Jameco, Magothy, and Lloyd aquifers. This study documents, for the first time, extensive saltwater intrusion of the Lloyd aquifer along the southwestern coast of Long Island, N.Y. Several public-supply wells in the southern parts of Nassau, Queens, and Kings Counties have been adversely affected by saltwater intrusion causing supply wells to be shutdown and abandoned. Due to the ongoing groundwater pumping in southern Nassau County, the freshwater/saltwater interface requires delineation and monitoring for any inland movement. In 2015–17, the U.S. Geological Survey collected time domain electromagnetic soundings at 12 locations and borehole electromagnetic induction conductivity logs at 9 wells within the study area to delineate several saltwater intrusion wedges. The upper glacial, Jameco, and Magothy aquifers were grouped into one aquifer complex within the study area to simplify interpretations. The coastal plain sediments increase in thickness from west to east and north to south because of their regional dip toward the southeast. Three separate wedges, shallow, intermediate, and deep, of saltwater intrusion were delineated in the upper glacial, Jameco, and Magothy aquifer complex. In addition, analysis of geophysical logs collected in an open borehole of a test well in southern Queens County in 1989 revealed the Lloyd aquifer was nearly completely intruded by saltwater with an estimated chloride concentration of 15,000 milligrams per liter. The geophysical logs from this well provides, for the first time, definitive proof of saltwater intrusion of the Lloyd aquifer on Long Island’s south shore, suggesting the freshwater/saltwater interface was at the coastline and not miles offshore as theorized by previous studies.

New York↗

USGS2008

Surface processes recorded by rocks and soils on Meridiani Planum, Mars: Microscopic Imager observations during Opportunity's first three extended missions

The Microscopic Imager (MI) on the Mars Exploration Rover Opportunity has returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Meridiani Planum landing site. Designed to simulate a geologist's hand lens, the MI is mounted on Opportunity's instrument arm and can resolve objects 0.1 mm across or larger. This paper provides an overview of MI operations, data calibration, and analysis of MI data returned during the first 900 sols (Mars days) of the Opportunity landed mission. Analyses of Opportunity MI data have helped to resolve major questions about the origin of observed textures and features. These studies support eolian sediment transport, rather than impact surge processes, as the dominant depositional mechanism for Burns formation strata. MI stereo observations of a rock outcrop near the rim of Erebus Crater support the previous interpretation of similar sedimentary structures in Eagle Crater as being formed by surficial flow of liquid water. Well‐sorted spherules dominate ripple surfaces on the Meridiani plains, and the size of spherules between ripples decreases by about 1 mm from north to south along Opportunity's traverse between Endurance and Erebus craters.

Journal of Geophysical Research E: Planets↗

USGS1998

Erosional valleys in the Thaumasia region of Mars: Hydrothermal and seismic origins

Analysis of erosional valleys, geologic materials and features, and topography through time in the Thaumasia region of Mars using co-registered digital spatial data sets reveals significant associations that relate to valley origin. Valleys tend to originate (1) on Noachian to Early Hesperian (stages 1 and 2) large volcanoes, (2) within 50–100 km of stages 1 and 2 rift systems, and (3) within 100 km of Noachian (stage 1) impact craters >50 km in diameter. These geologic preferences explain observations of higher valley-source densities (VSDs) in areas of higher elevations and regional slopes (>1°) because the volcanoes, rifts, and craters form high, steep topography or occur in terrain of high relief. Other stage 1 and stage 2 high, steep terrains, however, do not show high VSDs. The tendency for valleys to concentrate near geologic features and the overall low drainage densities in Thaumasia compared to terrestrial surfaces rule out widespread precipitation as a major factor in valley formation (as is proposed in wann, wet climate scenarios) except perhaps during the Early Noachian, for which much of the geologic record has been obliterated. Instead, volcanoes and rifts may indicate the presence of shallow crustal intrusions that could lead to local hydrothermal circulation, melting of ground ice and snow, and groundwater sapping. However, impact-crater melt would provide a heat source at the surface that might drive away water, fonning valleys in the process. Post-stage 1 craters mostly have low nearby VSDs, which, for valleys incised in older rocks, suggests burial by ejecta and, for younger valleys, may indicate desiccation of near-surface water and deepening of the cryosphere. Later Hesperian and Amazonian (stages 3 and 4) valleys originate within 100–200 km of three young, large impact craters and near rifts systems at Warrego Valles and the southern part of Coprates rise. These valleys likely developed when the cryosphere was a couple kilometers or more thick, inhibiting valley development by hydrothermal circulation. However, eruption of groundwater may have occurred from impact-induced fracturing and lateral and perhaps minor upward transport of water due to seismic pumping. The two smaller craters formed along the plateau margin where the highest potential hydraulic head would occur in aquifers beneath the plateau. In the case of the larger crater (Lowell, 200 km in diameter), potential aquifers would likely be at depths of kilometers below the cryosphere. Seismic energy generated by the Lowell impactor would have been much greater, pumping both groundwater and perhaps fluidized slurry to the surface from beneath the cryosphere to form the young valleys and flow deposit. Along the margin of Thaumasia, tectonic pressurization of groundwater also may have contributed to valley formation. Dissection of rim materials of the Argyre impact may relate to tectonic activity and the unconsolidated state of basin ejecta.

Journal of Geophysical Research E: Planets↗

USGS2022

Assessment of well yield, dominant fractures, and groundwater recharge in Wake County, North Carolina

A cooperative study led by the U.S. Geological Survey and Wake County Environmental Services was initiated to characterize the fractured-rock aquifer system and assess the sustainability of groundwater resources in and around Wake County. This report contributes to the development of a comprehensive groundwater budget for the study area, thereby helping to enable resource managers to make sound and sustainable water-supply and water-use decisions. Construction information was used to analyze the well depth, casing depth, and reported well yield of more than 7,500 inventoried wells. The median well depth and casing depth were 265 feet (ft) below land surface (bls) and 68 ft bls, respectively, and the median well yield was 10 gallons per minute. Generally, well yield increased with depth to around 200 ft bls and then began to decrease with depth within the fractured-rock aquifer. Borehole geophysical logging methods were used to characterize the fractured-rock aquifer by mapping the orientation of geologic structures within the subsurface. Structure measurements were made on resulting log data and mapped to observed general spatial trends within the regional groundwater system and more distinct hydrogeologic units. Many of the fractures observed within the borehole logs are steeply dipping across Wake County, although open fractures with shallow dip angles were also observed in most rock classes. Regional geologic structural trends were observed in proximity to the Jonesboro Fault. Potential groundwater recharge in the study area was estimated using a Soil-Water-Balance (SWB) model, as well as using base flow hydrograph separation. The SWB model calculated net infiltration below the root zone for 1981 through 2019 for a 5,402-square-mile area that extends to the counties surrounding Wake County. The mean annual net infiltration rate for the 39-year period was about 8.6 inches per year for the study area. The mean annual net infiltration results from the SWB model were comparable to annual base flow estimates using the PART hydrograph-separation method at six U.S. Geological Survey streamgages within the study area. Mean annual base flow for all six drainage basins was near 7.5 inches per year and estimates ranged from 2.9 to 8.9 inches. Comparisons of mean annual potential recharge from the SWB model and base flow estimates were generally within 2 inches, except during high flows for most of the drainage basins in the study area.

North Carolina↗

USGS1996

Use of ground-penetrating radar and continuous seismic-reflection profiling on surface-water bodies in environmental and engineering studies

Ground‐penetrating radar (GPR) and continuous seismic‐reflection profiling (CSP) on shallow rivers, lakes, and ponds are efficient and economical ways of obtaining subsurface hydrologic and geologic information for environmental and engineering studies. These methods are similar in that they produce continuous subsurface profiles, are easy to use in some applications, and the records can occasionally be straightforward to interpret. They are dissimilar in that GPR cannot penetrate electrically conductive water or subsurface sediments, and CSP usually cannot operate in water less than 5 feet (ft.) deep. GPR records collected on a lake in New Hampshire have been interpreted to estimate the depth to bedrock and to evaluate the grain‐size characteristics of the underlying stratified drift at the lakeshore boundary. In a pond in Massachusetts, CSP and GPR were used to determine depth to bedrock and the grain‐size characteristics of the subbottom materials in part of the pond. Water‐column multiple reflections, depth and conductivity of water and subsurface materials, and diffractions degraded the quality of the GPR records. CSP records collected in the Connecticut River near Hartford, Connecticut were used to estimate the depth of till and bedrock interfaces and to evaluate grain‐size characteristics of subsurface materials. Interpreted CSP records also can indicate bedding planes within consolidated rock units. Water‐column multiple reflections and very shallow water degraded the quality of the CSP records. GPR and CSP methods have been used to delineate infilled scour holes near bridge piers. Scour holes that were filled with up to 8 ft. of loose sand were mapped during engineering scour studies near a bridge in Connecticut. Because GPR and CSP operate on different physical principles, the two geophysical methods complement each other. Depending on the required depth of penetration and the degree of resolution needed, one or both of these methods can be used to acquire accurate and reliable subsurface hydrologic and geologic information critical to environmental and engineering studies.

Journal of Environmental & Engineering Geophysics↗

USGS2024

Chemistry, growth, and fate of the unique, short-lived (2019–2020) water lake at the summit of Kīlauea Volcano, Hawaii

Less than a year after the 2018 Kīlauea caldera collapse and eruption, water appeared in newly deepened Halemaʻumaʻu crater. The lake—unprecedented in the written record—grew to a depth of ∼50 m before lava from the December 2020 eruption boiled it away. Surface water heightened concerns of potential phreatic or phreatomagmatic explosions but also offered a new means of possibly identifying eruption precursors. The U.S. Geological Survey Hawaiian Volcano Observatory (HVO) monitored the lake via direct visual observation, webcams, thermal imaging, colorimetry, and laser rangefinders. HVO also employed uncrewed aircraft systems to sample the water and measure near-lake gas composition. The lake's δD and δ 18 O indicate a groundwater source with substantial evaporation. The initial sample had a salinity (total dissolved solids concentration) of 71,000 mg/L and was rich in sulfate (∼53,000 mg/L), iron (∼500 mg/L), and magnesium (∼10,000 mg/L). Subsequent samples were slightly more dilute. The water's pH (∼4), δ 34 S (+4.3‰), and surface temperatures (up to 85°C) suggest, rather than significant scrubbing of magmatic volatiles, leaching of basalt and reactions with sulfate minerals resulted in high concentrations of sulfate and other solutes. Thermodynamic modeling and precipitate mineralogy indicate that water composition was controlled by iron oxidation and sulfate dissolution. Although the lake exhibited no detectable precursors before the next eruption, and phreatic or phreatomagmatic explosions did not materialize, our multi-parameter approach to monitoring yielded an enhanced understanding of the hydrologic, geologic, and magmatic conditions that led to the formation of the unique and short-lived lake.

Hawaii↗

USGS2013

Evolution of dike opening during the March 2011 Kamoamoa fissure eruption, Kīlauea Volcano, Hawai`i

The 5–9 March 2011 Kamoamoa fissure eruption along the east rift zone of Kīlauea Volcano, Hawai`i, followed months of pronounced inflation at Kīlauea summit. We examine dike opening during and after the eruption using a comprehensive interferometric synthetic aperture radar (InSAR) data set in combination with continuous GPS data. We solve for distributed dike displacements using a whole Kīlauea model with dilating rift zones and possibly a deep décollement. Modeled surface dike opening increased from nearly 1.5 m to over 2.8 m from the first day to the end of the eruption, in agreement with field observations of surface fracturing. Surface dike opening ceased following the eruption, but subsurface opening in the dike continued into May 2011. Dike volumes increased from 15, to 16, to 21 million cubic meters (MCM) after the first day, eruption end, and 2 months following, respectively. Dike shape is distinctive, with a main limb plunging from the surface to 2–3 km depth in the up‐rift direction toward Kīlauea's summit, and a lesser projection extending in the down‐rift direction toward Pu`u `Ō`ō at 2 km depth. Volume losses beneath Kīlauea summit (1.7 MCM) and Pu`u `Ō`ō (5.6 MCM) crater, relative to dike plus erupted volume (18.3 MCM), yield a dike to source volume ratio of 2.5 that is in the range expected for compressible magma without requiring additional sources. Inflation of Kīlauea's summit in the months before the March 2011 eruption suggests that the Kamoamoa eruption resulted from overpressure of the volcano's magmatic system.

Hawai'i↗

USGS2000

Surface properties of Mars' polar layered deposits and polar landing sites

On December 3, 1999, the Mars Polar Lander and Mars Microprobes will land on the planet's south polar layered deposits near (76°S, 195°W) and conduct the first in situ studies of the planet's polar regions. The scientific goals of these missions address several poorly understood and globally significant issues, such as polar meteorology, the composition and volatile content of the layered deposits, the erosional state and mass balance of their surface, their possible relationship to climate cycles, and the nature of bright and dark aeolian material. Derived thermal inertias of the southern layered deposits are very low (50-100 J m -2 s -1/2 K -1 ), suggesting that the surface down to a depth of a few centimeters is generally fine grained or porous and free of an appreciable amount of rock or ice. The landing site region is smoother than typical cratered terrain on ∼1 km pixel -1 Viking Orbiter images but contains low-relief texture on ∼5 to 100 m pixel -1 Mariner 9 and Mars Global Surveyor images. The surface of the southern deposits is older than that of the northern deposits and appears to be modified by aeolian erosion or ablation of ground ice.

Journal of Geophysical Research E: Planets↗

USGS2012

Strain accumulation across the Wasatch Fault near Ogden, Utah

Deformation of a 70 by 40‐km trilateration network spanning the north trending Wasatch fault near Ogden, Utah, has been monitored from 1972 through 1990. All but nine of the 200 measurements are consistent with deformation that is linear in time. We presume that those nine observations are contaminated by some blunder in making the measurements and that deformation is linear in time. The strain rate over the 1972–1990 interval across a 40‐km‐wide zone west of the Wasatch fault (hanging wall block) is adequately approximated by a 0.04 ± 0.01 μstrain/yr N85°E±5° uniaxial extension. Adequate coverage of the footwall block is available only over the 1981–1990 interval, and the strain rate across a 30‐km‐wide zone there is approximated by a 0.07 ± 0.03 μstrain/yr N20°E±12° uniaxial extension. The observed extension of the hanging wall block in the direction normal to fault strike suggests that the Wasatch fault may be listric; a planar fault would produce a zone of contraction normal to the fault at the surface of the hanging wall block. The component of extension parallel to fault strike observed on the footwall block is unexplained.

Utah↗

USGS2022

Structural properties of the Southern San Andreas fault zone in northern Coachella Valley from magnetotelluric imaging

The Southern San Andreas fault (SSAF) poses one of the largest seismic risks in California. Yet, there is much ambiguity regarding its deeper structural properties around Coachella Valley, in large part due to the relative paucity of everyday seismicity. Here, we image a multistranded section of the SSAF using a non-seismic method, namely magnetotelluric (MT) soundings, to help inform depth-dependent fault zone geometry, fluid content and porosity. The acquired MT data and resultant inversion models highlight a conductive column encompassing the SSAF zone that includes a 2–3 km wide vertical to steeply northeast dipping conductor down to ∼4 km depth (maximum of ∼1 Ω·m at 2 km depth) and another prominent conductor in the ductile crust (∼1 Ω·m at 12 km depth and slightly southwest of the surface SSAF). We estimate porosities of 18–44 per cent for the conductive uppermost 500 m, a 10–15 per cent porosity at 2 km depth and that small amounts (0.1–3 per cent) of interconnected hypersaline fluids produce the deeper conductor. Located northeast of this conductive region is mostly resistive crust indicating dry crystalline rock that extends down to ∼20 km in places. Most of the local seismicity is associated with this resistive region. Located farther northeast still is a conductive region at >13 km depth and separate from the one to the southwest. The imaged anomalies permit two interpretations. The SSAF zone is vertical to steeply northeast dipping in the upper crust and (1) is near vertical at greater depth creating mostly an impermeable barrier for northeast fluid migration or (2) continues to dip northeast but is relatively dry and resistive up to ∼13 km depth where it manifests as a secondary deep ductile crustal conductor. Taken together with existing knowledge, the first interpretation is more likely but more MT investigations are required.

California↗

USGS1998

Evaluation of hydrologic data collected at the North Penn Area 12 Superfund Site, Montgomery County, Pennsylvania

The North Penn Area 12 Superfund Site is underlain by the Lockatong Formation, which consists of interbedded gray to black siltstone and shale. The beds of the Lockatong Formation strike northeast and dip about 10d to 20d to the northwest in the vicinity of the site. Ground water moves through fractures that are nearly vertical and horizontal in the shale and siltstone. Permeability and storage are very low. Borehole-geophysical logs were obtained from eight wells to determine the location of fractures, water-producing and water-receiving intervals, and intervals of borehole flow. The logs also were used to quantify fluid movement in the borehole, to characterize the lithology, and to obtain data on well construction. The logs indicate fractures at depths less than 100 feet are more frequent and generally are more productive than fractures at depths greater than 100 feet. The fluid resistivity of water in shallow intervals usually was greater than that in deeper intervals. The rate and direction of fluid movement under nonpumping conditions differs in the boreholes logged. In the northwest part of the site, no vertical flow was detected in three wells and very small amounts of flow were measured in two wells. In the southwest part of the site, downward flow was measured in two wells. Aquifer-isolation tests in three wells provided information on hydraulic heads and specific capacities in discrete vertical intervals and allowed collection of water samples form discrete water-bearing intervals. Natural annual fluctuations of water levels in 11 wells ranged form 11.4 to 28.3 feet. Seven of the 11 wells gave very similar water-level hydrographs. The four southernmost wells on the site show rises in water levels after precipitation much sooner than the other seven wells. Two other wells show daily fluctuations caused by pumping. A potentiometric-surface map of the site and vicinity was prepared from water-level measurements made in late July 1995. The map can be used to determine the approximate direction of ground-water flow.

Water-Resources Investigations Report↗

USGS2016

Tomographic image of a seismically active volcano: Mammoth Mountain, California

High-resolution tomographic P wave, S wave, and V P / V S velocity structure models are derived for Mammoth Mountain, California, using phase data from the Northern California Seismic Network and a temporary deployment of broadband seismometers. An anomalous volume (5.1 × 10 9 to 5.9 × 10 10 m 3 ) of low P and low S wave velocities is imaged beneath Mammoth Mountain, extending from near the surface to a depth of &sim;2 km below sea level. We infer that the reduction in seismic wave velocities is due to the presence of CO 2 distributed in oblate spheroid pores with mean aspect ratio α = 1.6 × 10 −3 to 7.9 × 10 −3 (crack-like pores) and mean gas volume fraction ϕ = 8.1 × 10 −4 to 3.4 × 10 −3 . The pore density parameter κ = 3 ϕ /(4π α ) = na 3 =0.11, where n is the number of pores per cubic meter and a is the mean pore equatorial radius. The total mass of CO 2 is estimated to be 4.6 × 10 9 to 1.9 × 10 11 kg. The local geological structure indicates that the CO 2 contained in the pores is delivered to the surface through fractures controlled by faults and remnant foliation of the bedrock beneath Mammoth Mountain. The total volume of CO 2 contained in the reservoir suggests that given an emission rate of 500 tons day −1 , the reservoir could supply the emission of CO 2 for &sim;25–1040 years before depletion. Continued supply of CO 2 from an underlying magmatic system would significantly prolong the existence of the reservoir.

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