bluuebunny/crossref_metadata_2025 huggingface dataset:
Tomographic constraints on the geodynamic evolution of the Italian region
Authors: Lucente Francesco Pio , Chiarabba Claudio , Cimini Giovanni B. , Giardini Domenico
Published: December 2003

In this paper we present P wave tomographic images of the mantle beneath Italy obtained by inverting ∼6000 teleseismic P and PKP wave arrival times, accurately repicked, recorded in the time period 1988–1994 by the stations of the National Seismic Network of the Istituto Nazionale di Geofisica. We pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set. The data were inverted with the well‐established Aki‐Christofferson‐Husebye tomographic technique; different reference models and residuals computation have been tried to verify the stability of the results. The high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the Alps and the Apennines, looking for their lateral and vertical continuity down to 800 km depth. The main finding of this study is a continuous high‐velocity body located between 250 and 670 km depth beneath the entire Apenninic system dipping toward the Tyrrhenian area, which continues upward segmented in two main anomalies in the northern Apenninic and the Calabrian Arcs. We interpret this high‐velocity feature as the subducted oceanic lithosphere between the Eurasian and African plates, dipping down to the upper‐lower mantle boundary beneath the Tyrrhenian Sea. The retrieved images of the lithosphere subducting beneath Apennines are reliable in terms of thickness (about 80–90 km) and P wave velocity contrast (2–4% higher than the normal mantle). Furthermore, our tomographic images, which focus on the deep geometry and continuity of the velocity structures, provide new keys to understanding the geodynamic evolution of the Italian region. The segmentation of the high‐velocity slab upward suggests a complex evolution of the arc‐trench system and the initially continuous subduction of the Ionian‐Adriatic plate progressively developed in subordinate arcs, probably due to lateral heterogeneity of the subducting lithosphere.
Effects of plate bending and fault strength at subduction zones on plate dynamics
Authors: Conrad Clinton P. , Hager Bradford H.
Published: December 2003

For subduction to occur, plates must bend and slide past overriding plates along fault zones. Because the lithosphere is strong, significant energy is required for this deformation to occur, energy that could otherwise be spent deforming the mantle. We have developed a finite element representation of a subduction zone in which we parameterize the bending plate and the fault zone using a viscous rheology. By increasing the effective viscosity of either the plate or the fault zone, we can increase the rates of energy dissipation within these regions and thus decrease the velocity of a plate driven by a given slab buoyancy. We have developed a simple physical theory that predicts this slowing by estimating a convecting cell's total energy balance while taking into account the energy required by inelastic deformation of the bending slab and shearing of the fault zone. The energy required to bend the slab is proportional to the slab's viscosity and to the cube of the ratio of its thickness to its radius of curvature. The distribution of dissipation among the mantle, lithosphere, and fault zone causes the speed of a plate to depend on its horizontal length scale. Using the observation that Earth's plate velocities are not correlated to plate size, we can constrain the lithosphere viscosity to be between 50 and 200 times the mantle viscosity, with higher values required if the fault zone can support shear tractions ≳50 MPa over 300 km. These subduction zone strengths imply that the mantle, fault zone, and lithosphere dissipate about 30%, 10%, and 60% of a descending slab's potential energy release if the slab is 100 km thick. The lithospheric component is highly dependent on slab thickness; it is smaller for thin plates but may be large enough to prevent bending in slabs that can grow thicker than 100 km. Subduction zone strength should be more stable than mantle viscosity to changes in mantle temperature, so the controlling influence of subduction zones could serve to stabilize plate velocities over time as the Earth cools. Because the “details” of convergent plate boundaries are so important to the dynamics of plate motion, numerical models of mantle flow should treat subduction zones in a realistic way.
Resolving the problem of shallow magnetizations of Tertiary age in Asia: insights from paleomagnetic data from the Qiangtang, Kunlun, and Qaidam blocks (Tibet, China), and a new hypothesis
Authors: Cogné J. P. , Halim N. , Chen Y. , Courtillot V.
Published: December 2003

We present new paleomagnetic results obtained at 39 sampling sites from five sections of Tertiary red bed formations: two Eocene formations from the Qiangtang block of Tibet (Xialaxiu locality; 32.8°N, 96.6°E) and the Xining basin of Qaidam (Xining locality; 36.5°N, 102.0°E) and three Neogene formations from the Xining basin (Jungong locality; 34.7°N, 100.7°E) and the Kunlun block (Tuoluo lake and West Yushu localities; 35.3°N, 98.6°E and 33.2°N, 96.7°E, respectively). Thermal demagnetization of the rocks isolated a high‐temperature component that we interpret as the primary magnetization in four localities. The paleopoles lie at 52.6°N/352°E (dp/dm = 6.0°/10.7°) for Xialaxiu, 61.6°N/211.3°E (dp/dm = 9.7°/16.1°) for Xining, 66.0°N/228.6°E (dp/dm = 3.6°/6.9°) for Jungong, and 53.9°N/205.4°E (dp/dm = 5.6°/10.0°) for West Yushu. As in previous studies of Tertiary formations from Asia, the inclinations we obtained are shallower (by 18° to 26°) than the magnetic field computed from the Eurasian apparent polar wander path (APWP) at 10 and 20 Ma for Neogene rocks and at 40 and 60 Ma for Eocene rocks. On the basis of a compilation of Eocene data from the South China Block, Tibet, central Asia and Kyrgyzstan, we conclude that this inclination anomaly reflects erroneous predictions of positions of the Siberian craton when based on the APWP of Eurasia. The main reason for this discrepancy might be nonrigid behavior of the Eurasian plate in the Tertiary. Combination of this with intracontinental shortening of Asia under the penetration of India provides a full explanation for the anomaly. Verification of this new interpretation of the “inclination anomaly” will require new geologic and paleomagnetic data from the northern parts of these remote regions in Mongolia and Siberia.
Streaming potential measurements: 2. Relationship between electrical and hydraulic flow patterns from rock samples during deformation
Authors: Lorne Benoit , Perrier Frédéric , Avouac Jean‐Philippe
Published: December 2003

Streaming potential and resistivity measurements have been performed on Fontainebleau sandstone and Villejust quartzite samples in a triaxial device during compaction, uniaxial compression, and rupture. Measurements on individual samples do not show any clear intrinsic dependence of the streaming potential coefficient with permeability. An apparent dependence of the streaming potential coefficient with permeability is, however, observed during deformation. The effect of surface conductivity is taken into account and is small compared with the observed changes in the streaming potential coefficient. The observed dependence is therefore interpreted in terms of a difference in the evolution of the electrical and hydraulic connectivity patterns during deformation. This effect causes the streaming potential coefficient, and consequently the inferred ξ potential, to be reduced by a geometrical factor RG representing the electrical efficiency of the hydraulic network. Estimates of the RG factor varying between 0.2 and 0.8 for electrolyte resistivity larger than 100 Ωm are obtained by comparing the values of the ξ potential inferred from intact rock samples with the values obtained from crushed rock samples, where the geometrical effects are assumed to be negligible. The reduction of the streaming potential coefficient observed during compaction or uniaxial compression suggests that the tortuosity of the hydraulic network increases faster than the tortuosity of the electrical network. Before rupture, an increase in the streaming potential coefficient associated with the onset of dilatancy was observed for three samples of Fontainebleau sandstone and one sample of Villejust quartzite. The changes in streaming potential coefficient prior to failure range from 30% to 50%. During one experiment, an increase in the concentration of sulfate ions was also observed before failure. These experiments suggest that observable streaming potential and geochemical variations could occur before earthquakes.
Crustal structure beneath the central Oregon convergent margin from potential‐field modeling: Evidence for a buried basement ridge in local contact with a seaward dipping backstop
Authors: Fleming Sean W. , Tréhu Anne M.
Published: December 2003

Models of magnetic and gravity anomalies along two E‐W transects offshore central Oregon, one of which is coincident with a detailed velocity model, provide quantitative limits on the structure of the subducting oceanic crust and the crystalline backstop. The models indicate that the backstop‐forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America ∼50 million years ago, has a seaward dip of less than 60°3. Seismic, magnetic, and gravity data are compatible with no more than 2 km of subducted sediments between the Siletz terrane and the underlying crystalline crust of the Juan de Fuca plate. The data also suggest the presence of a N‐S trending, 200‐km‐long basaltic ridge buried beneath the accretionary complex from about 43°N to 45°N. Although the height and width of this ridge probably vary along strike, it may be up to 4 km high and several kilometers wide in places and appears to be locally in contact with the Siletz terrane beneath Heceta Bank. Several models for the origin of this ridge are discussed. These include: a sliver of Siletz terrane detached from the main Siletz terrane during a late Eocene episode of strike‐slip faulting; imbrication and thickening of subducted oceanic crust in place; an aseismic ridge rafted in on the subducting oceanic crust during the past 1.2 million years; and a series of ridges and/or seamounts rafted in over a longer period of time and transferred from the subducting plate to the overlying plate. The last model is the most consistent with the complicated history of local uplift, subsidence, and slope instability recorded in the ridges, basins, and banks of this part of the margin. We speculate that the massive seaward dipping western edge of the Siletz terrane in this region inhibits subduction of seamounts and sediments, resulting in fomation of buried ridge as the accumulated flotsam and jetsom of subduction. This process may also be responsible for thickening of lower accretionary complex material, oversteepening of slopes leading to massive slumping, and north‐south extension through strike‐slip faulting in the accretionary complex to the west of the buried ridge. Regardless of its origin, the ridge may currently be acting as an asperity inhibiting subduction.
Strong crustal heterogeneity in the Bolivian Altiplano as suggested by attenuation of Lg waves
Authors: Baumont David , Paul Anne , Beck Susan , Zandt George
Published: December 2003

Using the seismological data of four temporary networks installed across the Andes of Bolivia and Chile between 1990 and 1995, we study the amplitudes of the Lg phase for 45 regional events located at crustal depths. This analysis reveals that Lg is strongly attenuated for most of the ray paths crossing the Altiplano independent of the azimuth of propagation. To map the anomalies and evaluate their frequency dependence, we conduct a regionalization of the crustal average Qs factor using a damped least squares inversion technique in the frequency range 0.6–5 Hz. The resulting maps delineate regions with contrasting values of apparent attenuation. The Altiplano is characterized by strong attenuation with an average Qs of 100 at 1 Hz, whereas the Cordilleras are regions of weaker attenuation. The frequency dependence of Qs is investigated in an attempt to separate the source of the apparent attenuation into scattering and anelastic absorption. Within the hypothesis of a weak frequency dependence of the intrinsic Q, we show that at 1 Hz, scattering plays a major part in the attenuation of Lg waves whereas at 4 Hz, both mechanisms are of equal importance. These results indicate that the attenuation of Lg waves in the Altiplano is due more to scattering by small‐scale heterogeneities than to a high percentage (>1%) of partial melt involving a large part of the crust.
Transitional geomagnetic impulse hypothesis: Geomagnetic fact or rock‐magnetic artifact?
Authors: Camps Pierre , Coe Robert S. , Prévot Michel
Published: December 2003

A striking feature of the Steens Mountain (Oregon) geomagnetic polarity reversal is the two (maybe three) extremely rapid field directional changes (6 degrees per day) proposed to account for unusual behavior in direction of remanent magnetization in a single lava flow. Each of these very fast field changes, or impulses, is associated with a large directional gap (some 90°) in the record. In order to check the spatial reproducibility of the paleomagnetic signal over distances up to several kilometers, we have carried out a paleomagnetic investigation of two new sections (B and F) in the Steens summit region which cover the second and the third directional gap. The main result is the description of two new directions, which are located between the pre second and post second impulse directions. These findings weigh against the hypothesis that the geomagnetic field cause the unusual intraflow fluctuations, which now appears to be more ad hoc as an explanation of the paleomagnetic data. However, the alternative baking hypothesis remains also ad hoc since we have to assume variable rock magnetic properties that we have not yet been able to detect within the flows at the original section Steens A and D 1.5 km to the north. In addition, new results for 22 transitional and normal lava flows in section B are presented that correlate well with earlier results from section A.
Lunar and solar torques on the oceanic tides
Authors: Ray Richard D. , Bills Bruce G. , Chao Benjamin Fong
Published: December 2003

A general framework for calculating lunar and solar torques on the oceanic tides is developed in terms of harmonic constituents. Axial torques and their associated angular momentum and Earth rotation variations are deduced from recent satellite‐altimeter and satellite‐tracking tide solutions. Torques on the prograde components of the tide produce the familiar secular braking of the rotation rate. The estimated secular acceleration is approximately −1300 arcseconds/century2 (less 4% after including atmospheric tides); the implied rate of change in the length of day is 2.28 milliseconds/century. Torques on the retrograde components of the tide produce periodic rotation variations at twice the tidal frequency. Interaction torques (for example, solar torques on lunar tides) generate a large suite of rotation‐rate variations at sums and differences of the original tidal frequencies. These are estimated for periods from 18.6 years to 6 hours. At subdaily periods the angular momentum variations are 5 to 6 orders of magnitude smaller than the variations caused by ocean tidal currents.
Shear wave splitting in the Appalachians and the Urals: A case for multilayered anisotropy
Authors: Levin Vadim , Menke William , Park Jeffrey
Published: December 2003

Observations of shear wave splitting in the northeastern U.S. Appalachians and in the foredeep of the Urals vary significantly with the back azimuth and incidence angle of the incoming phase. These variations suggest two or more layers within the upper mantle with different anisotropic properties. Synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach. Relying on a subset of back‐azimuth and incidence angle may strongly bias the model inferred, especially if the observations are averaged. On the other hand, the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the Earth. Using a new error estimation technique for splitting, we find that individual measurements from broadband data have errors of the order of δϕ = 3°–7°for the fast direction and 0.1–0.2 s for the delay of split shear waves. The azimuthal variation of splitting parameters is broadly consistent throughout the Appalachian terranes in the northeast United States, especially for two long‐running stations in the northeast United States, HRV (Harvard, Massachusetts) and PAL (Palisades, New York). Observations can be separated into two distinct populations, with mean fast‐axis azimuths of N60°E±4°and N119°E±2°. Delay values within each population range from near zero to ∼1 s. Azimuthal splitting variation for ARU (Arti, Russia) in the foredeep of Uralian mountains is characterized by sharp transitions between different groups of observations. Using synthetic seismograms in simple structures, we develop one‐dimensional anisotropic models under stations HRV and ARU. The model for HRV includes two layers of anisotropic material under an isotropic crust, with fast‐axis azimuths N53°E and N115°E for the bottom and the top layers, respectively. The model for the upper mantle under ARU includes a layer with a fast‐axis at N50°E atop a layer with fast axis azimuth N90°E. Our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under ARU, reported by Levin and Park [1997a]. Our results suggest that both Urals and Appalachians possess a relict anisotropy in the tectosphere, associated with past continental collision and accretion, underlain by anisotropy with orientation similar to the local absolute plate motion, suggesting an asthenospheric component to the signal.
Sequence stratigraphy, structure, and tectonic history of the southwestern Ontong Java Plateau adjacent to the North Solomon Trench and Solomon Islands Arc
Authors: Phinney Eric J. , Mann Paul , Coffin Millard F. , Shipley Thomas H.
Published: December 2003

The Ontong Java Plateau (OJP) is the largest and thickest oceanic plateau on Earth and one of the few oceanic plateaus actively converging on an island arc. We present velocity determinations and geologic interpretation of 2000 km of two‐dimensional, multi‐channel seismic data from the southwestern Ontong Java Plateau, North Solomon Trench, and northern Solomon Islands. We recognize three megasequences, ranging in age from early Cretaceous to Quaternary, on the basis of distinct interval velocities and seismic stratigraphic facies. Megasequence OJ1 is early Cretaceous, upper igneous crust of the OJP and correlates with basalt outcrops dated at 122–125 Ma on the island of Malaita. The top of the overlying megasequence OJ2, a late Cretaceous mudstone unit, had been identified by previous workers as the top of igneous basement. Seismic facies and correlation to distant Deep Sea Drilling Project/Ocean Drilling Program sites indicate that OJ2 was deposited in a moderately low‐energy, marine environment near a fluctuating carbonate compensation depth that resulted in multiple periods of dissolution. OJ2 thins south of the Stewart Arch onto the Solomon Islands where it is correlated with the Kwaraae Mudstone Formation. Megasequence OJ3 is late Cretaceous through Quaternary pelagic cover which caps the Ontong Java Plateau; it thickens into the North Solomon Trench, and seismic facies suggest that OJ3 was deposited in a low‐energy marine environment. We use seismic facies analysis, sediment thickness, structural observations, and quantitative plate reconstructions of the position of the OJP and Solomon Islands to propose a tectonic, magmatic, and sedimentary history of the southwestern Ontong Java Plateau. Prior to 125 Ma late Jurassic and early Cretaceous oceanic crust formed. From 125 to 122 Ma, the first mantle plume formed igneous crust (OJ1). Between 122 and 92 Ma, marine mudstone (OJ2 and Kwaraae mudstone of Malaita, Solomon Islands) was deposited on Ontong Java Plateau. At 92 Ma a second mantle plume caused widespread volcanism on the plateau. From 92 to 15 Ma, pelagic carbonate sediment (OJ3) was deposited. At ∼15 Ma the southern Ontong Java Plateau was deformed by normal faults during its approach toward the North Solomon Trench. Finally, from 4 to 0 Ma, the Malaita Accretionary Prism formed during collision between a substantially thicker portion of the Ontong Java Plateau and the Solomon Islands arc. Flexure of the Ontong Java Plateau near the trench caused coeval normal faulting.
Three‐dimensional numerical modeling of coarse‐grained clastic deposition in sedimentary basins
Authors: Ritchie Bryan D. , Hardy Stuart , Gawthorpe Rob L.
Published: December 2003

A three‐dimensional numerical model of clastic deposition in sedimentary basins is used to investigate the development of coarse‐grained deltas in response to relative sea level changes. The model incorporates fully three‐dimensional sediment delivery and deposition, together with incised channel formation and slope failure. A combined random‐walk/steepest‐descent algorithm is used to simulate sediment delivery from a drainage basin outlet to the depositional shoreline, together with a nonlinear three‐dimensional diffusion equation for modeling slope‐dependent sediment erosion, transport, and deposition. When sediment channels (transport pathways) are out of a specified grade, advective erosion occurs, thereby increasing the total sediment supply delivered to the basin. Multiple sediment sources into a basin can be simulated, together with tectonic subsidence and eustatic sea level changes. The model results presented display many of the key features observed in both modern and ancient coarse‐grained deltas. During periods of slowly changing and high relative sea level (highstands), a broad delta front progrades approximately radially away from the sediment source. Conversely, incised channels are developed during rapid falls of relative sea level, leading to localized deposition of deltaic lobes at the mouths of these channels. The incised channels initiate at the exposed delta front and propagate backward toward the sediment source. The rate of growth of incised channels is shown to be an important control on delta morphology, sediment distribution, and lowstand lobe size and location. Lobes are particularly well developed during periods of slowly changing low relative sea level (lowstands). Periods of rapid rise of relative sea level lead to flooding and infilling of incised channel systems and a rapid landward shift in deposition (transgressions). Incised channels and delta lobes are best developed in a model where tectonic subsidence is similar to that expected for hanging‐wall‐sourced deltas in extensional half‐graben settings. In contrast, both channels and lobes are less well developed in a model where tectonic subsidence is similar to that expected for footwall‐sourced deltas in such settings. Many of the features developed in the models are, by their nature, three dimensional, and two‐dimensional analysis of the model results can lead to erroneous interpretations of the causes of along‐strike variability. These aspects of sequence variability have important implications for the application of the sequence stratigraphic methodology to many sedimentary basins.
Obduction‐related remagnetization at the base of an ophiolite: Paleomagnetism of the Samail nappe lower sequence and of its continental substratum, southeast Oman Mountains
Authors: Feinberg Hugues , Horen Hélène , Michard André , Saddiqi Omar
Published: December 2003

This paper considers the effect of obduction tectonics on the magnetic properties of a fast spreading ridge ophiolite. The paleomagnetism of the ultramafics (seven sites) and layered gabbros (two sites) of the Samail, Wadi Tayin‐Ibra, and Muscat massifs of Oman has been investigated, along with that of the Permian metabasites from the subophiolitic Saih Hatat continental window (five sites). Of the 202 studied samples, 183 exhibit stable components with a dominant NNW trend and a shallow, positive or, less frequently, negative inclination. Superposition of normal and reverse polarities was observed in one gabbro, three peridotite, and two metabasite sites. The in situ characteristic directions are similar in both the nappe and metamorphic window, except for a lone, low‐grade metabasite with a component (D = 168°, I = 35°) referred to a preserved Permian magnetization. The moderate directional scatter can be explained by postnappe tilting along the NNE trending axis. The high‐grade metabasites in the window were completely remagnetized during the metamorphism associated with nappe emplacement and subsequent exhumation tectonics between 80 and 70 Ma. Since the overlying ophiolites have similar, normal, or reverse directions, they were remagnetized contemporaneously. The magnetic mineralogy of the ultramafic‐layered gabbro sequence was likely modified extensively at that time by serpentinization from the upward metamorphic fluid advection in the faulted ophiolite. Therefore the magnetic properties of the lower part of the southern Samail ophiolite cannot be used for modeling magnetic anomalies of oceanic lithosphere.
A new method for the estimation of cooling and denudation rates using paramagnetic centers in quartz: A case study on the Eldzhurtinskiy Granite, Caucasus
Authors: Grün Rainer , Tani Atsushi , Gurbanov Anatoly , Koshchug Dmitry , Williams Ian , Braun Jean
Published: December 2003

We present a new method for the assessment of the most recent cooling and denudation rates using paramagnetic centers in quartz measured by electron spin resonance (ESR) spectroscopy. These centers have a relatively low thermal stability. For cooling rates of 40° and 1000°C Myr−1, effective closure temperatures vary between 55° and 82°C (Ti center) and 49° and 64°C (Al center), respectively. Samples were collected from two cores that were drilled into the Eldzhurtinskiy Granite, which has an emplacement age of ∼2 Ma as measured by U/Pb analyses of zircons. One 1500 m core was taken from a drill hole into the dome of the granite, a second core of 4000 m from a drill hole at the base of the Baksan Valley. Our results yield cooling rates of between 160 and 250°C Myr−1 for the upper core and between 570° and 600°C Myr−1 for the lower core; the corresponding denudation rates are ∼2.5 (upper core) and 5.5 mm a−1 (lower core). The shape of the temperature profile of the lower core indicates recent erosion. When fitting the temperature data with a two‐dimensional heat‐transfer model, we obtain a net denudation rate of ∼10 mm a−1 and cooling rates in the range of 500°C Myr−1, thus confirming the cooling rates estimated by ESR. However, the ESR denudation rates underestimate the erosion rate of the Baksan Valley because the geothermal gradient is not equilibrated between the surface and the depth of the annihilation temperatures, 950 and 1800 m for the Al and Ti centers, respectively. We conclude that ESR measurements of paramagnetic centers in quartz will allow the reconstruction of landscape dynamics for the past 10–1000 kyr and that in conjunction with U/Pb, fission track, and Ar/Ar analyses it will be possible to develop dynamic models for Quaternary tectonic movements.
Climate instability during the penultimate glaciation: Evidence from two high‐resolution loess records, China
Authors: Ding Z. L. , Ren J. Z. , Yang S. L. , Liu T. S.
Published: December 2003

The Xinzhuangyuan and Lijiayuan loess sections in the northwestern part of the Chinese Loess Plateau were studied down to paleosol unit S2, which probably formed during marine oxygen isotopic stage 7, ∼200,000 years B.P. The thickness of the loess‐soil sequence above S2 is ∼63 m at Xinzhuangyuan and 43 m at Lijiayuan. A 2 cm sample spacing yields a mean depositional resolution of 50–80 years for the last and penultimate glacial loess units (L1 and L2) and ∼200 years for the last interglacial soil unit (Sl), thus enabling us to reconstruct high‐resolution climatic changes during the last two glacial‐interglacial cycles. The grain size records of the two sections are regarded as a proxy for changes in the monsoon‐desert system over northern China. Results show that frequent, large‐amplitude climatic oscillations on millennial timescales occurred during the penultimate glaciation in a manner similar to that during the last glaciation, suggesting that suborbital‐scale climatic variations may be a common feature of the climate system during glacial periods of the Pleistocene. The last interglacial soil (S1) is composed of three individual soils and two thin intervening loess horizons in both of the sections. Short‐term variations in grain size are not prominent within the soil complex, implying the absence of strong millennial‐scale climatic oscillations during the last interglaciation in northern China.
Rate‐ and state‐dependent friction of intact rock and gouge
Authors: Sleep Norman H.
Published: December 2003

The shear traction during frictional sliding depends on the instantaneous sliding strain rate
and on a state variable ψ which represents the previous history of the sample. A formulation of rate and state friction covers the continuum between gouge and intact rock, as well as the behavior at high and low normal tractions. A traditional formula for the instantaneous coefficient of friction is retained,
, where μ0 is the steady state coefficient of friction at the reference strain rate
, and a and b are small constants. The normalizing ψ0 for isothermal conditions is ΔPN/ΔPN0, where ΔP is the effective normal traction, ΔP0 is a reference normal traction, and N is an exponent. The coefficient of friction of intact rock is approximately μclose−bN ln(ΔP/ΔPclose), where the coefficient of friction is μclose, at the normal traction ΔPclose where essentially all the pore space is closed during steady state sliding. The formulation provides an acceptable fit to the observed coefficient of friction of Westerly Granite at normal tractions up to 1500 MPa including steady sliding and intact rock failure.
Regional waveform propagation in the Arabian Peninsula
Authors: Mellors Robert J. , Camp Victor E. , Vernon Frank L. , Al‐Amri Abdullah M. S. , Ghalib Ali
Published: December 2003

Regional waveform propagation is characterized in the Arabian Peninsula using data from a temporary network of broadband seismometers. Between November 1995 and March 1997, 332 regional (delta<15°) events were recorded from nine stations deployed across the Arabian Shield. Regional phase propagation was analyzed in two ways: by individual inspection of the waveforms and by stacking of waveforms. Inspection of the waveforms revealed consistent variations in individual seismograms according to the region of origin. Waveforms from events in the Gulf of Aqaba, northwest of the network, possess weak Pn, Pg, and Sn but show a prominent Lg phase. In contrast, clear Pn, Sn, and Lg are observed for events located in the Zagros, a region northeast of the network. Events near the Straits of Hormuz also display Pn and Sn but lack a strong high‐frequency Lg. Southern Red Sea and African earthquakes have moderate‐amplitude body phases with some Lg. For the stacks the data were high‐pass filtered at 1 Hz, rectified, binned, and then stacked by time/distance or by time/slowness. The time/distance stacks show clear differences between regions that correspond to the variations observed in individual seismograms. The time/slowness stacks allow comparison of relative phase velocities and amplitudes. Pn velocity under the network was estimated to be 8.0±0.2 km/s, consistent with data from prior refraction profiles. The area of inefficient Pn and Sn propagation coincides with an area of Holocene volcanism and suggests that anomalous upper mantle underlies much of the Arabian Shield.
Shallow structure of the Landers Fault Zone from explosion‐generated trapped waves
Authors: Li Yong‐Gang , Aki Keiiti , Vidale John E. , Xu Fei
Published: December 2003

With fault‐zone trapped waves generated by near‐surface explosions within the fault zone of the 1992 Landers, California, earthquake and recorded by linear seismic arrays deployed along and across the fault, we have delineated the shallow seismic structure near the 30‐km‐long southern rupture on the Johnson Valley fault. The explosion‐excited trapped waves with relatively large amplitude and long‐duration wave train after the S waves are similar to those generated by aftershocks [Li et al, 1994a, b] but have lower frequencies and travel more slowly. Coda‐normalized amplitude spectra of explosion‐excited trapped waves show a maximum at ∼2 Hz, which decreases rapidly with the station offset from the fault trace on the cross‐fault profiles. Normalized amplitudes of trapped waves on the along‐fault profile also decreased with distance between the explosion and station, giving an apparent Q of ∼18 at 1–2 Hz in the fault zone for the shot near the profile. The dispersion of trapped waves from 0.6 to 2.5 Hz recorded on this along‐fault profile implies a shear velocity of ∼1.0 km/s for the fault zone and ∼1.8 km/s for the wall rock, while the data from the farther shot show an increase in velocity and Q with depth. Measured group velocities and Q values were used as constraints in the numerical modeling of trapped waves on cross‐fault and along‐fault profiles. Results reveal that the shallow Johnson Valley fault is marked by a zone 250 m wide where the shear velocity is 1 km/s and Q is 20. Calculation of finite difference synthetics for a depth‐varying fault structure show that these model parameters apply to the depth of ∼1 km, below which the fault zone shear velocity increases to 1.9 km/s and Q increases to 30.
Subsurface propagation of thermomechanical kinks through hydrothermal fluid‐saturated porous horizons
Authors: Garcìa Ralph , Natale Giuseppe
Published: December 2003

In this paper a one‐dimensional nonlinear analytical model for the mechanism of subsurface fluid‐rock coupling dynamics in hydrothermal systems is proposed. The model is based upon the modern thermoporoelasticity theory and its two nonlinear heatlike equations, namely, the stress diffusion equation and the fluid‐rock energy equation. On the lower boundary of a subsurface fluid‐saturated porous horizon a buried thermomechanical source is supposed to build up so that coupled fluid‐rock temperature and pore fluid pressure changes are generated. These changes are thought of as being related to the Heaviside step function, which is here adopted as the initial condition to be combined with Burgers' equation, obtained in the case of small fluid diffusivity in the stress diffusion equation. By using the Hopf‐Cole transformation an analytical solution corresponding to a solitary wave such as a thermomechanical kink is found that propagates upward through the fluid‐saturated thermoelastically reactive porous horizon, thus supplying changes in the thermomechanical properties of the fluid emissions as the wave approaches the ground surface. Some cases of episodes of unrest are considered for some hydrothermal systems, showing the agreement between our theoretical curves of fluid surface signals and the enhanced activity in the fluid emissions. In this connection the upsurge of thermomechanical solitary shock waves such as kinks can be used as an indicator of thermal energy buildup and approaching mechanical destabilization at depth in both volcanic and geothermal systems.
A new statistical test for static stress triggering: Application to the 1987 Superstition Hills earthquake sequence
Authors: Anderson Greg , Johnson Hadley
Published: December 2003

Over the past several years, many investigators have argued that static stress changes caused by large earthquakes influence the spatial and temporal distributions of subsequent regional seismicity, with earthquakes occurring preferentially in areas of stress increase and reduced seismicity where stress decreases. Some workers have developed quantitative methods to test for the existence of such static stress triggering, but no firm consensus has yet been reached as to the significance of these effects. We have developed a new test for static stress triggering in which we compute the change in Coulomb stress on the focal mechanism nodal planes of a set of events spanning the occurrence of a large earthquake. We compare the statistical distributions of these stress changes for events before and after the mainshock to decide if we can reject the hypothesis that these distributions are the same. Such rejection would be evidence for stress triggering. We have applied this test to the November 24, 1987, Elmore Ranch/Superstition Hills earthquake sequence and find that those post‐mainshock events that experienced stress increases of at least 0.01–0.03 MPa (0.1–0.3 bar) or that occurred from 1.4 to 2.8 years after the mainshocks are consistent with having been triggered by mainshock‐generated static stress changes.
Interplate coupling in the Tohoku district, Japan, deduced from geodetic data inversion
Authors: El‐Fiky Gamal S. , Kato Teruyuki
Published: December 2003

The spatial distribution and the temporal change of the strength of interplate coupling between the subducting Pacific plate and overlying continental plate in the Tohoku district, northeast Japan, have been investigated through the inversion analysis of geodetic data. The geodetic data used for this analysis are the rates of vertical (1966–1995) and horizontal (1979–1991) displacements, which presumably represent average crustal movements during the interseismic period. We incorporate smoothing constraints on back slip distribution into the observed data. The degree of the constraints is controlled by hyperparameters whose optimal values are determined by minimizing Akaike's Bayesian Information Criterion (ABIC). Under the assumption of ABIC, we can obtain a unique solution. The result of the inversion analysis shows that there are two regions of high coupling, the first extending from 20 to 45 km in the north (40.5°N) and the second extending from 25 to 55 km in the south (38.5°N). The average back slip rate in the northern region reaches 55±11 mm yr−1 and in the southern region 41±14 mm yr−1. Also, the inversion analysis shows that the direction of the plate convergence at the Japan trench is N71°±3°W in the northern region and N48°±5°W in the southern region. The interplate coupling becomes weaker toward the shallower and deeper portions and exhibits significant temporal variation.
Equations of state for Fe32+Fe23+Si3O12“skiagite” garnet and Fe2SiO4‐Fe3O4spinel solid solutions
Authors: Woodland A. B. , Angel R. J. , Koch M. , Kunz M. , Miletich R.
Published: December 2003

The equations of state of Fe32+Fe23+Si3O12, or “skiagite” garnet, and two Fe2SiO4‐Fe3O4spinel solid solutions were determined from room temperature hydrostatic compression experiments up to 11 GPa performed at the European Synchrotron Radiation Facility. These data indicate that there are no first‐order phase transitions or changes in compression mechanism in skiagite garnet or Fe3+‐bearing silicate spinel solid solutions up to 11 GPa at room temperature. Skiagite garnet has an isothermal bulk modulusK0T= 157.4(3.0) GPa and a pressure derivative of the isothermal bulk modulusK0T= 6.7(8), where numbers in parentheses represent 1 estimated standard deviation of these values. Combination of this result with those for other silicate garnets reveals an inverse linear relationship between the octahedral cation‐oxygen (Y‐O) bond length and the bulk modulus. The dominating effect of the octahedral site on the bulk compression of garnet can be understood by considering that the octahedral‐tetrahedral network has no rigid unit modes and that no cation substitution occurs on the tetrahedral sites in these garnets. It is apparent that the incorporation of Fe3+and Cr acts to lower the bulk modulus of aluminosilicate garnets. Consideration of the effect of compression on the molar volume results in nonsystematic changes in calculated oxygen fugacity for garnet‐bearing mantle peridotites that equilibrated at high pressures. The pressure‐volume data obtained from two Fe3+‐bearing silicate spinel solid solutions with compositions Xfay= 0.45 and Xfay= 0.57 (fay, fayalite component) yielded similar enough values ofK0TandK′0Tto warrant a combined refinement with the data points scaled to the appropriate zero‐pressure volumes. This gaveK0T= 168.9(1.2) GPa andK′0T= 5.7(1.2) with χW2= 4.6 (weighted χ2) andK0T= 175.5(1.4) GPa whenK0Tis fixed equal to 4 (χW2= 4.9). Our solid solutions have significantly lower bulk moduli than either magnetite or Fe2SiO4spinel end‐members or indeed many other spinels.
Seismic anisotropy beneath the lower half of the North Island, New Zealand
Authors: Marson‐Pidgeon Katrina , Savage Martha Kane , Gledhill Ken , Stuart Graham
Published: December 2003

Teleseismic ScS and SKS events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the North Island, New Zealand. This area lies above the Hikurangi subduction zone, and the array provides ray paths which sample the mantle both above and below the slab. Shear wave splitting measurements give similar fast polarizations and delay times at each station. The average SKS fast polarization is approximately NE‐SW, subparallel to the strike of subduction and the major geological features, with an average SKS delay time of 1.6±0.1 s. This lack of variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle. The anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence. Any anisotropy in the slab is probably due to fossil mineral alignment. Anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow. The similar NE‐SW fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench‐parallel direction in both regions.
The relationship between synoptic weather systems and meteorological forcing on the North Carolina inner shelf
Authors: Austin Jay A. , Lentz Steven J.
Published: December 2003

A strong relationship is observed between synoptic weather systems and atmospheric forcing of the ocean as estimated from buoy measurements made on the North Carolina inner shelf during August and October‐November 1994 as part of the Coastal Ocean Processes (CoOP) Inner Shelf Study. Synoptic variation (timescales of days to weeks) in the meteorological time series was primarily associated with the passage of atmospheric frontal systems. The most common synoptic weather pattern observed was the passage of a low‐pressure center to the north of the study site, which caused the associated cold front to pass over the study region. Before passage of the cold front, warm, moist northeastward winds increased the heat flux into the ocean, whereas after the cold front passed, cold, dry southwestward winds decreased the heat flux into the ocean. In addition, in the presence of oceanic stratification, northeastward winds drove coastal upwelling, bringing colder water to the surface, further increasing the air‐sea temperature contrast and hence the heat flux into the ocean inshore of the surface front between cool upwelled water and warmer water offshore. The decrease in surface heat flux during the passage of a cold front was of order 400 W m−2, due primarily to a decrease in latent heat flux. Although other synoptic patterns were observed, including one warm front passage and two tropical storm systems, the dominance of cold fronts as a source of variability resulted in a strong positive correlation between the along‐shelf component of wind stress and the surface heat flux. To address the issue of spatial variation in the surface heat fluxes, data from several different sources located along a cross‐shelf transect were analyzed. This analysis suggests that the temperature of the atmospheric boundary layer undergoes adjustment when warm air blows over cold water but not when cold air blows over warm water. This produces cross‐shelf gradients in the bulk estimates of turbulent heat fluxes during offshore winds but not during onshore winds.
Atmospheric effects in the Angola‐Benguela frontal zone
Authors: Kostianoy A. G. , Lutjeharms J. R. E.
Published: December 2003

The Angola‐Benguela Front is the generic border between, on the one hand, the Benguela Current and the coastal upwelling regime along the southwest African coastline and, on the other hand, the warm, southward flowing Angola Current. This frontal zone has a variable morphology, geographic location, and thermal characteristics. It has been suggested that this variability may be related to changing wind patterns. We have studied the thermal characteristics of the surface expression of the frontal zone for a 74‐day period using daily thermal infrared images and compared these to wind patterns from synoptic weather maps. There is little correlation between the location of the northern and the southern front that form the envelope for this frontal zone. The location of the southern front largely determines the thermal gradient across the entire frontal zone. Large short‐term displacements in the geographic location of the frontal zone as well as short‐term variations in its thermal characteristics are shown to be strongly correlated to the atmospheric pressure gradient of the South Atlantic anticyclone with a 1‐day lag period. This is true irrespective of the latitude changes of the center of the South Atlantic anticyclone.
Interannual variability of South Atlantic circulation from 4 years of TOPEX/POSEIDON satellite altimeter observations
Authors: Witter Donna L. , Gordon Arnold L.
Published: December 2003

Variability of large‐scale and regional South Atlantic circulation is investigated using TOPEX/POSEIDON sea level observations. Interannual variations are identified from empirical orthogonal functions of gridded sea level fields, year‐to‐year fluctuations of root‐mean‐square sea level variability, and variability of Agulhas eddies evaluated from the along‐track data. Two modes of variability are identified. A basin‐scale mode indicates that sea level in the eastern South Atlantic underwent a transition from a state of high sea level and enhanced gyre‐scale geostrophic circulation in 1993 and 1994, to a state of lower sea level and more sluggish circulation in 1996. The dominant mode of basin‐scale zonal wind has the same temporal signature, suggesting a link between the observed variation of gyre‐scale circulation and the regional wind forcing. Time variations of this mode also coincide with a transition from a broad Agulhas eddy corridor observed in 1993 and 1994 to a narrower corridor observed in 1996. The input of salt and vorticity to the South Atlantic subtropical gyre via Agulhas eddies may therefore be partially controlled by interannual variations of the wind‐forced, large‐scale circulation. A second mode isolates interannual variations in the Brazil‐Malvinas Confluence region. During 1993, eddy variability along the Brazil Current extension was relatively strong and variability along the continental slope was weak. The opposite pattern was observed in 1995. These variations may be related to interannual variations of the latitude of the confluence. While variations associated with both modes are smaller than those observed on seasonal timescales, these interannual variations contribute significantly to the total South Atlantic variability.