bluuebunny/crossref_metadata_2025 huggingface dataset:
Tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases (PKP, Pdiff)
Authors: Kárason Hrafnkell , van der Hilst Rob D.
Published: February 2004

The mapping of variations in P wave speed in the deep mantle is restricted by the uneven sampling of P waves, in particular beneath the Southern Hemisphere. To enhance data coverage, we augmented the ∼1.6 million summary rays of P, pP, and pwP that we used in previous studies with differential travel times of diffracted and refracted core phases. For the core‐refracted differential travel time residuals (PKPAB‐PKPDF and PKPAB‐PKPBC) we used 1383 cross‐correlated digital waveforms as well as ∼27,000 routinely processed bulletin data. We used the waveform data to define quality criteria for the selection and reduction of the bulletin PKP data. For PKPDF‐Pdiff we only considered 543 records derived from waveform cross correlation. No PcP data were used in this study. We used optical ray theory to calculate the ray paths associated with the P, pP, pwP, and PKP data, which are measured at 1 Hz. However, to account for the large Fresnel zones of the low‐frequency (∼50 mHz) PKPDF‐Pdiff data we estimated the three‐dimensional shape of the Fréchet sensitivity kernels from kernels calculated by normal mode summation. The use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high‐frequency data to constrain small‐scale structure. The differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers, and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary (CMB). However, images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle. Here, we do not make a priori assumptions about the mantle source of anomalous time differentials. From test inversions we conclude that (both upper and lower) mantle structures that are poorly resolved by P data can be mapped into the core along PKP paths but that the effect of outer core structures, if any, on the mantle model is small. Compared to the inversion of the P, pP, and pwP alone, the inclusion of the PKPAB‐PKPDF and PKPAB‐PKPBC and PKPDF‐Pdiff data improves the resolution of structure beneath 2200 km depth. In particular, the joint inversion puts better constraints on the long‐wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the CMB that is in agreement with but still smaller than inferences from shear wave studies. Resolution tests indicate that in some regions the enhanced definition of structure is significant, but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle.
Spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting Pacific plate beneath the northeastern Japan arc: A triple‐planed deep seismic zone
Authors: Igarashi Toshihiro , Matsuzawa Toru , Umino Norihito , Hasegawa Akira
Published: February 2004

The northeastern Japan arc is located in one of the most seismically active subduction zones in the world. In this study, we relocated hypocenters and determined focal mechanisms of small earthquakes (M≤5) beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate. In the hypocenter relocation we adopted a “source region station correction method” in which station corrections vary with hypocenter locations. We have developed a new focal mechanism determination technique named “master solution method,” which is analogous to the “master event method” in hypocenter determination. We applied the method to P and SH wave amplitude data to obtain 1106 focal mechanism solutions. From the new mechanism solutions and relocated hypocenters we found that there occur both low‐angle thrust fault (LT) type and downdip compression (DC) type earthquakes at depths from 40 to 70 km near the aseismic front; the DC type events are underlying the LT‐type events. Almost all the earthquake clusters are composed of LT‐type events. The western limit of the region where LT‐type events have occurred is subparallel to the trench axis, although it undulates considerably; it delineates the westernend of the active region of interplate seismicity. Furthermore, we found that normal fault (NF) type events also occur at depths from 70 km in the upper plane of the double‐planed deep seismic zone, which is characterized mainly by DC‐type event. These NF‐type events are distributed only in a thin uppermost portion of the slab close to the plate boundary. Below these, in the lower plane, are downdip extension (DE) type events. This result indicates that the deep seismic zone in the northeastern Japan arc is not double‐planed but triple‐planed, even beneath the land area, which cannot be explained by any simple models.
Large‐scale crustal deformation of the Tibetan Plateau
Authors: Shen Feng , Royden Leigh H. , Burchfiel B. Clark
Published: February 2004

The topography, velocity, and strain fields calculated from a three‐dimensional Newtonian viscous model for large‐scale crustal deformation are generally in good agreement with results from geological, geodetic and earthquake studies in and around the Tibetan Plateau, provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau (equivalent to a viscosity of 1012 Pa s within a 250‐m‐thick channel or 1018 Pa s within a 15‐km‐thick channel). Model studies and observations show a plateau at ∼5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins. Rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins (at rates ∼20 mm/yr). Model results show north‐south shortening (∼10 mm/yr) in reasonable agreement with GPS data (5–8 mm/yr of north‐south shortening across the northern two thirds of the plateau) and east‐west stretching (10–15 mm/yr) across the eastern half of the high plateau, in reasonable agreement with seismic, geologic, and GPS data. Upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east. Clockwise rotation of material around the east Himalayan syntaxis (at rates up to ∼10 mm/yr) occurs partly as a result of dextral shear between Indian and Asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east. There is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust, except in southern and northern Tibet where margin‐perpendicular extension occurs only for the case of an extremely weak lower crust. Our results suggest that the Tibetan Plateau is likely to have gone through a two‐stage development. The first stage produced a long, narrow, high orogen whose height may have been comparable to the modern plateau. The second stage produced a plateau that grew progressively to the north and east. East‐west stretching, eastward plateau growth and dextral rotation around the east Himalayan syntaxis probably did not begin until well into the second stage of plateau growth, perhaps becoming significant after ∼20 m.y. of convergence.
Estimating the thickness of the free gas zone beneath Hydrate Ridge, Oregon continental margin, from seismic velocities and attenuation
Authors: Tréhu Anne M. , Flueh Ernst R.
Published: February 2004

Recent Ocean Drilling Program (ODP) results in the Oregon accretionary prism and on the Blake Ridge indicate that the zone containing free gas beneath the hydrate‐bearing near‐surface sediments is considerably thicker than previously thought. In this paper, we present results from travel time inversion of refracted seismic waves that show very low (<1.85 km/s) velocities extending for 500–600 m beneath the base of the gas hydrate stability zone in Hydrate Ridge on the Oregon continental margin near ODP site 892. The low‐velocity near‐surface layer extends across Hydrate Ridge and beneath the adjacent continental slope to the east. Because Pliocene sediments are exposed at the crest of Hydrate Ridge in an erosional setting, we suggest that these low velocities indicate the extent of a zone of dispersed free gas rather than recent sedimentation. Strong frequency‐dependent attenuation of amplitudes is observed for P waves crossing this zone. Amplitude spectra, referenced to spectra for similar accretionary complex paths that do not cross the interpreted gassy layer, indicate a very low P wave quality factor (Qp) within this zone, with Qp∼12 compared to Qp>100 in the “normal” accretionary complex sediments west of Hydrate Ridge. These results suggest that refraction seismic techniques are a powerful way to constrain the depth to which free gas is present in sediments beneath the hydrate stability zone. Defining the extent of the free gas zone is an important factor for estimating the total volume of gas present and for evaluating its impact on slope stability and potential contribution to global climate change.
Influence of pore fluid chemistry on the complex conductivity and induced polarization responses of Berea sandstone
Authors: Lesmes David P. , Frye Kevin M.
Published: February 2004

The spectral induced‐polarization (IP) response of rocks and soils is a complex function of pore solution chemistry, sample microgeometry, and surface chemical properties. We measure the complex conductivity and the time domain IP responses of Berea sandstone as a function of pore fluid ionic strength and pH. Complex conductivity is measured over the frequency range 10−3 to 106 Hz, and chargeability is computed using a time window of 0.16 to 1.74 s. The field IP parameters: phase, percent frequency effect, and chargeability are functions of both the surface and bulk electrical properties of the sample and are observed to decrease with increasing solution conductivity. Dividing these parameters by the sample resistivity yields normalized IP parameters (quadrature conductivity, metal factor, normalized chargeability) that are proportional to the imaginary component of the complex surface conductivity. Normalized IP parameters increase with ionic strength up to concentrations of 10−1 M NaCl and show a reduced response at pH 3, the point of zero charge for quartz‐dominated systems. For concentrations >10−1 M NaCl, the normalized parameters decrease with increasing concentration. This decrease in surface polarization may indicate a decrease in the effective mobility of polarizing charges at high solution concentration. Our data indicate that normalized IP parameters are directly related to the physiochemical parameters that control the surface conductivity responses of rocks and soils. Normalization of IP measurements in environmental investigations should increase the effectiveness of IP surveys, especially in high‐conductivity environments.
Variations in axial morphology, segmentation, and seafloor roughness along the Pacific‐Antarctic Ridge between 56°S and 66°S
Authors: Ondréas Hélène , Aslanian Daniel , Géli Louis , Olivet Jean‐Louis , Briais Anne
Published: February 2004

The spreading rate at the Pacific‐Antarctic Ridge (PAR) increases rapidly from 54 mm/yr near Pitman Fracture Zone (FZ) up to 76 mm/yr near Udintsev FZ, resulting in three domains of axial morphology: an axial valley south of Pitman FZ, an axial high north of Saint Exupéry FZ, and in between, the transitional domain extends over 650 km. It comprises sections of ridge with an axial valley or an axial high and generally displays a very low cross‐sectional relief. It is also characterized by two propagating rifts. Two domains of different seafloor roughness appear south of Udintsev FZ: east of 157°W these two domains are separated by a 1000‐km V‐shaped boundary. West of 157°W, the boundary approximately coincides with Chron 3a or Chron 4. The southward migration of the transitional area during the last 35 Myr explains the V‐shaped boundary: (1) increases in spreading rate above a threshold value produced changes in axial morphology; and (2) in the transition zone, rotations of the spreading direction were accommodated by the plate boundary, either by rift propagation or by transitions from fracture zones to non transform discontinuities, leaving trails on the seafloor that presently delineate the V. Seafloor roughness variations are not controlled by exactly the same spreading rate dependence as changes in axial morphology. The transition from rough to smooth seems to have occurred everywhere for spreading rates greater than 50 mm/yr, except in a domain presently centered on Saint‐Exupéry FZ, where it occurred for spreading rates >60 to 65 mm/yr. Independent results from melting model calculations of major elements [Vlastelic et al., 2000] indicate that the upper mantle temperature is likely to be cooler between Antipodes and La Rose FZs. The combination of these two results reveals the existence of a 700‐km‐long segmentation of the upper mantle, with a “cool” area centered on Saint‐Exupéry FZ.
Deformation across the Pacific‐North America plate boundary near San Francisco, California
Authors: Prescott William H. , Savage James C. , Svarc Jerry L. , Manaker David
Published: February 2004

We have detected a narrow zone of compression between the Coast Ranges and the Great Valley, and we have estimated slip rates for the San Andreas, Rodgers Creek, and Green Valley faults just north of San Francisco. These results are based on an analysis of campaign and continuous Global Positioning System (GPS) data collected between 1992 and 2000 in central California. The zone of compression between the Coast Ranges and the Great Valley is 25 km wide. The observations clearly show 3.8±1.5 mm yr−1 of shortening over this narrow zone. The strike slip components are best fit by a model with 20.8±1.9 mm yr−1 slip on the San Andreas fault, 10.3±2.6 mm yr−1 on the Rodgers Creek fault, and 8.1±2.1 mm yr−1 on the Green Valley fault. The Pacific‐Sierra Nevada‐Great Valley motion totals 39.2±3.8 mm yr−1 across a zone that is 120 km wide (at the latitude of San Francisco). Standard deviations are one σ. The geodetic results suggest a higher than geologic rate for the Green Valley fault. The geodetic results also suggest an inconsistency between geologic estimates of the San Andreas rate and seismologic estimates of the depth of locking on the San Andreas fault. The only convergence observed is in the narrow zone along the border between the Great Valley and the Coast Ranges.
Estimating source parameters from deformation data, with an application to the March 1997 earthquake swarm off the Izu Peninsula, Japan
Authors: Cervelli P. , Murray M. H. , Segall P. , Aoki Y. , Kato T.
Published: February 2004

We have applied two Monte Carlo optimization techniques, simulated annealing and random cost, to the inversion of deformation data for fault and magma chamber geometry. These techniques involve an element of randomness that permits them to escape local minima and ultimately converge to the global minimum of misfit space. We have tested the Monte Carlo algorithms on two synthetic data sets. We have also compared them to one another in terms of their efficiency and reliability. We have applied the bootstrap method to estimate confidence intervals for the source parameters, including the correlations inherent in the data. Additionally, we present methods that use the information from the bootstrapping procedure to visualize the correlations between the different model parameters. We have applied these techniques to GPS, tilt, and leveling data from the March 1997 earthquake swarm off of the Izu Peninsula, Japan. Using the two Monte Carlo algorithms, we have inferred two sources, a dike and a fault, that fit the deformation data and the patterns of seismicity and that are consistent with the regional stress field.
Increased probability of large earthquakes near aftershock regions with relative quiescence
Authors: Ogata Yosihiko
Published: February 2004

It is shown that after a large earthquake of M6 class or over has taken place, another event of similar size or larger is relatively more likely (in the sense of rate per unit area) to occur in the near field than in the far field. In particular, we show that the aftershock activity of the first event provides useful information for assessing the probability of a following large event in the neighborhood (within a distance of a few degrees). Namely, if the aftershock activity from the first event becomes relatively quiet compared to the normal decay, the occurrence rate of larger events in the neighborhood is several times higher during the first decade after the main shock than would be the case for normal aftershock activity. For measuring such phenomena precisely, we need to model the normal aftershock activity. In fact, many aftershock sequences are more complex than the simple inverse power decay represented by the modified Omori formula. The epidemic type aftershock sequences (ETAS) model is a generalized version of the modified Omori formula, which fits well with various aftershock sequences, including nonvolcanic type swarms. Using this model, aftershock sequences are investigated for the 76 main shocks that occurred in and around Japan during the last three quarters of a century. The focus is placed on objective examination of whether or not, in an aftershock sequence, there exists a significant change point followed by relative quiescence, that is, a significant lowering of the seismicity from that predicted by the ETAS model. Relative quiescence can take place regardless of the seismicity level. The extent of such quiescence, if it exists, is seen from the diagrams of cumulative numbers versus the transformed occurrence times based on the predicted occurrence rate by the estimated ETAS model. It is thus demonstrated that relative quiescence can be a helpful factor in detecting anomalous aftershock activity. This can then be used to forecast whether or not a large event is more likely to follow shortly (within ∼6 years) in the neighborhood (within a distance of 3°) of the initial event.
Dielectric spectroscopy of sedimentary rocks
Authors: Lesmes David P. , Morgan Frank Dale
Published: February 2004

A physiochemical model for the complex dielectric response of sedimentary rocks is used to invert broadband dielectric spectra for effective grain size distributions. The complex dielectric response of each grain within the “water‐wet” granular matrix is obtained by superimposing the polarization of the electrochemical double layer, which is assumed to surround each grain, with the complex dielectric response of the dry mineral grain. The effective complex dielectric response of the water‐wet matrix (grains and surface phase) is obtained by volume averaging over the entire distribution of particle sizes. The complex dielectric response of the total mixture (water‐wet matrix and bulk pore solution) is obtained using the Bruggeman‐Hanai‐Sen effective medium theory. Studies of Berea sandstone show that the grain size distribution, obtained by inverting the real part of the complex dielectric spectra, is similar to the grain size distribution obtained from optical images of the sample in thin section. The current model, however, does not account for surface roughness effects or the polarization of counterions over multiple grain lengths; therefore the grain size distribution obtained by dielectric spectroscopy is broader than the image‐derived distribution. The dielectric‐derived grain size distribution can be fit with two separate power laws that crossover at R ≅ 1 μm, which corresponds to a relaxation frequency of 2 kHz. The low‐frequency dielectric response (f < 2 kHz) is primarily controlled by the macroscopic grain fraction (mainly quartz grains), which has a fractal dimension of d = 1.84±0.05. The high‐frequency dielectric response (f > 2 kHz) is primarily controlled by the clay size grains and surface roughness, which has a fractal dimension of d = 2.48±0.07. At very low frequencies (f < 0.1 Hz) the dielectric response appears to be controlled by the electrochemical polarization of counterions over multiple grain lengths. A more general model should account for the effects of surface roughness and grain interactions on the dielectric response. It would also be useful to develop a simplified version of this model, perhaps similar in form to the empirically derived Cole‐Cole response, which could be more easily used to model and interpret electrical geophysical field surveys (e.g., induced‐polarization, ground‐penetrating radar, and time domain reflectometery measurements).
The 1994 Java tsunami earthquake: Slip over a subducting seamount
Authors: Abercrombie Rachel E. , Antolik Michael , Felzer Karen , Ekström Göran
Published: February 2004

On June 2, 1994, a large subduction thrust earthquake (Ms 7.2) produced a devastating tsunami on the island of Java. This earthquake had a number of unusual characteristics. It was the first recorded large thrust earthquake on the Java subduction zone. All of the aftershock mechanisms exhibit normal faulting; no mechanisms are similar to the main shock. Also, the large tsunami and the relatively low energy radiated by the main shock have led to suggestions that this earthquake might have involved slow, shallow rupture near the trench, similar to the 1992 Nicaragua earthquake. We first relocate the main shock and the aftershocks. We then invert long‐period surface waves and broadband body waves to determine the depth and spatial distribution of the main shock slip. A dip of 12°, hypocenter depth of 16 km and moment of 3.5×l020 N m (Mw 7.6) give the best fit to the combined seismic data and are consistent with the plate interface geometry. The source spectrum obtained from both body and surface waves has a single corner frequency (between 10 and 20 mHz) implying a stress drop of ∼0.3 MPa. The main energy release was preceded by a small subevent lasting ∼12 s. The main slip occurred at ∼20 km depth, downdip and to the NW of the hypocenter. This area of slip is collocated with a prominent high in the bathymetry that has been identified as a subducting seamount. We interpret the Java earthquake as slip over this subducting seamount, which is a locked patch in an otherwise decoupled subduction zone. We find no evidence for slow, shallow rupture. No thrust aftershocks are expected if the entire locked zone slipped during the main shock, but extension of the subducting plate behind the seamount would promote normal faulting as observed. It seems probable that such a source model could also explain the size and timing of the observed tsunami.
Thermal effects of gabbro accretion from a deeper second melt lens at the fast spreading East Pacific Rise
Authors: Chen Yongshun John
Published: February 2004

Motivated by recent observations, a ridge model has been developed which includes both a shallow melt lens and a second deeper melt lens above the Moho to assess the thermal effect of the gabbro accretion from the lower melt lens at the fast spreading East Pacific Rise. Modeling results argue for a limited role of the lower melt lens in constructing the layer 3 gabbros. If significant (>10%) amount of the layer 3 gabbros is emplaced near the Moho, then the liberated latent heat during crystallization would generate a large “molten” section of the lower crust due to the inability of cold seawater to penetrate into this hot/ductile region. A large molten section of the lower crust is not consistent with recent seismic tomographic results of the East Pacific Rise. Thus it is likely that the amount of the gabbro being emplaced from the lower melt lens at the EPR is significantly less than that suggested by Oman ophiolite studies, such a lower melt lens is short‐lived, or a large portion of the sill intrusions observed at Oman was emplaced at a few kilometers off axis (>3–4 km).
Transition from passive to active rifting: Relative importance of asthenospheric doming and passive extension of the lithosphere
Authors: Huismans Ritske S. , Podladchikov Yuri Y. , Cloetingh Sierd
Published: February 2004

We present quantitative modeling results of the dynamic interplay of passive extension and active convective thinning of the mantle lithosphere beneath intracontinental rift zones investigating the relative importance of thermal buoyancy forces associated with asthenospheric doming and far‐field intraplate stresses on the style of rifting. To this aim we employ a two‐dimensional numerical code based on a finite element method formulation for nonlinear temperature dependent viscoelastoplastic rheology. Brittle behavior is modeled using Mohr‐Coulomb plasticity. The models support a scenario in which passive stretching leads to an unstable lithospheric configuration. Thermal buoyancy related to this asthenospheric doming subsequently drives active upwelling in a lithosphere scale convection cell. In the late synrift to early postrift the lithospheric horizontal stresses caused by the active asthenospheric upwelling may start to compete with the far‐field intraplate stresses. At this stage the domal forces may dominate and even drive the system causing a change from passive to active rifting mode. If this transition occurs, the model predicts (1) drastic increase of subcrustal thinning beneath the rift zone, (2) lower crustal flow towards the rift flanks, (3) middle crustal flow towards the rift center, (4) the coeval occurrence of tensional stresses within and compressive stresses around the upwelling region, and (5) possible surface uplift. Late postrift thermal cooling removes the thermal buoyancy forces. At this stage the far‐field forces dominate the stress state again and the lithosphere becomes more sensitive to small changes in the intraplate stresses. The model results may explain several key observations that are characteristic of a large number of intracontinental rift basins. These features include differential thinning of extending lithosphere, the discrepancy between fault‐related extension and crustal thinning, late (end of synrift to early postrift) mantle related volcanism, surface domal uplift succeeding rifting, and rift flanks uplift associated with extension of a weak lithosphere.
Surface movements of emplaced lava flows measured by synthetic aperture radar interferometry
Authors: Stevens N. F. , Wadge G. , Williams C. A. , Morley J. G. , Muller J.‐P. , Murray J. B. , Upton M.
Published: February 2004

Lava flows continue to move after they have been emplaced by flow mechanisms. This movement is largely vertical and can be detected using differential synthetic aperture radar (SAR) interferometry. There are three main components to this motion: (1) movement of surface scatterers, resulting in radar phase decorrelation, (2) measurable subsidence of the flow surface due to thermal contraction and clast repacking, and (3) time‐dependent depression of the flow substrate. These effects act in proportion to the thickness of the lava flow and decay with time, although there is a time lag before the third component becomes significant. We explore these effects using SAR data from the ERS satellites over the Etna volcano, Sicily. Phase decorrelation on young, thick a'a lava flows persists for a few years and probably results from surface block rotations during flow contraction. Maximum measured subsidence rates of the 1991–1993 lava flow over a period of 70 days are about 0.7 mm day−1, but are potentially greater in areas of data decorrelation. These rates fall to <2.7 × 10−2 mm day−1 after about 20 years in flows about 50 m thick, sooner for thinner flows. Comparison with measured subsidence rates on Kilauean lava lakes suggests that thermal contraction only accounts for about one third of the observed subsidence. The remaining motion is thought to come from surface clast repacking during cooling and from creep mechanisms in the flow substrate. Measurements of postemplacement surface movement provide new constraints on the thermomechanical properties of lava flows and have cautionary implications for the interpretation of interferometric SAR data of volcanoes.
Dynamics of magma flow in volcanic conduits with variable fragmentation efficiency and nonequilibrium pumice degassing
Authors: Papale Paolo
Published: February 2004

Numerical models of volcanic conduit dynamics assume that the foamy structure of magma is totally destroyed at fragmentation, neglecting the presence of vesicular fragments in the above gas‐particle flow region. The abundance of pumice among the volcanic pyroclasts is, however, the outstanding evidence that such a complete atomization assumption is an oversimplification. The present one‐dimensional, multiphase, nonequilibrium magma ascent modeling rejects such an assumption, taking into account the presence of pumice, ash, and loose crystals in the turbulent flow region above fragmentation. A parametric study is conducted on the role of pumice abundance, related to a fragmentation efficiency parameter, in affecting the magma ascent dynamics. Additionally, different hypotheses on pumice degassing behavior are evaluated. The numerical results show that the presence of even relatively small amounts of pumice translates into a large decrease of continuous gas volume fraction, while other flow quantities are mainly affected by the efficiency of pumice degassing. A decreased degassing efficiency translates into significant velocity and pressure decrease, mixture density increase, and further gas volume fraction decrease, that overall result in a decrease of the mechanical energy content of the erupted magma by an amount comparable to that due to water content decrease by several weight percent. Postprocessing of numerical results based on the dynamics of porous flow of gas in pumice allows one to relate the efficiency of pumice degassing to a measurable pumice texture parameter, namely the degree of gas bubble coalescence, which is therefore identified as a major factor affecting the ascent dynamics of silicic magma.
A Poisson model for identifying characteristic size effects in frequency data: Application to frequency‐size distributions for global earthquakes, “starquakes”, and fault lengths
Authors: Leonard Thomas , Papasouliotis Orestis , Main Ian G.
Published: February 2004

The standard Gaussian distribution for incremental frequency data requires a constant variance which is independent of the mean. We develop a more general and appropriate method based on the Poisson distribution, which assumes different unknown variances for the frequencies, equal to the means. We explicitly include “empty bins”, and our method is quite insensitive to the choice of bin width. We develop a maximum likelihood technique that minimizes bias in the curve fits, and penalizes additional free parameters by objective information criteria. Various data sets are used to test three different physical models that have been suggested for the density distribution: the power law; the double power law; and the “gamma” distribution. For the CMT catalog of global earthquakes, two peaks in the posterior distribution are observed at moment magnitudes m* = 6.4 and 6.9 implying a bimodal distribution of seismogenic depth at around 15 and 30 km, respectively. A similar break at a characteristic length of 60 km or so is observed in moment‐length data, but this does not outperform the simpler power law model. For the earthquake frequency‐moment data the gamma distribution provides the best overall fit to the data, implying a finite correlation length and a system near but below the critical point. In contrast, data from soft gamma ray repeaters show that the power law is the best fit, implying infinite correlation length and a system that is precisely critical. For the fault break data a significant break of slope is found instead at characteristic scale of 44 km, implying a typical seismogenic thickness of up to 22 km or so in west central Nevada. The exponent changes from 1.5 to −2.1, too large to be accounted for by changes in sampling for an ideal, isotropic fractal set.
Resolution analysis of finite fault source inversion using one‐ and three‐dimensional Green's functions: 1. Strong motions
Authors: Graves Robert W. , Wald David J.
Published: February 2004

We develop a methodology to perform finite fault source inversions from strong motion data using Green's functions (GFs) calculated for a three‐dimensional (3‐D) velocity structure. The 3‐D GFs are calculated numerically by inserting body forces at each of the strong motion sites and then recording the resulting strains along the target fault surface. Using reciprocity, these GFs can be recombined to represent the ground motion at each site for any (heterogeneous) slip distribution on the fault. The reciprocal formulation significantly reduces the required number of 3‐D finite difference computations to at most 3NS, where NS is the number of strong motion sites used in the inversion. Using controlled numerical resolution tests, we have examined the relative importance of accurate GFs for finite fault source inversions which rely on near‐source ground motions. These experiments use both 1‐D and 3‐D GFs in inversions for hypothetical rupture models in order (1) to analyze the ability of the 3‐D methodology to resolve trade‐offs between complex source phenomena and 3‐D path effects, (2) to address the sensitivity of the inversion results to uncertainties in the 3‐D velocity structure, and (3) to test the adequacy of the 1‐D GF method when propagation effects are known to be three‐dimensional. We find that given “data” from a prescribed 3‐D Earth structure, the use of well‐calibrated 3‐D GFs in the inversion provides very good resolution of the assumed slip distribution, thus adequately separating source and 3‐D propagation effects. In contrast, using a set of inexact 3‐D GFs or a set of hybrid 1‐D GFs allows only partial recovery of the slip distribution. These findings suggest that in regions of complex geology the use of well‐calibrated 3‐D GFs has the potential for increased resolution of the rupture process relative to 1‐D GFs. However, realizing this full potential requires that the 3‐D velocity model and associated GFs should be carefully validated against the true 3‐D Earth structure before performing the inverse problem with actual data.
Estimating the crustal power spectrum from vector Magsat data
Authors: Lowe David A. J. , Parker Robert L. , Purucker Michael E. , Constable Catherine G.
Published: February 2004

The Earth's magnetic field can be subdivided into core and crustal components and we seek to characterize the crustal part through its spatial power spectrum, Rl. We process vector Magsat data to isolate the crustal field and then invert power spectral densities of flight‐local components along‐track for Rl following O'Brien et al. [1999]. Our model, designated LPPC, is accurate up to approximately spherical harmonic degree 45 (λ = 900 km): this is the resolution limit of our data and suggests that global crustal anomaly maps constructed from vector Magsat data should not contain features with wavelengths less than 900 km. We find continental power spectra to be greater than oceanic ones and attribute this to the relative thicknesses of continental and oceanic crust.
Spatial variability in the flow of a valley glacier: Deformation of a large array of boreholes
Authors: Harper Joel T. , Humphrey Neil F. , Pfeffer W. Tad , Huzurbazar Snehalata V. , Bahr David B. , Welch Brian C.
Published: February 2004

Measurements of the deformation of a dense array of boreholes in Worthington Glacier, Alaska, show that the glacier moves with generally bed‐parallel motion. Strain in the 200 m deep valley glacier is constant near the surface but follows a nonlinear vertical profile below a depth of about 120 m. By a depth of 180 m, the octahedral strain rate reaches 0.35 yr−1. The three‐dimensional velocity field shows spatial complexity with significant deviations from plane strain, despite relatively simple valley geometry in the vicinity of the 6×106 m3 study volume. No evidence was found for time‐varying deformation or movement along localized shear planes. Observations were made by repeatedly measuring the long‐axis geometry of 31 closely spaced boreholes over a 70 day period, and three additional holes after 1 full year of deformation. The holes were spaced 15 to 30 m apart. Installation and measurement of such a large number of boreholes required the development of a semiautomated hot water drilling system that creates straight and vertical boreholes with uniform walls. The equipment and procedures enables borehole profiles to be measured without the use of hole casing. Inclinometry measurements collected in the holes were processed, analyzed for error, and visualized as a fully three‐dimensional data set. The new methods offer unique insight into small‐scale spatial and temporal variations in the pattern of flow in a valley glacier.
Structural and chemical characterization of shear zones in the freshly activated Nojima fault, Awaji Island, southwest Japan
Authors: Tanaka Hidemi , Fujimoto Koichiro , Ohtani Tomoyuki , Ito Hisao
Published: February 2004

Behavior and role of each shear zone in a shallow fault zone of granitic origin during seismic cycles are revealed by comprehensive examinations of petrographic and chemical characterization on a fault zone in the Geological Survey of Japan (GSJ) drill core penetrating the Nojima fault which was activated during the 1995 Hyogoken‐Nanbu earthquake (M = 7.2). The GSJ core consists of granodiorite and porphyritic intrusive rocks including a Nojima fault zone which involves seven thin shear zones: main shear zone (MSZ, 625 m depth), upper cataclasite zone (UCZ), upper shear zone (USZ), lower shear zones (LSZ‐1 and LSZ‐2), and lower cataclasite zones (LCZ‐1 and LCZ‐2). These shear zones are generally surrounded by weakly pulverized and altered (fault‐related) rocks (WPAR) which generally show volume gain. The fault zone architecture is clarified as follows: (1) Total thickness of the Nojima fault zone is ∼70 m. (2) All shear zones except the older cataclasite zones (UCZ‐1, LCZ‐1, and LCZ‐2) were evolved from WPAR, indicating that pulverization and alteration of recent activity were more diffused at the initial stage of faulting and gradually localized to each shear zone. (3) The MSZ (2 m thick) can be regarded as a high‐velocity frictional zone with accompanying volume loss (compaction) and possibly with heat generation during coseismic periods. (4) The LSZ‐1 (3.6 m thick), located just beneath the MSZ and typically showing explosion brecciation texture, is also regarded as a coseismic shear zone. This zone could function as a trap zone for fluid or gas during postseismic/interseismic periods. (5) The LSZ‐2 (2.7 m thick), located around 710 m depth, contains foliated fault gouge enriched with clay minerals and characterized by a large degree of volume gain, possibly a result of slow velocity motion or creep during the interseismic and/or postseismic periods.
Magnetostratigraphy and sedimentology of the Jingchuan red clay section and correlation of the Tertiary eolian red clay sediments of the Chinese Loess Plateau
Authors: Ding Z. L. , Yang S. L. , Hou S. S. , Wang X. , Chen Z. , Liu T. S.
Published: February 2004

A 325‐m‐thick loess and red clay sequence was recently discovered at Jingchuan, which is situated about 45 km north of the previously studied Lingtai section in the middle part of the Chinese Loess Plateau. The Jingchuan section is composed of 199 m of Pleistocene loess and 126 m of Tertiary red clay deposits. Paleomagnetic studies show that the Jingchuan red clay has a basal age of at least 7.7 Ma, which represents the oldest red clay deposits presently known in the Loess Plateau. Field observations and grain size analyses both suggest an eolian origin of the red clay, thus extending available records of the eolian deposits in the Loess Plateau from about 7.0 Ma to 7.7 Ma. Correlation of both pedostratigraphy and magnetic susceptibility records suggests that the late Cenozoic red clay records in the Loess Plateau have a general continuous nature, thereby providing potential for deriving long‐term climatic evolution signals from this eolian loess and red clay sequence.
Low‐temperature oxidation in MORB of titanomagnetite to titanomaghemite: A gradual process with implications for marine magnetic anomaly amplitudes
Authors: Zhou Weiming , Van der Voo Rob , Peacor Donald R. , Wang Daming , Zhang Youxue
Published: February 2004

Mid‐ocean ridge basalt (MORB) samples, varying in age from Recent to Jurassic, were selected for electron microscopic and rock magnetic studies. Our observations indicate that the degree of oxidation of titanomagnetite in MORB increases only gradually with sample age. The titanomagnetite in recent MORB (<20,000 years) shows no sign of alteration (z∼0). Quaternary samples near the ridge (<2 Ma) typically have z values of less than 0.35, indicating a low degree of oxidation, whereas samples with ages of tens of millions of years have z values of up to 0.9. Some older samples show lower z values, but the upper envelope of our observations in a z versus age plot can be represented by the function z = p + q log t, where p ≈ 0.38, q ≈ 0.38, and t is in millions of years (for t>100 ka). Both electron microscopic observations and rock magnetic data support the notion that low‐temperature oxidation of titanomagnetite to titanomaghemite in MORB is a gradual process. Moreover, the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales, including regional oceanic crustal structures, lithological features, grain size, and surrounding matrices. Pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows, owing to higher porosity and permeability of the former. In contrast, interstitial glass can protect fine titanomagnetite grains from alteration. The natural remanent magnetization (NRM) intensity of MORB varies as a function of age, magnetic granulometry, concentration of magnetic materials, and the degree of alteration. Fine‐grained MORB samples typically have higher NRM intensity. The NRM intensity appears to decrease substantially with increasing degree of maghemitization and, hence, with age. The envelope of the smoothed magnetic anomaly amplitudes resembles the change in NRM intensity for MORB samples of the last 30 million years, but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported. Maghemitization of the micrometer‐sized and larger grains is probably only partly responsible, whereas a significant contribution to NRM intensities is inferred from those submicrometer‐sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass.
BEDMAP: A new ice thickness and subglacial topographic model of Antarctica
Authors: Lythe Matthew B. , Vaughan David G.
Published: February 2004

Measurements of ice thickness on the Antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database. From these data, a seamless suite of digital topographic models have been compiled for Antarctica and its surrounding ocean. This includes grids of ice sheet thickness over the grounded ice sheet and ice shelves, water column thickness beneath the floating ice shelves, bed elevation beneath the grounded ice sheet, and bathymetry to 60°S, including the sub‐ice‐shelf cavities. These grids are consistent with a recent high‐resolution surface elevation model of Antarctica. While the digital models have a nominal spatial resolution of 5 km, such high resolution is justified by the original data density only over a few parts of the ice sheet. The suite does, however, provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps. The total volume of the Antarctic ice sheet calculated from the BEDMAP grid is 25.4 million km3, and the total sea level equivalent, derived from the amount of ice contained within the grounded ice sheet, is 57 m, comprising 52 m from the East Antarctic ice sheet and 5 m from the West Antarctic ice sheet, slightly less than earlier estimates. The gridded data sets can be obtained from the authors.
A magnetotelluric profile across the German Deep Drilling Project (KTB) area: Two‐ and three‐dimensional modeling results
Authors: Eisel Markus , Haak Volker , Pek Josef , Červ Václav
Published: February 2004

Previous interpretations of magnetotelluric data from the vicinity of the German Deep Drilling Project (KTB) revealed two major structures: a midcrustal layer of increased conductivity and large, regional extent and a highly anisotropic upper crust. Nevertheless, a satisfactory combination of both structures explaining all measurements has not yet been achieved, mostly due to incomplete and qualitatively poor data. Simplified superposition of both structures could not yield an explanation of the observations. On the basis of a carefully processed new data set we apply different modeling approaches to verify the existence of both structures. Models calculated with a two‐dimensional modeling program, which allows for general anisotropy, as well as a full three‐dimensional code show that an anisotropic upper crust is overlaying a regional east‐west striking high‐conductivity structure. Nevertheless, this continuous conductive midcrustal layer with a conductance decreasing from north to south must be replaced by a quasi‐anisotropic one, at least in the region of the KTB. The final model may still be oversimplified, considering the complexity of the true but unknown geology in this particular area, but it demonstrates which major electrically effective structures could be resolved.
Conceptual and physical clarification of rate and state friction: Frictional sliding as a thermally activated rheology
Authors: Nakatani Masao
Published: February 2004

We observed slow frictional slip occurring at a constant shear stress below the nominal friction level and compared it with the time‐dependent strengthening of the frictional interface, which was also tracked experimentally. It was found that slip velocity decreases as the interface strengthens due to aging, while it increases with the applied shear stress. These dependencies were both exponential and were of similar magnitudes, as implied by the framework law of rate‐ and state‐dependent friction. In the spirit of the adhesion theory of friction the dependence of slip velocity on interface strength is understood to be the result of the change of the shear stress acting on frictional junctions due to the change of junction population, though the observed dependence was somewhat stronger than a simple model based on this idea predicts. By correcting the observed slip velocity for the effect of the change of the interface strength, we could obtain a unique relationship between stress and slip velocity, which may be readily compared with a standard rheological formulation. Thus the obtained relationship between stress and slip velocity showed a reasonable agreement with the absolute rate theory over a temperature range of 25–800°C for the present experimental condition (fine albite powder, 20 MPa normal stress, no pore water).