bluuebunny/crossref_metadata_2025 huggingface dataset:
Laboratory measurements of ultrasonic wave velocities in rocks from the Campi Flegrei volcanic system and their relation to other field data
Authors: Zamora M. , Sartoris G. , Chelini W.
Published: February 2004

The Campi Flegrei caldera, located near the city of Naples, Italy, has shown signs of unrest, characterized by large ground deformation, seismic activity, and changes in the hydrothermal system since 1970. Any attempt to model the processes giving rise to these phenomena requires some knowledge of the subsurface structure and of the physical properties of the rocks at depth. In this study we report the results of laboratory measurements of ultrasonic compressional and shear wave velocities in rock samples cored in 4 different geothermal wells reaching a maximun depth of 3 km. The wells are located in the San Vito (SV) and Mofete (MF) areas, at about 3 km north and 4 km west, respectively, from the center of the caldera. We selected 10 core samples, 7 from the SV wells and 3 from the MF wells, the attention being focused on the most important rock formations underneath Campi Flegrei and on the lower sections of the wells where data had been lacking. The measurements involved the transmission of ultrasonic waves along three orthogonal directions through dry and water‐saturated rocks at room conditions. The samples are only moderately anisotropic (13% for one SV sample and <8% for all the other samples). Wave velocities generally increase with the depth of the sample. The range of velocities is 3–5.4 km s−1 for P waves and 1.5–2.9 km s−1 for S waves. For the SV samples, water saturation generally implies slightly larger (<10%) Vp values and smaller (<20%) Vs values as compared with “dry” velocities. For the MF samples, water saturation implies both higher VP and VS velocities (up to 35% and 18%, respectively). These results may be explained by the different type of porosity of the SV samples (vesicular) and the MF samples (fissurai). The ultrasonic measurements on water‐saturated samples are slightly higher than the available in situ VP sonic measurements (25% for one SV sample and <15% for all the other samples). These differences can be explained by velocity dispersion between ultrasonic and sonic frequencies and by the different pressure‐temperature conditions existing at depth.
Rheology and rupture of homogeneous silicate liquids at magmatic temperatures
Authors: Bottinga Y.
Published: February 2004

Homogeneous silicate liquids have a Newtonian rheology under conditions of modest stress, but under relatively high stress or otherwise imposed high rates of deformation their viscosity may become non‐Newtonian before the onset of rupture. This behavior has been discussed semi‐empirically as well as in terms of the Maxwell model liquid. Here it is shown that the configurational entropy theory for the occurrence of the glass transition can also explain this non‐Newtonian rheology of silicate liquids ranging in composition from silica‐rich to silica‐poor. The pressure and temperature dependence of the transition from Newtonian to non‐Newtonian flow is discussed qualitatively. Geological applications are indicated. Fragmentation of silicate liquids takes place when the glass transition is crossed and the liquid has become solid like. Experimental evidence indicates that the rupture strength is only slightly larger than the stress needed to cause the onset of non‐Newtonian rheology.
Numerical simulation of fracture set formation: A fracture mechanics model consistent with experimental observations
Authors: Renshaw Carl E. , Pollard David D.
Published: February 2004

A physically based model for the evolution of a single set of planar, parallel fractures subject to a constant remote stress is presented. The model simulates the mechanical interaction between fractures using a recently developed approximation technique for stress analysis in elastic solids with many fractures. A comparison between experimental and numerical results shows that the model can accurately simulate the development of experimentally generated fracture sets. Once the flaw geometry is specified, only one parameter controls the geometric evolution of the fracture set. This parameter, the velocity exponent, relates fracture propagation velocity to stress concentration at the fracture tip. Monte Carlo sensitivity analyses suggest that this parameter also controls the extent to which fracture growth is concentrated within zones or clusters. Similar analyses suggest that the extent of fracture clustering is less sensitive to the flaw density.
Active tectonics and human survival strategies
Authors: King Geoffrey , Bailey Geoffrey , Sturdy Derek
Published: February 2004

Tectonic movements continuously remould the surface of Earth in response to plate motion. Yet such deformation is rarely taken into account when assessing landscape change and its impact on human land use, except perhaps as an occasional hazard to human life or a temporary disruption in the longer term patterns of human history. However, active tectonics also create and sustain landscapes that can be beneficial to human survival, forming a complex topography of potentially fertile sedimentary basins enclosed by mountain barriers that can facilitate the control and exploitation of food resources, especially animal prey. We discuss the tectonic history of northwest Greece and show how the Paleolithic sites of the region are located to take advantage of tectonically created features at both a local and a regional scale. We suggest that the association of significant concentrations of early Paleolithic sites with tectonically active regions is not coincidental and that on the longer time spans of human biological evolution, active tectonics has been an important selective agent contributing to the development of the human species as an intelligent predator.
Mesozoic granitoid magmatism in southeast British Columbia: Implications for the origin of granitoid belts in the North American Cordillera
Authors: Brandon Alan D. , Smith Alan D.
Published: February 2004

The trace element and isotopic compositions of Mesozoic granitoids in southeast British Columbia constrain the timing and mechanisms for granitoid production in the western North American Cordillera. The granitoids were emplaced in Middle Jurassic and mid‐Cretaceous pulses into Paleozoic‐Mesozoic accreted terranes, Paleozoic metasediments, and Proterozoic metasediments of the Purcell‐Windermere Group. Middle Jurassic compositions range from quartz diorite to tonalite to granite, with high Ba and low Rb and Nb abundances similar to those in granitoids found within Mesozoic arc complexes of western North America. Isotopic compositions range for εSrT from −27 to +33 and εNdT from 0 to −7 with initial 206Pb/204Pb, 208Pb/204Pb, and 207Pb/204Pb lying in an array between upper and lower crust reservoirs. These compositions can be produced by addition of 5–25% of Precambrian crust to basalt. The Mid‐Jurassic granitoids formed as part of an arc magmatic complex during subduction of ocean crust and as accretion of outboard terranes occurred west of the North American continent. In contrast, mid‐Cretaceous batholiths are composed of weakly peraluminous hornblende‐biotite granitoids and strongly peraluminous two‐mica granites. These granitoids have lower εNdT, and εSrT, 206Pb/204Pb, 208Pb/204Pb, and 207Pb/204Pb ratios extending to more radiogenic compositions than the Middle Jurassic granitoids and have complete overlap with isotopic compositions of southeast British Columbia Precambrian crust. Their trace element and isotopic compositions are consistent with a model in which crustal anatexis of Precambrian basement gneisses produced the hornblende‐biotite granitoids and anatexis of Proterozoic metapelites produced the two‐mica granites as a melt zone migrated up through the crust. Collision and accretion of exotic terranes resulted in a maximum in crustal thickening in the Late Jurassic to Early Cretaceous (∼150–130 Ma) in the Canadian Cordillera, 20–40 m.y. prior to mid‐Cretaceous (∼110 Ma) granitoid magmatism. The timing of these events are consistent with anatexis being a response to crustal thickening.
Measurement of diurnal and semidiurnal rotational variations and tidal parameters of Earth
Authors: Herring Thomas A. , Dong Danan
Published: February 2004

We discuss the determination of diurnal and semidiurnal variations in the rotation rate and the direction of rotation axis of Earth from the analysis of 8 years of very long baseline interferometry (VLBI) data. This analysis clearly show that these variations are largely periodic and tidally driven; that is, the periods of the variations correspond to the periods of the largest lunar and solar tides. For rotation rate variations, expressed in terms of changes in universal time (UT), the tidal lines with the largest observed signals are O1 (amplitude 23.5 microseconds in time (μs), period 25.82 solar hours); K1 (18.9 μs, 23.93 hours); M2 (17.9 μs, 12.54 hours); and S2 (8.6 μs, 12.00 hours). For variations in the direction of the rotation axis (polar motion), significant signals exist in the retrograde semidiurnal band at the M2 and S2 tides (amplitudes 265 and 119 microarc seconds (μas), respectively); the prograde diurnal band at the O1, K1, and P1 tides (amplitudes 199,152, and 60 μas, respectively); and the prograde semidiurnal band at the M2 and K2 tides (amplitudes 58 and 39 μas, respectively). Variations in the retrograde diurnal band are represented by corrections to the nutations of Earth's body axes, and our estimates here are consistent with previous estimates except that a previously noted discrepancy in the 13.66‐day nutation (corresponding to the O1 tide) is largely removed in this new analysis. We estimate that the standard deviations of these estimates are 1.0 μs for the UTl variations and 14–16 μas for the polar motion terms. These uncertainties correspond to surface displacements of ∼0.5 mm. From the analysis of atmospheric angular momentum data we conclude that variations in UTl excited by the atmosphere with subdaily periods are small (∼1 μs). We find that the average radial tidal displacements of the VLBI sites in the diurnal band are largely consistent with known deficiencies in current tidal models, i.e., deficiencies of up to 0.9 mm in the treatment of the free core nutation resonance. In the semidiurnal band, our analysis yields estimates of the second‐degree harmonic radial Love number h2 at the M2 tide of 0.604+i0.005±0.002. The most likely explanation for the rotational variations are the effects of ocean tides, but there may also be some contributions from atmospheric tides, the effects of triaxiality of Earth, and the equatorial second‐degree‐harmonic components of the core‐mantle boundary.
Erosion and tectonics at the margins of continental plateaus
Authors: Masek Jeffrey G. , Isacks Bryan L. , Gubbels Timothy L. , Fielding Eric J.
Published: February 2004

The topography across the eastern margin of the central Andean plateau north of 18°S (Beni region) bears a strong resemblance to the topography of the southern margin of the Tibetan plateau (Nepal Himalaya), with both regions featuring a steep frontal slope and high peaks at the plateau edge. In contrast, the topography of the eastern margin of the central Andean plateau south of 18°S (Pilcomayo region) tapers toward the foreland more gently and has no line of high peaks at the margin. Both the Himalayan and the Beni regions have been the sites for large amounts of denudation, and in both regions, geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the Neogene. We hypothesize that the steep frontal slope and high peaks of the Beni region and Himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid Pilcomayo region reflects a tectonic landform only slightly modified by erosion. We propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope, eroding back the plateau while enhancing or maintaining the steep long‐wavelength slope. A numerical model coupling orographic precipitation, erosion, and tectonic uplift demonstrates the plausibility of this hypothesis. The erosional efflux in both the Beni and Nepal Himalaya have been considerable, and simple mass balance calculations for the Himalaya suggest that during the Neogene, the erosional mass efflux has generally outpaced the tectonic mass influx. This contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period, and/or increased late Cenozoic erosion rates.
Reconstruction of topography and related depositional systems during active thrusting
Authors: Burbank Douglas W. , Vergés Jaume
Published: February 2004

Reliable reconstruction of former topography in deformed regions is commonly difficult, due to degradation of former erosional and depositional surfaces. In contrast to most modem landscapes, however, ancient localities can sometimes provide clearer insights on subsurface geometries of deposition, deformation, and erosion and on their variations through time. In some exceptional circumstances, ancient depositional sequences are preserved in direct juxtaposition with the structures that controlled their geometrical and sedimentological character. We describe here the evolving topography and depositional responses caused by the late Eocene growth of a detachment fold and related thrusts in the southern Pyrenees. Topography within these deforming systems can be reconstructed on the basis of (1) relief associated with paleovalleys, (2) geometric relationships of syntectonic strata with adjacent structures, and (3) relief of hanging walls above depositional or erosional surfaces of the same age. Onlapping, offlapping, and overlapping stratigraphic relationships are interpreted in the context of the relative rate of sediment accumulation versus the rate of uplift of the crest of the fold. In the study area, two contrasting fluvial systems provided sediment to the deforming area: a large longitudinal system, flowing parallel to the fold axes and carrying detritus from the distant hinterland, and a smaller transverse system that carried locally derived clasts. During fold growth, syntectonic sedimentary beds (growth strata) were progressively rotated in the forelimb of the fold. Proximal unconformities developed in the forelimb growth strata, when accumulation rates were low. Topographic relief on the backlimb of the growing fold caused transverse paleovalleys (>150 m deep) to be incised at high angles to the fold axis. A switch from incision to infilling of the paleovalleys appears to be controlled by relative rates of subsidence, sediment supply and accumulation, and uplift. During an interval of rapid accumulation and low rates of subsidence and uplift, the effects of rising local base levels propagated up the transverse valleys, where they initiated backfilling of the paleovalleys. As deformation began on an adjacent, more hinterlandward thrust, waning growth of the detachment fold permitted depositional overlap of its crest, as sedimentation shifted toward the hinterland. Subsequently, as the new footwall was folded, longitudinal rivers filled the space formerly occupied by transverse rivers, and a new detachment fold grew in the very shallow (<25 m) subsurface. Although similar examples are scarce in the geological record, the synthesis from this Pyrenean locale illustrates how stratal geometries, reconstructed river patterns, precise stratigraphic ages, and preserved erosional surfaces can be combined to reconstruct evolving topography during active folding and faulting in terrestrial environments.
Chemical and isotopic gas emissions at Campi Flegrei: Evidence for an aborted period of unrest
Authors: Tedesco D.
Published: February 2004

The hydrothermal system beneath Campi Flegrei, an explosive caldera in southern Italy, is quantified by analyzing gas samples collected since 1983, during and after 2 years of intense earthquake swarms and dramatic uplift of the caldera center. These gas samples were obtained from both land and submarine fumaroles, as well as a few samples from water wells. At the Bocca Grande fumarole in Solfatara Crater, near the center of Campi Flegrei, large variations were measured in the amount of water vapor, hydrogen sulfide, nitrogen and hydrogen gases, methane, and hydrogen chloride. Some variations could be controlled partly by a change in heat flux from a deep source to a surficial hydrothermal system. Possible evidence of a change in heat flux comes from subsurface temperatures, calculated from a water‐carbon monoxide reaction. The calculated temperatures changed most in 1989–1990. Concurrent changes were measured in concentrations of hydrogen sulfide, nitrogen, and hydrogen. No change was noted in helium or carbon isotopes, which could have indicated a change in emission of magmatic gases. I suggest the changes in gas chemistry since 1985 could be evidence of an aborted period of unrest, which was characterized as a change in gas emissions but did not progress far enough to produce uplift or earthquake swarms. As pointed out by other researchers, the ratio of helium isotopes, which implies the presence of magma, is uniform across an area that includes Campi Flegrei. The distribution of carbon isotopes, which is nonuniform, correlates with limestone outcrops and the presence of shallow thermal sources.
Effects of the core cooling on the internal dynamics and thermal evolution of terrestrial planets
Authors: Arkani‐Hamed Jafar
Published: February 2004

The effects of thermal coupling of the core and mantle on the thermal evolution and mantle dynamics of terrestrial planets are investigated using three‐dimensional thermal convection models in a spherical shell. Three pairs of models are presented to study mantle models of different viscosities and internal heating. In each pair, one model adopts a constant temperature at the core‐mantle boundary, and the other allows the core to thermally couple to the mantle and cool. In the first and second pairs, the mantle is heated only from below, whereas in the third pair, internal heating is due to radioactive elements. The mantle viscosity of the first and third pairs is 1021 Pa s, while the mantle of the second pair has a viscosity of 1022 Pa s. The thermal coupling of the core and mantle has strong effects on the thermal boundary layer near the core‐mantle boundary but only minor effects on the thermal boundary layer near the surface. It cools the core, substantially weakens the thermal boundary layer near the core‐mantle boundary, and thus reduces the vigor of convection in the mantle. The models with cooling cores result in a substantially colder and relatively less dynamic mantle than the models with a fixed temperature at the core‐mantle boundary which overestimates temperatures in the mantle.
Measurements of thermal infrared spectral reflectance of frost, snow, and ice
Authors: Salisbury John W. , D'Aria Dana M. , Wald Andrew
Published: February 2004

Because much of Earth's surface is covered by frost, snow, and ice, the spectral emissivities of these materials are a significant input to radiation balance calculations in global atmospheric circulation and climate change models. Until now, however, spectral emissivities of frost and snow have been calculated from the optical constants of ice. We have measured directional hemispherical reflectance spectra of frost, snow, and ice from which emissivites can be predicted using Kirchhoff s law (e = 1‐R). These measured spectra show that contrary to conclusions about the emissivity of snow drawn from previously calculated spectra, snow emissivity departs significantly from blackbody behavior in the 8–14 μm region of the spectrum; snow emissivity decreases with both increasing particle size and increasing density due to packing or grain welding; while snow emissivity increases due to the presence of meltwater.
The seismic velocity structure of the Newfoundland Appalachian orogen
Authors: Hughes Stephen , Hall Jeremy , Luetgert James H.
Published: February 2004

The deep structure of the Newfoundland Appalachian orogen is investigated by analyses of three intersecting seismic refraction/wide‐angle reflection profiles which traverse the Gander and Dunnage zones or central mobile belt of Newfoundland. A simultaneous travel time inversion for velocity and interface was applied to the in‐line seismic refraction/wide‐angle reflection data and constrained by synthetic amplitude models. The results of the modeling procedure show a subhorizontally layered crust with upper crustal velocities ranging from 5.4 to 6.2 km/s, a midcrustal velocity of 6.25–6.35 km/s, and a lower crustal velocity of 6.7±0.2 km/s. The top of the lower crust is marked by a series of prominent reflections between 18 and 23 km depth which suggest a complex layered velocity interface. Strong laterally coherent Moho reflections indicate a sharp crust‐mantle transition at 35 ± 3 km. The uppermost mantle has a velocity of 8.0±0.2 km/s, and a reflecting horizon at 55 km depth suggests an increase to velocities approaching 8.5 km/s. Normal moveout corrections applied to fan profiles provide constraining evidence for the reflecting horizon at the top of the lower crust and laterally continuous Moho reflections at 11–12 s two‐way travel time. Comparisons with a coincident deep seismic reflection profile show that the refraction and reflection Mohos match to better than 2–3 km. Bulk Poisson's ratios of 0.23–0.24 for the whole crust calculated from PmP/SmS travel times suggest a crust dominated by quartzofeldspathic lithologies and a notable absence of voluminous mafic additions to the lower crust. The absence of a deep crustal root, coupled with the bulk intermediate composition inferred for the lower crust from the seismic refraction/wide‐angle data, implies that the crust beneath central Newfoundland has undergone multiple periods of reactivation and equilibration following successive orogenic episodes.
Fracture growth under compression
Authors: Wei K. , De Bremaecker J.‐C.
Published: February 2004

Under uniaxial compression, a planar fracture occurs in a cylindrical specimen of rock, while the extension of a preexisting crack is deflected toward the compression axis, creating open kinks. Our numerical models show that the maximum strain energy release rate criterion (Gmax) provides a physical basis for these seemingly contradictory observations. On a natural fracture where the friction coefficient is the same on the crack face and on its extension, the strain energy reduction is maximum if the crack extends in its own plane. This maximum is even more pronounced if the residual strain energy after the crack initiation is taken into account. In the case of an artificial crack the friction coefficient is normally smaller on the artificial cut than it would be on its natural extension; sliding is thus relatively difficult, and the maximum strain energy reduction occurs when the extension is deflected toward the compressional axis and opens.
Mapping the Piute Mountains, California, with thermal infrared multispectral scanner (TIMS) images
Authors: Hook Simon J. , Karlstrom Karl E. , Miller Calvin F. , McCaffrey Kenneth J. W.
Published: February 2004

Thermal infrared multispectral scanner (TIMS) data were acquired in 1990 over the Piute Mountains, California, to evaluate their usefulness for lithologic mapping in an area of metamorphosed, structurally complex, igneous and sedimentary rocks. The data were calibrated and atmospherically corrected, and emissivity variations in the form of alpha residuals were extracted from which color composite images were made. There was an excellent visual correlation between the units revealed in the color composite image and the lithologic units mapped in the eastern side of the area. It was also possible to correct, improve, and extend the recent map. For example, several areas mapped as granodioritic gneiss had TIMS alpha residual spectra consistent with mafic rocks and were subsequently mapped as amphibolite. The presence of a swarm of mafic dikes, of which only a few had previously been identified, was also revealed. The images also showed color variations in granitoid plutons that correlated with compositional variations previously determined by extensive field and geochemical work. The western Piute Mountains is an area that has proven to be especially difficult to map with field and air photographic methods, due to heterogeneous nature of certain units and various levels of desert varnish. The images permitted the extremely heterogeneous Proterozoic schists, gneisses, and granites to be easily mapped because the small‐scale compositional variability was averaged to the TIMS pixel size (12 m × 12 m square). Areal measurements from the images show that the Proterozoic rocks in the Piute Mountains consist of granitoids (∼50%), biotite gneiss (∼15%), pelitic gneiss (∼15%), quartzite (∼10%), and amphibolite (∼10%). TIMS data can dramatically increase the efficiency and the quality of geologic mapping in well‐exposed heterogeneous areas with minimal vegetation cover where detailed mapping of lithologic contacts by traditional methods is unusually difficult.
Seismic structure of the U.S. Mid‐Atlantic continental margin
Authors: Holbrook W. Steven , Purdy G. M. , Sheridan R. E. , Glover L. , Talwani M. , Ewing J. , Hutchinson D.
Published: February 2004

Multichannel and wide‐angle seismic data collected off Virginia during the 1990 EDGE Mid‐Atlantic seismic experiment provide the most detailed image to date of the continent‐ocean transition on the U.S. Atlantic margin. Multichannel data were acquired using a 10,800 in3 (177 L) airgun array and 6‐km‐long streamer, and coincident wide‐angle data were recorded by ten ocean bottom seismic instruments. A velocity model constructed by inversion of wide‐angle and vertical‐incidence travel times shows strong lateral changes in deep‐crustal structure across the margin. Lower‐crustal velocities are 6.8 km/s in rifted continental crust, increase to 7.5 km/s beneath the outer continental shelf, and decrease to 7.0 km/s in oceanic crust. Prominent seaward‐dipping reflections within basement lie within layers of average velocity 6.3–6.5 km/s, consistent with their interpretation as basalts extruded during rifting. The high‐velocity lower crust and seaward‐dipping reflections comprise a 100‐km‐wide, 25‐km‐thick ocean‐continent transition zone that consists almost entirely of mafic igneous material accreted to the margin during continental breakup. The boundary between rifted continental crust and this thick igneous crust is abrupt, occupying only about 20 km of the margin. Appalachian intracrustal reflectivity largely disappears across this boundary as velocity increases from 5.9 km/s to >7.0 km/s, implying that the reflectivity is disrupted by massive intrusion and that very little continental crust persists seaward of the reflective crust. The thick igneous crust is spatially correlated with the East Coast magnetic anomaly, implying that the basalts and underlying intrusives cause the anomaly. The details of the seismic structure and lack of independent evidence for an appropriately located hotspot in the central Atlantic imply that nonplume processes are responsible for the igneous material.
Modeling fluvial erosion on regional to continental scales
Authors: Howard Alan D. , Dietrich William E. , Seidl Michele A.
Published: February 2004

The fluvial system is a major concern in modeling landform evolution in response to tectonic deformation. Three stream bed types (bedrock, coarse‐bed alluvial, and fine‐bed alluvial) differ in factors controlling their occurrence and evolution and in appropriate modeling approaches. Spatial and temporal transitions among bed types occur in response to changes in sediment characteristics and tectonic deformation. Erosion in bedrock channels depends upon the ability to scour or pluck bed material; this detachment capacity is often a power function of drainage area and gradient. Exposure of bedrock in channel beds, due to rapid downcutting or resistant rock, slows the response of headwater catchments to downstream baselevel changes. Sediment routing through alluvial channels must account for supply from slope erosion, transport rates, abrasion, and sorting. In regional landform modeling, implicit rate laws must be developed for sediment production from erosion of sub‐grid‐scale slopes and small channels.
On the handling of uncertainties in estimating the hazard of rupture on a fault segment
Authors: Rhoades D. A. , Van Dissen R. J. , Dowrick D. J.
Published: February 2004

Uncertainties in data and parameter values have often been ignored in hazard estimates based on historic and prehistoric records of rupture on fault segments. A mixture of distributions approach is appropriate to handle uncertainties in parameters of recurrence time distributions estimated from the geological and historical earthquake record of a fault segment, and a mixture of hazards approach is appropriate for data uncertainties and for uncertainties in parameters estimated from a set of similar faults. The former approach admits updating of the distributions for uncertainty as time passes. The aim is to present the hazard as a single value which takes account of both data and parameter uncertainties, conditional only on modeling assumptions. The proposed methods are described in detail for the exponential and lognormal recurrence time models for fault‐rupturing earthquakes and applied, by way of illustration, to selected fault segments, namely, the Mojave segment of the San Andreas fault, California, and the Wellington‐Hutt Valley segment of the Wellington fault, New Zealand.
Asymmetric upwarp of the asthenosphere beneath the Baikal rift zone, Siberia
Authors: Gao S. , Davis P. M. , Liu H. , Slack P. D. , Zorin Y. A. , Logatchev N. A. , Kogan M. , Burkholder P. D. , Meyer R. P.
Published: February 2004

In the summer of 1991 we installed 27 seismic stations about lake Baikal, Siberia, aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the Baikal rift zone and adjacent regions. Sixty‐six teleseismic events with high signal‐to‐noise ratio were recorded. Travel time and Q analysis of teleseisms characterize an upwarp of the lithosphere‐asthenosphere boundary under Baikal. Theoretical arrival times were calculated by using the International Association of Seismology and Physics of the Earth's interior 1991 Earth model, and travel time residuals were found by subtracting computed arrival times from observed ones. A three‐dimensional downward projection inversion method is used to invert the P wave velocity structure with constraints from deep seismic sounding data. Our results suggest that (1) the lithosphere‐asthenosphere transition upwarps beneath the rift zone, (2) the upwarp has an asymmetric shape, (3) the velocity contrast is −4.9% in the asthenosphere, (4) the density contrast is −0.6%, and (5) the P wave attenuation contrast t* is 0.1 s.
Atmospheric pressure loading parameters from very long baseline interferometry observations
Authors: MacMillan D. S. , Gipson John M.
Published: February 2004

Atmospheric mass loading produces a primarily vertical displacement of the Earth's crust. This displacement is correlated with surface pressure and is large enough to be detected by very long baseline interferometry (VLBI) measurements. Using the measured surface pressure at VLBI stations, we have estimated the atmospheric loading term for each station location directly from VLBI data acquired from 1979 to 1992. Our estimates of the vertical sensitivity to change in pressure range from 0 to −0.6 mm/mbar depending on the station. These estimates agree with inverted barometer model calculations (Manabe et al., 1991; vanDam and Herring, 1994) of the vertical displacement sensitivity computed by convolving actual pressure distributions with loading Green's functions. The pressure sensitivity tends to be smaller for stations near the coast, which is consistent with the inverted barometer hypothesis. Applying this estimated pressure loading correction in standard VLBI geodetic analysis improves the repeatability of estimated lengths of 25 out of 37 baselines that were measured at least 50 times. In a root‐sum‐square (rss) sense, the improvement generally increases with baseline length at a rate of about 0.3 to 0.6 ppb depending on whether the baseline stations are close to the coast. For the 5998‐km baseline from Westford, Massachusetts, to Wettzell, Germany, the rss improvement is about 3.6 mm out of 11.0 mm. The average rss reduction of the vertical scatter for inland stations ranges from 2.7 to 5.4 mm.
Constraints on inner core anisotropy from PKP(DF) travel times
Authors: Shearer Peter M.
Published: February 2004

PKP(DF) travel times reported by the International Seismological Centre (ISC) are analyzed to study the depth dependence of inner core anisotropy. Plots of PKP(DF) travel time residuals versus range for different subsets of the data clearly show large anomalies at ranges beyond about 149° for those paths with inner core ray angles within 30° of the rotation axis. On average, the times are about 3 to 5 s earlier for these paths than for rays at other angles, consistent with recent waveform measurements of anomalous PKP(DF) times by Creager (1992) and Song and Helmberger (1993), and indicative of inner core anisotropy in which the P velocity is about 3% faster for N‐S paths than E‐W paths. The size of these apparent anomalies in the ISC data are much larger than those inferred from several previous analyses of ISC PKP(DF) data which indicated 0.5 to 2 s anomalies at these ranges. The earlier studies applied binning and averaging schemes to the data using windows that were overly tight, thus excluding some of the most anomalous data points and biasing the results toward smaller anomalies. To avoid this problem, I apply a method for computing summary ray residuals that finds the maxima in smoothed histograms of the residuals and does not rely on a fixed interval to window the data points. Standard errors on each summary ray residual are estimated using a bootstrap technique that randomly resamples the data. Plots of the summary ray residuals for several range intervals reveal a small number of independent paths nearly parallel to the rotation axis for which the arrivals are 3 to 5 s early. Although the data are sparse for paths within 30° of the rotation axis, anisotropy does not appear to diminish with depth within the inner core and may cause a much larger anomaly near 180° than the 2‐s value obtained in several prior studies of ISC data. The uniform anisotropy models of Creager (1992) and Song and Helmberger (1993) fit the bulk of the data reasonably well. However, the observed residuals at ranges between 132° and 140° are smaller than those predicted by most anisotropy models, suggesting that the anisotropy may diminish within the outermost 50 km of the inner core.
Strain localization along the San Andreas Fault: Consequences for loading mechanisms
Authors: Gilbert Lewis E. , Scholz Christopher H. , Beavan John
Published: February 2004

The orientation of the near‐fault strain accumulation associated with the relative motion between the Pacific and North American plates is better predicted by the orientation of the local faulting than by the orientation of the regional plate motion. This observation follows from consideration of shear strain results from geodetic studies ranging from the Imperial Valley to Shelter Cove along the San Andreas fault system. This set of results includes calculations from regions where the faulting and plate motion are considerably oblique to each other. Both the localization of the strain associated with Pacific‐North American motion and the orientation of the associated strain can be simply caused by failure of the lithosphere (as opposed to localization of plate driving forces). Two types of models for the rheology and geometry of lithospheric failure have been prevalent in the geodetic literature: (1) viscoelastic models which are dominated by horizontal layering of the crust, and (2) elastic half‐space models with geometry which is dominantly vertical. Horizontal models can accommodate the observed strain results only if postseismic relaxation times are long compared to the repeat times of large earthquakes. Vertical models produce strains which are parallel to the local structure at all times. If the horizontal detachments of the Viscoelastic models exist, they must be at depths 2–3 times the seismogenic thickness. Geologic studies also indicate that major transcurrent features extend, as ductile shear zones, to depths much larger than would be expected in horizontal models. These considerations lead us to favor models with vertical geometries. The vertical structure of these models can be reconciled with laboratory measurements of rock strength through the strain‐softening processes associated with flow of composite materials such as quartzo‐feldspathic rocks. Independent of the horizontal rheological layering in the starting (low strain) material, progressive deformation will lead to preferential weakening in the plane of shear and to the development of a shear zone which will always be weaker than the surrounding material; thus shear zones such as the San Andreas system develop vertical tabular geometries which extend far below the brittle‐plastic transition and which are mechanically persistent.
Modeling thermal infrared (2–14 μm) reflectance spectra of frost and snow
Authors: Wald Andrew E.
Published: February 2004

Existing theories of radiative transfer in close‐packed media assume that each particle scatters independently of its neighbors. For opaque particles, such as are common in the thermal infrared, this assumption is not valid, and these radiative transfer theories will not be accurate. A new method is proposed, called “diffraction subtraction”, which modifies the scattering cross section of close‐packed large, opaque spheres to account for the effect of close packing on the diffraction cross section of a scattering particle. This method predicts the thermal infrared reflectance of coarse (>50 μm radius), disaggregated granular snow. However, such coarse snow is typically old and metamorphosed, with adjacent grains welded together. The reflectance of such a welded block can be described as partly Fresnel in nature and cannot be predicted using Mie inputs to radiative transfer theory. Owing to the high absorption coefficient of ice in the thermal infrared, a rough surface reflectance model can be used to calculate reflectance from such a block. For very small (<50 μm), disaggregated particles, it is incorrect in principle to treat diffraction independently of reflection and refraction, and the theory fails. However, for particles larger than 50 μm, independent scattering is a valid assumption, and standard radiative transfer theory works.
Extensional transform zones and oblique spreading centers
Authors: Taylor Brian , Crook Keith , Sinton John
Published: February 2004

Extensional transform zones (ETZs) are plate boundary segments of order 100 km long that strike at angles between 15° and 45° to the extension direction. They are characterized by neovolcanic/tectonic zones comprising overlapping en echelon volcanic systems and/or faults that trend 30°–75° to the extension direction, sometimes accompanied by a Riedel shear. Below these surficial en echelon structures the deformation is aseismic and ductile, and the plate boundary is probably continuous. ETZs occur in fast and slow spreading and rifting environments and may persist in a stable configuration for several million years. ETZs link oblique spreading segments to transform faults in the Manus and probably the Lau backarc basins. The Reykjanes Peninsula and Tjornes Fracture Zone in Iceland and the Mak'Arrasou in Afar are ETZs that link subaerial to submarine spreading or rifting segments. The Brawley and Cerro‐Prieto seismic zones appear to be ETZs in the Imperial and Mexicali valleys that link the San Andreas, Imperial, and Cerro‐Prieto transform faults. Experimental and analytical models of transtensional deformation in brittle‐ductile systems match many of the observed characteristics of ETZs and oblique spreading centers, including variably sigmoidal to straight en echelon faults that are not parallel to the extension direction. The contrasting fault patterns reflect the rheology of the models and lithosphere: they are more sigmoidal when the strain in the lower ductile layer is more focused, causing the axial faults to curve as they propagate toward parallelism with the less ductile rift margins. The angle (Ø) between the faults and the extension direction decreases with the angle (α) between the strike of the zone and the extension direction. ETZs occur in the range 15° ≤ α ≤ 45°, whereas oblique spreading centers have 45° < α < 90° and transform relay zones have 0° < α < 15°. Oblique fast spreading segments exhibit ridge‐parallel faults and volcanic systems (Ø = α), presumably reflecting locally rotated stress fields, whereas at oblique slow spreading centers, Ø is closer to orthogonal (α < Ø < 90°).
MgSiO3‐FeSiO3‐Al2O3 in the Earth's lower mantle: Perovskite and garnet at 1200 km depth
Authors: O'Neill Bridget , Jeanloz Raymond
Published: February 2004

Natural pyroxene and garnet starting materials are used to study the effects of joint Fe and Al substitution into MgSiO3 perovskite at ∼50 GPa. Garnet is found to coexist with perovskite in samples containing both Fe and Al to pressures occurring deep into the lower mantle (∼1200 km depth). The volume of the perovskite unit cell is Vo(Å3) = 162.59 + 5.95xFeSiO3 + 10.80xAl2O3 with aluminum causing a significant increase in the distortion from the ideal cubic cell. On the basis of a proposed extension of the MgSiO3‐Al2O3 high‐pressure phase diagram toward FeSiO3, Fe is shown to partition preferentially into the garnet phase. The stability of garnet deep into the lower mantle may hinder the penetration of subducted slabs below the transition zone.
Paleomagnetic correlation of Late Quaternary lava flows in the lower east rift zone of Kilauea Volcano, Hawaii
Authors: Hagstrum Jonathan T. , Champion Duane E.
Published: February 2004

Paleomagnetic data reflecting paleosecular variation (PSV) of the geomagnetic field are used to correlate individual lava flows in the lower east rift zone of Kilauea Volcano. This area has been recently mapped based on petrography, stratigraphy, soil development, and degree of weathering of the flows; eight 14C dates also provide a framework for the relative age assignments. Site‐mean directions of remanent magnetization for historic flows in this region indicate that local magnetic anomalies do not mask the PSV signal, that within‐flow angular differences are generally <5°, and that temporal resolution of the paleomagnetic directions is of the order of 100 years. The paleomagnetic correlations do not conflict with the observed stratigraphic relationships, and PSV reference curves from dated lava flows and lake sediments have also been used to help determine the sequence of directional groups. Paleomagnetic grouping of flows into eruptive events indicates a different and perhaps more refined eruptive history for the lower east rift zone than implied by the geologic mapping alone. The number of individual flows based on petrographic distinctions is likely the same, but the frequency of events (including petrographically distinct flows) is apparently lower than previously thought. Flows from ridge crest vents bifurcated by the rift indicate formation of the central graben in this region after ∼300 years ago, possibly related to the 1790 eruptions along dual fractures.
Atmospheric pressure loading effects on Global Positioning System coordinate determinations
Authors: vanDam Tonie M. , Blewitt Geoffrey , Heflin Michael B.
Published: February 2004

Earth deformation signals caused by atmospheric pressure loading are detected in vertical position estimates at Global Positioning System (GPS) stations. Surface displacements due to changes in atmospheric pressure account for up to 24% of the total variance in the GPS height estimates. The detected loading signals are larger at higher latitudes where pressure variations are greatest; the largest effect is observed at Fairbanks, Alaska (latitude 65°), with a signal RMS of 5 mm. Out of 19 continuously operating GPS sites (with a mean of 281 daily solutions per site), 18 show a positive correlation between the GPS vertical estimates and the modeled loading displacements. Accounting for loading reduces the variance of the vertical station positions on 12 of the 19 sites investigated. Removing the modeled pressure loading from GPS determinations of baseline length for baselines longer than 6000 km reduces the variance on 73 of the 117 baselines investigated. The slight increase in variance for some of the sites and baselines is consistent with expected statistical fluctuations. The results from most stations are consistent with ∼65% of the modeled pressure load being found in the GPS vertical position measurements. Removing an annual signal from both the measured heights and the modeled load time series leaves this value unchanged. The source of the remaining discrepancy between the modeled and observed loading signal may be the result of (1) anisotropic effects in the Earth's loading response, (2) errors in GPS estimates of tropospheric delay, (3) errors in the surface pressure data, or (4) annual signals in the time series of loading and station heights. In addition, we find that using site dependent coefficients, determined by fitting local pressure to the modeled radial displacements, reduces the variance of the measured station heights as well as or better than using the global convolution sum.
Tectonic model for the evolution of oceanic crust in the northeastern Indian Ocean from the Late Cretaceous to the Early Tertiary
Authors: Krishna K. S. , Rao D. Gopala , Ramana M. V. , Subrahmanyam V. , Sarma K. V. L. N. S. , Pilipenko A. I. , Shcherbakov V. S. , Murthy I. V. Radhakrishna
Published: February 2004

Bathymetry and magnetic studies (part of the Trans Indian Ocean Geotraverse investigations) in the northeastern Indian Ocean revealed seafloor topographic features, magnetic lineations (19 through 32B) and abandoned spreading centers. The seafloor topography of the Ninetyeast Ridge is relatively wider and shallower south of 15°S. The magnetic anomalies indicate nine fracture zones. Two of them are newly identified. Some of the fracture zones are reflected in the bathymetry. Abandoned spreading centers between 86°E Fracture Zone (FZ) and 92°E FZ are interpreted as the western extensions of the Wharton Ridge. They ceased spreading along with other spreading centers in the Wharton Basin soon after the formation of magnetic anomaly 19 (around 42 Ma) and merged the Indian and Australian plates as single Indo‐Australian plate. The pattern of magnetic lineations between 86°E FZ and 90°E FZ indicate a series of southerly ridge jumps at anomalies 30, 26 (Royer et al., 1991 and other workers) and 19. These ridge jumps transferred portions of the Antarctic plate to the Indian plate. The captured portions and offset along 86°E FZ between India‐Antartica Ridge and Wharton Ridge resulted in an anomalous extra oceanic crust between 86°E FZ and Ninetyeast Ridge spanning 11° in latitude.