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At least 1,315 records · Page 73Linked to original sources
USGS2004

Interseismic strain and rotation rates in the northeast Mojave domain, eastern California

The northeast Mojave domain, a type locality for bookshelf faulting, is a region of east striking, left-lateral faults in the northeast corner of the Mojave block, a block otherwise dominated by ∼N40°W striking, right-lateral faults. Paleomagnetic evidence suggests that blocks within the domain have rotated clockwise about a vertical axis as much as 60° since 12.8 Ma [Schermer et al., 1996] . In 1994, and again in 2002, the U.S. Geological Survey surveyed an array of 14 geodetic monuments distributed across the northeast Mojave domain. The 2002 survey results were adjusted to remove the coseismic offsets imposed by the nearby Hector Mine earthquake (16 October 1999, M w = 7.1). The adjusted deformation across the array appears to be uniform and can be approximated by the principal strain rates ε 1 = 28.9 ± 9.1 N77.2°W ± 4.8° and ε 2 = −48.2 ± 8.9 N12.8°E ± 4.8° nstrain yr −1 ; extension reckoned positive, and quoted uncertainties are standard deviations. That strain accumulation could be released by slip on faults striking N32°W but not by bookshelf faulting on the east striking faults alone. The vertical axis rotation rate of the northeast Mojave domain as a whole relative to fixed North America is 71.0 ± 6.4 nrad yr −1 (4.07° ± 0.37° Myr −1 ) clockwise, about twice the maximum tensor shear strain rate. The observed rotation rate acting over 12.8 Myr would produce a clockwise rotation of 52.1° ± 4.7°, exclusive of possible coseismic rotations. That rotation is in rough agreement with the paleomagnetic rotation accumulated in the individual fault blocks within the northeast Mojave domain since 12.8 Ma.

California↗
USGS2009

Instrumental record of debris flow initiation during natural rainfall: Implications for modeling slope stability

The middle of a hillslope hollow in the Oregon Coast Range failed and mobilized as a debris flow during heavy rainfall in November 1996. Automated pressure transducers recorded high spatial variability of pore water pressure within the area that mobilized as a debris flow, which initiated where local upward flow from bedrock developed into overlying colluvium. Postfailure observations of the bedrock surface exposed in the debris flow scar reveal a strong spatial correspondence between elevated piezometric response and water discharging from bedrock fractures. Measurements of apparent root cohesion on the basal ( C b ) and lateral ( C l ) scarp demonstrate substantial local variability, with areally weighted values of C b = 0.1 and C l = 4.6 kPa. Using measured soil properties and basal root strength, the widely used infinite slope model, employed assuming slope parallel groundwater flow, provides a poor prediction of hydrologic conditions at failure. In contrast, a model including lateral root strength (but neglecting lateral frictional strength) gave a predicted critical value of relative soil saturation that fell within the range defined by the arithmetic and geometric mean values at the time of failure. The 3‐D slope stability model CLARA‐W, used with locally observed pore water pressure, predicted small areas with lower factors of safety within the overall slide mass at sites consistent with field observations of where the failure initiated. This highly variable and localized nature of small areas of high pore pressure that can trigger slope failure means, however, that substantial uncertainty appears inevitable for estimating hydrologic conditions within incipient debris flows under natural conditions.

Oregon↗
USGS2026

Multi-site evaluation of a postfire debris-flow runout forecast method

Postfire debris flows pose a hazard to human life, property, and infrastructure when they travel from steep source areas to urbanized alluvial fans or other developed areas. Existing methods for rapid (<1 week) postfire debris-flow hazard assessment document the increase in the likelihood and size of debris flows as the magnitude of high-intensity rain necessary to initiate debris flows increases but do not indicate the extent of downstream debris-flow runout. Although many models for the simulation of debris-flow motion are available, there is no established approach for using these models to delineate locations susceptible to postfire debris-flow runout that (a) is feasible to use at the spatial scale of an entire fire; (b) is appropriate for runout onto unconfined areas; (c) reproduces observed relations between runout and rainfall intensity; and (d) characterizes inherent uncertainty in runout, even without spatiotemporally variable rainfall. We propose and evaluate a method for generating postfire debris-flow runout hazard maps that has all the above qualities. Selection of case studies prioritized events triggered by a range of rainfall intensities, locations within and outside of southern California, and observed runout onto unconfined topography. Qualitative and quantitative assessment of performance for four events indicate that simulation results broadly match observations albeit with some discrepancies at a scale larger than structure or land parcel level (approximately 20-m by 20-m). The method may be used to identify potentially hazardous areas immediately following a fire and to provide approximate runout forecasts when a storm is imminent.

Arizona, California↗
USGS2012

The sea slope problem revisited

Discrepancies in sea surface topography based on comparisons between the results of steric leveling and repeated geodetic levelings have identified what is known as the ‘sea slope problem.’ This problem is actually twofold: (1) the sea surface relief based on steric leveling differs significantly from that based on geodetic leveling along several generally north-south coasts, and (2) successively propagated levelings between several widely separated tide stations indicate that the stationary sea slope seemingly has been changing with time, whereas differenced sea level means between these stations indicate that the sea surface relief has remained virtually invariant during the same intervals. Reexamination of the three reported discrepancies between geodetically and sterically determined sea slopes indicates that the Australian example is based on leveling of a quality inappropriate to the comparison. The discrepancy developed along the Atlantic coast of the United States is limited to the reach between Portsmouth–Hampton Roads, Virginia, and Key West, Florida, where the accuracy of steric leveling may be especially vulnerable owing to the dynamic effects of the Gulf Stream. Reconsideration of the example along the Pacific coast of the United States indicates that the various discrepancies are due chiefly to intrasurvey movement and resultant distortion of geodetically defined height differences between tide stations. Agreement between the results of steric and geodetic leveling along tectonically inactive north-south coasts devoid of strong boundary currents is generally good. This observation supports the conclusions (1) that any directionally dependent systematic error in geodetic leveling is measurably insignificant and (2) that where allowance is made for the possible effects of major boundary currents or intrasurvey movement during levelings between tide stations, the sea slope problem tends to vanish.

Journal of Geophysical Research B: Solid Earth↗
USGS1993

Seismological mapping of fine structure near the base of the Earth's mantle

The Earth's core–mantle boundary (CMB) juxtaposes liquid iron and crystalline silicates, and is a region of large vertical thermal gradients. The D" region, which extends up to 200–300 km above the CMB, often has elevated shear-wave velocity and suggestions of lateral variations in structure 1 . Recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times, amplitudes and waveforms from more than a thousand short-period seismometers 2 . We observe, across Canada and the United States, P waves that have grazed the CMB from the powerful nuclear test in Lop Nor, China, on 21 May 1992. First-arrival travel times and large secondary arrivals are consistent with a 1.5% compressional velocity increase with depth ∼ 130 km above the CMB—about half the thickness of D" in this locality 3 . Our observations, together with evidence for the absence of such a thin, fast layer in neighbouring regions, suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle.

Nature↗
USGS2004

Emplacement, rapid burial, and exhumation of 90-Ma plutons in southeastern Alaska

In southeastern Alaska, granodiorite-tonalite plutons of the Admiralty-Revillagigedo belt intruded the Jurassic-Cretaceous Gravina belt along the eastern side of the Alexander terrane around 90 Ma. These plutons postdate some deformation related to a major contractional event between the previously amalgamated Wrangellia and Alexander terranes and the previously accreted terranes of the North American margin. We studied the aureole mineral assemblages of these plutons near Petersburg, Alaska, determined pressure and temperature of equilibration, and examined structures that developed within and adjacent to these plutons. Parallelism of magmatic and submagmatic fabrics with fabrics in the country rock indicates synchroneity of pluton emplacement with regional deformation and suggests that magma transport to higher crustal levels was assisted by regional deformation. Replacement of andalusite by kyanite or sillimanite indicates crustal thickening soon after pluton emplacement. Regional structural analysis indicates the crustal thickening was accomplished by thrust burial. Thermobarometric analyses indicate the aureoles reached near-peak temperatures of 525 to 635 ??C at pressures of 570 to 630 MPa. Consideration of the rate of thermal decay of the aureoles suggests that burial was rapid and occurred at rates around 5 to 8 mm/year. Structural observations indicate there was contractional deformation before, during, and after emplacement of the 90-Ma plutons. Initial exhumation of the Admiralty-Revillagedo belt in the Petersburg area may have occurred along a thrust west of the pluton belt within the Gravina belt. ?? 2004 NRC Canada.

Canadian Journal of Earth Sciences↗
USGS2015

Comment on “Models of stochastic, spatially varying stress in the crust compatible with focal‐mechanism data, and how stress inversions can be biased toward the stress rate” by Deborah Elaine Smith and Thomas H. Heaton

Smith and Heaton (2011) propose a model in which stress in the crust is fractal‐like and highly variable on a range of length scales, including short length‐scales of ~1 km. Smith and Heaton (2011) motivate the need for stress heterogeneity on short length‐scales by citing observations such as short length‐scale changes in stress directions inferred from borehole breakouts, short length‐scale changes in earthquake slip, and the success of numerical models that include short‐wavelength stress heterogeneity. The heterogeneous part of the stress field in their model is more than twice as large as the homogeneous part. The stress field in this model frequently reverses itself over short distances, as can be seen in figure14 a of Smith and Heaton (2011). The modeled stress field contains at least 10 areas of reversed shear stress direction over the length of a 100 km long profile, with the length of the reversed areas ranging from <1 to ~5 km. This model makes specific predictions about the orientations and heterogeneity of earthquake focal mechanisms. Smith and Heaton (2011) attempt to validate this heterogeneous stress model using observations of earthquake focal‐mechanism variability from Hardebeck (2006). They then demonstrate that the model predicts a bias in the orientations of earthquake focal mechanisms, which are biased away from the background stress and toward the stressing rate. They suggest the focal‐mechanism bias in this model invalidates the large body of work over the last several decades, that has inferred stress orientations from the inversion of earthquake focal mechanisms. The question of whether or not the Smith and Heaton (2011) model is applicable to the real Earth is therefore important not only for understanding spatial stress variability but also for evaluating the numerous studies that have inferred crustal stress orientations from earthquake focal mechanisms (e.g., as compiled by Heidbach et al. , 2008).

Bulletin of the Seismological Society of America↗
USGS1998

Bayesian statistics applied to the location of the source of explosions at Stromboli Volcano, Italy

We present a method for determining the location and spatial extent of the source of explosions at Stromboli Volcano, Italy, based on a Bayesian inversion of the slowness vector derived from frequency-slowness analyses of array data. The method searches for source locations that minimize the error between the expected and observed slowness vectors. For a given set of model parameters, the conditional probability density function of slowness vectors is approximated by a Gaussian distribution of expected errors. The method is tested with synthetics using a five-layer velocity model derived for the north flank of Stromboli and a smoothed velocity model derived from a power-law approximation of the layered structure. Application to data from Stromboli allows for a detailed examination of uncertainties in source location due to experimental errors and incomplete knowledge of the Earth model. Although the solutions are not constrained in the radial direction, excellent resolution is achieved in both transverse and depth directions. Under the assumption that the horizontal extent of the source does not exceed the crater dimension, the 90% confidence region in the estimate of the explosive source location corresponds to a small volume extending from a depth of about 100 m to a maximum depth of about 300 m beneath the active vents, with a maximum likelihood source region located in the 120- to 180-m-depth interval.

Stromboli Volcano↗
USGS1991

Use of longitudinal strain in identifying driving and resisting elements of landslides

Observations of deformation at the surfaces of landslides in Utah and Hawaii indicate that the upslope parts of the land-slides have stretched and the downslope parts have shortened parallel with the direction of movement. The maximum displacement of each landslide occurs in a relatively undeformed zone between the zones of shortening and stretching. The pattern of deformation at the surface of these landslides may be useful in analyzing their mechanics by helping to constrain the longitudinal forces in limit-equilibrium stability analysis. We used earth-pressure calculations to determine the range of possible longitudinal forces (per unit width) for active failure in the zone of stretching and for passive failure in the zone of shortening of one of the Hawaiian landslides. Longitudinal forces computed by stability analysis, assuming homogeneous strength, exceeded the possible forces in much of the upslope half of the landslide. Consequently, we assumed inhomogeneous strength and adjusted shear-strength parameters at each segment of the slip surface until the longitudinal forces computed by stability analysis agreed with those computed by earth-pressure theory, and the factor of safety approached unity. The distribution of longitudinal forces computed for inhomogeneous strength indicated that the boundary between driving and resisting elements of the landslide is near the thickest part of the slide, in agreement with a simple formula for the location of the boundary.

Geological Society of America Bulletin↗
USGS2017

New insight into the origin of manganese oxide ore deposits in the Appalachian Valley and Ridge of northeastern Tennessee and northern Virginia, USA

Manganese oxide deposits have long been observed in association with carbonates within the Appalachian Mountains, but their origin has remained enigmatic for well over a century. Ore deposits of Mn oxides from several productive sites located in eastern Tennessee and northern Virginia display morphologies that include botryoidal and branching forms, massive nodules, breccia matrix cements, and fracture fills. The primary ore minerals include hollandite, cryptomelane, and romanèchite. Samples of Mn oxides from multiple localities in these regions were analyzed using electron microscopy, X-ray analysis, Fourier transform infrared spectroscopy, and trace and rare earth element (REE) geochemistry. The samples from eastern Tennessee have biological morphologies, contain residual biopolymers, and exhibit REE signatures that suggest the ore formation was due to supergene enrichment (likely coupled with microbial activity). In contrast, several northern Virginia ores hosted within quartz-sandstone breccias exhibit petrographic relations, mineral morphologies, and REE signatures indicating inorganic precipitation, and a likely hydrothermal origin with supergene overprinting. Nodular accumulations of Mn oxides within weathered alluvial deposits that occur close to breccia-hosted Mn deposits in Virginia show geochemical signatures that are distinct from the breccia matrices and appear to reflect remobilization of earlier-emplaced Mn and concentration within supergene traps. Based on the proximity of all of the productive ore deposits to mapped faults or other zones of deformation, we suggest that the primary source of all of the Mn may have been deep seated, and that Mn oxides with supergene and/or biological characteristics resulted from the local remobilization and concentration of this primary Mn.

Tennessee, Virginia↗
USGS2020

Sediment record of mining legacy and water quality from a drinking-water reservoir, Aztec, New Mexico, USA

The record of mining legacy and water quality was investigated in sediments collected in 2018 from four trenches in the Aztec, New Mexico, drinking-water reservoir #1. Bulk chemical analysis of sediments with depth in the reservoir revealed variable trace-element (uranium, vanadium, arsenic, copper, sulfur, silver, lead, and zinc) concentrations, which appear to coincide with historical mining and milling operations. Cesium-137 age dating, which identified the location of the 1963 radioactive fallout maximum, combined with the known age of the bottom and top of the sediment trenches, was used to estimate a polynomial sedimentation rate (average rate = 1.7 cm/yr). The clay size fraction (< 0.004 mm) was the dominant grain-size fraction of the sediments. Abundant fine-grained phyllosilicate (clay) minerals, predominantly montmorillonite and kaolinite, may explain sorption properties of trace elements. Scanning electron microscopy evaluation of sediments from two trenches showed copper and zinc associated with sulfur, and arsenic associated with iron and aluminum oxides. Results from laboratory batch experiments indicated that uranium, vanadium, and arsenic were released when sediments were reacted with a 150 mg/L sodium bicarbonate solution whereas copper was released when sediments were reacted with 2 mMol/L acetic acid. Observed concentrations from the two leach tests were below regulatory thresholds for delivery of solids to a landfill and were below drinking-water standards. Diatom relative abundance indicates that the water quality in the reservoir was not impaired by high metal concentrations.

Colorado, New Mexico↗
USGS2005

Macropolygon morphology, development, and classification on North Panamint and Eureka playas, Death Valley National Park CA

Panamint and Eureka playas, both located within Death Valley National Park, exhibit a host of surficial features including fissures, pits, mounds, and plant-covered ridges, representing topographic highs and lows that vary up to 2 m of relief from the playa surface. Aerial photographs reveal that these linear strands often converge to form polygons, ranging in length from several meters to nearly a kilometer. These features stand out in generally dark contrast to the brighter intervening expanse of flat, plant-free, desiccated mud of the typical playa surface. Ground-truth mapping of playa features with differential GPS (Global Positioning System) was conducted in 1999 (North Panamint Valley) and 2002 (Eureka Valley). High-resolution digital maps reveal that both playas possess macropolygons of similar scale and geometry, and that fissures may be categorized into one of two genetic groups: (1) shore-parallel or playa-interior desiccation and shrinkage; and (2) tectonic-induced cracks. Early investigations of these features in Eureka Valley concluded that their origin may have been related to agricultural activity by paleo-Indian communities. Although human artifacts are abundant at each locale, there is no evidence to support the inference that surface features reported on Eureka Playa are anthropogenic in origin. Our assumptions into the genesis of polygons on playas is based on our fortuitous experience of witnessing a fissure in the process of formation on Panamint Playa after a flash flood (May 1999); our observations revealed a paradox that saturation of the upper playa crusts contributes to the establishment of some desiccation features. Follow-up visits to the same feature over 2 yrs' time are a foundation for insight into the evolution and possible longevity of these features. ?? 2005 Elsevier B.V. All rights reserved.

Earth-Science Reviews↗
USGS2018

Unraveling the dynamics of magmatic CO2 degassing at Mammoth Mountain, California

The accumulation of magmatic CO 2 beneath low-permeability barriers may lead to the formation of CO 2 -rich gas reservoirs within volcanic systems. Such accumulation is often evidenced by high surface CO 2 emissions that fluctuate over time. The temporal variability in surface degassing is believed in part to reflect a complex interplay between deep magmatic degassing and the permeability of degassing pathways. A better understanding of the dynamics of CO 2 degassing is required to improve monitoring and hazards mitigation in these systems. Owing to the availability of long-term records of CO 2 emissions rates and seismicity, Mammoth Mountain in California constitutes an ideal site towards such predictive understanding. Mammoth Mountain is characterized by intense soil CO 2 degassing (up to ∼1000 t d −1 ) and tree kill areas that resulted from leakage of CO 2 from a CO 2 -rich gas reservoir located in the upper ∼4 km. The release of CO 2 -rich fluids from deeper basaltic intrusions towards the reservoir induces seismicity and potentially reactivates faults connecting the reservoir to the surface. While this conceptual model is well-accepted, there is still a debate whether temporally variable surface CO 2 fluxes directly reflect degassing of intrusions or variations in fault permeability. Here, we report the first large-scale numerical model of fluid and heat transport for Mammoth Mountain. We discuss processes (i) leading to the initial formation of the CO 2 -rich gas reservoir prior to the occurrence of high surface CO 2 degassing rates and (ii) controlling current CO 2 degassing at the surface. Although the modeling settings are site-specific, the key mechanisms discussed in this study are likely at play at other volcanic systems hosting CO 2 -rich gas reservoirs. In particular, our model results illustrate the role of convection in stripping a CO 2 -rich gas phase from a rising hydrothermal fluid and leading to an accumulation of a large mass of CO 2 (∼10 7 –10 8 t) in a shallow gas reservoir. Moreover, we show that both, short-lived (months to years) and long-lived (hundreds of years) events of magmatic fluid injection can lead to critical pressures within the reservoir and potentially trigger fault reactivation. Our sensitivity analysis suggests that observed temporal fluctuations in surface degassing are only indirectly controlled by variations in magmatic degassing and are mainly the result of temporally variable fault permeability. Finally, we suggest that long-term CO 2 emission monitoring, seismic tomography and coupled thermal–hydraulic–mechanical modeling are important for CO 2 -related hazard mitigation.

California↗
USGS2005

Forecasting the evolution of seismicity in southern California: Animations built on earthquake stress transfer

We develop a forecast model to reproduce the distibution of main shocks, aftershocks and surrounding seismicity observed during 1986-200 in a 300 ?? 310 km area centered on the 1992 M = 7.3 Landers earthquake. To parse the catalog into frames with equal numbers of aftershocks, we animate seismicity in log time increments that lengthen after each main shock; this reveals aftershock zone migration, expansion, and densification. We implement a rate/state algorithm that incorporates the static stress transferred by each M ??? 6 shock and then evolves. Coulomb stress changes amplify the background seismicity, so small stress changes produce large changes in seismicity rate in areas of high background seismicity. Similarly, seismicity rate declines in the stress shadows are evident only in areas with previously high seismicity rates. Thus a key constituent of the model is the background seismicity rate, which we smooth from 1981 to 1986 seismicity. The mean correlation coefficient between observed and predicted M ??? 1.4 shocks (the minimum magnitude of completeness) is 0.52 for 1986-2003 and 0.63 for 1992-2003; a control standard aftershock model yields 0.54 and 0.52 for the same periods. Four M ??? 6.0 shocks struck during the test period; three are located at sites where the expected seismicity rate falls above the 92 percentile, and one is located above the 75 percentile. The model thus reproduces much, but certainly not all, of the observed spatial and temporal seismicity, from which we infer that the decaying effect of stress transferred by successive main shocks influences seismicity for decades. Finally, we offer a M ??? 5 earthquake forecast for 2005-2015, assigning probabilities to 324 10 ?? 10 km cells.

Journal of Geophysical Research B: Solid Earth↗
USGS2020

Planetary sensor models interoperability using the community sensor model specification

This paper presents the photogrammetric foundations upon which the Community Sensor Model specification depends, describes common coordinate system and reference frame transformations that support conversion between image sensor (charge‐coupled device) coordinates to some arbitrary body coordinate, and describes the U.S. Geological Survey Astrogeology Community Sensor Model implementation ( https://github.com/USGS-Astrogeology/usgscsm ). We present a new image support data specification that provides the position, pointing, timing, and metadata information necessary to properly locate a pixel or observations location on a body and describe a system architecture designed to explicitly identify the responsibilities of software components within a larger pipeline or analytical environment. This paper concludes with a set of experiments that illustrate positional and pointing error in the sensor location and the impact on the computed surface location.

Earth and Space Science↗
USGS2023

Vulnerability of estuarine systems in the contiguous United States to water quality change under future climate and land-use

Changes in climate and land-use and land-cover (LULC) are expected to influence surface water runoff and nutrient characteristics of estuarine watersheds, but the extent to which estuaries are vulnerable to altered nutrient loading under future conditions is poorly understood. The present work aims to address this gap through the development of a new vulnerability assessment framework that accounts for (a) estuarine exposure to projected changes in total nitrogen (TN) and total phosphorus (TP) loads as a function of LULC and climate change under several scenarios, (b) sensitivity, and (c) adaptive capacity. The framework was applied to 112 estuaries and their contributing watersheds across the contiguous U.S., specifically to look at regional variability in estuarine vulnerability to nutrient loading. Study findings revealed that the largest increases in estuarine nutrient loads are expected in the North and South Atlantic regions and eastern Gulf of Mexico, while the lowest increases are expected in the North and South Pacific regions and the western Gulf of Mexico. However, the North Atlantic and the South Pacific had the highest adaptive capacity, which could potentially counteract the effects of LULC and climate change on nutrient loads. Strong variation in predicted estuarine nutrient loads was observed as a function of climate model projections, while projected LULC changes were more consistently associated with elevated loads. Our findings illustrate the benefits of integrating natural and socio-ecological factors to identify opportunities to develop adaptation plans and policies to mitigate ecological degradation in vitally important estuaries.

Earth's Future↗
USGS2016

Short-period volcanic gas precursors to phreatic eruptions: Insights from Poás Volcano, Costa Rica

Volcanic eruptions involving interaction with water are amongst the most violent and unpredictable geologic phenomena on Earth. Phreatic eruptions are exceptionally difficult to forecast by traditional geophysical techniques. Here we report on short-term precursory variations in gas emissions related to phreatic blasts at Po&aacute;s volcano, Costa Rica, as measured with an in situ multiple gas analyzer that was deployed at the edge of the erupting lake. Gas emitted from this hyper-acid crater lake approaches magmatic values of SO 2 /CO 2 1&ndash;6 days prior to eruption. The SO 2 flux derived from magmatic degassing through the lake is measureable by differential optical absorption spectrometry (sporadic campaign measurements), which allows us to constrain lake gas output and input for the major gas species during eruptive and non-eruptive periods. We can further calculate power supply to the hydrothermal system using volatile mass balance and thermodynamics, which indicates that the magmatic heat flux into the shallow hydrothermal system increases from &sim;27 MW during quiescence to &sim;59 MW during periods of phreatic events. These transient pulses of gas and heat from the deeper magmatic system generate both phreatic eruptions and the observed short-term changes in gas composition, because at high gas flux scrubbing of sulfur by the hydrothermal system is both kinetically and thermodynamically inhibited whereas CO 2 gas is always essentially inert in hyperacid conditions. Thus, the SO 2 /CO 2 of lake emissions approaches magmatic values as gas and power supply to the sub-limnic hydrothermal system increase, vaporizing fluids and priming the hydrothermal system for eruption. Our results suggest that high-frequency real-time gas monitoring could provide useful short-term eruptive precursors at volcanoes prone to phreatic explosions.

Poás Volcano↗
USGS1984

Crustal structure of the Appalachian Highlands in Tennessee

Crustal structure of the southern Appalachians and adjacent Interior Low Plateaus in Tennessee is derived from seismic-refraction measurements observed by the U.S. Geological Survey in 1965 along reversed lines, normal (NW-SE) and parallel (NE-SW) to the structure of the Appalachian Highlands' major geologic divisions. Its easternmost part is located approximately 80 km southwest of the westernmost part of the COCORP seismic-reflection traverse within the Blue Ridge province. The velocity-depth models derived for both observational directions consist of three crustal layers with surprisingly high velocities, being about 6.1-6.2 km/s in the upper crust down to 7-10 km depth, 6.7-6.8 km/s for the middle crust between about 17 and 34 km and varying from 7.1 to 7.4 km/s for the lower crust at about 40-47 km depth. The boundaries between the three crustal layers as well as the crust-mantle boundary are transition zones of up to 11 km thickness. Similar to old orogens in other parts of the earth, the main result is a thick crust, at places in excess of 50 km, with high average velocity and a broad crust-mantle transition zone.

Tennessee↗