bluuebunny/crossref_metadata_2025 huggingface dataset:
REVEL: A model for Recent plate velocities from space geodesy
Authors: Sella Giovanni F. , Dixon Timothy H. , Mao Ailin
Published: October 2002

We present a new global model for Recent plate velocities, REVEL, describing the relative velocities of 19 plates and continental blocks. The model is derived from publicly available space geodetic (primarily GPS) data for the period 1993–2000. We include an independent and rigorous estimate for GPS velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates. The velocity fields for North America, Eurasia, and Antarctica clearly show the effects of glacial isostatic adjustment, and Australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field. Two thirds of tested plate pairs agree with the NUVEL‐1A geologic (3 Myr average) velocities within uncertainties. Three plate pairs (Caribbean–North America, Caribbean–South America, and North America–Pacific) exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in NUVEL‐1A due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities. Most other differences probably reflect real velocity changes over the last few million years. Several plate pairs (Arabia–Eurasia, Arabia–Nubia, Eurasia–India) move more slowly than the 3 Myr NUVEL‐1A average, perhaps reflecting long‐term deceleration associated with continental collision. Several other plate pairs, including Nazca–Pacific, Nazca–South America and Nubia–South America, are experiencing slowing that began ∼25 Ma, the beginning of the current phase of Andean crustal shortening.
Hypocenter location by pattern recognition
Authors: Nicholson T. , Sambridge M. , Gudmundsson Ó.
Published: October 2002

A novel approach to hypocenter location is proposed on the basis the concept of pattern recognition. A new data misfit criterion for location is introduced which measures discrepancies between the observed arrival times of an event and those of “nearby” previous events. In the arrival pattern misfit measure, travel times predicted by an Earth model are effectively replaced by information from an ensemble of previous observations. Thin‐plate spline interpolation and generalized cross validation are applied to interpolate and smooth the resulting misfit function which may then be used in standard location algorithms. Synthetic experiments show that in certain circumstances, it is possible to achieve locations with errors smaller than those in the underlying database. It is suggested that the arrival pattern approach exploits information on lateral heterogeneous Earth structure contained in the database to constrain locations. The arrival pattern approach is illustrated by relocating 395 ground truth events from the Nevada Test Site, 482 earthquakes from the Marianas subduction zone, and 457 earthquakes from the Atlantic mid‐ocean ridge. It is shown that picking errors and unmodeled, small‐scale lateral heterogeneity are the most significant sources of event mislocation and that errors in the original locations of the database events make a much smaller contribution.
Paleomagnetic results indicate pre‐Permian counter‐clockwise rotation of the southern Tamworth Belt, southern New England Orogen, Australia
Authors: Geeve Richard J. , Schmidt Phillip W. , Roberts John
Published: October 2002

Visean ignimbrites from three fault‐bounded blocks within the southern Tamworth Belt, southern New England Orogen, Australia, retain a prefolding high‐temperature component of remanence, carried mainly by magnetite. Paleomagnetic data indicate that the southern Tamworth Belt occupied low to moderate paleolatitudes during the early Carboniferous. Normal and reversed polarities with low to moderate inclinations suggest that the steep, reverse polarity latest Carboniferous to earliest Permian overprint is absent from the high‐temperature component data set. The mean characteristic remanence direction of three early to middle Visean ignimbrite flows (32 sites) from the Rouchel block is: declination (dec) = 154.2° and inclination (inc) = 38.8° (α95 = 12.5°; k = 98.8), which corresponds to a paleo south pole at latitude 64.9°S, longitude 258.1°E (dp = 5.5°; dm = 9.2°, where dp is the angular length of the semiaxis of the ellipse of confidence of the calculated paleopole that lies along the pole to site great circle, and dm is the angular length of the semiaxis perpendicular to dp). Six middle to late Visean ignimbrite flows (29 sites) from the Gresford block retain a mean remanence of dec = 159.6° and inc = 54.1° (α95 = 13.7°; k = 24.7), corresponding to a paleo south pole at latitude 72.8°S, longitude 228.4°E (dp = 13.4°; dm = 19.2°). Despite widespread overprinting throughout much of the Myall block, reliable information was obtained from three late Visean ignimbrite flows (five sites), with a mean direction of dec = 118.1° and inc = 42.5° (α95 = 12.0°; k = 41.6), from which a paleo south pole of latitude 35.7°S, longitude 233.4°E (dp = 9.1°; dm = 14.7°) is calculated. The calculated paleo south poles do not fall on the early Carboniferous segment of the published apparent polar wander path for Australia and suggest that the southern Tamworth Belt has undergone significant counter‐clockwise rotation relative to cratonic Australia. Inferred rotations for the Rouchel, Gresford, and Myall blocks are approximately 80°, 80°, and 120°, respectively. Stratigraphic and geological evidence suggest rotation took place between the mid‐Namurian and latest Carboniferous and is thought to have resulted from sinistral strike‐slip during compression at the eastern margin of Australia.
Atmospheric excitation of the Earth's nutation: Comparison of different atmospheric models
Authors: Yseboodt Marie , de Viron Olivier , Chin Toshio M. , Dehant Véronique
Published: October 2002

Using the angular momentum approach, the effect of the atmosphere on the Earth's nutation is evaluated from five atmospheric circulation models. Five nutation frequencies are considered: the annual and semiannual prograde and retrograde and the terannual prograde. The atmospheric effect on the 18.6‐year nutation is also evaluated from the only model that gives a long enough homogeneous series. The results obtained from the different models and a noninverted barometer ocean model are intercompared and compared with the residuals between the observed and computed nutations. The evaluation of the atmospheric effect on the Earth's nutation is shown to be above the observation precision, particularly at the annual frequency (prograde and retrograde) and the semiannual prograde frequency but is highly dependent on the model used, strengthening the importance of having a correct estimation for the atmospheric effects on nutation. The time evolution of the atmospheric excitation of the nutation is also studied, using a wavelet analysis and a sliding‐window least squares method. We show that the fluctuations of the excited amplitude are above the level of precision of the observation.
Implications of sediment‐flux‐dependent river incision models for landscape evolution
Authors: Whipple Kelin X. , Tucker Gregory E.
Published: October 2002

Developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate, erosion, and tectonics. Current models of long‐term river network incision differ significantly in their treatment of the role of sediment flux. We analyze the implications of various sediment‐flux‐dependent incision models for large‐scale topography, in an attempt (1) to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and (2) to explain the apparent ubiquity of mixed bedrock‐alluvial channels in active orogens. Although certain forms of the various models can be discarded as inconsistent with morphological data, we find that the relative intrinsic concavity indices of detachment‐ and transport‐limited systems (defined herein) largely dictate whether the various models can be tied to distinctive steady state morphologies. Preliminary data suggest that no such diagnostic differences may exist, and other methods must be developed to test models. Accordingly, we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment‐ versus transport‐limited channels as a function of rock uplift rate that may allow discrimination among various models. Further, we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate. In addition to general differences between detachment‐ and transport‐limited systems our analysis identifies an interesting hysteresis in landscape evolution: “hybrid” channels at the threshold between detachment‐ and transport‐limited conditions are expected to act as detachment‐limited systems in response to an increase in rock uplift rate (or base level fall) and as transport‐limited systems in response to a decrease in rock uplift rate, especially during postorogenic topographic decline. The analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed.
Three‐dimensional attenuation model of the shallow Hikurangi subduction zone in the Raukumara Peninsula, New Zealand
Authors: Eberhart‐Phillips Donna , Chadwick Mark
Published: October 2002

The material properties of the shallow subduction zone are imaged with data from a passive seismic deployment in the Raukumara Peninsula. Attenuation images are obtained using t* values measured from 2848 spectra of P wave arrivals. The t* values are inverted for frequency‐independent three‐dimensional (3‐D) Qp, using a 3‐D velocity model. Linked nodes in a preliminary 2‐D inversion are used to obtain the most reliable initial model for 3‐D inversions. The results show the benefits of obtaining 3‐D Q in addition to 3‐D Vp and Vp/Vs. Q is related to localized permeability and heterogeneity, and thus 3‐D Q is an important tool for interpreting active tectonic regions. Qp in the shallow overlying plate generally corresponds to the mapped geology. Two high Qp features (>600) represent subducted seamounts or Cretaceous volcanics within the overlying plate. The gradient above the subducting plate is the strongest feature in the Qp model, with the slab having high Qp (600–900). Below 20‐km depth and above the plate interface, there is a zone of 50% decrease in Qp and high Vp/Vs about 10 km thick which represents actively subducting sediment and has distributed microearthquake activity associated with active underplating.
Determining surface wave arrival angle anomalies
Authors: Larson Erik W. F. , Ekström Göran
Published: October 2002

A new method for measuring arrival angles of teleseismic Love and Rayleigh waves is developed. The new method utilizes estimates of surface wave dispersion to create a phase‐matched filter to isolate the Love or Rayleigh wave in three‐component recordings. The polarization of the filtered wave group is determined in the time domain by application of a variation of the complex polarization method of Vidale [1986]. Orientation, linearity, and ellipticity of particle motion are estimated in several frequency bands to determine the frequency‐dependent polarization. The method employs an iterative scheme, by which a predicted Love wave, based on the estimated dispersion and polarization, is subtracted from the three‐component data prior to the estimation of Rayleigh wave polarization, and vice versa. The method is applied to an extensive set of Global Seismographic Network data covering the years 1989–1998. Between 4244 and 15,075 measurements are collected for fundamental mode Love and Rayleigh waves at nine different periods (37 to 150 s). Measurement uncertainties are estimated using the statistics of observations for pairwise similar paths and are generally of the order of 15–50% of the total signal, depending on the period and the wave type. Large and azimuthally invariant angle anomalies are documented for several stations and are consistent with misorientation of the horizontal seismometers. Two schemes are employed to determine the misorientations: (1) an azimuthally weighted average at each station, and (2) a joint inversion for seismometer misorientation and globally heterogeneous phase velocities. The determined corrections are robust and correlate well with those reported in earlier studies. Azimuthally varying arrival angle anomalies are shown to agree qualitatively with predictions of wave refraction calculated for recent phase velocity maps, which explain up to 30% of the variance in the new measurements.
Thermal structure of the North American uppermost mantle inferred from seismic tomography
Authors: Goes Saskia , van der Lee Suzan
Published: October 2002

We map the thermal state of the North American mantle between depths of 50 and 250 km by inverting P and S velocities of three recent seismic tomographic models. In the well‐resolved regions, temperatures derived from P velocities agree with those derived from S velocities within the estimated uncertainties, and generally, the seismic temperatures are in agreement with those inferred from surface heat flow. Adiabatic mantle temperatures are found as shallow as 50 km under most of the Basin and Range. Warm, subsolidus mantle and known crustal structure can account for the high average elevation and large‐scale variations in topography of western North America. In the cratonic mantle beneath the stable eastern part of North America, temperatures at 50–100 km are on average 500°C cooler than under the tectonic western part of the continent and adiabatic mantle temperatures are not reached until 200–250 km depth. To balance the effect on topography of the thermally implied density increase for the North American craton, we infer a compositionally induced density decrease equivalent to a 1% depletion in iron over a depth interval of 50–250 km. In regions where TP differs significantly from TS we drop our assumption that variations in seismic velocity are only due to thermal structure. A discrepancy between TP and TS between 50 and 150 km depth under the Cascades and the Gulf of California can be accounted for by the presence of 1 to 2 vol % of fluids and/or melt. Another such discrepancy beneath Wyoming remains enigmatic.
Internal structure of uppermost oceanic crust along the Western Blanco Transform Scarp: Implications for subaxial accretion and deformation at the Juan de Fuca Ridge
Authors: Karson Jeffrey A. , Tivey Maurice A. , Delaney John R.
Published: October 2002

The uppermost 2 km of oceanic crust created near the southern end of the Juan de Fuca Ridge (∼60 mm/yr, full spreading rate) is exposed in a “tectonic window” created along the north wall of the Blanco Transform Fault. A total of 27 submersible dives transect this escarpment over a distance of >30 km, corresponding to ∼1 m.y. of spreading history. In this exposure, weakly deformed basaltic lavas grade downward into much more intensely fractured lavas cut by basaltic dikes. Lava flow surfaces dip persistently to the northwest (toward the spreading center) and the dikes dip southeast (away from the spreading center). The underlying sheeted dike complex is composed of subparallel dikes that dip moderately to the southeast. Discrete faults and more distributed cataclastic deformation zones separate the panels of sheeted dikes from one another. Disruption of the lava and dike units are interpreted as a result of dramatic, subaxial, vertical mass transport that created the “accommodation space” for the accumulation of a thickness of 1000–1500 m of basaltic lava. The observed patterns of tilting of lavas and dikes imply subsidence beneath the region of active volcanism (∼3 km wide) that is most rapid near the axis and diminishes quickly off‐axis. Deformation that accommodates this subsidence has generated extensive fracture porosity in the subaxial region that could strongly influence upper crustal hydrothermal pathways, alteration history, seismic properties, and the distribution of subsurface biological communities.
Equivalent channel network model for permeability and electrical conductivity of fracture networks
Authors: Van Siclen Clinton D.
Published: October 2002

It is remarkable that the permeability k and electrical conductivity σ of saturated, fractured rock exhibit a power law relationship with exponent r as pressure is applied to the rock. To understand this behavior, the fracture network is viewed as a collection of connected, planar fractures. This allows the construction of algebraic expressions for the transport properties of the fracture network, in which the local effective properties, namely, the hydraulic aperture dh and the electric aperture de of the representative planar fracture (the “equivalent channel”), are distinguished from the network properties (e.g., fracture connectivity) parameterized by the tortuosity factors τh and τe. This “equivalent channel network model” reproduces the observed power law behavior on the conditions that dh3 ∝ der and τh ∝ τer over the range of applied pressures. The first condition is met, as demonstrated by calculations for a variety of simulated planar fractures using the Reynolds equations for fluid and current flow. The value of the exponent r is found to indicate the degree to which the fracture resembles a porous medium but cannot otherwise identify fracture surface or aperture characteristics. No direct evidence currently exists to support the second condition; however, such a power law relationship has been demonstrated elsewhere for simulated porous media.
New images of the Earth's upper mantle from measurements of surface wave phase velocity anomalies
Authors: Boschi Lapo , Ekström Göran
Published: October 2002

The sensitivity of a surface wave to the lateral structure of the Earth's mantle depends on the local structure and thickness of the crust; particularly at short periods, the strong lateral heterogeneity of the crust has a significant nonlinear effect on the surface wave “sensitivity functions.” Thus far, this effect has not been quantified, and tomographic studies of the upper mantle based on surface wave observations have not taken it into account. On the basis of a Jeffreys‐Wentzel‐Kramers‐Brillouin (JWKB) description of surface wave propagation we find and discuss the lateral variations of the sensitivity of Love and Rayleigh waves to mantle seismic anomalies, associated with a realistic model of the crust [Mooney et al., 1998]. We then use the resulting laterally varying sensitivity kernels to determine a new tomographic image of horizontally and vertically polarized shear velocity in the upper mantle from Love and Rayleigh wave phase velocity measurements (periods from 35 to 300 s). In terms of large‐scale patterns of high and low velocity our model is similar to previously published ones, confirming, among other features, the anomalous anisotropy underlying the central Pacific [Ekström and Dziewonski, 1998]; we find, however, that the lateral dependence of the surface wave sensitivity kernels has a significant effect on the absolute amplitude of the anomalies, which should not be neglected as we attempt to obtain higher‐resolution images of the upper mantle.
Slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the Mid‐Atlantic Ridge south of the Kane fracture zone (MARK) area
Authors: Gee J. , Meurer W. P.
Published: October 2002

The remanent magnetization of gabbroic material of the Mid‐Atlantic Ridge south of the Kane fracture zone (MARK) area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment. The remanence of these gabbroic samples is often complex, with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales (tens of centimeters to a few meters). Moreover, several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components. Given the nominal age (∼1 Ma) of the crust, we suggest that this pattern of normal/reversed‐normal polarity most plausibly reflects emplacement and/or cooling through three successive polarity intervals, Jaramillo normal polarity interval (1.07–0.99 Ma), a portion of the Matuyama reversed polarity interval (0.99–0.78 Ma), and the Brunhes normal polarity interval (0.78 Ma to present). A small number of samples with three well‐defined magnetization components have magnetic characteristics compatible with a remanence carried by fine‐grained, possibly single domain, magnetite. Laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the Jaramillo/Matuyama (0.99 Ma) and Matuyama/Brunhes (0.78 Ma) polarity transitions. Together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as ∼350°–475°C for a minimum of 0.21 m.y. after emplacement. We suggest that continued injection of liquid, in the form of sills or small magma bodies, over a broad region (half width of ∼3 km) is responsible for this slow cooling. In addition, inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole, suggesting that little, if any, resolvable tilt occurred during uplift of these rocks to the seafloor.
Small‐scale convection in the D″ layer
Authors: Solomatov V. S. , Moresi L.‐N.
Published: October 2002

Small‐scale convection has been suggested as a possible explanation for seismic heterogeneities in the D″ layer of the Earth's mantle. Recently developed scaling laws for convection with realistic viscosities allow quantitative assessment of this hypothesis. Large temperature and viscosity contrasts across the thermal boundary layer at the core‐mantle boundary suggest that small‐scale convection starts at the bottom of the thermal boundary layer and propagates upward before the thermal boundary layer as a whole becomes unstable. The convection boundary is likely to become a chemical boundary as a result of mixing within the convective layer. This implies that the D″ discontinuity can represent both convective and chemical boundary and that the presence or absence of small‐scale convection can be responsible for the observed intermittent nature of the D″ discontinuity. Most lateral heterogeneities in the D″ layer are concentrated near the convection boundary, consistent with seismic data. The length scale of lateral temperature variations, the topography of the core‐mantle boundary, and the viscosity of the D″ layer are in agreement with observational constraints. The thickness of the secondary thermal boundary layer formed at the bottom of the convective layer is similar to the thickness of the ultralow‐velocity zone. The magnitude of lateral variations in temperature can only marginally be responsible for lateral variations in seismic velocities. Variations in the topography of the convection boundary and chemical heterogeneities are likely to be more important factors. A large seismic velocity drop in the ultralow‐velocity zone cannot be explained by thermal effects alone and must be caused by other factors such as partial melting.
Estimation of the 13.63‐day lunar tide effect on length of day
Authors: Bellanger E. , Blanter E. M. , Le Mouël J.‐L. , Shnirman M. G.
Published: October 2002

Using a simple nonlinear analysis method (E. M. Blanter et al., submitted manuscript, 2001), we are able to observe in the length of day the 18.6‐year beating of the two main fortnightly lunar tides and to measure the amplitude of the smallest one with a great accuracy. We can thus estimate the associated zonal response coefficient κ for this period. We find κ = 0.311 ± 0.010.
Effect of density on electrical conductivity of chemically laden polar ice
Authors: Barnes P. R. F. , Wolff E. W. , Mulvaney R. , Udisti R. , Castellano E. , Röthlisberger R. , Steffensen J.‐P.
Published: October 2002

Electrical conductivity measurements made using the dielectric profiling technique (DEP) are compared to chemical data from the top 350 m of the Dome C ice core in Antarctica. The chemical data are used to calculate the concentration of the major acidic impurities in the core: sulphuric acid and hydrochloric acid. The conductivity coefficients in solid ice for sulphuric acid (βH2SO4) and hydrochloric acid (βHCl) are found to be 4.9 and 4.5 S m−1M−1. These are consistent with previously found values for the acid conductivity coefficient at different sites and suggest that the same conductivity mechanisms are important in all polar ice. A method of rolling regression analysis is used to find the variation of the pure ice conductivity (σ∞ pure) and the conductivity coefficient of sulphuric acid, βH2SO4, with depth. Then σ∞ pure and βH2SO4are assessed against changes in core density and hence volume fraction of ice, v, due to the inclusion of air bubbles in the firn. Looyenga's model for dielectric mixtures applied to conduction in firn broadly predicts the variation observed in σ∞ purebut does not fit well for ice above 110 m. A previous application of the theory of percolation in random lattices is used to model the conductivity coefficient in firn. The coefficient βH2SO4 is linked to vby the power law: βH2SO4(v) ∝ βH2SO4(1) (v − vc)t; where vc is a threshold volume fraction below which no conduction can take place and is related to the geometry of the conducting lattice being modeled. The value of the exponent tis also dependent on the structure of the lattice and is here found to be t = 2.5, which is slightly lower than the previously obtained value of t = 2.7 for a structure where each grain has between 14 and 16 nearest neighbors. This model is consistent with the concept of conduction, via liquid H2SO4, taking place at two grain boundaries for firn. The model does not, however, preclude conduction taking place via acid situated at three grain boundaries or in an interconnected vein network at densities above 640 kg m−3.
Three‐dimensional finite difference method and hybrid modeling of earthquake ground motion
Authors: Oprsal Ivo , Zahradnik Jiri
Published: October 2002

A new two‐step hybrid technique (hybrid) has been developed to estimate the earthquake ground motion in three‐dimensional (3‐D) local models excited by external sources. The main use of the technique is fast evaluation of site effects and seismic hazard because our technique needs less computer memory and time than all‐in‐one source‐path‐site computational methods. The first step of the hybrid requires an arbitrary 3‐D method (e.g., finite difference (FD), discrete wave number, ray, or analytical solution) to compute three‐component excitation on a grid box surrounding a site of interest. A particular excitation contains information on the source and path effects and is saved on disk as a time history. Depending on the method used, the first step may also contain nonplanar topography. The second step (3‐D FD) employs local structure and topography inside the excitation box and a procedure that exactly couples the first step results with the second step. The speedup of the hybrid method is made possible through the computation of the excitation (saved on disk) in the first step and the use of FD on irregular grids in the second step of the hybrid. The second step computational model can be just a small spatial fraction of the original source‐path‐site model. A practical parameter study example of a point double‐couple source and regional topography model (applied in the first step) with a local low‐velocity basin structure and ridge (both applied in the second step only) is presented.
Fault structure and mechanics of the Hayward Fault, California, from double‐difference earthquake locations
Authors: Waldhauser Felix , Ellsworth William L.
Published: October 2002

The relationship between small‐magnitude seismicity and large‐scale crustal faulting along the Hayward Fault, California, is investigated using a double‐difference (DD) earthquake location algorithm. We used the DD method to determine high‐resolution hypocenter locations of the seismicity that occurred between 1967 and 1998. The DD technique incorporates catalog travel time data and relative P and S wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events. The relocated seismicity reveals a narrow, near‐vertical fault zone at most locations. This zone follows the Hayward Fault along its northern half and then diverges from it to the east near San Leandro, forming the Mission trend. The relocated seismicity is consistent with the idea that slip from the Calaveras Fault is transferred over the Mission trend onto the northern Hayward Fault. The Mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the Calaveras Fault at Calaveras Reservoir. In some locations, discrete structures adjacent to the main trace are seen, features that were previously hidden in the uncertainty of the network locations. The fine structure of the seismicity suggests that the fault surface on the northern Hayward Fault is curved or that the events occur on several substructures. Near San Leandro, where the more westerly striking trend of the Mission seismicity intersects with the surface trace of the (aseismic) southern Hayward Fault, the seismicity remains diffuse after relocation, with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation. The seismicity is highly organized in space, especially on the northern Hayward Fault, where it forms horizontal, slip‐parallel streaks of hypocenters of only a few tens of meters width, bounded by areas almost absent of seismic activity. During the interval from 1984 to 1998, when digital waveforms are available, we find that fewer than 6.5% of the earthquakes can be classified as repeating earthquakes, events that rupture the same fault patch more than one time. These most commonly are located in the shallow creeping part of the fault, or within the streaks at greater depth. The slow repeat rate of 2–3 times within the 15‐year observation period for events with magnitudes around M = 1.5 is indicative of a low slip rate or a high stress drop. The absence of microearthquakes over large, contiguous areas of the northern Hayward Fault plane in the depth interval from ∼5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large‐magnitude earthquakes.
Crustal structure of the Java margin from seismic wide‐angle and multichannel reflection data
Authors: Kopp Heidrun , Klaeschen Dirk , Flueh Ernst R. , Bialas Jörg , Reichert Christian
Published: October 2002

Seismic investigations across the convergent Sunda margin off Indonesia provide a detailed image of the crustal architecture of the Sunda plate boundary. The combined analysis and interpretation of wide‐angle and reflection seismic data along two coincident profiles across the subduction zone are complemented by additional lines within the forearc domain, which yield some three‐dimensional (3‐D) constraints on the velocity‐depth structure across the margin. A detailed cross section of the subduction zone is presented, which is confirmed by supplementary gravity modeling. The Sunda convergence zone is a prime example of an accretionary margin, where sediment accretion has led to the formation of a massive accretionary prism, with a total width of >110 km between the trench and the forearc basin. It is composed of a frontal wedge which documents ongoing accretion and a fossil part behind the present backstop structure which constitutes the outer high. Moderate seismic velocities derived from wide‐angle modeling indicate a sedimentary composition of the outer high. The subducting oceanic slab is traced to a depth of almost 30 km underneath the accretionary prism. The adjacent forearc domain is characterized by a pronounced morphological basin which is underlain by a layer of increased seismic velocities and a shallow upper plate Moho at 16 km depth. We speculate that remnant fragments of oceanic crust might be involved in the formation of this oceanic‐type crust found at the leading edge of the upper plate beneath the forearc basin.
Slip rates of the Karakorum fault, Ladakh, India, determined using cosmic ray exposure dating of debris flows and moraines
Authors: Brown E. T. , Bendick R. , Bourlès D. L. , Gaur V. , Molnar P. , Raisbeck G. M. , Yiou F.
Published: October 2002

We measure an average slip rate of 4 ± 1 mm yr−1 along the Karakorum fault, heretofore considered one of Earth's greatest strike‐slip faults and thought by many to play a key role in Asian deformation kinematics. Levees of a debris flow, and contours of the fan on which it was deposited, have been displaced 40 ± 5 m. Concentrations of 10Be in boulders from the debris flow yield ages of 11–14 ka, implying a slip rate of 4 ± 1 mm yr−1 during that period. A fresher debris flow has been offset 2–2.5 m since 1–2 ka, implying the occurrence of an earthquake with M ∼ 7 since that time. Concentrations of 10Be in boulders on the crest of the most extensive moraine near Leh imply that the most recent major glacial advance occurred at 90 ± 15 ka. This is consistent with the inference of others that alpine glaciers in this region have not necessarily expanded in concert with Northern Hemisphere continental ice sheets. If features, including lateral moraines, that Liu inferred to have been offset 300–350 m by the Karakorum fault date from the same period, they too imply a slip rate of 3–4 mm yr−1. This slip rate is comparable to rates of extension across grabens within Tibet. With recent evidence that slip along the Altyn Tagh fault occurs at ∼10 mm yr−1, our rate suggests that slip along the boundaries of Tibet is not significantly more rapid than extension within the plateau. Hence, plate tectonics, in the strictest sense, ought not be applied to Tibet, because Tibet does not behave as a rigid plate.
Models for computing geomechanical constants of double‐porosity materials from the constituents' properties
Authors: Berryman James G. , Pride Steven R.
Published: October 2002

In previous work, phenomenological equations for the poroelastic behavior of a double‐porosity dual‐permeability medium (i.e., low‐permeability storage porosity and high‐permeability transport porosity both present in the same three‐dimensional (3‐D) volume) were formulated, and the coefficients in these equations were identified. In the present work, models are given instead that allow all of these coefficients to be determined from the underlying constituents' properties. Two different models are provided for the six geomechanical constants in the isotropic theory. In one model the low‐permeability storage porosity is assumed to be a mixture of two porous materials in nonwelded or partially welded contact, and the high‐permeability joint porosity develops as a misfit porosity at the interface between the two partially welded constituents. In this model the joint phase is 100% fluid. In the other model the low‐permeability storage porosity is modeled as a single uniform porous material, while the high‐permeability joint porosity is modeled as a second distinct porous material in welded contact with the first. In this model the joint phase is a porous continuum possessing a skeletal framework representing the asperities and gouge material that are present in real joints. The complete set of formulas needed for forward modeling the geomechanical constants is obtained for both scenarios. Examples are evaluated with these formulas, and comparisons are made to previous results on double‐porosity systems analysis.
How remote can the far remote reference site for magnetotelluric measurements be?
Authors: Shalivahan  , Bhattacharya Bimalendu B.
Published: October 2002

Remote reference (RR) magnetotelluric (MT) measurements are made to reduce the bias caused by noise in electric field E and magnetic field H at a local site. RR only works when noise at a local and remote sites are uncorrelated. A study has been undertaken to find the farthest distance of a far remote site for crustal study which maintains the effectiveness of the RR technique. From theoretical studies the conditions for valid RR estimates have been obtained. The study shows that the remote site can be kept at a considerably large distance from the local site. In a field experiment a fixed local site and several remote sites have been selected at distances of 80, 115, and 215 km away in the frequency range of 30 Hz (∼0.03 s) to 0.00055 Hz (∼1800 s) at midlatitudes where the wavelengths of the source magnetic fields are long compared to the site separation. The remote sites are distributed over diverse geological settings. All the data of the fixed local site have been remote reference processed with the corresponding remote sites. The study reveals that using a remote site located at as large a distance as 215 km results in unbiased observations and remains effective in improving the MT data quality for all frequency ranges. If the data were acquired at long periods and or in the high latitudes, then to extract the stable uniform field estimates of the impedance one has to carry out robust processing technique.
Infrared satellite monitoring at Popocatépetl: Explosions, exhalations, and cycles of dome growth
Authors: Wright Robert , De La Cruz‐Reyna Servando , Harris Andrew , Flynn Luke , Jose Gomez‐Palacios Juan
Published: October 2002

Since December 1994, activity at Popocatépetl has evolved through two main phases. The first was characterized by elevated seismic activity, degassing, and emissions of predominantly lithic ash. However, in March 1996 the volcano entered a magmatic phase, which has involved the emplacement of at least eight lava domes within its summit crater. We have used infrared radiance data acquired by the GOES meteorological satellite alongside ground‐based geophysical data sets to analyze aspects of this activity. We develop a technique to identify magmatic activity at the volcano. In the absence of a volcanic heat source the radiance emitted from the “hottest” pixel (Pr) contained within a 10 × 10 pixel GOES subscene centered on Popocatépetl's summit is well correlated with the average radiance emitted by the remaining pixels (Br) because at any given time all pixels within the target box have similar temperatures, varying as a function of season, cloud cover, and solar irradiance. However, during periods of heightened volcanic activity the radiance emitted by the hottest pixel varies as a function of a time‐independent volcanic heat source and changes in Pr and Br become decorrelated, allowing volcanic activity to be identified. Exhalations (intermittent ash emissions) cannot be identified by routine analysis of GOES data because of the low intensity of the associated thermal anomalies. Explosions, however, produce distinctive radiance signatures and can be reliably documented. Two dome growth episodes occurred during our study period, one in November–December 1998 and the other during February 2000. Although the techniques we describe identified the 1998 dome event, the 2000 dome went undetected. This is explained in terms of their contrasting emplacement styles. The 1998 dome was apparent to GOES due to the explosions that disrupted its cool carapace, scattered hot bombs around the summit cone, and exposed its hotter interior to the satellite sensor. In contrast, the February 2000 dome was not apparent because elevated explosive activity did not accompany its emplacement.
Form and flow of the Academy of Sciences Ice Cap, Severnaya Zemlya, Russian High Arctic
Authors: Dowdeswell J. A. , Bassford R. P. , Gorman M. R. , Williams M. , Glazovsky A. F. , Macheret Y. Y. , Shepherd A. P. , Vasilenko Y. V. , Savatyuguin L. M. , Hubberten H.‐W. , Miller H.
Published: October 2002

The 5,575‐km2 Academy of Sciences Ice Cap is the largest in the Russian Arctic. A 100‐MHz airborne radar, digital Landsat imagery, and satellite synthetic aperture radar (SAR) interferometry are used to investigate its form and flow, including the proportion of mass lost through iceberg calving. The ice cap was covered by a 10‐km‐spaced grid of radar flight paths, and the central portion was covered by a grid at 5‐km intervals: a total of 1,657 km of radar data. Digital elevation models (DEMs) of ice surface elevation, ice thickness, and bed elevation data sets were produced (cell size 500 m). The DEMs were used in the selection of a deep ice core drill site. Total ice cap volume is 2,184 km3 (∼5.5 mm sea level equivalent). The ice cap has a single dome reaching 749 m. Maximum ice thickness is 819 m. About 200 km, or 42%, of the ice margin is marine. About 50% of the ice cap bed is below sea level. The central divide of the ice cap and several major drainage basins, in the south and east of the ice cap and of up to 975 km2, are delimited from satellite imagery. There is no evidence of past surge activity on the ice cap. SAR interferometric fringes and phase‐unwrapped velocities for the whole ice cap indicate slow flow in the interior and much of the margin, punctuated by four fast flowing features with lateral shear zones and maximum velocity of 140 m yr−1. These ice streams extend back into the slower moving ice to within 5–10 km of the ice cap crest. They have lengths of 17–37 km and widths of 4–8 km. Mass flux from these ice streams is ∼0.54 km3 yr−1. Tabular icebergs up to ∼1.7 km long are produced. Total iceberg flux from the ice cap is ∼0.65 km3 yr−1 and probably represents ∼40% of the overall mass loss, with the remainder coming from surface melting. Driving stresses are generally lowest (<40 kPa) close to the ice cap divides and in several of the ice streams. Ice stream motion is likely to include a significant basal component and may involve deformable marine sediments.
Modeling hydrofracture
Authors: Flekkøy Eirik G. , Malthe‐Sørenssen Anders , Jamtveit Bjørn
Published: October 2002

We have developed a model to simulate the formation and evolution of hydraulic fractures. The model relies on a discrete spring network representation of the fracturing media and a continuum description of the fluid pressure that diffuses within it. The model is both versatile and efficient due to the lack of redundant components in the description of the fluid dynamics and the simple mechanics of the spring network. The model is validated quantitatively in a simple test case where it is shown that the pressure forces on the solid agree with theoretical predictions. Subsequently, it is applied to some simplified geological scenarios in order to explore some generic effects. While no attempt has been made to apply representative parameters for real geological systems, the model demonstrates that the fracture formation depends crucially on the way pressure diffuses into the rock, both in the case of a point pressure source and in the case of a sandwich or “caprock” geometry where high fluid pressure causes hydraulic fracturing of a low‐permeability “lid” layer. Finally, the continuum limit of the elastic system is obtained analytically in order to identify the proper macroscopic parameters that are needed when simulations are to be matched to real geological systems.
Pore pressure and poroelasticity effects in Coulomb stress analysis of earthquake interactions
Authors: Cocco Massimo , Rice James R.
Published: October 2002

Pore pressure changes are rigorously included in Coulomb stress calculations for fault interaction studies. These are considered changes under undrained conditions for analyzing very short term postseismic response. The assumption that pore pressure is proportional to fault‐normal stress leads to the widely used concept of an effective friction coefficient. We provide an exact expression for undrained fault zone pore pressure changes to evaluate the validity of that concept. A narrow fault zone is considered whose poroelastic parameters are different from those in the surrounding medium, which is assumed to be elastically isotropic. We use conditions for mechanical equilibrium of stress and geometric compatibility of strain to express the effective normal stress change within the fault as a weighted linear combination of mean stress and fault‐normal stress changes in the surroundings. Pore pressure changes are determined by fault‐normal stress changes when the shear modulus within the fault zone is significantly smaller than in the surroundings but by mean stress changes when the elastic mismatch is small. We also consider an anisotropic fault zone, introducing a Skempton tensor for pore pressure changes. If the anisotropy is extreme, such that fluid pressurization under constant stress would cause expansion only in the fault‐normal direction, then the effective friction coefficient concept applies exactly. We finally consider moderately longer timescales than those for undrained response. A sufficiently permeable fault may come to local pressure equilibrium with its surroundings even while that surrounding region may still be undrained, leading to pore pressure change determined by mean stress changes in those surroundings.
Complex earthquake rupture and local tsunamis
Authors: Geist Eric L.
Published: October 2002

In contrast to far‐field tsunami amplitudes that are fairly well predicted by the seismic moment of subduction zone earthquakes, there exists significant variation in the scaling of local tsunami amplitude with respect to seismic moment. From a global catalog of tsunami runup observations this variability is greatest for the most frequently occurring tsunamigenic subduction zone earthquakes in the magnitude range of 7 <Mw< 8.5. Variability in local tsunami runup scaling can be ascribed to tsunami source parameters that are independent of seismic moment: variations in the water depth in the source region, the combination of higher slip and lower shear modulus at shallow depth, and rupture complexity in the form of heterogeneous slip distribution patterns. The focus of this study is on the effect that rupture complexity has on the local tsunami wave field. A wide range of slip distribution patterns are generated using a stochastic, self‐affine source model that is consistent with the falloff of far‐field seismic displacement spectra at high frequencies. The synthetic slip distributions generated by the stochastic source model are discretized and the vertical displacement fields from point source elastic dislocation expressions are superimposed to compute the coseismic vertical displacement field. For shallow subduction zone earthquakes it is demonstrated that self‐affine irregularities of the slip distribution result in significant variations in local tsunami amplitude. The effects of rupture complexity are less pronounced for earthquakes at greater depth or along faults with steep dip angles. For a test region along the Pacific coast of central Mexico, peak nearshore tsunami amplitude is calculated for a large number (N= 100) of synthetic slip distribution patterns, all with identical seismic moment (Mw= 8.1). Analysis of the results indicates that for earthquakes of a fixed location, geometry, and seismic moment, peak nearshore tsunami amplitude can vary by a factor of 3 or more. These results indicate that there is substantially more variation in the local tsunami wave field derived from the inherent complexity subduction zone earthquakes than predicted by a simple elastic dislocation model. Probabilistic methods that take into account variability in earthquake rupture processes are likely to yield more accurate assessments of tsunami hazards.