bluuebunny/crossref_metadata_2025 huggingface dataset:
The superparamagnetism of Yucca Mountain Tuff
Authors: Worm Horst‐Ulrich , Jackson Mike
Published: September 2002

Yucca Mountain Tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range [Schlinger et al., 1988]. Magnetic measurements on three samples (comprising hysteresis loops at low and ambient temperatures, acquisition and demagnetization of isothermal remanent magnetization, thermal demagnetization of the saturation remanence and of a low‐temperature thermoremanence, and frequency‐ and temperature‐dependent susceptibilities) allow evaluation of the magnetic properties in terms of Néel's [1949] single‐domain theory. Precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results. In contrast, an attempt to derive mean particle volumes by fitting a Langevin curve to the room temperature magnetization curves fails, probably because the precondition for the Langevin function, KV/kT≪1, is not met. It is only for the sample with the smallest grains and in weak fields (<20 mT) that a Langevin fit provides a reasonable volume estimate. There is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility, thus verifying that Néel's theory is sufficient for the magnetic description of single‐domain assemblages spanning the superparamagnetic/stable single‐domain boundary. However, some deviations between modeled and measured susceptibilities exist, and the physical causes may include size‐dependent anisotropy, nonuniform magnetizations, and also an uncertain preexponential time “constant” τ0. While τ0 = 10−11 s gives the best fit for the sample with the largest grains, τ0 = 10−9 s is more reasonable for the others. Thus τ0 may indeed be size‐ and temperature‐dependent as predicted by Brown [1959]. The commonly cited parameter χfd (frequency dependence of susceptibility) reaches 30% at room temperature (RT) for one sample with a blocking temperatures just below RT, while χfd = 0 at RT for a superparamagnetic sample with smaller grains. These results thus exemplify that χfd is not limited to 15%, as a number of studies suggest, and that χfd = 0 must not be taken to imply the absence of superparamagnetic grains.
Locating hydrothermal vents by detecting buoyant, advected plumes
Authors: Veirs Scott R. , McDuff Russell E. , Lilley Marvin D. , Delaney John R.
Published: September 2002

An improved method of detecting buoyant hydrothermal plumes and locating their source vents is introduced. Plumes are detected by computing fluid stability from conductivity, temperature, and depth measurements acquired during navigated, towed, vertically oscillating casts over the Endeavour Segment of the Juan de Fuca Ridge. For each instability detected, the maximum range to its hydrothermal source is estimated by multiplying a theoretical plume equilibration time by a measured current velocity. Using an estimate of current direction, the method reliably locates plume sources where they are known to exist: in all of the focused vent fields mapped by submersible and in several isolated, diffuse flow sites. The method generates a distribution of hydrothermal sources that is more consistent with variations in surface permeability than with circulation cells spaced evenly along a uniformly permeable axis. Axial instabilities are nearly continuous along the heavily fissured and fractured western wall of the axial valley. Beyond the axial valley, instabilities evidence that hydrothermal upflow penetrates the outer slopes of the ridge crest, probably along a boundary between sheet and pillow flows.
Active folding of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal
Authors: Lavé J. , Avouac J. P.
Published: September 2002

We analyze geomorphic evidence of recent crustal deformation in the sub‐Himalaya of central Nepal, south of the Kathmandu Basin. The Main Frontal Thrust fault (MFT), which marks the southern edge of the sub‐Himalayan fold belt, is the only active structure in that area. Active fault bend folding at the MFT is quantified from structural geology and fluvial terraces along the Bagmati and Bakeya Rivers. Two major and two minor strath terraces are recognized and dated to be 9.2, 2.2, and 6.2, 3.7 calibrated (cal) kyr old, respectively. Rock uplift of up to 1.5 cm/yr is derived from river incision, accounting for sedimentation in the Gangetic plain and channel geometry changes. Rock uplift profiles are found to correlate with bedding dip angles, as expected in fault bend folding. It implies that thrusting along the MFT has absorbed 21±1.5 mm/yr of N‐S shortening on average over the Holocene period. The ±1.5 mm/yr defines the 68% confidence interval and accounts for uncertainties in age, elevation measurements, initial geometry of the deformed terraces, and seismic cycle. At the longitude of Kathmandu, localized thrusting along the Main Frontal Thrust fault must absorb most of the shortening across the Himalaya. By contrast, microseismicity and geodetic monitoring over the last decade suggest that interseismic strain is accumulating beneath the High Himalaya, 50–100 km north of the active fold zone, where the Main Himalayan Thrust (MHT) fault roots into a ductile décollement beneath southern Tibet. In the interseismic period the MHT is locked, and elastic deformation accumulates until being released by large (Mw> 8) earthquakes. These earthquakes break the MHT up to the near surface at the front of the Himalayan foothills and result in incremental activation of the MFT.
Identifying goethite and hematite from rock magnetic measurements of soils and sediments
Authors: France D. E. , Oldfield F.
Published: September 2002

We describe a sequence of experiments involving the acquisition of isothermal remanent magnetization (IRM) in high (>2 T) fields, followed by orthogonal demagnetization and the cooling and heating of remanent magnetization (Mr) over the temperature range −196°C to +680°C. The main aim of these experiments is to improve the basis for identifying the imperfect antiferromagnetic minerals contributing to the magnetic properties of natural soil and sediment samples, which often contain complex mineral mixtures. The procedures developed are first applied to a series of ore and synthetic samples of hematite and goethite, each characterized by X‐ray diffraction (XRD) spectra, and second to controlled mixtures of the two minerals. The results confirm the mineral‐diagnostic nature of the experiments used. They are subsequently used to identify goethite and hematite contributions to the rock magnetic properties of a variety of soils and recent sediments. Goethite can be unambiguously identified, often accompanied by hematite, in the majority of the samples studied, despite indications from the synthetic mixtures that the ratio of goethite to hematite may have to exceed an order of magnitude before the goethite signatures are readily recognizable. The rock magnetic tests are a much more sensitive basis for recognizing the presence of goethite in natural samples than are XRD analyses. The results suggest that goethite's contribution to the rock magnetic properties of soil and sediment samples may often have been underestimated.
Statistical analysis of dislocation dynamics during viscoplastic deformation from acoustic emission
Authors: Weiss Jérôme , Lahaie Franz , Grasso Jean Robert
Published: September 2002

We present experimental data of acoustic emission (AE) induced by dislocation motion during “pure” viscoplastic (ductile) deformation of singlecrystals and polycrystals of ice which provide opportunity to revisit collective dislocation dynamics as a critical phenomenon, as recently proposed for brittle fracturing. The data were recorded during compression and torsion creep experiments. AE statistics of power law type were systematically obtained under different experimental conditions. Among the possible candidates for such a system with threshold dynamics exhibiting power law statistics, critical points, disordered first‐order transitions, and self‐organized criticality should be considered. The revisitation of dislocation dynamics as a critical phenomenon allows rationalization of collective effects as well as of the heterogeneity and complexity of viscoplastic deformation of crystalline materials. Such critical behavior implies that dislocation avalanches and strain localizations are unpredictible, in a deterministic sense, in space, time, and energy domains and that large plastic instabilities account for most of the viscoplastic deformation.
Geophysical evidence for the evolution of the California Inner Continental Borderland as a metamorphic core complex
Authors: ten Brink Uri S. , Zhang Jie , Brocher Thomas M. , Okaya David A. , Klitgord Kim D. , Fuis Gary S.
Published: September 2002

We use new seismic and gravity data collected during the 1994 Los Angeles Region Seismic Experiment (LARSE) to discuss the origin of the California Inner Continental Borderland (ICB) as an extended terrain possibly in a metamorphic core complex mode. The data provide detailed crustal structure of the Borderland and its transition to mainland southern California. Using tomographic inversion as well as traditional forward ray tracing to model the wide‐angle seismic data, we find little or no sediments, low (≤6.6 km/s) P wave velocity extending down to the crust‐mantle boundary, and a thin crust (19 to 23 km thick). Coincident multichannel seismic reflection data show a reflective lower crust under Catalina Ridge. Contrary to other parts of coastal California, we do not find evidence for an underplated fossil oceanic layer at the base of the crust. Coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge, which represents the transition to the western Transverse Ranges. On the shelf the Palos Verdes Fault merges downward into a landward dipping surface which separates “basement” from low‐velocity sediments, but interpretation of this surface as a detachment fault is inconclusive. The seismic velocity structure is interpreted to represent Catalina Schist rocks extending from top to bottom of the crust. This interpretation is compatible with a model for the origin of the ICB as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks. The basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until ∼13 m.y. ago. Subduction of the young and hot Monterey and Arguello microplates under the Continental Borderland, followed by rotation and translation of the western Transverse Ranges, may have provided the necessary thermomechanical conditions for this extension and crustal inflow.
Seismic signature of Variscan and Alpine tectonics in NW Iberia: Crustal structure of the Cantabrian Mountains and Duero basin
Authors: Fernández‐Viejo Gabriela , Gallart Josep , Pulgar Javier A. , Córdoba Diego , Dañobeitia Juan José
Published: September 2002

The seismic structure of the crust of the NW Iberian Peninsula, along the Cantabrian Mountains and their southern foreland, the Duero basin, is investigated from the first data set available from refraction and wide‐angle reflection profiles. The velocity‐depth distributions obtained along E‐W and N‐S transects evidence lateral variations in crustal structure that can be associated to Variscan and Alpine tectonic episodes. The western part of the range, toward the Variscan hinterland zones, as well as the Duero basin below the sediments, show a crustal structure similar to that of the Variscan belt elsewere in Europe. The total crustal thickness is 30–32 km, and three main layers (upper, middle, and lower crust with average velocities 6.0, 6.25, and 6.8 km/s) are resolved. Some Paleozoic structures such as the basal detachment of the Cantabrian Zone are expressed in the seismic models. The crust appears to vary eastward in the external zones. The most outstanding feature is a crustal root beneath the highest Cantabrian summits where the Moho is found at 47 km depth. A prominent Alpine reworking of the crust in the eastern part of the Cantabrian Zone is thus revealed, and the N‐S seismic transects delineate a geometry similar to the one found across the central Pyrenees in the Etude Continentale et Océanique par Reflexion et Refraction Sismique (ECORS) profile, revealing the importance of the Alpine deformations in the northern part of the Iberian plate.
Shear wave anisotropy beneath the Andes from the BANJO, SEDA, and PISCO experiments
Authors: Polet J. , Silver P. G. , Beck S. , Wallace T. , Zandt G. , Ruppert S. , Kind R. , Rudloff A.
Published: September 2002

We present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western South America: the Broadband Andean Joint experiment (BANJO), a 1000‐km‐long east‐west line at 20°S, and the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) and Seismic Exploration of the Deep Altiplano (SEDA), deployed several hunderd kilometers north and south of this line. We determined the splitting parameters ϕ (fast polarization direction) and δt (splitting delay time) for waves that sample the above‐ and below‐slab regions: teleseismic *KS and S, ScS waves from local deep‐focus events, as well as S waves from intermediate‐focus events that sample only the above‐slab region. All but one of the *KS stacks for the BANJO stations show E‐W fast directions with δt varying between 0.4 and 1.5 s. However, for *KS recorded at most of the SEDA and PISCO stations, and for local deep‐focus S events north and south of BANJO, there is a rotation of ϕ to a more nearly trench parallel direction. The splitting parameters for above‐slab paths, determined from events around 200 km deep to western stations, yield small delay times (≤0.3 s) and N‐S fast polarization directions. Assuming the anisotropy is limited to the top 400 km of the mantle (olivine stability field), these data suggest the following spatial distribution of anisotropy. For the above‐slab component, as one goes from east (where *KS reflects the above‐slab component) to west, ϕ changes from E‐W to N‐S, and delay times are substantially reduced. This change may mark the transition from the Brazilian craton to actively deforming (E‐W shortening) Andean mantle. We see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion. The small delay times for above‐slab paths in the west require the existence of significant, spatially varying below‐slab anisotropy to explain the *KS results. The implied anisotropic pattern below the slab is not easily explained by a simple model of slab‐entrained shear flow beneath the plate. Instead, flow induced by the retrograde motion of the slab, in combination with local structural variations, may provide a better explanation.
Basal mechanics of Ice Stream B, west Antarctica: 2. Undrained plastic bed model
Authors: Tulaczyk Slawek , Kamb W. Barclay , Engelhardt Hermann F.
Published: September 2002

Based on the results of our studies of the physical conditions beneath Ice Stream B, we formulate a new analytical ice stream model, the undrained plastic bed model (henceforth the UPB model). Mathematically, the UPB model is represented by a non‐linear system of four coupled equations which express the relationships among ice sliding velocity, till strength, water storage in till, and basal melt rate. We examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant (less than hundreds of years). Temporal variability is introduced into the UPB model only by the direct dependence of till void ratio changes (ė = ∂e/∂t) on the basal melting rate mr. Since till strength τb{e} and ice stream velocity Ub{τb} change as long as till void ratio varies, the first condition for ice stream stability is that of constant till water storage ė = 0. The second condition for ice stream stability arises from the feedback between ice stream velocity, till strength, and the basal melting rate which depends on shear heating mr{ Ub τb}. This is the “weak till” condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress τb < (n + 1)−1 τd. The salient feature of the UPB model is its ability to produce two thermo mechanically controlled equilibrium states, one with a strong bed and slow ice velocities (“ice sheet” mode) and one with a weak bed and fast ice velocities (“ice‐stream” mode). This bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in West Antarctica. Basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes. The UPB model demonstrates that ice streams may be prone to thermally triggered instabilities, during which small perturbations in the basal thermal energy balance grow, leading to generation or elimination of the basal conditions which cause ice streaming.
CO2 emissions related to the Yellowstone volcanic system: 2. Statistical sampling, total degassing, and transport mechanisms
Authors: Werner C. , Brantley S. L. , Boomer K.
Published: September 2002

A stratified adaptive sampling plan was designed to estimate CO2 degassing in Yellowstone National Park and was applied in the Mud Volcano thermal area. The stratified component of the sampling design focused effort in thermal areas and the adaptive component in high‐flux regions, yet neither sampling technique biased the estimate of total degassing. Both diffuse soil fluxes (up to ∼30,000 g m−2d−1) and emission rates from thermal vents (up to 1.7×108 mol yr−1) were measured in thermal areas. Soil fluxes observed in most nonthermal regions were similar to values reported for conifer forests (≤ 15 g m−2 d−1). However, through adaptive sampling, high‐flux vegetated sites were identified in Mud Volcano that likely would not have been found if sampling was focused in obvious thermal or altered regions. A simple model applied to flux measurements suggests that ∼40% of the analyzed measurements were dominated by possible advective transport and ∼30% by diffusive transport. Isotopic signatures of soil CO2 generally suggest a deep origin (δ13C = −2.3 to 0.0) in thermal areas and biogenic origin (δ13C = −20.5) in nonthermal, low‐flux areas. Vent emissions accounted for ∼32–63% of the total degassing observed at Mud Volcano (2.4 to 4.0×109 mol yr−1). The largest source of error in the estimation of total degassing (factor of ∼2) resulted because the population distribution of thermal feature emissions was indeterminate. Total CO2 emissions at Mud Volcano are comparable to other hydrothermal regions worldwide, suggesting that the Yellowstone volcanic system is likely a large contributor to global volcanic/metamoohic/hydrothermal (VMH) emissions.
Strain partitioning and interplate friction in oblique subduction zones: Constraints provided by experimental modeling
Authors: Chemenda A. , Lallemand S. , Bokun A.
Published: September 2002

Physical modeling of oblique subduction is performed to study the mechanism of strain partitioning. The model is two‐layer and includes the elasto‐plastic lithosphere (the overriding and subducting plates) and the low‐viscosity liquid asthenosphere. The subduction is driven by a push force from a piston and a pull force when the density contrast Δρ between the subducting plate and the asthenosphere is positive. We vary both Δρ and the interplate friction (frictional stresses). Slip partitioning is obtained only in the models with high interplate friction and only when the overriding plate contains a weak zone. This zone in the models corresponds either to locally thinned lithosphere or to cut (fault). The horizontal, trench‐normal component of the interplate friction force |Fph| can be comparable with the absolute value of the horizontal component of the nonhydrostatic interplate pressure force |Fph | in the subduction zone. Ffh is always negative (compression), while Fph can be either negative (compressional subduction regime) or positive (extensional regime). High friction, which promotes partitioning, can completely cancel the extensional (suction) force Fph. Back arc tension and strike‐slip faulting appear thus as conflicting processes, although they can coexist in the same subduction zone, depending on the relative values of relevant forces. It appears that high friction can exist only in compressional subduction zones where partitioning should develop more easily. This conclusion is supported by the comparison of two oblique subduction zones, having similar geometry: the compressional southern Kurile zone (strong partitioning) and extensional southern Ryukyu zone (no lithospheric‐scale partitioning). Other factors controlling the strain partitioning are the length of the oblique subduction zone, the boundary conditions at the transverse limits of the forearc sliver, and of course, the obliquity of subduction.
Influence of friction and fault geometry on earthquake rupture
Authors: Nielsen Stefan B. , Carlson J. M. , Olsen Kim B.
Published: September 2002

We investigate the impact of variations in the friction and geometry on models of fault dynamics. We focus primarily on a three‐dimensional continuum model with scalar displacements. Slip occurs on an embedded two‐dimensional planar interface. Friction is characterized by a two‐parameter rate and state law, incorporating a characteristic length for weakening, a characteristic time for healing, and a velocity‐weakening steady state. As the friction parameters are varied, there is a crossover from narrow, self‐healing slip pulses to crack‐like solutions that heal in response to edge effects. For repeated ruptures the crack‐like regime exhibits periodic or aperiodic systemwide events. The self‐healing regime exhibits dynamical complexity and a broad distribution of rupture areas. The behavior can also change from periodicity or quasi‐periodicity to dynamical complexity as the total fault size or the length‐to‐width ratio is increased. Our results for the continuum model agree qualitatively with analogous results obtained for a one‐dimensional Burridge‐Knopoff model in which radiation effects are approximated by viscous dissipation.
Time‐dependent triggered afterslip following the 1989 Loma Prieta earthquake
Authors: Segall Paul , Bürgmann Roland , Matthews Mark
Published: September 2002

We employ a modified version of the Network Inversion Filter to investigate time‐dependent slip following the 1989 Loma Prieta earthquake. Previous analysis of Global Positioning System (GPS) and leveling data suggests afterslip on the Loma Prieta rupture as well as aseismic slip on a thrust fault northeast of the San Andreas fault which we identify with the Foothills thrust belt. We analyzed 173 daily GPS solution files at 62 stations collected from 1989.8 to 1998.3 (a total of 1,134 three‐dimensional relative baseline determinations). The observed position changes are assumed to result from secular deformation, random benchmark motions, and temporally varying fault slip. The data reveal temporal variations in slip rate but poorly resolve spatial variations in fault slip. The amount of temporal smoothing is estimated by maximum likelihood. Conditional on this estimate, reverse slip on the Foothills thrust decays from 45±12 mm/yr immediately after the earthquake to zero by 1992. Reverse slip on the Loma Prieta rupture surface decays from 57±11 mm/yr to zero by 1994. Right‐lateral slip on the Loma Prieta rupture surface decays monotonically from 30±10 mm/yr to zero by 1994. These results suggest that (1) triggered afterslip can occur off the main rupture zone on adjacent faults, (2) shallow afterslip dominated the postseismic deformation for the 8 years following the earthquake, and (3) postseismic slip on the Foothills thrust may account for a significant portion of its total slip budget.
Rock magnetic properties related to thermal treatment of siderite: Behavior and interpretation
Authors: Pan Yongxin , Zhu Rixiang , Banerjee Subir K. , Gill J. , Williams Q.
Published: September 2002

Detailed analyses of rock magnetic experiments were conducted on the oxidation products of high‐purity natural crystalline siderite that were thermally treated in air atmosphere. Susceptibilities increase sharply between 400° and 530°C indicative of some new ferrimagnetic mineral phase generation. Both a drop (between 540° and 590°C) on the heating cycle and a dramatic increase (from 590°C to 520°C) on the cooling cycle occurred and are well consistent with the characteristic of magnetite. A distinct Hopkinson‐type susceptibility peak indicates that hematite is the terminal product if siderite is heated to 700°C over and over. It has been revealed in detail that the original inverse magnetic susceptibility fabric contributed by the crystalline anisotropy of siderite in siderite‐bearing specimens is changed to a normal magnetic fabric during incremental heating over 410°–490°C. This is a result of dominant contributions from the distribution anisotropy of newly transformed ferromagnetic minerals. A strong chemical‐viscous remanent magnetization could be produced during siderite oxidation in an external field. Rock magnetic experimental results show that magnetite, maghemite, and hematite are the transformation products of high‐temperature oxidation of siderite in air. Maghemite was not completely inverted to hematite even at temperature as high as 690°C during incremental thermal treatments. The mineral transformation processes were confirmed by conventional optical microscopic observation, X‐ray diffractometry and Mössbauer spectroscopic analyses. These results indicate that the rock magnetic methods used here are reliable and highly sensitive in detecting very small magnetic phase changes in rocks. We conclude that these temperaturedependent variations of magnetic properties can be used as criteria for identification of siderite in rocks and sediments. Furthermore, it is clear that great care should be exercised in thermal demagnetization of siderite‐bearing rocks in paleomagnetic, magnetic anisotropy, and rock magnetic studies.
Shear wave splitting, continental keels, and patterns of mantle flow
Authors: Fouch Matthew J. , Fischer Karen M. , Parmentier E. M. , Wysession Michael E. , Clarke Timothy J.
Published: September 2002

In this study we investigated the origin of seismic anisotropy in the mantle beneath North America. In particular, we evaluated whether shear wave splitting patterns in eastern North America are better explained by anisotropy caused by lithospheric deformation, anisotropy due to mantle flow beneath the lithosphere, or a combination of both. We examined new measurements of shear wave splitting from the Missouri to Massachusetts broadband seismometer array (MOMA), the North American Mantle Anisotropy and Discontinuity experiment (NOMAD), as well as splitting parameters from several previous studies. We developed a simple finite difference model that approximates mantle flow around a complex, three‐dimensional continental lithospheric keel. To evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern North America, we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model. Our results indicate that a significant portion of observed shear wave splitting in eastern North America can be explained by mantle flow around the continental keel. However, shear wave splitting patterns in a few regions of eastern North America indicate that a component of lithospheric anisotropy must exist, particularly in regions containing the largest keel thicknesses. For eastern North America, as well as for splitting observations in Australia, Europe, and South America, we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow.
Crustal magnetization of the Reykjanes Ridge and implications for its along‐axis variability and the formation of axial volcanic ridges
Authors: Lee Sang‐Mook , Searle Roger C.
Published: September 2002

We have inverted the sea surface total magnetic field measurements of the Reykjanes Ridge, between 57°N and 63°N, for crustal magnetization intensity assuming a uniformly thick magnetic source layer and direction of magnetization corresponding to the geocentric axial dipole field. This section of the north Mid‐Atlantic Ridge (10 mm/yr half spreading rate) is unique in that it lies within the influence of the Iceland mantle plume, which causes it to spread obliquely and in the north to resemble morphologically a fast‐spreading ridge. On a regional scale the axial magnetization anomalies of the Reykjanes Ridge can be divided into three regimes from south to north. These regimes correspond to sections of the ridge with different magmatic and tectonic styles which are believed to represent the effects of a pulsing plume beneath Iceland. The axis south of 59°10′N has the highest magnetization anomaly with the greatest variability and exhibits the characteristics of a slow‐spreading ridge unaffected by a plume. The axis between 59°10′N and 60°50′N has the lowest magnetization anomaly on average and is where we presume the present day plume front is located. The absence of large fault scarps and low magnetization are consistent with the idea of a midcrustal magma reservoir, where mixing causes the magma to become less highly fractionated and produces less magnetic lavas. Finally, a moderate increase in the magnetization anomaly and degree of faulting occurs north of 60°50′N, signaling a gradual recovery in the crustal production and underlying structure after the passage of the plume front. On the local scale, with a few exceptions, the loci of high magnetization anomalies coincide with the axial volcanic ridges. Notable lateral offsets in axial magnetization and decay in magnitude were found at 57°50′N, 58°05′N, and 59°12′N. Comparisons with seismic layer 2A show that variations in the magnetic source layer thickness may account for as much as half of the observed variability in the along‐axis magnetization. There are also indications that the relative age of the extrusives and the degree of faulting and fissuring may be factors affecting the magnetization intensity of volcanic edifices on the seafloor.
Western Nankai Trough seismogenic zone: Results from a wide‐angle ocean bottom seismic survey
Authors: Kodaira Shuichi , Takahashi Narumi , Park Jin‐Oh , Mochizuki Kimihiro , Shinohara Masanao , Kimura Shozo
Published: September 2002

The Nankai Trough, southwestern Japan, is recognized as a vigorous seismogenic zone with well‐studied historic earthquakes. This paper presents results of a wide‐angle ocean bottom seismographs (OBS) study at the western Nankai Trough seismogenic zone. The OBS data used were acquired on a profile (250 km long) across the presumed coseismic slip zone of the 1946 Nankaido earthquake (Ms = 8.2). The main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake. The crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge. P wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities (Vp = 5.0–5.6 km/s and 6.6–6.8 km/s in oceanic layers 2 and 3, respectively). The maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with Vp = 3.4–4.6 km/s in the deeper part. The subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward: 3.2°and 7.2° at 0–50 km and 50–100 km from the trough axis, respectively. The oceanic crust is smooth to the hypocenter zone, down to 40 km depth beneath Shikoku Island. Our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust‐island arc crust contact zone. Even though there is large uncertainty about the seaward limit of the coseismic slip zone, the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism.
On the anisotropy of secular variation deduced from paleomagnetic volcanic data
Authors: Love J. J.
Published: September 2002

We seek to establish whether or not secular variation, the rate at which the magnetic field is changing in time, is a function of field direction: Is secular variation anisotropic? From a large number of paleomagnetic lava data, many of which record polarity transitions, we find that vectors from stratigraphically adjacent flows are most (least) correlated during nontransitional (transitional) periods. Since volcanic activity is unrelated to, and therefore uncorrelated with, magnetic secular variation, this relationship indicates that secular variation is enhanced during transitions, perhaps by a factor of ∼2. Thus secular variation is anisotropic, being a function of the deviation of field direction from an axial dipole. Despite this spatial dependence, we find little convincing evidence that transitional fields exhibit persistent recurring quasi‐stationary states which manifest themselves in terms of midlatitude virtual geomagnetic poles (VGPs), as has sometimes been hypothesized. On the other hand, we identify tentatively some relative quiescence in secular variation during midtransition when VGPs fall near the equator. Numerous subsets of the database are examined for consistency and statistical errors are estimated by bootstrap analyses. Statistical models of secular variation need to incorporate the information content in serial correlations, like those analyzed here, from stratigraphie sections if the lava data are to be fully exploited.
Kinematics of active deformation in the Sulaiman Lobe and Range, Pakistan
Authors: Bernard M. , Shen‐Tu B. , Holt W. E. , Davis D. M.
Published: September 2002

The western margin of the Indian plate is highly oblique to the direction of convergence between India and Asia and represents an excellent example of large‐scale oblique continent‐continent collision. Determining the strain field in western Pakistan and how it relates to the plate motion and plate margin geometry affords an exceptional opportunity for understanding oblique margin processes in general. Through the inversion of regional and teleseismic body waves, we have determined the source parameters of 10 moderate‐sized earthquakes that occurred between 1964 and 1985 in and around the Sulaiman Range, Pakistan. The earthquakes are dominantly thrust events with slip vectors that are approximately perpendicular to the lobate Sulaiman mountain front. Slip vector orientations rotate 60°–70° from a N‐S to a WNW‐ESE direction of compression, consistent with the geometries of the complex, festoon‐shaped mountain belts of this margin. We have estimated the spatial variation of the horizontal strain rate and velocity fields within Sulaiman using vertically averaged models that accommodate plate motion constraints within a deforming layer. The most important factors determining the style of strain rotation in the Sulaiman Lobe and Range are the presence of pure strike‐slip motion along the Chaman Fault, and the relatively rigid and undeformed Katawaz Basin that is therefore allowed to translate obliquely relative to India. This same conclusion is obtained using either a three‐dimensional, frictional, analogue model with significant basal tractions or a thin sheet viscous numerical model without basal tractions. Thrusting in a predominantly NW‐SE direction in the Sulaiman Range accommodates 5–14 mm/yr of N‐S motion between India‐Eurasia and 3–6 mm/yr of E‐W shortening. Seismic moment release this century within the India‐Eurasia plate boundary zone, west of the western Himalayan Syntaxis, constitutes roughly 40% of the expected total seismic moment release for this time period. Particularly significant moment rate deficits exist within the Sulaiman Range and along the Chaman Fault.
Induced earthquakes accompanying the water injection experiment at the Nojima fault zone, Japan: Seismicity and its migration
Authors: Tadokoro Keiichi , Ando Masataka , Nishigami Kin'ya
Published: September 2002

The 1995 Hyogo‐ken Nanbu (Kobe) earthquake of M7.2 occurred on January 17, 1995. After the earthquake, a scientific drilling program called the Nojima Fault Zone Probe was carried out at the Nojima fault which ruptured during the mainshock. Water was injected during two periods, February 9–13 and March 16–25, 1997. The pumping pressure at the surface was about 4 MPa. Pressurized water was injected into a 1800‐m‐deep borehole and supplied to the surrounding rock at depths between 1480 and 1670 m. The total amount of injected water was 258 m3. An increase in earthquake activity was observed 4 or 5 days after the beginning of each water injection. The seismicity increased in the region around 3 or 4 km from the injection point. This suggests that these earthquakes are likely to have been induced by the water injections. The induced earthquakes were located between 2 and 4 km in depth and had magnitudes ranging from −2 to +1. The hypocenters of the induced earthquakes migrated with speeds of ∼2–40 m/h. The speeds decreased with time, suggesting a relationship with the diffusion of water. Values of intrinsic permeability of 10−14–10−15 m2 were obtained from the time dependency of induced seismicity change. The coefficient of friction in the area where the induced earthquakes occurred was estimated to be less than 0.3. Twenty earthquake clusters were found, and cross‐spectrum analysis was applied to them. We could distinguish between the induced and noninduced earthquakes from the analysis. The induced earthquakes forming each cluster migrated with speeds of 20–80 m/h, which means microscale water migration or permeation. The present water injection experiment revealed that the Nojima fault zone was highly permeable and could slip with small (∼10% or less) increases in pore fluid pressure or shear stress.
Fission track analysis of the footwall of the Catalina detachment fault, Arizona: Tectonic denudation, magmatism, and erosion
Authors: Fayon Annia K. , Peacock Simon M. , Stump Edmund , Reynolds Stephen J.
Published: September 2002

New apatite and zircon fission track ages obtained from the footwall of the Catalina metamorphic core complex record a complicated cooling history associated with mid‐Tertiary extension. Zircon fission track ages record the progressive unroofing of the Catalina metamorphic core complex along the Catalina detachment fault. Zircon fission track ages range from 31.9 to 19.4 Ma, generally decrease in the hanging‐wall slip direction, and yield slip rates along the Catalina detachment fault ranging from 1.2 to 12 km Myr−1. In contrast, apatite fission track ages increase in the hanging wall slip direction. Samples from the main range of the Santa Catalina Mountains yield apatite fission track ages of 20.5 to 14.6 Ma; samples from the Santa Catalina Mountain forerange, located closer to the detachment fault, yield apatite fission track ages of 21.4 to 18.8 Ma. Rapid cooling (40° to 60 °C Myr−1) related to detachment faulting is best recorded by zircon fission track ages and higher‐temperature thermochronometers in the main range and by nearly concordant zircon and apatite fission track ages in the forerange. Slower cooling (3°–7°C Myr−1) of the footwall is recorded by shortened mean confined fission track lengths (<14 μm) and is related to erosional unroofing. Approximately 2 km of late‐Tertiary erosion played a significant role in the unroofing of the footwall of the Catalina metamorphic core complex in contrast to metamorphic core complexes in western Arizona, where detachment faulting is the dominant unroofing mechanism.
Crustal attenuation and site effects at Parkfield, California
Authors: Abercrombie Rachel E.
Published: September 2002

At Parkfield, California, the San Andreas fault separates strongly contrasting rock types: high velocity Salinian granite to the SW and lower velocity Franciscan basement to the NE. I use the recordings of nine earthquakes in 10 boreholes (≤1000 m deep) and calculate P and S wave spectral ratios between borehole recordings at different depths to eliminate the source. I then invert for Q on each side of the San Andreas fault. I find that the attenuation on the NE side of the fault (QNE ∼100) is approximately twice that on the SW side (QSW ∼200) in the depth range ∼200 m to 5 km. QP and QS are similar, but their ratio is not well resolved. I estimate QP ∼50 and QS ∼80 in the fault zone itself by combining recordings at a site within the fault zone with the inversion results and confirm that it is a low‐Q zone. Q is also observed to increase with depth, and the rate of this increase decreases as the depth increases. In the upper 1 km at the Varian well (on the NE side of the San Andreas fault), QP ∼30 and QS ∼20, with QP increasing from ∼20 between 0 and 300 m to ∼55 between 600 and 900 m. This study demonstrates that the borehole seismometers at Parkfield are not deep enough for the effects of attenuation to be ignored in analysis of small earthquakes. Also, the attenuation below 1 km on the NE side of the fault is actually larger than that below ∼200 m on the SW side. The results of this study suggest that the attenuation at depths of a few hundred to a few thousand meters should be considered when estimating site effects in seismic hazard studies.
Origin of Lüders' bands in deformed rock
Authors: Olsson William A.
Published: September 2002

Lüders' bands are shear deformation features commonly observed in rock specimens that have been deformed experimentally in the brittle‐ductile transition regime. For specimens that contain both faults (shear fractures that separate the specimen) and bands the bands form earlier in the deformation history, and their orientations are often different from the fault. These differences pose the question of the relationship between these two structures. Understanding the origin of these features may shed light on the genesis of apparent natural analogues and on the general process of rock deformation and fracture in the laboratory. This paper presents a hypothesis for the formation of Lüders' bands in laboratory specimens based on deformation localization theory considered in the context of the nonuniform stress distribution of the conventional triaxial experiment. Lüders' bands and faults appear to be equivalent reflections of the localization process as it is controlled by nonuniform distributions of stress and evolution of incremental constitutive parameters resulting from increasing damage. To relate conditions for localization in laboratory specimens to natural settings, it will be necessary to design new experiments that create uniform stress and deformation fields or to extract constitutive data indirectly from standard experiments using computational means.
On rate‐state and Coulomb failure models
Authors: Gomberg J. , Beeler N. , Blanpied M.
Published: September 2002

We examine the predictions of Coulomb failure stress and rate‐state frictional models. We study the change in failure time (clock advance) Δtdue to stress step perturbations (i.e., coseismic static stress increases) added to “background” stressing at a constant rate (i.e., tectonic loading) at timet0. The predictability of Δtimplies a predictable change in seismicity rater(t)/r0, testable using earthquake catalogs, wherer0is the constant rate resulting from tectonic stressing. Models ofr(t)/r0, consistent with general properties of aftershock sequences, must predict an Omori law seismicity decay rate, a sequence duration that is less than a few percent of the mainshock cycle time and a return directly to the background rate. A Coulomb model requires that a fault remains locked during loading, that failure occur instantaneously, and that Δtis independent oft0. These characteristics imply an instantaneous infinite seismicity rate increase of zero duration. Numerical calculations ofr(t)/r0for different state evolution laws show that aftershocks occur on faults extremely close to failure at the mainshock origin time, that these faults must be “Coulomb‐like,” and that the slip evolution law can be precluded. Real aftershock population characteristics also may constrain rate‐state constitutive parameters;amay be lower than laboratory values, the stiffness may be high, and/or normal stress may be lower than lithostatic. We also compare Coulomb and rate‐state models theoretically. Rate‐state model fault behavior becomes more Coulomb‐like as constitutive parameteradecreases relative to parameterb. This is because the slip initially decelerates, representing an initial healing of fault contacts. The deceleration is more pronounced for smaller a, more closely simulating a locked fault. Even when the rate‐state Δthas Coulomb characteristics, its magnitude may differ by some constant dependent onb. In this case, a rate‐state model behaves like a modified Coulomb failure model in which the failure stress threshold is lowered due to weakening, increasing the clock advance. The deviation from a non‐Coulomb response also depends on the loading rate, elastic stiffness, initial conditions, and assumptions about how state evolves.
Depth‐dependent structure of the Landers fault zone from trapped waves generated by aftershocks
Authors: Li Yong‐Gang , Vidale John E. , Aki Keiiti , Xu Fei
Published: September 2002

We delineate the internal structure of the Johnson Valley and Kickapoo faults (Landers southern rupture) at seismogenic depth using fault zone trapped waves generated by aftershocks. Trapped waves recorded at the dense linear seismic arrays deployed across and along the surface breaks of the 1992 M7.5 Landers earthquake show large amplitudes and dispersive wave trains following the S waves. Group velocities of trapped waves measured from multiple band‐pass‐filtered seismograms for aftershocks occurring at different depths between 1.8 km and 8.2 km show an increase in velocity with depth. Velocities range from 1.9 km/s at 4 Hz to 2.6 km/s at 1 Hz for shallow events, while for deep events, velocities range from 2.3 km/s at 4 Hz to 3.1 km/s at 1 Hz. Coda‐normalized amplitude spectra of trapped waves peak in amplitudes at 3–4 Hz for stations located close to the fault trace. The amplitude decays rapidly with the station offset from the fault zone. Normalized amplitudes also decrease with distance along the fault, giving an apparent Q of 30 for shallow events and 50 for deep events. We evaluated depth‐dependent fault zone structure and its uncertainty from these measurements plus our previous results from near‐surface explosion‐excited trapped waves [Li et al., 1999] in a systematic model parameter‐searching procedure using a three‐dimensional (3‐D) finite difference computer code [Graves, 1996]. Our best model of the Landers fault zone is 250 m wide at the surface, tapering to 100–150 m at 8.2 km depth. The shear velocity within the fault zone increases from 1.0 to 2.5 km/s and Q increases from 20 to 60 in this depth range. Fault zone shear velocities are reduced by 35 to 45% from those of the surrounding rock and also vary along the fault zone with an increase of ∼10% near ends of the southern rupture zone.