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At least 163 records · Page 9Linked to original sources
USGS2012

Partition coefficients of Hf, Zr, and ree between phenocrysts and groundmasses

Partition coefficients of Hf, Zr, and REE between olivine, orthopyroxene, clinopyroxene, plagioclase, garnet, amphibole, ilmenite, phlogopite, and liquid are presented. Samples consist of megacrysts in kimberlite, phenocrysts in alkaline basalts, tholeiitic basalts and andesitic to dacitic rocks, and synthetic garnet and clinopyroxene in Hawaiian tholeiites. The Hf-Lu and Zr-Lu elemental fractionations are as large as the Lu-Sm or Lu-Nd fractionation. The Hf and Zr partition coefficients between mafic phenocrysts and liquids are smaller than the Lu partition coefficients, but are similar to the Nd or Sm partition coefficients. The Hf and Zr partition coefficients between ilmenite, phlogopite, and liquid are larger than the Lu partition coefficients for these minerals and their corresponding liquids. The Hf-Zr elemental fractionation does not occur except for extreme fractionation involving Zr-minerals and extremely low fo 2 . These data have an important bearing on chronological and petrogenetic tracer studies involving the Lu-Hf isotopic system.

Journal of Geophysical Research Solid Earth↗
USGS2012

Forecast model for moderate earthquakes near Parkfield, California

Earthquake instability models have possible application to earthquake forecasting because the models simulate both preseismic and coseismic changes of fault slip and ground deformation. In the forecast procedure proposed here, repeated measurements of preseismic fault slip and ground deformation constrain the values of model parameters. The early part of the model simulation corresponds to the available field data, and the subsequent part constitutes an estimate of future faulting and ground deformation. In particular, the time, location, and size of unstable faulting are estimates of the pending earthquake parameters. The forecast accuracy depends on the model realism and parameter resolution. The forecast procedure is applied to fault creep and trilateration data measured near Parkfield, California, where at least five magnitude 5.5 to 6 earthquakes have occurred regularly since 1881, the last in 1966. The quasi-static model consists of a flat vertical plane embedded in an elastic half space. Spacially variable fault slip of strike-slip sense is driven by an increasing regional shear stress but is impeded by a relatively strong patch of brittle, strain-softening fault. The field data are consistent with these approximate values of patch parameters: radius of 3 km, patch center 5 km deep and 8 km southeast of the 1966 epicenter, and maximum brittle strength of 26 bars. Fluctuations in the available field data prevent estimating the earthquake time with any more precision than use of the 21±8 year recurrence interval. However, the model may later give a more precise estimate of the earthquake time if the fault slip rate near the inferred patch increases before the earthquake, as predicted by the model.

Journal of Geophysical Research Solid Earth↗
USGS2012

Excitation of a buried magmatic pipe: A seismic source model for volcanic tremor

Recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so-called long-period or low-frequency events. Accordingly, we can view the long-period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor-generating system. We present a seismic model in which the source of tremor is the acoustic resonance of a fluid-filled volcanic pipe triggered by excess gas pressure. The model consists of three elements, namely, a triggering mechanism, a resonator, and a radiator. For simplicity, we assume a hemispherical trigger, cylindrial resonator, and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk-shaped radiator shutting off its bottom. Considering the simple case of a source buried in a homogeneous half space, we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances. The results demonstrate that the displacement attributed to the pipe dominates the near-field motion, while that due to the disk is representative of the intermediate and far fields. The trigger itself has a smaller contribution, mainly limited to the field in the proximity of the source. The characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well-developed harmonic wave train dominated by Rayleigh waves at larger distances. No clear shear arrival can be detected in the synthetic seismograms. The displacement spectrum reflects the organ-pipe modes of the conduit, and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid. In the case of the cylindrical magma column considered, the radiation loss is proportional to the square of the pipe radius, while the loss related to viscous damping is inversely proportional to the same factor, indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir. The relative importance of the pipe and disk elements, likewise, is a function of the conduit cross section. This suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near- and far-field observations.

Journal of Geophysical Research Solid Earth↗
USGS2012

Recent movement on the Garlock Fault as suggested by water level fluctuations in a well in Fremont Valley, California

Water levels have been continuously recorded since March 1978 in a well in Fremont Valley, where several strands of the adjacent Garlock fault zone have exhibited both left-lateral displacement and components of normal displacement. Differences in water levels indicate that a fault segment lies between the observation well and a nearby irrigation well. During the 4-year recording period, six sharp fluctuations, or “spikes,” were noted. These fluctuations, occurring over 2- to 4-day periods, have amplitudes of 15–30 cm. They appear to be the result of creep events on a nearby fault. Two types of creep events are plausible: (1) normal slip on an en echelon trace of the Garlock fault less than 300 m south of the well, with the north side up relative to Fremont Valley or (2) left-lateral slip on the same fault. Because of the nature of the fluctuations we favor the latter interpretation. Dislocation models utilizing exponential, arc tangent, and skewed cosine functions were used to analyze the water level fluctuations, associated pressure distribution, and fault displacements. The results suggest that creep on the fault ranges from several millimeters to a centimeter for individual events. Estimates of cumulative creep for the period 1978–1982 range from 20 to 50 mm, depending on the particular model employed.

Journal of Geophysical Research Solid Earth↗
USGS2012

Global map of eolian features on Mars

Ten basic categories of eolian features on Mars were identified from a survey of Mariner 9 and Viking orbiter images. The ten features mapped are (1) light streaks (including frost streaks), (2) dark streaks, (3) sand sheets or splotches, (4) barchari dunes, (5) transverse dunes, (6) crescentic dunes, (7) anomalous dunes, (8) yardangs, (9) wind grooves, and (10) deflation pits. The features were mapped in groups, not as individual landforms, and recorded according to their geographic positions and orientations on maps of 1:12.5 million or 1:25 million scale. In the north polar region, light and dark streaks indicate winds from the west and northwest. Frost streaks show northeast and northwest winds. Barchan dunes show southwest and west winds. Transverse dunes show east and west winds. Local erosional features show winds from the northwest or southeast and northeast or southwest. In the middle and low northern latitudes, streaks show northeast winds; yardangs and deflation pits show mostly easterly and northeasterly winds. In the low southern latitudes, light streaks and dunes record northwest and northeast winds; dark streaks record southeast winds. In high southern latitudes, most streaks and dunes record southeast and east winds. In the south polar region, light and frost streaks record southwest and northwest winds, whereas dark streaks and transverse dunes show southeast winds. The patterns recorded for ephemeral features conform to global conditions of strong southern spring and summer (northern fall and winter) wind circulation. Erosional features in bedrock indicate long-term and perhaps ancient wind trends, whereas depositional features may record relatively more recent winds. Wind directions indicated by yardangs, pits, and other erosional features seldom correspond to those shown by streaks. These erosional features indicating winds of different directions than those that formed the streaks may have been carved when the effective regional winds on Mars were different due to relative hemisphere insolation differences, which are related to obliquity variations. Alternatively, many yardangs and pits may be largely controlled by structure and carved along surface structural patterns or features that channeled both ancient and present-day winds. Deflation pits in the mantled terrain may contain the best record of ancient wind trends. These pits seem to have stratigraphically related orientations. Different stratigraphic units composing the mantle contain wind erosion pits that possibly record large-scale wind direction changes because of long-term changes in the obliquity of the Mars.

Journal of Geophysical Research Solid Earth↗
USGS2012

Scarp degraded by linear diffusion: Inverse solution for age

Under the assumption that landforms unaffected by drainage channels are degraded according to the linear diffusion equation, a procedure is developed to invert a scarp profile to find its “diffusion age.” Diffusion age, having dimension [length] 2 , is the product of diffusivity times chronological age. The second moment of scarp slope grows linearly with age. This fact, together with an assumption about initial scarp shape, allows the inverse determination of diffusion age. Age found assuming a vertical initial scarp is termed “apparent age”; any nonvertical initial scarp profile has a nonzero initial apparent age. True diffusion age differs from apparent age by a fraction of scarp offset squared. The inverse procedure applied to synthetic data yields the following rules of thumb. Evidence of initial scarp shape has been lost when apparent age reaches twice its initial value. If a scarp is formed by two events, the inversion gives their offset-weighted-mean age with an error that is a fraction of offset squared. A scarp that appears to have been formed by one event may have been formed by two with an interval between them as large as apparent age. After scarps of two fault traces have diffused to appear as one, the error in inferred age may be as large as half the apparent age. Variation of apparent age along strike would indicate multiple fault traces. The simplicity of scarp profile measurement and this inversion makes profile analysis attractive. If linearity of the flow law, time for a free face to be reduced to the angle of repose, and variation of diffusivity with climate and material could be established, profile analysis would become a reliable dating technique.

Journal of Geophysical Research Solid Earth↗
USGS2012

The relocation of microearthquakes in the northern Mississippi Embayment

Three-component seismograms, recorded by a small array of digital instruments in the northern Mississippi embayment, consistently show a high-amplitude phase on the vertical component that arrives approximately 0.8 s before the shear wave. On the basis of its timing and apparent velocity, this phase is identified as an S-P conversion from the boundary between the unconsolidated Cenozoic sediments and the underlying Paleozoic rocks. Synthetic models of ground motion in the Mississippi embayment indicate that vertical displacement amplitudes are higher for S-P conversions than for transmitted shear waves at all angles of incidence. The models and their agreement with observations of the three-component seismograms suggest that true shear waves cannot be reliably identified from vertical component seismograms recorded in this area. The travel times of converted phases are used, together with crustal velocity models derived from a recent seismic refraction survey, to relocate approximately 500 microearthquakes recorded by the Central Mississippi Valley regional seismic network. Since the network data are recorded by vertical component seismographs, we assume that the S phases, observed at the stations sited on unconsolidated sediments, are S-P conversions. This assumption significantly reduces the average rms residual and provides well-constrained hypocentral locations for 350 earthquakes. The relocated microearthquakes cluster tightly along previously identified epicentral trends. Focal depths range from 0.5 to 22 km, but events deeper than 14 km are rare, and events shallower than 3 km are confined, almost exclusively, to the area between Ridgely, Tennessee, and New Madrid, Missouri. Focal mechanisms are generally consistent with the results of previous studies. The axial seismicity trend, which extends from Caruthersville, Missouri, to Marked Tree, Arkansas, is characterized by right-lateral strike-slip motion on a northeast trending, nearly vertical fault plane. The events between Ridgely, Tennessee, and New Madrid, Missouri, commonly show strike-slip focal mechanisms, with minor components of both normal and reverse dip slip.

Northern Mississippi Embayment↗
USGS2012

Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview

Geological, chronological, and structural studies of the Long Valley-Mono/Inyo Craters area document a long history of related volcanic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km 3 prior to its climatic caldera-forming eruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8–10 km deep with smaller cupolas as shallow as 4–5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km 3 of lava, the record over the past 0.5 m.y. suggests that eruptions of 1 km 3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1–0.2 km 3 by additional magma since mid-1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8–6.2 earthquakes that occurred in the region beginning in 1978 had non-double-couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low-viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500-year intervals over the past 2000–3000 years, whereas the Long Valley magma chamber has erupted at about 200,000-year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity.

Journal of Geophysical Research Solid Earth↗
USGS2012

Links between magma-tectonic rate balances, plutonism, and volcanism

With Smith's (1979) model of ash flow magmatism as the point of departure, quantitative regimes of crustal magmatic evolution are outlined. The average extrusive regime of ash flow magmatism operates at a rate of about 0.001 km 3 /yr; progressively larger episodes of increasing repose times characterize the evolution of caldera-forming eruptions. The intrusive regime operates at an inferred average rate of about 0.01 km 3 /yr expressed in terms of the volumetric influx of mafic magma from the mantle, based on the record of silicic rates of extrusion and a 10:1 intrusive to extrusive ratio derived from evidence of zoned silicic systems. This rate is almost identical with the mean cumulative rate for the Hawaiian-Emperor system of basaltic volcanism, which is determined to be 0.015 km 3 /yr. Fluctuations about a mean rate of 10 −2 km 3 /yr lead to a threefold classification of magmatic regimes in terms of source power, depending on whether they are persistently greater than (HHER regime), about equal to (MHER regime), or lower than (LHER regime) the mean Hawaiian-Emperor rate. Ash flow magmatism correlates with the MHER regime, averaged over a sufficiently long time. Thus the conditions of Hawaiian-Emperor magmatism are identified with a characteristic power of magma transfer rates that optimize mechanisms for creation of high-level chambers of derivative and fractionated silicic magma (terminology of Smith (1979)) within sufficiently localized regions of high residence times. These conditions represent a balance point, and a bifurcation, in dynamic paths of magmatic evolution leading to widely divergent types of plutonic-volcanic associations. Categorical examples of the principal types of igneous path are discussed relative to continental basalt, silicic ash flow, batholithic granite, and intrusive porphyry systems. The Henry Mountains, Utah, are thought to represent an example of an intrusive porphyry generated in the LHER regime of a mafic source system lacking sufficient power for volcanic expression or significant mineralization. Long Valley, California (and possibly other systems along the eastern front of the Sierra Nevada), is an example operating at the mean rate with transient excursions to higher and lower rates. In regions of rapid extension and/or volcanic propagation (whether linear or areal) these regimes give rise to basaltic eruptions ranging from isolated cinder cones to flood basalts.

Journal of Geophysical Research Solid Earth↗
USGS2012

An ocean bottom seismometer study of shallow seismicity near the Mid- America Trench offshore Guatemala

Five ocean bottom seismometers recorded seismicity near the Mid-America Trench offshore Guatemala for 27 days in 1979. The array was emplaced in the lower slope region, just above the topographic trench, in the area investigated during Deep Sea Drilling Project legs 67 and 84. Approximately 170 events were recorded by three or more seismometers, and almost half were located with statistical hypocentral errors of less than 10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone northwest of the array. In terms of depth, most events were located within the subducting Cocos plate rather than in the overlying plate or at the plate-plate boundary. Their apparent concentration in the lower crust and upper mantle may suggest that the upper crust does not have the strength to support earthquake-generating stresses. The data permit construction of a magnitude-duration scale, calibrated with m b magnitudes for events located by the World-Wide Standard Seismograph Network (WWSSN) and recorded by our array and by the network recording foreshocks and aftershocks of the 1979 Petatlan earthquake. Most magnitudes ranged between 3.0 and 4.0 m b , and the threshold magnitude of locatable events was about 2.8 m b . Two distinct composite focal mechanisms were determined. One appears to indicate high-angle reverse faulting in the subducting plate, in a plane parallel to trench axis strike. The other, constructed for some earthquakes in the zone northwest of the array, seems to show normal faulting along possible fault planes oriented quasi-perpendicular to the trench axis. The normal faulting is consistent with the segmentation of the Cocos plate that has been proposed from land evidence. Such segmentation might be evidenced offshore by normal faulting along planes subperpendicular to trench strike. Alternatively, the seismicity zone and associated normal faulting mechanism may be the subsurface expression of the tectonics responsible for the San Jose Canyon, a prominent submarine canyon located farther upslope. Finally, projection of our seismicity sample and of well-located WWSSN events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array, and the deeper Wadati-Benioff zone seismicity located by the WWSSN. We tentatively ascribe this gap to inadequate sampling, but we suggest that it requires further investigation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Upper mantle structure from teleseismic P wave arrivals in Washington and northern Oregon

Teleseismic P wave travel time residuals are used to detect lateral velocity heterogeneities in the upper mantle beneath Washington and northern Oregon. The results of an inversion for three-dimensional velocity variations resolves an east dipping high-velocity zone that we interpret as the subducting Juan de Fuca plate. The plate is characterized by 3–8% higher velocities than those in the surrounding upper mantle. Inversion of the travel time data and ray trace modeling indicate that the plate extends to a depth of 200–300 km. The plate dips at a moderate angle of 45° to the east-northeast beneath the central Washington Cascade Range north of Mount Rainier, with 5% faster velocities than the surrounding upper mantle. Beneath the North Cascade Range of Washington, the plate strikes to the northwest and has 6–8% faster velocities than the upper mantle to the west. South of 47°N, beneath the Cascade Range in southern Washington and northern Oregon, the plate dips steeply to the east and has 3–4% faster velocities than the surrounding upper mantle. Based on changes in the geometry and velocity structure of the subducted Juan de Fuca plate east of about 123°W, we propose that the subducted slab is segmented into three sections beneath Washington and northern Oregon.

Journal of Geophysical Research Solid Earth↗
USGS2012

Usefulness of weak bands in midinfrared remote sensing of particulate planetary surfaces

Midinfrared (2.5–25 μm) reflectance spectra of minerals are often used to predict emittance qualitatively. These spectra display weak overtone and combination tone bands, which may be as diagnostic of composition as the strong fundamental molecular vibration bands usually considered for remote sensing applications, but which have been widely ignored. However, unlike the strong bands, the contrast of weak bands relative to the continuum in the midinfrared usually does not decrease with decreasing particle size, but typically increases. To illustrate this behavior, transmittance and reflectance spectra of calcite and quartz are presented for the wavelength range from 4000 to 400 cm −1 (2.5–25 μm). It is the purpose of this paper to point out that these weak bands are potentially useful for compositional remote sensing of particulate planetary surfaces; this will require completion of supporting laboratory studies to document the occurrence of weak bands.

Journal of Geophysical Research Solid Earth↗
USGS2012

Age of tilted reefs, Hawaii

Submerged carbonate reefs are preserved as a series of submarine terraces between Molokai and Hawaii along a 200-km span of the southeastern Hawaiian Ridge. Limestones from two of the terraces have been sampled from submersibles and dated radiometrically at 13 and 120 ka. Recognition that the terraces are tilted permits assignment of about a dozen terraces from 150 to 1300 m depth to eight general reef platforms. These reefs were drowned by the combined effects of island subsidence and sea level rise at the end of successive glacial stages from 13 to 647 ka. The platforms are tilted 5 m/km southeast toward the locus of volcanic loading centered on the island of Hawaii. The end of the active period of volcanic shield building is represented on each nonactive volcano by a submerged change in slope. Dating of this slope change by reference to the dated reefs provides independent evidence that shield building ended at about the same time that the volcano changed from eruption of tholeiitic to alkalic basalt. The variation in depth of the dated tilted reefs along the ridge indicates that the end of subsidence follows the end of shield building by about one half million years. The present northwest margin of major subsidence probably crosses southeast Maui. During the last half million years the rate of southeast migration of the beginning of both subsidence and of tholeiitic volcanism does not appear to have increased. However, the rate of southeast migration of the end of both subsidence and of tholeiitic volcanism has apparently increased. This shortening of the life history of the younger volcanoes implies an increase in eruption rates.

Journal of Geophysical Research Solid Earth↗
USGS2012

Global petrologic variations on the moon: A ternary-diagram approach.

A ternary-diagram approach for determination of global petrologic variations on the lunar surface is presented that incorporates valuable improvements in our previous method of using geochemical variation diagrams. This new approach uses a ternary diagram that is subdivided into equally spaced segments along each of its three sides and has a triangular area in its center. The segments are assigned distinct colors that cover the range of the visible spectrum. The apexes are assigned the three primary colors, and the center triangle, which represents equal proportions of the three apexes, is assigned the color gray. The apexes are assigned the average Fe and (Th/Ti) c (ratio normalized to chondrites) compositions of KREEP/Mg-suite rocks, mare basalts, and ferroan anorthosites. The composition of each picture element (pixel) within these Apollo orbital gamma ray databases is used to determine its position within the ternary diagram. The color corresponding to this ternary position is then placed on a classification map at that pixel's position within the orbital databases. Error databases produced for the Fe and (Th/Ti) c data were used to exclude pixels having high errors. The resultant classification map shows spatial transitions among petrologic units, allows direct determination of the relative proportions of each end-member composition in a pixel, and increases the geologic interpretability of these data over that of the individual elemental databases. The classification units correspond remarkably well to observed geologic units, when we consider the spatial resolution (100 km) of the gamma ray spectrometer. Our results are as follows: (1) The highlands contain large areas of relatively pure ferroan anorthosite. (2) The average composition of the upper lunar crust is represented by an “anorthositic gabbro” composition, which supports the “magma-ocean” hypothesis for lunar crustal genesis. (3) KREEP/Mg-suite rocks are a minor fraction of the upper lunar crust. (4) Within the farside highlands, areas of KREEP/Mg-suite rocks coincide mostly with areas of crustal thinning, which are probably areas of KREEP basalt extrusions or localized excavations of Mg-suite rocks or KREEP-rich rocks. (5) Portions of the east limb and farside highlands have considerable amounts of a mafic, chondritic Th/Ti component (like mare basalt) whose occurrences coincide with mapped concentrations of light plains that display dark-halo craters; the presence of this component supports the hypothesis that mare volcanism had occurred within this highland region before the end of final heavy bombardment.

Journal of Geophysical Research Solid Earth↗
USGS2012

Character and distribution of borehole breakouts and their relationship to in situ stresses in deep Columbia River basalts

The character and distribution of borehole breakouts in deeply buried basalts at the Hanford Site in south central Washington State are examined in light of stress indicator data and hydraulic-fracturing stress data by means of acoustic televiewer and acoustic waveform logging systems. A series of boreholes penetrating the Grande Ronde Basalt of the Columbia River Basalt Group were logged to examine the extent of breakouts at depths near 1000 m. Breakouts occur discontinuously throughout the interiors of most flows. In some boreholes the distribution of borehole wall breakouts closely correlates with the incidence of core disking. Differences in the distribution of breakouts and disking are attributed to differences in failure mechanisms. A thin interval of breakout-free basalt occurs near the upper and lower limits of flow interiors, with many intervals of breakouts terminating at the intersection of oblique fractures with the borehole. Hydraulic-fracturing stress measurement results obtained from four deep boreholes indicate anisotropic horizontal principal stresses, with maximum principal stress along an approximate north trending axis, consistent with the east trending orientation of breakouts. Acoustic waveform logs indicate that there is no measurable difference between the seismic properties of breakout-free flow tops and flow interiors. The highly coherent waveforms obtained in almost all flow interiors indicate that damage to the borehole wall associated with breakout formation remains confined to the thin annulus of stress concentration.

Journal of Geophysical Research Solid Earth↗
USGS2012

Reflections from midcrustal rocks within the Mesozoic subduction complex near the eastern Aleutian Trench

Seismic reflection data collected in 1973 by Western Geophysical Company show that highly reflective rocks make up the midcrust of the convergent margin adjacent to the eastern Aleutian Trench. These rocks form an arch that strikes obliquely across the strongly expressed northeast-southwest structural grain of exposed Mesozoic rocks. In an earlier report we proposed that the deep events mark the location of either the presently subducting plate or the top of underplated rocks. However, the short radius of curvature of this arch precludes the possibility that the deep events indicate the igneous oceanic crust. Instead, the deep reflections could be from underplated rocks that have been arched by the imbrication or underplating of strata below the reflective rocks. High-amplitude water layer and interbed multiples prevent precise connection of shallow and deep geology. For example, the Border Ranges fault, a suture between major tectonostratigraphic terranes, is not revealed on any of our seismic sections. We speculate, however, that one band of reflections that rises toward but does not reach the surface is from the Eagle River thrust fault, which separates Late Cretaceous melange from deformed turbidite sequences of the same age.

Journal of Geophysical Research Solid Earth↗
USGS2012

Analysis of time series of glacier speed: Columbia Glacier, Alaska

During the summers of 1984 and 1985, laser measurements were made of the distance from a reference location to markers on the surface of the lower reach of Columbia Glacier, Alaska. The data have numerous gaps, mostly because of inclement weather. The laser measurements were corrected for variations in atmospheric temperature and pressure and for the trajectory of the marker. The marker speed was calculated by using a combination of cubic splines and digital filters to arrive at a data set with 1-hour time intervals. The speed varies from 7 to 15 m/d and has three noteworthy components: (1) a low-frequency perturbation in speed with a time scale of days related to increased precipitation, (2) semidiurnal and diurnal variations related to sea tides, and (3) diurnal variations related to glacier surface melt. The low frequency and tidal period variations are separated by low-pass filtering and subtracting the filtered data from the original data. The variations from melt runoff and tides have the same frequency range and are examined using harmonic analysis and multiple regression with the various forcing functions. The high-frequency portion of the ice speed signal is dominated by tidal influences and to a lesser extent by meltwater influences. The low-frequency portion of the signal is dominated by the effects of precipitation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Analysis of two-color geodimeter measurements of deformation within the Long Valley caldera: June 1983 to October 1985

Line length changes from several baselines in a trilateration network within the Long Valley caldera clearly define a decrease in strain rate from June 1983 through October 1985. The data consist of more than 1600 length measurements on 23 baselines using a two-color geodimeter, which has a precision of 0.2 ppm of the line length. Initial measurements made during the summer of 1983 show extension rates were as high as 6 ppm/yr on some baselines. The rates subsequently decreased steadily, to less than 1 ppm/yr during the summer of 1985. Ih contrast, the strain rates on a few other baselines were less than 0.2 ppm/yr averaged over the 2.3-year interval. A model is constructed using these observations as well as yearly observations of a trilateration network within and near the caldera. The model contains two points of inflation located at 5 and 10 km beneath the resurgent dome of the caldera plus dextral slip on a fault plane within the south moat within an elastic half-space. To mimic the observed time dependence in the two-color geodimeter data, we conclude that the rates of both slip and inflation have steadily decreased over the survey interval. Because the two-color geodimeter network is closer to all three sources than the single-color geodimeter network, these data are important for defining the factor of 2–3 rate decrease of slip and inflation. Comparing our estimates of slip and inflation with similar estimates for the year prior to July 1983, we conclude that the rate decrease is probably a result of postseismic and postinflation relaxation following the earthquake swarm in the south moat in January 1983.

Journal of Geophysical Research Solid Earth↗