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Results for “Journal of Geophysical Research: Earth Surface”

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USGS2012

Relation of the spectroscopic reflectance of olivine to mineral chemistry and some remote sensing implications

Using high-resolution visible and near-infrared diffuse spectral reflectance, we have systematically investigated apparent wavelength shifts as a function of mineral chemistry in the Fe/Mg olivine series from Fo 11 to Fo 91 . The study also shows that trace amounts of nickel can be spectrally detected in the olivine structure. We show that significant compositional information can only be extracted at relatively high resolution, because the overall spectral characteristics of the olivines change only subtly as a function of the Fe/Mg ratio. Significant spectral variation as a function of grain size is also demonstrated, adding a further complication to the interpretation of remotely sensed data from olivine-rich surfaces. This laboratory study is expected to aid in the interpretation of remotely sensed data from both terrestrial and extraterrestrial bodies. Terrestrial applications may include the recognition of ultramafic, ultrabasic, and basaltic terrains which in themselves may have mineral potential. Among extraterrestrial applications, the asteroids are obvious candidates for further examination if instrumentation can provide the necessary wavelength coverage, resolution, and signal-to-noise ratio so that spectra can be compared to those laboratory data discussed here. Some permutations of Fe-Mg-Ni relations in olivines are discussed as they apply to the interpretation of asteroid surfaces and other extraterrestrial bodies.

Journal of Geophysical Research Solid Earth↗

USGS2012

Crustal strain near the Big Bend of the San Andreas Fault: Analysis of the Los Padres-Tehachapi Trilateration Networks, California

In the region of the Los Padres-Tehachapi geodetic network, the San Andreas fault (SAF) changes its orientation by over 30° from N40°W, close to that predicted by plate motion for a transform boundary, to N73°W. The strain orientation near the SAF is consistent with right-lateral shear along the fault, with maximum shear rate of 0.38±0.01 μrad/yr at N63°W. In contrast, away from the SAF the strain orientations on both sides of the fault are consistent with the plate motion direction, with maximum shear rate of 0.19±0.01 μrad/yr at N44°W. The strain rate does not drop off rapidly away from the fault, and thus the area is fit by either a broad shear zone below the SAF or a single fault with a relatively deep locking depth. The fit to the line length data is poor for locking depth d less than 25 km. For d of 25 km a buried slip rate of 30 ± 6 mm/yr is estimated. We also estimated buried slip for models that included the Garlock and Big Pine faults, in addition to the SAF. Slip rates on other faults are poorly constrained by the Los Padres-Tehachapi network. The best fitting Garlock fault model had computed left-lateral slip rate of 11±2 mm/yr below 10 km. Buried left-lateral slip of 15±6 mm/yr on the Big Pine fault, within the Western Transverse Ranges, provides significant reduction in line length residuals; however, deformation there may be more complicated than a single vertical fault. A subhorizontal detachment on the southern side of the SAF cannot be well constrained by these data. We investigated the location of the SAF and found that a vertical fault below the surface trace fits the data much better than either a dipping fault or a fault zone located south of the surface trace.

Journal of Geophysical Research Solid Earth↗

USGS1998

A generalized law for brittle deformation of Westerly granite

A semiempirical constitutive law is presented for the brittle deformation of intact Westerly granite. The law can be extended to larger displacements, dominated by localized deformation, by including a displacement-weakening break-down region terminating in a frictional sliding regime often described by a rate- and state-dependent constitutive law. The intact deformation law, based on an Arrhenius type rate equation, relates inelastic strain rate to confining pressure P c , differential stress σ Δ , inelastic strain ε i and temperature T . The basic form of the law for deformation prior to fault nucleation is where σ o and ε o are normalization constants (dependent on confining pressure), a is rate sensitivity of stress, and α is a shape parameter. At room temperature, eight experimentally determined coefficients are needed to fully describe the stress-strain-strain rate response for Westerly granite from initial loading to failure. Temperature dependence requires apparent activation energy ( E * ∼ 90 kJ/mol) and one additional experimentally determined coefficient. The similarity between the prefailure constitutive law for intact rock and the rate- and state-dependent friction laws for frictional sliding on fracture surfaces suggests a close connection between these brittle phenomena.

Journal of Geophysical Research B: Solid Earth↗

USGS2003

Apparent break in earthquake scaling due to path and site effects on deep borehole recordings

We reexamine the scaling of stress drop and apparent stress, rigidity times the ratio between seismically radiated energy to seismic moment, with earthquake size for a set of microearthquakes recorded in a deep borehole in Long Valley, California. In the first set of calculations, we assume a constant Q and solve for the corner frequency and seismic moment. In the second set of calculations, we model the spectral ratio of nearby events to determine the same quantities. We find that the spectral ratio technique, which can account for path and site effects or nonconstant Q, yields higher stress drops, particularly for the smaller events in the data set. The measurements determined from spectral ratios indicate no departure from constant stress drop scaling down to the smallest events in our data set (Mw 0.8). Our results indicate that propagation effects can contaminate measurements of source parameters even in the relatively clean recording environment of a deep borehole, just as they do at the Earth's surface. The scaling of source properties of microearthquakes made from deep borehole recordings may need to be reevaluated.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Short-period strain (0.1–105 s): Near-source strain field for an earthquake (ML 3.2) near San Juan Bautista, California

Measurements of dilational earth strain in the frequency band 25–10 −5 Hz have been made on a deep borehole strainmeter installed near the San Andreas fault. These data are used to determine seismic radiation fields during nuclear explosions, teleseisms, local earthquakes, and ground noise during seismically quiet times. Strains of less than 10 −10 on these instruments can be clearly resolved at short periods (< 10 s) and are recorded with wide dynamic range digital recorders. This permits measurement of the static and dynamic strain variations in the near field of local earthquakes. Noise spectra for earth strain referenced to 1 (strain) 2 /Hz show that strain resolution decreases at about 10 dB per decade of frequency from −150 dB at 10 −4 Hz to −223 dB at 10 Hz. Exact expressions are derived to relate the volumetric strain and displacement field for a homogeneous P wave in a general viscoelastic solid as observed on colocated dilatometers and seismometers. A rare near-field recording of strain and seismic velocity was obtained on May 26, 1984, from an earthquake ( M L 3.2) at a hypocentral distance of 3.2 km near the San Andreas fault at San Juan Bautista, California. While the data indicate no precursory strain release at the 5 × 10 −11 strain level, a coseismic strain release of 1.86 nanostrain was observed. This change in strain is consistent with that calculated from a simple dislocation model of the event. Ground displacement spectra, determined from the downhole strain data and instrument-corrected surface seismic data, suggest that source parameters estimated from surface recordings may be contaminated by amplification effects in near-surface low-velocity materials.

California↗

USGS2012

Formation of left-lateral fractures within the Summit Ridge Shear Zone, 1989 Loma Prieta, California, Earthquake

The 1989 Loma Prieta, California, earthquake is characterized by the lack of major, throughgoing, coseismic, right-lateral faulting along strands of the San Andreas fault zone in the epicentral area. Instead, throughout the Summit Ridge area there are zones of tension cracks and left-lateral fracture zones oriented about N45°W, that is, roughly parallel to the San Andreas fault in this area. The left-lateral fractures zones are enigmatic because their left-lateral slip is opposite to the right-lateral sense of the relative motion between the Pacific and North American plates. We suggest that the enigmatic fractures can be understood if we assume that coseismic deformation was by right-lateral shear across a broad zone, about 0.5 km wide and 4 km long, beneath Summit Ridge. The shear zone has an orientation of about N70° W and is oblique to the San Andreas fault zone, which has a trend of N45° to 50°W, so that right-lateral shearing, along with some dilation, can account for the orientation of the fracture zones. Using an analog, kinematic model of the Summit Ridge shear zone and the observable geometric parameters (a shear zone about 0.5 km wide, an acute angle of 25° between the tension cracks and the shear-zone walls, left-lateral slip of 5 cm, and spacing of about 100 m in the tension cracks) we estimate that the amount of right-lateral shift across the Summit Ridge shear zone was on the order of 1.4 m. This is comparable to the amount of slip for coseismic faulting at depth, 1.9 to 2.4 m, as computed by several investigators. Thus contrary to most previous reports on the Loma Prieta earthquake, which assert that coseismic, right-lateral ground rupture was restricted to considerable (greater than 4 km) depths in the epicentral area, we find that nearly all the right-lateral offset is represented at the ground surface by the Summit Ridge shear zone.

Journal of Geophysical Research B: Solid Earth↗

USGS2000

Geochemical evidence for a magmatic CO 2 degassing event at Mammoth Mountain, California, September-December 1997

Recent time series soil CO 2 concentration data from monitoring stations in the vicinity of Mammoth Mountain, California, reveal strong evidence for a magmatic degassing event during the fall of 1997 lasting more than 2 months. Two sensors at Horseshoe Lake first recorded the episode on September 23, 1997, followed 10 days later by a sensor on the north flank of Mammoth Mountain. Direct degassing from shallow intruding magma seems an implausible cause of the degassing event, since the gas released at Horseshoe Lake continued to be cold and barren of other magmatic gases, except for He. We suggest that an increase in compressional strain on the area south of Mammoth Mountain driven by movement of major fault blocks in Long Valley caldera may have triggered an episode of increased degassing by squeezing additional accumulated CO 2 from a shallow gas reservoir to the surface along faults and other structures where it could be detected by the CO 2 monitoring network. Recharge of the gas reservoir by CO 2 emanating from the deep intrusions that probably triggered deep long-period earthquakes may also have contributed to the degassing event. The nature of CO 2 discharge at the soil-air interface is influenced by the porous character of High Sierra soils and by meteorological processes. Solar insolation is the primary source of energy for the Earth atmosphere and plays a significant role in most diurnal processes at the Earth surface. Data from this study suggest that external forcing due largely to local orographic winds influences the fine structure of the recorded CO 2 signals.

California↗

USGS1997

A test of the double-shearing model of flow for granular materials

The double-shearing model of flow attributes plastic deformation in granular materials to cooperative slip on conjugate Coulomb shears (surfaces upon which the Coulomb yield condition is satisfied). The strict formulation of the double-shearing model then requires that the slip lines in the material coincide with the Coulomb shears. Three different experiments that approximate simple shear deformation in granular media appear to be inconsistent with this strict formulation. For example, the orientation of the principal stress axes in a layer of sand driven in steady, simple shear was measured subject to the assumption that the Coulomb failure criterion was satisfied on some surfaces (orientation unspecified) within the sand layer. The orientation of the inferred principal compressive axis was then compared with the orientations predicted by the double-shearing model. The strict formulation of the model [ Spencer , 1982] predicts that the principal stress axes should rotate in a sense opposite to that inferred from the experiments. A less restrictive formulation of the double-shearing model by de Josselin de Jong [1971] does not completely specify the solution but does prescribe limits on the possible orientations of the principal stress axes. The orientations of the principal compression axis inferred from the experiments are probably within those limits. An elastoplastic formulation of the double-shearing model [ de Josselin de Jong , 1988] is reasonably consistent with the experiments, although quantitative agreement was not attained. Thus we conclude that the double-shearing model may be a viable law to describe deformation of granular materials, but the macroscopic slip surfaces will not in general coincide with the Coulomb shears.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

The Galapagos Spreading Centre at 86° W: A detailed geothermal field study

We report here measurements of the heat flow field of the Galapagos Spreading Center on crust of age less than 1.0 m.y. The 443 measurements in an area of about 570 km 2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near-axis conductive heat flux. The intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern, with circulation apparently continuing beyond the limits of our survey. The areal average of the conductive heat flux is 7.1 ± 0.8 HFU (295 ± 33 mW/m 2 ), about one-third the heat flux predicted by plate models. The remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis. The pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography. Sharp lateral gradients in the heat flow, greater than 10 HFU/km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys, may indicate a local concentration of the circulation by surface fault systems and/or variable sediment thickness.

Journal of Geophysical Research Solid Earth↗

USGS2012

Upper crustal structure of the Mount Hood, Oregon, region as revealed by time term analysis

Seismic refraction data with a dense areal distribution were collected to study the seismic structure of Mount Hood and the surrounding region. This area is typical of Cascade volcanoes and is geologically quite complex. The prime goals of this project were to search for velocity variations in the upper crustal rocks and to determine if the velocity of these rocks is anisotropic. A new system, including 100 remote recording units, was developed to facilitate the collection of data in this type of survey. The data collected in this study reveal a large variation in velocity and thickness of the uppermost crustal rocks that is probably typical of the High Cascade province. A regional structural pattern surrounding Mount Hood, where there is a marked thinning of low-velocity near-surface rocks, suggests that the present edifice of Mount Hood lies on top of a much larger structure, possibly the roof of a large batholith that was emplaced prior to the eruption of the volcanic rocks that form the modern mountain. A straightforward time term analysis of the data reveals this structure. When the time terms are used to correct the travel time observations for the variation in structure under each station, the remaining set of residuals indicates a variation of travel time with azimuth. While a systematic error in the time term reduction cannot be ruled out, this variation is probably the result of anisotropic velocity structure in the underlying refractor. The alignment of the direction of maximum velocity with the direction of maximum principal stress and the decrease in anisotropy with depth suggest that the anisotropic velocity structure is related to the opening and closing of fractures in the present stress field. The direction of maximum velocity is about N25°W, and the degree of anisotropy appears to vary with depth from about 3% near the surface to zero at depths below 8 km.

Journal of Geophysical Research Solid Earth↗

USGS2014

Time-averaged discharge rate of subaerial lava at Kīlauea Volcano, Hawai‘i, measured from TanDEM-X interferometry: Implications for magma supply and storage during 2011-2013

Differencing digital elevation models (DEMs) derived from TerraSAR add-on for Digital Elevation Measurements (TanDEM-X) synthetic aperture radar imagery provides a measurement of elevation change over time. On the East Rift Zone (EZR) of Kīlauea Volcano, Hawai‘i, the effusion of lava causes changes in topography. When these elevation changes are summed over the area of an active lava flow, it is possible to quantify the volume of lava emplaced at the surface during the time spanned by the TanDEM-X data—a parameter that can be difficult to measure across the entirety of an ~100 km 2 lava flow field using ground-based techniques or optical remote sensing data. Based on the differences between multiple TanDEM-X-derived DEMs collected days to weeks apart, the mean dense-rock equivalent time-averaged discharge rate of lava at Kīlauea between mid-2011 and mid-2013 was approximately 2 m 3 /s, which is about half the long-term average rate over the course of Kīlauea's 1983–present ERZ eruption. This result implies that there was an increase in the proportion of lava stored versus erupted, a decrease in the rate of magma supply to the volcano, or some combination of both during this time period. In addition to constraining the time-averaged discharge rate of lava and the rates of magma supply and storage, topographic change maps derived from space-based TanDEM-X data provide insights into the four-dimensional evolution of Kīlauea's ERZ lava flow field. TanDEM-X data are a valuable complement to other space-, air-, and ground-based observations of eruptive activity at Kīlauea and offer great promise at locations around the world for aiding with monitoring not just volcanic eruptions but any hazardous activity that results in surface change, including landslides, floods, earthquakes, and other natural and anthropogenic processes.

Hawai'i↗

USGS2009

Premonitory acoustic emissions and stick-slip in natural and smooth-faulted Westerly granite

A stick-slip event was induced in a cylindrical sample of Westerly granite containing a preexisting natural fault by loading at constant confining pressure of 150 MPa. Continuously recorded acoustic emission (AE) data and computer tomography (CT)-generated images of the fault plane were combined to provide a detailed examination of microscale processes operating on the fault. The dynamic stick-slip event, considered to be a laboratory analog of an earthquake, generated an ultrasonic signal that was recorded as a large-amplitude AE event. First arrivals of this event were inverted to determine the nucleation site of slip, which is associated with a geometric asperity on the fault surface. CT images and AE locations suggest that a variety of asperities existed in the sample because of the intersection of branch or splay faults with the main fault. This experiment is compared with a stick-slip experiment on a sample prepared with a smooth, artificial saw-cut fault surface. Nearly a thousand times more AE were observed for the natural fault, which has a higher friction coefficient (0.78 compared to 0.53) and larger shear stress drop (140 compared to 68 MPa). However at the measured resolution, the ultrasonic signal emitted during slip initiation does not vary significantly between the two experiments, suggesting a similar dynamic rupture process. We propose that the natural faulted sample under triaxial compression provides a good laboratory analogue for a field-scale fault system in terms of the presence of asperities, fault surface heterogeneity, and interaction of branching faults. ?? 2009.

Journal of Geophysical Research B: Solid Earth↗

USGS1996

Geyser periodicity and the response of geysers to deformation

Numerical simulations of multiphase fluid and heat transport through a porous medium define combinations of rock properties and boundary conditions which lead to geyser‐like periodic discharge. Within the rather narrow range of conditions that allow geyser‐like behavior, eruption frequency and discharge are highly sensitive to the intrinsic permeabilities of the geyser conduit and the surrounding rock matrix, to the relative permeability functions assumed, and to pressure gradients in the matrix. In theory, heat pipes (concomitant upward flow of steam and downward flow of liquid) can exist under similar conditions, but our simulations suggest that the periodic solution is more stable. Simulated time series of geyser discharge are chaotic, but integrated quantities such as eruption frequency and mass discharge per eruption are free of chaos. These results may explain the observed sensitivity of natural geysers to small strains such as those caused by remote earthquakes, if ground motion is sufficient to induce permeability changes. Changes in geyser behavior caused by minor preseismic deformation, periodic surface loading, and Earth tides are more difficult to explain in the context of our current model.

Journal of Geophysical Research B: Solid Earth↗

USGS1996

A migratory mantle plume on Venus: Implications for Earth?

A spatially fixed or at least internally rigid hotspot reference frame has been assumed for determining relative plate motions on Earth. Recent 1:5,000,000 scale mapping of Venus, a planet without terrestrial-style plate tectonics and ocean cover, reveals a systematic age and dimensional progression of corona-like arachnoids occurring in an uncinate chain. The nonrandom associations between arachnoids indicate they likely formed from a deep-seated mantle plume in a manner similar to terrestrial hotspot features. However, absence of expected convergent "plate" margin deformation suggests that the arachnoids are the surface expression of a migratory mantle plume beneath a stationary surface. If mantle plumes are not stationary on Venus, what if any are the implications for Earth?

Journal of Geophysical Research B: Solid Earth↗

USGS2022

Off-fault deformation in regions of complex fault geometries: the 2013, Mw7.7, Baluchistan rupture (Pakistan)

Observations of recent earthquake surface ruptures show that ground deformations include a localized component occurring on faults, and an off-fault component affecting the surrounding medium. This second component is also referred to as off-fault deformation (OFD). The localized component generally occurs on complex networks of faults that connect at depth onto a unique fault plane, whereas OFD consists of distributed fracturing and diffuse deformation of the bulk volume, and occurs over scales of hundreds of meters to kilometers around the faults. High-resolution optical image correlation presents a unique potential to characterize the complexity of the surface displacements, including on-fault displacements and OFDs. In this study, we used sub-pixel correlation of 0.5-m resolution optical images to measure the surface displacement field with a <20 cm accuracy for a 30-km long section of the 2013 M w 7.7 Baluchistan, Pakistan, rupture. Our results document significant variability in the fault displacements, associated with large proportions of OFD in regions of fault geometrical complexity. Conversely, in regions where the fault geometry is simple, surface deformation is entirely accommodated by the primary faults with 0% OFD. When combining the localized deformation on faults with the OFD, we show that the total surface displacement budget is constant along the strike of the rupture, despite strong variations observed in the rupture geometry. Based on this analysis, we propose an idealized scenario of earthquake surface deformation as a function of the rupture geometrical variations.

Journal of Geophysical Research, Solid Earth↗

USGS2012

Seismic evidence for active magmatic underplating beneath the Basin and Range Province, western United States

Near-vertical and wide-angle seismic reflection data provide evidence for the presence of a magma body at the base of the crust beneath Buena Vista Valley in northwestern Nevada. The seismic response of this hypothesized magma body is characterized by high-amplitude, near-vertical P wave reflections and a comparably strong P -to- S converted phase. The magma body, referred to here as the Buena Vista Magma Body, is probably a single sill with thickness no greater than 200 m and length no greater than 1.8 km. The melt fraction in the sill is undoubtedly greater than 20–30%, and probably exceeds 50%. Melt composition is unconstrained. Although the age of the Buena Vista Magma Body is difficult to determine precisely, it is probably no older than 500,000 years. This suggests that magmatism in the Basin and Range Province is an ongoing process, despite the relative paucity of volcanic rocks erupted at the surface during the last 6 m.y.

Nevada, Idaho↗

USGS2012

Stress measurements at depth in the vicinity of the San Andreas fault. Implications for the magnitude of shear stress at depth

Using the hydraulic fracturing technique, we have made a systematic series of in situ stress measurements in wells drilled near the San Andreas fault. In an attempt to provide constraints for the magnitude of shear stress on the San Andreas fault at depth we have measured both the variation of stress with distance from the fault in relatively shallow (∼230 m) wells and the variation of stress with depth in a ∼1-km-deep well located 4 km from the fault. The shallow wells are located along profiles roughly perpendicular to the fault in the western Mojave desert near Palmdale and in central California where the fault is creeping. In both areas the direction of maximum compression was found to be approximately 45° from the local trend of the San Andreas. The two stress profiles show very similar results: (1) shear stress (on planes parallel to the San Andreas) increases with distance from the fault, more markedly in the western Mojave, (2) the far-field shear stress at ∼200 m depth is ∼50 bars, and (3) the horizontal principal stresses as well as shear stress increase with depth more rapidly in the wells farthest from the fault. The ∼1-km-deep well, also located in the western Mojave desert, shows increases of both horizontal principal stresses and shear stress with depth. Shear stress increases from about 25 bars at 150–300 m to about 80 bars at 750–850 m. Although this rapid increase of shear stress with depth suggests that the mean shear stress on the fault at seismogenic depths exceeds several hundred bars, the principal stresses increase with depth in a steplike manner. As this may be a near-surface effect, extrapolation of the measurements to much greater depths may not be warranted.

Journal of Geophysical Research Solid Earth↗

USGS2012

Earthquake swarm in Long Valley caldera, California, January 1983: Evidence for dike inflation

The 1982–1983 deformation observed by trilateration and leveling surveys across the Long Valley caldera is apparently related to the 8.5‐km‐long by 8‐km‐deep vertical rupture surface defined by the January 1983 earthquake swarm that occurred in the south moat of the caldera. The observed deformation can be explained as follows. In late 1982, 0.03 km 3 of magma was injected into a dike that dips 30° northward from the bottom of the rupture surface. The downdip dimension of this dike is 8 km. The dike inflation accounts for the uplift observed across the caldera as well as some of the horizontal deformation. Inflation of the dike generated a tension of about 3 bars across the vertical plane that was to become the rupture surface of the January swarm. This reduced the frictional stress on the rupture plane and perhaps triggered the slip that caused the January swarm. Right‐lateral slip (0.22 m) on the uppermost 2 km of the rupture plane during and after the January swarm accounts for the additional horizontal deformation observed. The model can be improved marginally if strike slip is admitted over the entire rupture surface and 0.006 km 3 of magma is injected along that surface in the depth interval 3–8 km. The improvement in the model fit, however, is not sufficient to require shallow injection of magma. Thus we conclude that inflation of a dike at depth (8–12 km) dipping northward beneath the resurgent dome plus shallow right‐lateral slip on the rupture surface is a simple, but not unique, explanation of the observed deformation and seismicity.

California↗