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Results for “Journal of Interpretation Research”

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USGS1977

Clay-mineral variability in the suspended sediments of the San Francisco Bay system, California

Semiquantitative determinations of the clay-mineral composition have been made on nearly synoptic samples of surface suspended sediments collected seasonally throughout the San Francisco Bay system. The relative amounts of chlorite + kaolinite are generally highest in the northern reach of the system, whereas illite is dominant in the southern reach. The proportion of montmorillonite is low throughout the bay. Time-series and replicate samples collected at individual stations show that the difference in clay-mineral content between reaches is real and reflects a change in the source of clay-mineral particles within the bay. The Sacramento-San Joaquin river system supplies the northern reach, whereas most clay-mineral particles come from resuspension by waves and tidal currents in the southern reach. Analyses of bottom sediments and the spatial variability in the northern reach suggest that the relationship between the abundance and sources of clay minerals may, in turn, be a function of particle size. This study demonstrates the utility of suspended clay minerals in the interpretation of sediment-dispersal patterns in estuaries.

California↗

USGS2015

Changes in seismic velocity during the first 14 months of the 2004–2008 eruption of Mount St. Helens, Washington

Mount St. Helens began erupting in late 2004 following an 18 year quiescence. Swarms of repeating earthquakes accompanied the extrusion of a mostly solid dacite dome over the next 4 years. In some cases the waveforms from these earthquakes evolved slowly, likely reflecting changes in the properties of the volcano that affect seismic wave propagation. We use coda-wave interferometry to quantify small changes in seismic velocity structure (usually <1%) between two similar earthquakes and employed waveforms from several hundred families of repeating earthquakes together to create a continuous function of velocity change observed at permanent stations operated within 20 km of the volcano. The high rate of earthquakes allowed tracking of velocity changes on an hourly time scale. Changes in velocity were largest near the newly extruding dome and likely related to shallow deformation as magma first worked its way to the surface. We found strong correlation between velocity changes and the inverse of real-time seismic amplitude measurements during the first 3 weeks of activity, suggesting that fluctuations of pressure in the shallow subsurface may have driven both seismicity and velocity changes. Velocity changes during the remainder of the eruption likely result from a complex interplay of multiple effects and are not well explained by any single factor alone, highlighting the need for complementary geophysical data when interpreting velocity changes.

Journal of Geophysical Research B: Solid Earth↗

USGS2019

Lithospheric signature of late Cenozoic extension in electrical resistivity structure of the Rio Grande rift, New Mexico, USA

We present electrical resistivity models of the crust and upper mantle from two‐dimensional (2‐D) inversion of magnetotelluric (MT) data collected in the Rio Grande rift, New Mexico, USA. Previous geophysical studies of the lithosphere beneath the rift identified a low‐velocity zone several hundred kilometers wide, suggesting that the upper mantle is characterized by a very broad zone of modified lithosphere. In contrast, the surface expression of the rift (e.g., high‐angle normal faults and synrift sedimentary units) is confined to a narrow region a few tens of kilometers wide about the rift axis. MT data are uniquely suited to probing the depths of the lithosphere that fill the gap between surface geology and body wave seismic tomography, namely the middle to lower crust and uppermost mantle. We model the electrical resistivity structure of the lithosphere along two east‐west trending profiles straddling the rift axis at the latitudes of 36.2 and 32.0°N. We present results from both isotropic and anisotropic 2‐D inversions of MT data along these profiles, with a strong preference for the latter in our interpretation. A key feature of the anisotropic resistivity modeling is a broad (~200‐km wide) zone of enhanced conductivity (<20 Ωm) in the middle to lower crust imaged beneath both profiles. We attribute this lower crustal conductor to the accumulation of free saline fluids and partial melt, a direct result of magmatic activity along the rift. High‐conductivity anomalies in the midcrust and upper mantle are interpreted as fault zone alteration and partial melt, respectively.

New Mexico↗

USGS2003

Visible/near-infrared spectra of experimentally shocked plagioclase feldspars

High shock pressures cause structural changes in plagioclase feldspars such as mechanical fracturing and disaggregation of the crystal lattice at submicron scales, the formation of diaplectic glass (maskelynite), and genuine melting. Past studies of visible/ near-infrared spectra of shocked feldspars demonstrated few spectral variations with pressure except for a decrease in the depth of the absorption feature near 1250-1300 nm and an overall decrease in reflectance. New visible/near-infrared spectra (400-2500 nm) of experimentally shocked (17-56 GPa) albite- and anorthite-rich rock powders demonstrate similar trends, including the loss of minor hydrated mineral bands near 1410, 1930, 2250, and 2350 nm. However, the most interesting new observations are increases in reflectance at intermediate pressures, followed by subsequent decreases in reflectance at higher pressures. The amount of internal scattering and overall sample reflectance is controlled by the relative proportions of micro-fractures, submicron grains, diaplectic glass, and melts formed during shock metamorphism. We interpret the observed reflectance increases at intermediate pressures to result from progressively larger proportions of submicron feldspar grains and diaplectic glass. The ensuing decreases in reflectance occur after diaplectic glass formation is complete and the proportion of genuine melt inclusions increases. The pressure regimes over which these reflectance variations occur differ between albite and anorthite, consistent with thermal infrared spectra of these samples and previous studies of shocked feldspars. These types of spectral variations associated with different peak shock pressures should be considered during interpretation and modeling of visible/near-infrared remotely sensed spectra of planetary and asteroidal surfaces.

Journal of Geophysical Research E: Planets↗

USGS1974

Evidence for the presence of a heart in Paleozoic ostracodes inconclusive

The presence of a heart in some Paleozoic ostracodes has been inferred by analogy with some living myodocopids that have this organ, and an ordinal classification of the leperditiids has been proposed because of this inference. A review of the present knowledge of the internal shell structure of living myodocopids indicates that there is no basis for interpreting certain markings preserved on fossil molds and steinkerns as indicating the presence of a heart.

Journal of Research of the U.S. Geological Survey↗

USGS1973

Formation of joints in bedrock by moving glacial ice

The orientation of joints in igneous and sedimentary rocks was measured at 21 localities in California, Maine, and New York to investigate the hypothesis that glaciation may open joints in bedrock. A summary of strikes of all joint sets shows the following pattern relative to the direction of glacial advance: two sets, thought to be extension joints, flank the direction of advance and are separated from it by about 10°; two sets, thought to be shear joints, flank the direction of advance by about 40°; and two sets, thought to be release joints, are about 70° to 80° from the direction of advance, or nearly perpendicular to the extension joints. An average of five joint sets, including three or four in this pattern, was found at each locality. The joints are believed to have been opened, in response to stress applied by the moving ice, along preexisting zones of weakness (potential regional joints) in the rock. Some new joints may have formed where no such zone was present near the position required for fracture under the applied stress. Additional investigation of the hypothesis is desirable because of its possible bearing on glacial erosion by quarrying, on interpretation of joint patterns in glaciated regions, and on the nature of aquifers in glaciated bedrock.

California, Maine, New York↗

USGS1975

Early Ordovician gastropod opercula and epicontinental seas

Collections made for some years from Lower Ordovician rocks by many geologists have established that the heavy calcareous operculum of the gastropod Ceratopea is almost never found associated with a shell and that commonly this operculum is secondarily silicified. Ceratopea may have lived in a stressed habitat and may have been essentially the only shelled invertebrate present. Presumably these animals grazed on algal mats and mounds growing in areas of water slightly more saline than open marine water and were killed by an influx of hypersaline water. Subsequent decay of soft parts would permit the operculum to rot off and would also lighten the shell so that it could float away. Pene-contemporaneous silicification of calcareous opercula on the Ordovician sea bottom is postulated. Some Middle Ordovician opercula of the Middle to Late Ordovician genus Maclurites serve to test this speculation. These opercula also occur in geologic settings which may be interpreted in terms of kill of animal, wafting away of shell, and secondary silicification of operculum.

Alabama, Arkansas, Colorado, Georgia, Maryland, Ne↗

USGS2001

Resolution analysis of finite fault source inversion using one- and three-dimensional Green's functions 2. Combining seismic and geodetic data

Using numerical tests for a prescribed heterogeneous earthquake slip distribution, we examine the importance of accurate Green's functions (GF) for finite fault source inversions which rely on coseismic GPS displacements and leveling line uplift alone and in combination with near-source strong ground motions. The static displacements, while sensitive to the three-dimensional (3-D) structure, are less so than seismic waveforms and thus are an important contribution, particularly when used in conjunction with waveform inversions. For numerical tests of an earthquake source and data distribution modeled after the 1994 Northridge earthquake, a joint geodetic and seismic inversion allows for reasonable recovery of the heterogeneous slip distribution on the fault. In contrast, inaccurate 3-D GFs or multiple 1-D GFs allow only partial recovery of the slip distribution given strong motion data alone. Likewise, using just the GPS and leveling line data requires significant smoothing for inversion stability, and hence, only a blurred vision of the prescribed slip is recovered. Although the half-space approximation for computing the surface static deformation field is no longer justifiable based on the high level of accuracy for current GPS data acquisition and the computed differences between 3-D and half-space surface displacements, a layered 1-D approximation to 3-D Earth structure provides adequate representation of the surface displacement field. However, even with the half-space approximation, geodetic data can provide additional slip resolution in the joint seismic and geodetic inversion provided a priori fault location and geometry are correct. Nevertheless, the sensitivity of the static displacements to the Earth structure begs caution for interpretation of surface displacements, particularly those recorded at monuments located in or near basin environments.

Journal of Geophysical Research B: Solid Earth↗

USGS2006

Aftershock decay, productivity, and stress rates in Hawaii: Indicators of temperature and stress from magma sources

We examined dozens of aftershock sequences in Hawaii in terms of Gutenberg-Richter and modified Omori law parameters. We studied p, the rate of aftershock decay; A p , the aftershock productivity, defined as the observed divided by the expected number of aftershocks; and c, the time delay when aftershock rates begin to fall. We found that for earthquakes shallower than 20 km, p values >1.2 are near active magma centers. We associate this high decay rate with higher temperatures and faster stress relaxation near magma reservoirs. Deep earthquakes near Kilauea's inferred magma transport path show a range of p values, suggesting the absence of a large, deep magma reservoir. Aftershock productivity is >4.0 for flank earthquakes known to be triggered by intrusions but is normal (0.25 to 4.0) for isolated main shocks. We infer that continuing, post-main shock stress from the intrusion adds to the main shock's stress step and causes higher A p . High A p in other zones suggests less obvious intrusions and pulsing magma pressure near Kilauea's feeding conduit. We calculate stress rates and stress rate changes from pre-main shock and aftershock rates. Stress rate increased after many intrusions but decreased after large M7–8 earthquakes. Stress rates are highest in the seismically active volcano flanks and lowest in areas far from volcanic centers. We found sequences triggered by intrusions tend to have high A p , high (>0.10 day) c values, a stress rate increase, and sometimes a peak in aftershock rate hours after the main shock. We interpret these values as indicating continuing intrusive stress after the main shock.

Hawai'i↗

USGS1996

Frictional constraints on crustal faulting

We consider how variations in fault frictional properties affect the phenomenology of earthquake faulting. In particular, we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes. We model these variations using a rate- and state-dependent friction law, where we differentiate velocity-weakening behavior into two fields: the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening. Similarly, we differentiate velocity-strengthening behavior into two fields: the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening. The strong seismic field comprises the seismic slip concentrations, or asperities. The two “intermediate” fields, weak seismic and compliant, have frictional velocity dependences that are close to velocity neutral: these fields modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the weak seismic and compliant regions slip aseismically, while the strong seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically. This “mixed” frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance. The model also provides a mechanism for rupture arrest: dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas, producing broad areas of accelerated afterslip. Aftershocks occur on both the weak seismic and compliant areas around a fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the seismicity and the coseismic slip for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes. The interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip: these regions are interpreted as either weak seismic or compliant, depending on whether or not they manifest interevent seismicity.

Journal of Geophysical Research B: Solid Earth↗

USGS1987

Regional heat flow variations in the northern Michigan and Lake Superior region determined using the silica heat flow estimator

Conventional heat flow data are sparse for northern Michigan. The groundwater silica heat flow estimator expands the database sufficiently to allow regional variations in heat flow to be examined. Heat flow shows a pattern of alternating highs and lows trending ESE across the Upper Peninsula and Lake Superior. The informal names given to these features, their characteristic heat flow and inferred causes are listed: {A table is presented} The results suggest that, for the study area, regional variations in heat flow cannot be interpreted solely in terms of regional variations of the heat generation rate of basement rocks. ?? 1987.

Journal of Volcanology and Geothermal Research↗

USGS1978

Iron-titanium oxide minerals and associated alteration phases in some uranium-bearing sandstones

Detrital iron-titanium (Fe-Ti) oxide minerals of the ulvospinel-magnetite (titanomagnetite) and ilmenite-hematite (titanohematite) solid solution series are common in uranium-bearing sandstones. Alteration of Fe-Ti oxide minerals in oxidizing environments formed secondary products (primarily hematite) that are distinct from those produced under reducing conditions (iron disulfide minerals). Oxidation of sulfidized Fe-Ti oxide minerals, by the processes that formed uranium rolls, produced ferric oxide minerals (limonite) having textures that mimic those of the iron disulfides. Titanomagnetite and titanohematite have been severely depleted in the ore-bearing zones of some uranium deposits. The alteration of detrital Fe-Ti oxide minerals near uranium ore deposits may produce characteristic signatures in the magnetization of the sandstone. Knowledge of the distribution and magnetic properties of these minerals can aid in interpreting data from total magnetic-field and magnetic-susceptibility surveys of uranium deposits.

Colorado, New Mexico, Texas, Wyoming↗

USGS2015

Postearthquake relaxation evidence for laterally variable viscoelastic structure and water content in the Southern California mantle

I reexamine the lower crust and mantle relaxation following two large events in the Mojave Desert: the 1992 M 7.3 Landers and 1999 M 7.1 Hector Mine, California, earthquakes. Time series from continuous GPS sites out to 300 km from the ruptures are used to constrain models of postseismic relaxation. Crustal motions in the Mojave Desert region are elevated above background for several years following each event. To account for broadscale relaxation of the lower crust and mantle, the Burgers body model is employed, involving transient and steady state viscosities. Joint afterslip/postseismic relaxation modeling of the GPS time series up to one decade following the Hector Mine earthquake reveals a significant rheological contrast between a northwest trending “southwest domain” (that envelopes the San Andreas fault system and western Mojave Desert) and an adjacent “northeast domain” (that envelopes the Landers and Hector Mine rupture areas in the central Mojave Desert). The steady state viscosity of the northeast domain mantle asthenosphere is inferred to be ∼4 times greater than that of the southwest domain. This pattern is counter to that expected for regional heat flow, which is higher in the northeast domain, but it is explicable by means of a nonlinear rheology that includes dependence on both strain rate and water concentration. I infer that the southwest domain mantle has a relatively low steady state viscosity because of its high strain rate and water content. The relatively low mantle water content of the northeast domain is interpreted to result from the continual extraction of water through igneous and volcanic activity over the past ∼20 Myr.

California↗

USGS2002

High-resolution image of Calaveras Fault seismicity

By measuring relative earthquake arrival times using waveform cross correlation and locating earthquakes using the double difference technique, we are able to reduce hypocentral errors by 1 to 2 orders of magnitude over routine locations for nearly 8000 events along a 35-km section of the Calaveras Fault. This represents ∼92% of all seismicity since 1984 and includes the rupture zone of the M 6.2 1984 Morgan Hill, California, earthquake. The relocated seismicity forms highly organized structures that were previously obscured by location errors. There are abundant repeating earthquake sequences as well as linear clusters of earthquakes. Large voids in seismicity appear with dimensions of kilometers that have been aseismic over the 30-year time interval, suggesting that these portions of the fault are either locked or creeping. The area of greatest slip in the Morgan Hill main shock coincides with the most prominent of these voids, suggesting that this part of the fault may be locked between large earthquakes. We find that the Calaveras Fault at depth is extremely thin, with an average upper bound on fault zone width of 75 m. Given the location error, however, this width is not resolvably different from zero. The relocations reveal active secondary faults, which we use to solve for the stress field in the immediate vicinity of the Calaveras Fault. We find that the maximum compressive stress is at a high angle, only 13° from the fault normal, supporting previous interpretations that this fault is weak.

California↗

USGS2012

Along-shelf current variability on the Catalan inner-shelf (NW Mediterranean)

We examine the circulation over the inner shelf of the Catalan Sea using observations of currents obtained from three ADCPs within the inner-shelf (24 and 50 m depth) during March-April 2011. The along-shelf current fluctuations during that period are mainly controlled by the local wind stress on short time scales and by remote pressure gradients on synoptic time scales. Different forcing mechanisms are involved in the along-shelf momentum balance. During storm conditions, wind stress, sea level gradients and the non-linear terms dominate the balance. During weak wind conditions, the momentum balance is controlled by the pressure gradient, while during periods of moderate wind in the presence of considerable stratification, the balance is established between the Coriolis and wind stress terms. Vertical variations of velocity are affected by the strong observed density gradient. The increased vertical shear is accompanied by the development of stratified conditions due to local heating when the wind is not able to counteract (and destroy) stratification. The occasional influence of the Besòs river plume is observed in time scales of hours to days in a limited area in front of Barcelona. The area affected by the plume depends on the vertical extend of the fresher layer, the fast river discharge peak, and the relaxation of cross-shore velocities after northeast storm events. This contribution provides a first interpretation of the inner-shelf dynamics in the Catalan Sea.

Journal of Geophysical Research↗

USGS1993

Fluvial response to late Quaternary climatic fluctuations, central Kobuk Valley, northwestern Alaska

Much of northwestern Alaska remained unglaciated during the Pleistocene and thus offers a favorable setting for examining long-term records of high-latitude geological and biological change. Epiguruk, a large cut bank 3.5 km long and up to 36 m high on the Kobuk River south of the Brooks Range in eastern Beringia, exposes complex sedimentary successions representing cycles of upper Quaternary alluviation and eolian sedimentation, downcutting, and soil formation. A rich record of plants and mammals is also preserved in the section. Deposits of fluvial channels and flood plains, eolian dunes, sand sheets, loess, and ponds, as well as organic soils (Histosols) are represented. Parallel-bedded fine sand and coarse silt couplets that commonly contain root structures, ripple cross-lamination, and silt drapes are flood-plain sediments apparently deposited at the interface of fluvial and eolian environments. Multiple fluvial-to-eolian depositional sequences were caused by influx of eolian sediment to the river from intermittently active dune fields south of the Kobuk River. Alluviation in the Kobuk Valley was coeval with glaciation in the Brooks Range, whereas downcutting occurred during interstadials when dune stabilization limited sediment supply. The depositional model developed at Epiguruk may be useful in interpreting some of the widespread subhorizontally stratified late-glacial deposits of Europe and North America.

Alaska↗

USGS1976

Age and origin of the Darrough Felsite, southern Toiyabe Range, Nevada

The name Darrough Felsite was originally assigned to a body of fine-grained quartzofeldspathic igneous rock probably greater than 3 km thick that is exposed continuously over 100 km 2 in the southern Toiyabe Range of central Nevada. The Darrough was supposed to consist mostly of intrusive rocks of probable Permian age. Reexamination of parts of the Darrough Felsite indicates that it is Tertiary and has no genetic affiliation to pre-Tertiary sedimentary and volcanic rocks of the Toiyabe Range. The Darrough constitutes an enormously thick succession of crystal-lithic tuff, tuff breccia, crystal tuff, and volcanogenic sedimentary breccia and sandstone. The tuffaceous rocks are strongly compacted where ash particles are the dominant constituent but are much less compacted where lithic fragments and crystals constitute a rigid framework. Much of the Darrough Felsite may have undergone extensive siliciflcation, probably as a postcompaction event. Contact relations of the Darrough Felsite indicate that it is younger than pre-Tertiary rocks of the southern Toiyabe Range and the Ophir pluton, here dated at 53.9 m.y. (K-Ar, biotite). Three K-Ar ages of minerals from tuff of the Darrough are 26.1, 22.7, and 22.3 m.y., suggesting a middle Tertiary age. A large dike intruding the Darrough. previously assigned a Jurassic age, gives a K-Ar-biotite age of 29.4 m.y. We tentatively interpret the Darrough Felsite to represent an accumulation of ash and breccia flows in a volcanotectonic depression.

Nevada↗

USGS1996

Linkages between chemical contaminants and tumors benthic Great Lakes fish

Cutaneous papilloma have been observed at high prevalences in populations of brown bullheads ( Ameiurus nebulosus ) and white suckers ( Catostomus commersoni ) distributed throughout the Great Lakes, with no clear-cut delineation between prevalences in fish populations from industrialized sites in comparison to more pristine sites. However, bullhead papilloma prevalences greater than 20% were never observed at pristine reference sites, while high prevalences of sucker papilloma (> 20%) were only observed in populations from the lower Great Lakes. It is probable that there is a viral etiology for papilloma affecting bullheads and suckers, although chemical contaminants may influence the development of these neoplasms through some mechanism. Hepatic neoplasms of both cholangiocytic and hepatocytic origin have been observed at relatively high prevalences in Great Lakes populations of brown bullheads and white suckers. Prevalences of total hepatic neoplasms of > 9% in bullheads and > 3% in suckers were usually only observed in populations from industrialized areas of the Great Lakes. However, relatively high prevalences of hepatic neoplasms were also observed in suckers and bullheads from pristine areas where there is a preponderance of older fish in the population. In the case of both bullheads and suckers, high prevalences of hepatic neoplasms appear to be associated with severe biliary disease (e.g. cholangiofibrosis, cholangiohepatitis). It is highly probable that the etiology of hepatic cancers in bullheads and suckers from the Great Lakes is associated with exposure to chemical contaminants, and in some areas, specifically with exposure to polynuclear aromatic hydrocarbons. However, other chemical contaminants may indirectly influence hepatic tumor rates by increasing the severity of biliary disease in fish from contaminated areas. Overall, cutaneous papilloma prevalences above approximately 25% and hepatic neoplasm prevalences above approximately 5% in these species should be interpreted as an indicator of environmental degradation in the area where fish were surveyed.

Journal of Great Lakes Research↗