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At least 55 records · Page 3Linked to original sources
USGS2004

Remotely triggered seismicity on the United States west coast following the Mw 7.9 Denali fault earthquake

The M w 7.9 Denali fault earthquake in central Alaska of 3 November 2002 triggered earthquakes across western North America at epicentral distances of up to at least 3660 km. We describe the spatial and temporal development of triggered activity in California and the Pacific Northwest, focusing on Mount Rainier, the Geysers geothermal field, the Long Valley caldera, and the Coso geothermal field. The onset of triggered seismicity at each of these areas began during the Love and Raleigh waves of the M w 7.9 wave train, which had dominant periods of 15 to 40 sec, indicating that earthquakes were triggered locally by dynamic stress changes due to low-frequency surface wave arrivals. Swarms during the wave train continued for ∼4 min (Mount Rainier) to ∼40 min (the Geysers) after the surface wave arrivals and were characterized by spasmodic bursts of small ( M ≤ 2.5) earthquakes. Dynamic stresses within the surface wave train at the time of the first triggered earthquakes ranged from 0.01 MPa (Coso) to 0.09 MPa (Mount Rainier). In addition to the swarms that began during the surface wave arrivals, Long Valley caldera and Mount Rainier experienced unusually large seismic swarms hours to days after the Denali fault earthquake. These swarms seem to represent a delayed response to the Denali fault earthquake. The occurrence of spatially and temporally distinct swarms of triggered seismicity at the same site suggests that earthquakes may be triggered by more than one physical process.

North America↗
USGS1948

Mineralogy and petrology of the currant creek magnesite deposits and associated rocks of Nevada

Cryptocrystalline magnesite occurs as relatively small dense, white masses in a Tertiary volcanic tuff, here named the Currant tuff, in White Pine and Nye counties, 29 miles southwest of Ely, Nevada. The deposits are small, but some of the magnesite is of very high quality with almost no iron or aluminum. However, much of the magnesite is mixed with dolomite and calcite, and a serpentinelike mineral occurs in one of the deposits in sufficient abundance to be of possible use in ceramics. In scattered areas the tuff has been altered by solutions rich in magnesium and bicarbonic acid to dolomite, magnesite, magnesium silicate, and calcite. Mineral and chemical characteristics are the same throughout the entire rock assemblage of flows and tuffs, suggesting that all the rocks have been derived from the same parent magma. The tuff formation, which ranges in thickness from a few feet to over 400 feet, occurs between two groups of volcanic flows here referred to as the lower volcanics and the upper volcanics. The lower volcanics is composed mainly of flows of hypersthene dacite, but in the southwestern part of the area flows of a hypersthene andesite of balsaltic habit occur between the dacite and the overlying Currant tuff. The upper volcanics which overlie the Currant tuff consist of porphyritic quartz latite and an overlying massive latite crystal tuff. The Currant tuff containing the magnesite has both unmodified and reworked tuff members. The formation of hydrothermal dolomite, magnesite, and deweylite is discussed in detail, and the physical chemistry of solutions containing calcium, magnesium, and carbon dioxide is applied in an explanation of the origin of magnesium-rich solutions.

Nevada↗
USGS2015

Ground motion simulation for the 23 August 2011, Mineral, Virginia earthquake using physics-based and stochastic broadband methods

Three broadband simulation methods are used to generate synthetic ground motions for the 2011 Mineral, Virginia, earthquake and compare with observed motions. The methods include a physics‐based model by Hartzell et al. (1999, 2005), a stochastic source‐based model by Boore (2009), and a stochastic site‐based model by Rezaeian and Der Kiureghian (2010, 2012). The ground‐motion dataset consists of 40 stations within 600 km of the epicenter. Several metrics are used to validate the simulations: (1) overall bias of response spectra and Fourier spectra (from 0.1 to 10 Hz); (2) spatial distribution of residuals for GMRotI50 peak ground acceleration (PGA), peak ground velocity, and pseudospectral acceleration (PSA) at various periods; (3) comparison with ground‐motion prediction equations (GMPEs) for the eastern United States. Our results show that (1) the physics‐based model provides satisfactory overall bias from 0.1 to 10 Hz and produces more realistic synthetic waveforms; (2) the stochastic site‐based model also yields more realistic synthetic waveforms and performs superiorly for frequencies greater than about 1 Hz; (3) the stochastic source‐based model has larger bias at lower frequencies (<0.5&thinsp;&thinsp;Hz) and cannot reproduce the varying frequency content in the time domain. The spatial distribution of GMRotI50 residuals shows that there is no obvious pattern with distance in the simulation bias, but there is some azimuthal variability. The comparison between synthetics and GMPEs shows similar fall‐off with distance for all three models, comparable PGA and PSA amplitudes for the physics‐based and stochastic site‐based models, and systematic lower amplitudes for the stochastic source‐based model at lower frequencies (<0.5&thinsp;&thinsp;Hz).

Bulletin of the Seismological Society of America↗
USGS1981

Lithology, reservoir properties, and burial history of portion of Gammon Shale (Cretaceous), southwestern North Dakota

In the northern Great Plains, large quantities of biogenic methane are contained at shallow depths in Cretaceous marine mudstones. The Gammon Shale and equivalents of the Milk River Formation in Canada, which comprise most sediments deposited offshore during the Eagle-Telegraph Creek regression, are typical of such gas-bearing rocks. At Little Missouri field, southwestern North Dakota, Gammon reservoirs consist of discontinuous lenses and laminae of siltstone, less than 10 mm thick, enclosed by silty clay shale. Large amounts of allogenic clay, including highly expansible mixed-layer illite-smectite cause great water sensitivity and high measured and calculated water-saturation values. Reconstructed burial depths, clay mineralogy, and organic matter maturation studies show that the Gammon has not undergone thermal conditions sufficient for oil or thermal gas generation. Scarce authigenic minerals such as pyrite, siderite, and calcite probably formed as a result of bacterial metabolism early in the burial history. The scarcity of authigenic silicates suggests that diagenesis has been inhibited during much of the burial history by the presence of free methane. Shale layers are practically impermeable whereas siltstone microlenses are porous (30 to 40%) and have permeabilities on the order of 3 to 30 md. Reservoir continuity between siltstone layers is poor and, overall, reservoir permeability is probably less than 0.4 md. Connecting passageways between siltstone lenses are 0.1 µm or less in diameter. Organic matter in the low-permeability reservoirs served as the source of biogenic methane, and capillary forces acted as the trapping mechanism for gas accumulation. At Little Missouri field, reservoirs and non-reservoirs cannot be distinguished on the basis of lithology, and much of the Gammon interval is potentially economic. Future research should be directed toward determining the physical basis of log response in the low-permeability reservoirs and toward the development or application of water-free recovery technology.

North Dakota↗
USGS2023

Seafloor observations eliminate a landslide as the source of the 1918 Puerto Rico Tsunami

The 11 October 1918 devastating tsunami in northwest Puerto Rico had been used as an example for earthquake‐induced landslide tsunami hazard. Three pieces of evidence pointed to a landslide as the origin of the tsunami: the discovery of a large submarine landslide scar from bathymetry data collected by shipboard high‐resolution multibeam sonar, reported breaks of submarine cable within the scar, and the fit of tsunami models to flooding observations. Newly processed seafloor imagery collected by remotely operated vehicle (ROV) show, however, pervasive Fe–Mn crust (patina) on the landslide walls and floor, indicating that the landslide scar is at least several hundred years old. C 14 "> 14 C dates of sediment covering the landslide floor verify this interpretation. Although we have not searched the region systematically for an alternative tsunami source, we propose a possible source—a two‐segment normal‐fault rupture along the eastern wall of Mona rift. The proposed fault location matches the published normal faults with steep bathymetry and is close to the International Seismological Center–Global Earthquake Model catalog locations of the 1918 mainshock and aftershocks. The ROV observations further show fresh vertical slickensides and rock exposure along the proposed fault trace. Hydrodynamic models from an M w "> M w 7.2 earthquake rupture along the eastern wall of the rift faithfully reproduce the reported tsunami amplitudes, polarities, and arrival times. Our analysis emphasizes the value of close‐up observations and physical samples to augment remote sensing data in natural hazard studies.

Puerto Rico↗
USGS2010

Influence of Physiochemical and watershed characteristics on mercury concentration in walleye, Sander vitreus , M.

Elevated mercury concentration has been documented in a variety of fish and is a growing concern for human consumption. Here, we explore the influence of physiochemical and watershed attributes on mercury concentration in walleye ( Sander vitreus, M . ) from natural, glacial lakes in South Dakota. Regression analysis showed that water quality attributes were poor predictors of walleye mercury concentration (R 2 = 0.57, p = 0.13). In contrast, models based on watershed features (e.g., lake level changes, watershed slope, agricultural land, wetlands) and local habitat features (i.e., substrate composition, maximum lake depth) explained 81% ( p = 0.001) and 80% ( p = 0.002) of the variation in walleye mercury concentration. Using an information theoretic approach we evaluated hypotheses related to water quality, physical habitat and watershed features. The best model explaining variation in walleye mercury concentration included local habitat features (W i = 0.991). These results show that physical habitat and watershed features were better predictors of walleye mercury concentration than water chemistry in glacial lakes of the Northern Great Plains.

Bulletin of Environmental Contamination and Toxico↗
USGS2016

Linking silicate weathering to riverine geochemistry—A case study from a mountainous tropical setting in west-central Panama

Chemical analyses from 71 watersheds across an ∼450 km transect in west-central Panama provide insight into controls on weathering and rates of chemical denudation and CO 2 consumption across an igneous arc terrain in the tropics. Stream and river compositions across this region of Panama are generally dilute, having a total dissolved solute value = 118 ± 91 mg/L, with bicarbonate and silica being the predominant dissolved species. Solute, stable isotope, and radiogenic isotope compositions are consistent with dissolution of igneous rocks present in Panama by meteoric precipitation, with geochemical signatures of rivers largely acquired in their upstream regions. Comparison of a headwater basin with its entire watershed observed considerably more runoff production from the high-elevation upstream portion of the catchment than in its much more spatially extensive downstream region. Rock alteration profiles document that weathering proceeds primarily by dissolution of feldspar and pyroxene, with base cations effectively leached in the following sequence: Na > Ca > Mg > K. Control on water chemistry by bedrock lithology is indicated through a linking of elevated ([Na + K]/[Ca + Mg]) ratios in waters to a high proportion of catchment area silicic bedrock and low ratios to mafic bedrock. Sr-isotope ratios are dominated by basement-derived Sr, with only very minor, if any, contribution from other sources. Cation weathering of Ca sil + Mg sil + Na + K spans about an order in magnitude, from 3 to 32 tons/km 2 /yr. Strong positive correlations of chemical denudation and CO 2 consumption are observed with precipitation, mean watershed elevation, extent of land surface forest cover, and physical erosion rate.

Chagras and Pacora watersheds↗
USGS2020

The predictive skills of elastic Coulomb rate-and-state aftershock forecasts during the 2019 Ridgecrest, California, earthquake sequence

Operational earthquake forecasting protocols commonly use statistical models for their recognized ease of implementation and robustness in describing the short-term spatiotemporal patterns of triggered seismicity. However, recent advances on physics-based aftershock forecasting reveal comparable performance to the standard statistical counterparts with significantly improved predictive skills when fault and stress field heterogeneities are considered. Here, we perform a pseudo-prospective forecasting experiment during the first month of the 2019 Ridgecrest (California) earthquake sequence. We develop seven Coulomb rate-and-state models that couple static stress change estimates with continuum mechanics expressed by the rate-and-state friction laws. Our model parametrization supports a gradually increasing complexity; we start from a preliminary model implementation with simplified slip distributions and spatially homogeneous receiver faults to reach an enhanced one featuring optimized fault constitutive parameters, finite-fault slip models, secondary triggering effects, and spatially heterogenous planes informed by pre-existing ruptures. The data-rich environment of Southern California allows us to test whether incorporating data collected in near real-time during an unfolding earthquake sequence boosts our predictive power. We assess the absolute and relative performance of the forecasts by means of statistical tests used within the Collaboratory for the Study of Earthquake Predictability (CSEP) and compare their skills against a standard benchmark ETAS model for the short (24 hours after the two Ridgecrest mainshocks) and intermediate-term (one month). Stress-based forecasts expect heightened rates along the whole near-fault region and increased expected seismicity rates in Central Garlock Fault. Our comparative model evaluation supports that faulting heterogeneities coupled with secondary triggering effects are the most critical success components behind physics-based forecasts, but also underlines the importance of model updates incorporating near real-time available aftershock data reaching better performance than ETAS models. We explore the physical basis behind our results by investigating the localized shut down of pre-existing normal faults in the Ridgecrest near-source area.

California↗
USGS1999

Professional conduct of scientists during volcanic crises

Stress during volcanic crises is high, and any friction between scientists can distract seriously from both humanitarian and scientific effort. Friction can arise, for example, if team members do not share all of their data, if differences in scientific interpretation erupt into public controversy, or if one scientist begins work on a prime research topic while a colleague with longer-standing investment is still busy with public safety work. Some problems arise within existing scientific teams; others are brought on by visiting scientists. Friction can also arise between volcanologists and public officials. Two general measures may avert or reduce friction: (a) National volcanologic surveys and other scientific groups that advise civil authorities in times of volcanic crisis should prepare, in advance of crises, a written plan that details crisis team policies, procedures, leadership and other roles of team members, and other matters pertinent to crisis conduct. A copy of this plan should be given to all current and prospective team members. (b) Each participant in a crisis team should examine his or her own actions and contribution to the crisis effort. A personal checklist is provided to aid this examination. Questions fall generally in two categories: Are my presence and actions for the public good? Are my words and actions collegial, i.e., courteous, respectful, and fair? Numerous specific solutions to common crisis problems are also offered. Among these suggestions are: (a) choose scientific team leaders primarily for their leadership skills; (b) speak publicly with a single scientific voice, especially when forecasts, warnings, or scientific disagreements are involved; (c) if you are a would-be visitor, inquire from the primary scientific team whether your help would be welcomed, and, in general, proceed only if the reply is genuinely positive; (d) in publications, personnel evaluations, and funding, reward rather than discourage teamwork. Models are available from the fields of particle physics and human genetics, among others.

Bulletin of Volcanology↗
USGS2006

Basaltic thermals and Subplinian plumes: Constraints from acoustic measurements at Shishaldin volcano, Alaska

The 1999 basaltic eruption of Shishaldin volcano (Alaska, USA) included both Strombolian and Subplinian activity, as well as a “pre-Subplinian” phase interpreted as the local coalescence within a long foam in the conduit. Although few visual observations were made of the eruption, a great deal of information regarding gas velocity, gas flux at the vent and plume height may be inferred by using acoustic recordings of the eruption. By relating acoustic power to gas velocity, a time series of gas velocity is calculated for the Subplinian and pre-Subplinian phases. These time series show trends in gas velocity that are interpreted as plumes or, for those signals lasting only a short time, thermals. The Subplinian phase is shown to be composed of a thermal followed by five plumes with a total expelled gas volume of &#x2248; 1.5 &#x00D7; 10 7 m 3 "> ≈ 1.5 × 10 7 m 3 . The initiation of the Subplinian activity is probably related to the arrival of a large overpressurised bubble close to the top of the magma column. A gradual increase in low-frequency (0.01–0.5 Hz) signal prior to this “trigger bubble” may be due to the rise of the bubble in the conduit. This delay corresponds to a reservoir located at ≈3.9 km below the surface, in good agreement with studies on other volcanoes. The presence of two thermal phases is also identified in the middle of the pre-Subplinian phase with a total gas release of &#x2248; 4.3 &#x00D7; 10 6 m 3 "> ≈ 4.3 × 10 6 m 3 and &#x2248; 3.6 &#x00D7; 10 6 m 3 "> ≈ 3.6 × 10 6 m 3 . Gas velocity at the vent is found to be &#x2248; 82 m . s &#x2212; 1 "> ≈ 82 m . s − 1 and &#x2248; 90 m . s &#x2212; 1 "> ≈ 90 m . s − 1 for the Subplinian plumes and the pre-Subplinian thermals respectively. The agreement is very good between estimates of the gas flux from modelling the plume height and those obtained from acoustic measurements, leading to a new method by which eruption physical parameters may be quantified. Furthermore, direct measurements of gas velocity can be used for better estimates of the S O 2 "> S O 2 flux released during the eruption.

Alaska↗
USGS2022

Enumerating plausible multifault ruptures in complex fault systems with physical constraints

We propose a new model for determining the set of plausible multifault ruptures in an interconnected fault system. We improve upon the rules used in the Third Uniform California Earthquake Rupture Forecast (UCERF3) to increase connectivity and the physical consistency of ruptures. We replace UCERF3’s simple azimuth change rules with new Coulomb favorability metrics and increase the maximum jump distance to 15 km. Although the UCERF3 rules were appropriate for faults with similar rakes, the Coulomb calculations used here inherently encode preferred orientations between faults with different rakes. Our new rules are designed to be insensitive to discretization details and are generally more permissive than their UCERF3 counterparts; they allow more than twice the connectivity compared to UCERF3, yet heavily penalize long ruptures that take multiple improbable jumps. The set of all possible multifault ruptures in the California fault system is near-infinite, but our model produces a tractable set of 326,707 ruptures (a modest 29% increase over UCERF3, despite the greatly increased connectivity). Inclusion in the rupture set does not dictate that a rupture receives a significant rate in the final model; rupture rates are subsequently determined by data constraints used in an inversion. We describe the rupture building algorithm and its components in detail and provide comparisons with ruptures generated by a physics-based multicycle earthquake simulator. We find that greater than twice as many ruptures generated by the simulator violate the UCERF3 rules than violate our proposed model.

California↗
USGS1971

Contrasting behavior of P, Ti, and Nb in a differentiated high-alumina olivine tholeiite and a calc-alkaline andesitic suite

Crystallization differentiation in a low-K 2 O, high-Al 2 O 3 olivine tholeiite (Hat Creek, California) yields segregation veins of basaltic andesite composition, and residual, interstitial glasses of dacite and rhyolite composition. P, Ti, and probably Nb, are progressively enriched in segregation veins and residual dacitic glass by crystallization of olivine, plagioclase, augite, and magnetite. P and Ti are depleted in residual rhyolitic glass by crystallization of magnetite, ilmenite, and apatite. By contrast, in a typical orogenic calc-alkaline suite (from Mount Jefferson, Oregon), P, Ti, and Nb are depleted with increasing K and Si. The most likely minerals capable of producing decreasing P, Ti, and Nb with increasing K are apatite, Fe-Ti oxides, amphibole, biotite, and sphene. There is no direct evidence of these minerals in Mount Jefferson basalt and andesite but they are present in calc-alkaline gabbro and tonalite and may occur at a shallow (crustal) depth beneath Mount Jefferson. Alternatively, mixing of dacite-rhyolite with basalt-andesite may account for the Mount Jefferson trend. By physical and chemical analogy with segregation veins and residual interstitial glasses, basaltic andesite and dacite-rhyolite magmas may segregate from complementary rock at depth, and mix during ascent to the surface.

California, Oregon↗
USGS1990

Coda duration magnitudes in central California: An empirical approach

A new empirical coda magnitude M D is presented for the central California seismic network (CALNET), in which station corrections are introduced to reduce the influence of hypocenter and station distributions. The new magnitude scale is obtained by relating the lapse-time τ^ to local magnitudes M L using data from 55 earthquakes with 1.1 ≦ M L ≤ 5.6 from June 1977 through 1981. Magnitude estimates are significantly improved by introducing corrections for the seismograph instrument attenuation setting α ij and a site correction δ j . The magnitude of the i th event 〈 M D 〉, is the median value of individual M D ij = -1.03 + 2.10log 10 τˆ ij + 0.00268 τˆ ij + α ij + δ j , where τ^ ij is the lapse-time measured at the j th station. Local site effects influence the duration of the coda and cause stations to either overestimate or underestimate 〈 M D 〉, typically up to 0.5 magnitude units. The distribution of the site corrections is spatially correlated, changing coherently across major tectonic and geologic features such as the San Andreas fault. It appears that the site corrections are influenced by the physical attenuation properties near the station, seemingly contradicting the concept that the coda is related to back-scattering and attenuation in a volume with dimensions on the order of the hypocenter to receiver distance.

California↗
USGS1953

Fluorspar deposits of the Eagle Mountains, Trans-Pecos Texas

The Eagle Mountains are in the southeastern part of Hudspeth County, Tex., about 17 miles southwest of Van Horn and 100 miles southeast of El Paso, Tex. The fluorspar deposits are in the northern and northeastern parts of the mountains, except for the Rocky Ridge deposits, which are near the center of the mountainous mass. A good all-weather road leads south from Allamoore on U. S. Highway 80 to the mine and mill at Spar Valley. With the exception of the Rocky Ridge deposits, all deposits can be reached by ranch roads from the main Spar Valley road. Fluorspar was first found in the Eagle Mountains in 1919, but no development was undertaken until 1942. Since then, mining has been done at Eagle Spring and at the various deposits in Spar Valley. Many other deposits have been found in the area. About 12,000 tons of fluorspar had been shipped previous to January 1949, most of which came from the North ore body in Spar Valley. A mill was built near the deposits In 1945. With the exception of the Eagle Spring and Tank Canyon deposits, both of minor importance, the fluorspar deposits as of 1950 are all controlled by Texas Fluorspar Mines, Inc., of Van Horn, Tex. Cretaceous sedimentary rocks, which crop out on the flanks of the mountains, are overlain by a thick series of Tertiary volcanics that make up much of the central part of the mountains. Low on the northeast side, the Cretaceous rocks are underlain by Permian (?) limestones and the pre-Cambrian Carrizo Mountain schist. The Cretaceous sedimentary rocks range from the Yucca formation of Early Cretaceous (lower Trinity) age through the Eagle Ford formation of Late Cretaceous age. The rocks on the northeast side of the mountains dip southwest, and those on the southwest and west sides dip east-northeast and northeast. The axis of the large syncline thus formed roughly parallels the axis of the range. The igneous rocks occur within the trough of the syncline. Both intrusive and extrusive rocks are present. The extrusive rocks have been separated into three divisions : the lower rhyolitic series, trachyte porphyry, and the upper rhyolitic series. Both rhyolitic series consist of flows, flow breccias, volcanic breccias, and tuffaceous sediments, all predominantly of rhyolitic composition, although tending toward andesite locally. These volcanics have been intruded by a small stock of syenite, named in this report the Eagle Peak syenite, which crops out in the central, higher parts of the mountains. Rhyolite sills have invaded the sedimentary rocks near the margin of the volcanics, and diabase and late rhyolite dikes have intruded both sedimentary and volcanic rocks. Faults are common in the area, and six series of faults have been recognized. Thrusting from the southwest occurred both before and after the igenous activity and the subsequent downwarping of the central part of the mountains. The early thrust faults were followed by high-angle normal and reverse faults that trend northeastward and cut the volcanics. Later normal and reverse faults trending northwestward, and faults with large horizontal displacements trending roughly eastward, also are present, in addition to very late faults trending in a general northwesterly direction. Fluorspar occurs in the Eagle Mountains both as replacement deposits in limestone and as fissure veins, chiefly in rhyolite. Chief among the fissure veins are those occurring along the Rhyolite fault, the Wind Canyon fault, the vein at Shaft 4, and the veins on Fox claims 9 and 10. The most important replacement deposits are in the Rocky Ridge area and in Spar Valley. • At the North ore body in Spar Valley, the fluorine-bearing solutions replaced a series of sandy limestones in the upper beds of the lower part of the Finlay formation. Structural conditions limited the extent of the replaceable beds and consequently of the fluorspar mineralization. The fluorine-bearing solutions represent a very late stage of the igneous activity of the area. The large east-trending faults with their wide zones of gouge and breccia, typified by the Rhyolite and Wind Canyon faults, acted as the major channels for the solutions in their upward course. From these faults, the solutions spread outward into other faults and fractures, chiefly those with a northeasterly trend, and into the adjoining limestones. The physical and chemical nature of the surrounding rock, as well as structural conditions affecting the presence of openings in the rock, were the major controlling factors governing the size, extent, and position of the fluorspar deposits.

Texas↗
USGS2001

Molluscan faunal distribution in Florida Bay, past and present: An integration of down-core and modern data

Statistical comparison of modern molluscan fauna to down-core molluscan assemblages in four cores elucidates changes in the Florida Bay ecosystem during the past 100 to 200 years. Fluctuations within molluscan faunal dominance and diversity patterns suggest a response to changing environmental conditions. Faunal dominance patterns indicate an increase in salinity in the northern transitional zone, and possibly the eastern portion of Florida Bay. Distinctive faunal shifts recorded at Russell Bank occur approximately between 1913 and 1933 and at Bob Allen mudbank between approximately 1900 and 1910. The period from approximately 1930 to 1980 within these cores shows rapid and dramatic fluctuations in species dominance and faunal richness. Beginning around 1980, the mussel Brachidontes exustus , which can tolerate diminished water quality and a wide range of salinities, increases in percent abundance in the upper portion of all four cores and becomes the dominant species at Russell Bank and Bob Allen Mudbank. While these fluctuations within assemblages are distinctive, they are not so profound that they represent a major shift in estuarine zonations within northern, eastern, and central Florida Bay during the past 100 to 200 years. The majority of the molluscan fauna that are present at the core sites today are generally present throughout the period of deposition. Fluctuations in the molluscan faunal record down-core primarily express changes in dominance and diversity within assemblages and do not reflect substantial changes in overall assemblages. It is these fluctuations in dominance and the appearance or disappearance of critical indicator species that are indicative of salinity changes. Understanding the dynamics of an ecosystem and the natural range of variation in the system over an extended period of time is a critical component of effective restoration. Analysis of the modern environment provides a means to interpret biological data preserved in cores, and to determine the physical and chemical variations in the environment indicated by the biota. Knowledge of the past provides the best insight to predicting the impact of future change on the environment.

Florida↗
USGS2003

Gravity and magnetic expression of the San Leandro gabbro with implications for the geometry and evolution of the Hayward Fault zone, northern California

The Hayward Fault, one of the most hazardous faults in northern California, trends north-northwest and extends for about 90 km along the eastern San Francisco Bay region. At numerous locations along its length, distinct and elongate gravity and magnetic anomalies correlate with mapped mafic and ultramafic rocks. The most prominent of these anomalies reflects the 16-km-long San Leandro gabbroic block. Inversion of magnetic and gravity data constrained with physical property measurements is used to define the subsurface extent of the San Leandro gabbro body and to speculate on its origin and relationship to the Hayward Fault Zone. Modeling indicates that the San Leandro gabbro body is about 3 km wide, dips about 75??-80?? northeast, and extends to a depth of at least 6 km. One of the most striking results of the modeling, which was performed independently of seismicity data, is that accurately relocated seismicity is concentrated along the western edge or stratigraphically lower bounding surface of the San Leandro gabbro. The western boundary of the San Leandro gabbro block is the base of an incomplete ophiolite sequence and represented at one time, a low-angle roof thrust related to the tectonic wedging of the Franciscan Complex. After repeated episodes of extension and attenuation, the roof thrust of this tectonic wedge was rotated to near vertical, and in places, the strike-slip Hayward Fault probably reactivated or preferentially followed this pre-existing feature. Because earthquakes concentrate near the edge of the San Leandro gabbro but tend to avoid its interior, we qualitatively explore mechanical models to explain how this massive igneous block may influence the distribution of stress. The microseismicity cluster along the western flank of the San Leandro gabbro leads us to suggest that this stressed volume may be the site of future moderate to large earthquakes. Improved understanding of the three-dimensional geometry and physical properties along the Hayward Fault will provide additional constraints on seismic hazard probability, earthquake modeling, and fault interactions that are applicable to other major strike-slip faults around the world.

California↗
USGS1999

Effects of wind turbines on upland nesting birds in Conservation Reserve Program grasslands

Grassland passerines were surveyed during summer 1995 on the Buffalo Ridge Wind Resource Area in southwestern Minnesota to determine the relative influence of wind turbines on overall densities of upland nesting birds in Conservation Reserve Program (CRP) grasslands. Birds were surveyed along 40 m fixed width transects that were placed along wind turbine strings within three CRP fields and in three CRP fields without turbines. Conservation Reserve Program grasslands without turbines and areas located 180 m from turbines supported higher densities (261.0-312.5 males/100 ha) of grassland birds than areas within 80 m of turbines (58.2-128.0 males/100 ha). Human disturbance, turbine noise, and physical movements of turbines during operation may have disturbed nesting birds. We recommend that wind turbines be placed within cropland habitats that support lower densities of grassland passerines than those found in CRP grasslands.

The Wilson Bulletin↗