bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence for charge drift modulation at intermediate solar activity from the flux variation of protons and α particles
Authors: Boella G. , Gervasi M. , Mariani S. , Rancoita P. G. , Usoskin I. G.
Published: February 2004

We have studied the flux of galactic protons and helium nuclei measured at 1 AU along two consecutive solar activity cycles. We have correlated cosmic ray fluxes measured on IMP 8 satellite at low energy (50 – 300 MeV amu−1) with neutron monitor counts registered at Climax station (at energies above few GeV) during the period 1973 – 1995. We have found a systematic excess in the flux of the positive charged particles during the periods with positive solar magnetic field polarity (A > 0) with respect to the flux during the periods of negative polarity (A < 0). This flux excess gives an experimental evidence that charge drift effect plays an important role in the modulation of galactic cosmic rays. A systematic investigation of charge drift modulation is presented. The dependence of the drift effect on the solar activity phase arid the particle energy has been also studied. A variation of the proton (as well as helium) flux might be as large as 40%, at the solar activity minimum of two contiguous cycles, at energy ≤ 100 MeV.
Numerical modeling of magma withdrawal during explosive caldera‐forming eruptions
Authors: Folch A. , Codina R. , Martí J.
Published: February 2004

We propose a simple physical model to characterize the dynamics of magma withdrawal during the course of caldera‐forming eruptions. Simplification involves considering such eruptions as a piston‐like system in which the host rock is assumed to subside as a coherent rigid solid. Magma behaves as a Newtonian incompressible fluid below the exsolution level and as a compressible gas‐liquid mixture above this level. We consider caldera‐forming eruptions within the frame of fluid‐structure interaction problems, in which the flow‐governing equations are written using an arbitrary Lagrangian‐Eulerian (ALE) formulation. We propose a numerical procedure to solve the ALE governing equations in the context of a finite element method. The numerical methodology is based on a staggered algorithm in which the flow and the structural equations are alternatively integrated in time by using separate solvers. The procedure also involves the use of the quasi‐Laplacian method to compute the ALE mesh of the fluid and a new conservative remeshing strategy. Despite the fact that we focus the application of the procedure toward modeling caldera‐forming eruptions, the numerical procedure is of general applicability. The numerical results have important geological implications in terms of magma chamber dynamics during explosive caldera‐forming eruptions. Simulations predict a nearly constant velocity of caldera subsidence that strongly depends on magma viscosity. They also reproduce the characteristic eruption rates of the different phases of an explosive caldera‐forming eruption. Numerical results indicate that the formation of vortices beneath the ring fault, which may allow mingling and mixing of parcels of magma initially located at different depths in the chamber, is likely to occur for low‐viscosity magmas. Numerical results confirm that exsolution of volatiles is an efficient mechanism to sustain explosive caldera‐forming eruptions and to explain the formation of large volumes of ignimbrites.
Velocity structure of the upper mantle discontinuities beneath North America from waveform inversion of broadband seismic data using a genetic algorithm
Authors: Neves F. A. , Singh S. C. , Priestley Keith
Published: February 2004

We present a velocity structure of the mantle discontinuities beneath the United States from a full waveform inversion of broadband seismic data. The data consist of 10 moderate earthquakes in the western United States that were recorded on 13 broadband seismometers across the United States operated by the U.S. National Seismic Network. A total of 36 seismograms with distance ranging from 1150 to 2700 km were used. A full waveform inversion based on two‐dimensional modeling and one‐dimensional inversion methods using a genetic algorithm was adopted. The inversion method is objective, provides error bounds on the model, and does not require any a priori information about these discontinuities. We find that the upper discontinuity is ∼10 km thick, from 395 to 405 km, where the velocity increases from 8.80 to 9.15 km s−1 with a velocity gradient of 0.035 s−1. The lower discontinuity is ∼5 km thick, occurring over a depth range of 650–655 km, where the velocity increases from 10.20 to 10.70 km s−1. Our results are in agreement with previous studies of the lower discontinuity but do not agree for a sharp (<5 km) upper mantle discontinuity.
Spatial variations in Te in the southern Appalachians, eastern United States
Authors: Armstrong G. D. , Watts A. B.
Published: February 2004

Various studies in the oceans have shown that the flexural rigidity, or equivalently effective elastic thickness (Te), of the lithosphere is determined by the load and plate age. The results of studies in the continents have, however, been more controversial. Determinations of Te made using spectral studies and based on the Bouguer anomaly coherence technique and surface and subsurface loading suggest that North America has a high Te “core” (>100 km) which is flanked by lower values. In contrast, studies based on the free air admittance suggest that Te of North America is <25 km, and of the order of the seismogenic layer thickness. It has been proposed that this discrepancy results from the fact that estimates based only on Bouguer coherence may be biased upward due to topographical erosion which introduces “noise”, especially at short wavelengths. In order to address this question further, we have used a maximum entropy based coherence method to determine the wavelength relationship between gravity and topography in the southern Appalachians, a region where the Bouguer coherence, free‐air admittance, and forward modeling techniques have already been applied. Our studies reveal a variable Te structure with a mean Te of 51 km and values which have a range 20 to 100 km. The mapped Te fabric has a distinct NE‐SW trend which appears to follow the tectonic elements of the southern Appalachians. In particular, the foreland is generally associated with higher Te values than the flanking orogenic belt. Correlations at smaller scales are difficult, however, to establish. The Te fabric does not reflect the complex terrains that make up highly deformed regions within the orogenic belt. Our spectrally determined Te estimates are in close agreement with ones based on forward modeling. They are a factor of 4 higher, however, than results previously based on free‐air admittance. We attribute this to the fact that we have used the Bouguer coherence technique which accounts for both surface and subsurface loading. Our results suggest that while other factors such as erosion may upwardly bias Te in some regions, buried loads satisfactorily explain the spectral estimates in the southern Appalachians, irrespective of whether they are based on Bouguer coherence or free‐air admittance. There is solid geological evidence in the southern Appalachians which is readily attributable to the presence of subsurface loads. This is a strong argument, we believe, to suggest that Te in the continents is high and can have values which range from 20 to 100 km.
Tectonic synthesis of the Olympic Mountains segment of the Cascadia wedge, using two‐dimensional thermal and kinematic modeling of thermochronological ages
Authors: Batt Geoffrey E. , Brandon Mark T. , Farley Kenneth A. , Roden‐Tice Mary
Published: February 2004

A fully coupled two‐dimensional kinematic and thermal model of a steady state accretionary wedge, constrained by an extensive data set of fission track and (U‐Th)/He ages for apatite and zircon, is here used to investigate the development of the Olympic Mountains segment of the Cascadia accretionary wedge. The model has two main free parameters:
, the maximum rate of erosion for a generic erosion function operating at the top of the wedge, and α, the distribution of sedimentary accretion into the wedge. The best fit values for
and α and their confidence limits are determined through an iterative search of parameter space. This study represents the first time that such inversion methods have been used to quantify the thermal‐kinematic evolution of an accretionary wedge. Our results suggest that horizontal transport plays an important role in the exhumation trajectories experienced by material passing through the Cascadia wedge. At a 95% confidence interval, 80 to 100% of the sedimentary sequence from the subducting Juan de Fuca Plate has been accreted at the front of the wedge offshore of the Olympics over the past 14 m.y. This frontally accreted material must then traverse the entire width of the wedge prior to its eventual exposure in the Olympic forearc high. Assessed in this two‐dimensional framework, the fission track and (U‐Th)/He age data sets from the Olympic Mountains are all best fit by
of 0.9–1.0 mm yr−1, despite variation in the timescales relevant to the three chronometers. This result supports the hypothesis that the Olympic Mountains segment of the Cascadia accretionary wedge has been in a flux steady‐state since ∼14 Ma. The demonstration of a flux balance across the Cascadia margin also suggests that margin‐parallel transport has not had a significant role in driving uplift of the Olympic Mountains.
Finite element models of stress orientations in well‐developed strike‐slip fault zones: Implications for the distribution of lower crustal strain
Authors: Lynch John C. , Richards Mark A.
Published: February 2004

Finite element models are used to examine the effects of strike‐slip earthquakes on stresses in an elastic layer overlying a finite width viscoelastic shear zone in the lower crust. The overall dimensions of the model are 300 km wide, 400 km long, and 50 km deep. Three geometries for the lower crustal shear zone are considered: (1) a viscoelastic half‐space approximation, with a shear zone that extends to the model boundaries (300 km in width); (2) a wide shear zone model (70 km in width); and (3) a narrow shear zone model (10 km in width). Earthquakes are simulated with a fault plane that slips without friction at the desired time step and is centered above the shear zone, extending to a depth of 15 km and running the length of the mesh. Far‐field plate velocity boundary conditions are enforced at the model edges so that stress on the fault evolves naturally. A Coulomb‐type failure criterion based on the average shear stress on the fault is set such that the earthquake cycle is ∼250 years. The models are run until a limit cycle is reached and transient stresses have decayed away. We focus on the maximum changes in the stress field during the earthquake cycle by examining stresses before and immediately after each earthquake. In addition to comparisons of the separate components of the stress tensor we present results in the form of maximum compressive stress orientations, plunge angles of the principal stress axes, and beach ball diagrams that facilitate visualizing the full tensor. Stresses are concentrated in the upper crust where it overlies the finite width viscoelastic shear zone, which causes the plunge angles of the principal stress axes to rotate from Andersonian orientations of 90° and 0° to angles that approach 45° in the lower crust. Our results suggest that an examination of stress orientations in the upper and middle crust from borehole breakouts and focal mechanisms may provide insight as to the distribution of strain in the lower crust and may eventually allow us to distinguish between localized and distributed deformation models for the lower crust in active strike‐slip zones.
Crustal stress field in southern California and its implications for fault mechanics
Authors: Hardebeck Jeanne L. , Hauksson Egill
Published: February 2004

We present a new, high spatial resolution image of stress orientation in southern California based on the inversion of earthquake focal mechanisms. We use this image to study the mechanics of faulting in the plate boundary region. The stress field contains significant spatial heterogeneity, which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause. Temporal changes in the stress field are also observed, primarily related to major earthquakes. The observed 15° (±10°) rotation of the stress axes due to the 1992M7.3 Landers mainshock implies that the deviatoric stress magnitude in the crust is low, of the order of 10 MPa. This suggests that active faults in southern California are weak. The maximum principal stress axis near the San Andreas Fault is often at ∼50° to the fault strike, indicating that the shear stress on the fault is comparable to the deviatoric stress. The San Andreas in southern California may therefore be a weak fault in a low‐strength crust.
Inferring rate and state friction parameters from a rupture model of the 1995 Hyogo‐ken Nanbu (Kobe) Japan earthquake
Authors: Guatteri Mariagiovanna , Spudich Paul , Beroza Gregory C.
Published: February 2004

We consider the applicability of laboratory‐derived rate‐and state‐variable friction laws to the dynamic rupture of the 1995 Kobe earthquake. We analyze the shear stress and slip evolution ofIde and Takeo's[1997] dislocation model, fitting the inferred stress change time histories by calculating the dynamic load and the instantaneous friction at a series of points within the rupture area. For points exhibiting a fast‐weakening behavior, the Dieterich‐Ruina friction law, with values ofdc= 0.01–0.05 m for critical slip, fits the stress change time series well. This range ofdcis 10–20 times smaller than the slip distance over which the stress is released,Dc, which previous studies have equated with the slip‐weakening distance. The limited resolution and low‐pass character of the strong motion inversion degrades the resolution of the frictional parameters and suggests that the actualdcis less than this value. Stress time series at points characterized by a slow‐weakening behavior are well fitted by the Dieterich‐Ruina friction law with values ofdc≥0.01–0.05 m. The apparent fracture energyGccan be estimated from waveform inversions more stably than the other friction parameters. We obtain aGc≈ 1.5×l06J m−2for the 1995 Kobe earthquake, in agreement with estimates for previous earthquakes. From this estimate and a plausible upper bound for the local rock strength we infer a lower bound forDcof about 0.008 m.
Influence of compression‐induced fluid pressures on rock strength in the brittle crust
Authors: Simpson Guy
Published: February 2004

Fluid pressures have an important influence on rock strength in the brittle crust. This study investigates the development of fluid overpressures and the subsequent decrease in rock strength (differential stress at failure), induced by tectonic compression of a fluid‐saturated poroelastic medium limited by a Mohr‐Coulomb failure surface. It is shown that the pore pressure induced by compression of a poroelastic medium is controlled by the rate of loading relative to the rate at which the fluid pressure can dissipate by diffusion. Excess pore pressures may be generated by this loading mechanism if permeabilities are low, if the boundary deformation rate is fast, or if the distance between drained boundaries is large. The magnitude of the pore pressures developed, and the subsequent strength decrease, is controlled by Skempton's coefficient B, the undrained Poisson ratio νu, and the friction angle ϕ. Numerical results for realistic parameter values indicate that in upper crustal settings characterized by rapid stress increases (5×10−10 MPa s−1), drainage length scales on the order of a few kilometers, and moderate to low permeabilities (10−18 to 10−16 m2), the strength may be reduced to ∼60% of the strength of rock containing a hydrostatic fluid pressure and to 40% of the strength of dry rock. Under undrained conditions, fluid pressures are generated in proportion to the magnitude of the differential stress. Even so, fluid pressures generated by compressive (plane strain) deformation under undrained conditions are unlikely to attain the lithostatic pressure (i.e., pf/(ρrgz) < 1). Approximate analytic expressions are presented which enable a priori determination of compression‐induced fluid pressures, and resulting rock strength modifications, in the brittle upper crust.
Vertical tectonics in northern Escanaba Trough as recorded by thick late Quaternary turbidites
Authors: Normark William R. , Serra Francesca
Published: February 2004

Escanaba Trough, the southernmost segment of the Gorda Ridge, is filled by as much as 500 m of late Quaternary turbidite and hemipelagic sediment. Coring at Deep Sea Drilling Project Site 35 and Ocean Drilling Program (ODP) Sites 1037 and 1038 together with 4.5‐kHz deep‐tow and 3.5‐kHz surface‐ship seismic reflection profiles enable a distinct pattern of reflections to be mapped throughout Escanaba Trough in the upper part of this sediment fill. The uppermost 80 m of turbidite sediment, which includes at least 11 turbidity current events, were deposited in <7 kyr. Nine of these turbidites are found throughout Escanaba Trough at water depths >3200 m. The turbidity currents were trapped upon entering Escanaba Trough, resulting in all of the sediment in suspension in the flows being deposited. The thickness of the turbidite layers reflects both the flow thickness and the vertical grain concentration within the flow that deposited the layer. Variations in the turbidite thickness with respect to water depth can be used to estimate the degree of relative vertical movement within the floor of Escanaba Trough. In the area of hydrothermal activity near ODP Site 1038, uplift of as much as 140 m has occurred over the past 8 kyr.
Viscoelastic shear zone model of a strike‐slip earthquake cycle
Authors: Pollitz Fred F.
Published: February 2004

I examine the behavior of a two‐dimensional (2‐D) strike‐slip fault system embedded in a 1‐D elastic layer (schizosphere) overlying a uniform viscoelastic half‐space (plastosphere) and within the boundaries of a finite width shear zone. The viscoelastic coupling model of Savage and Prescott [1978] considers the viscoelastic response of this system, in the absence of the shear zone boundaries, to an earthquake occurring within the upper elastic layer, steady slip beneath a prescribed depth, and the superposition of the responses of multiple earthquakes with characteristic slip occurring at regular intervals. So formulated, the viscoelastic coupling model predicts that sufficiently long after initiation of the system, (1) average fault‐parallel velocity at any point is the average slip rate of that side of the fault and (2) far‐field velocities equal the same constant rate. Because of the sensitivity to the mechanical properties of the schizosphere‐plastosphere system (i.e., elastic layer thickness, plastosphere viscosity), this model has been used to infer such properties from measurements of interseismic velocity. Such inferences exploit the predicted behavior at a known time within the earthquake cycle. By modifying the viscoelastic coupling model to satisfy the additional constraint that the absolute velocity at prescribed shear zone boundaries is constant, I find that even though the time‐averaged behavior remains the same, the spatiotemporal pattern of surface deformation (particularly its temporal variation within an earthquake cycle) is markedly different from that predicted by the conventional viscoelastic coupling model. These differences are magnified as plastosphere viscosity is reduced or as the recurrence interval of periodic earthquakes is lengthened. Application to the interseismic velocity field along the Mojave section of the San Andreas fault suggests that the region behaves mechanically like a ∼600‐km‐wide shear zone accommodating 50 mm/yr fault‐parallel motion distributed between the San Andreas fault system and Eastern California Shear Zone.
Quantitative characterization and elevation estimation of Lake Lahontan shoreline terraces from high‐resolution digital elevation models
Authors: Hare Jennifer L. , Ferguson John F. , Aiken Carlos L. V. , Oldow John S.
Published: February 2004

The precision with which the elevation of a feature, such as a terrace, can be measured depends on the characterization of the noise contaminating the measurement. A method for identification and extraction of terrace feature elevations is presented and the topographic noise, due to erosion, as well as measurement error, is quantified. High‐resolution digital elevation models (DEM) are acquired at six wave‐cut, volcanic bedrock terrace sites from around the highstand of paleo‐Lake Lahontan in the western Great Basin. Local DEMs, which are tied to regional geodetic control, were acquired using conventional total station, rapid postprocessed and real‐time kinematic Global Positioning System methods. The topographic signal is processed with derivative filters for geomorphic feature recognition and averaging for noise reduction. Results indicate that noise levels for identifiable features such as riser crest, knickpoint, and slope inflection point are statistically equivalent and on the order of 0.5 m standard deviation. Averaging within topographic bins spanning ∼50 m along terrace strike yields feature elevation estimates with standard errors on the order of 0.12 m. The mean bench window elevation (between the riser crest and knickpoint) has the lowest standard error and is systematically related to water level. Propagation of surveying, geoid estimation, and terrace feature elevation estimation errors indicates that displacements on the order of 0.5 m may be resolved using these methods. Elevation estimate interpretation involves terrace development, degradation, and neotectonics, but this new methodology has significant advantages in studies of neotectonic or geomorphic processes using local terrace elevation measurements.
Rheologic control of buoyancy‐driven extension of the Rio Grande rift
Authors: Townsend David A. , Sonder Leslie J.
Published: February 2004

Extensional deformation in the western United States, particularly in the Basin and Range, has frequently been ascribed to buoyancy forces arising from horizontal density contrasts. The Rio Grande rift is subject to buoyancy forces 2–3 times larger than those in the tectonically active northern Basin and Range, suggesting that it should be extending considerably faster than the northern Basin and Range, yet paradoxically is extending more slowly. Using a thin viscous sheet model, we test the hypothesis that regions surrounding the Rio Grande rift, which have rheologically stronger lithosphere, resist deformation and reduce the rate of extensional deformation in the Rio Grande rift to values consistent with observations. We find that these regions must have lithospheric strength at least twice that of the Rio Grande rift. This modest contrast in lithospheric strength is consistent with independent estimates derived from observations of heat flow, seismicity, and crustal thickness.
Creep rate changes at Parkfield, California 1966–1999: Seasonal, precipitation induced, and tectonic
Authors: Roeloffs Evelyn A.
Published: February 2004

Measured fault creep at Parkfield contains information about the slip rate on the San Andreas fault but also is affected by rainfall and other seasonal effects. Seven creep meters record a repeatable seasonal variation that can be removed from the data. Four of the creep meters exhibit surges in creep rate during the rainy season. For these instruments a function of rainfall is developed that crosses a threshold within a few days of the onset of accelerated creep. In addition to the seasonal variations and surges, most Parkfield creep meters record higher creep rates more frequently during the wet season than during the dry season. Characterizing this behavior makes it possible to distinguish between rainfall‐influenced creep and creep rate changes of tectonic origin. Accelerated creep beginning in January 1983 was probably induced by rainfall, rather than by tectonic sources related to the Coalinga earthquake in May 1983. Four creep meters in the central part of the Parkfield fault segment are recording higher creep rates since 1993. A strong case can be made that the increased creep rate at CRR1 is tectonic, but for the other three creep meters an influence of rainfall is difficult to rule out. Strain rate changes recorded on three‐component borehole strain meters suggest that the rate of subsurface creep to the northwest of these instruments is even greater. The 1993 creep rate increase is more abrupt and farther south than the expected signal from accelerating slip near the hypocenter of the 1966 Parkfield earthquake but is in the same location as rapid fault slip reported prior to that event.
Can relative paleointensities be determined from the normalized magnetization of the wind‐blown loess of China?
Authors: Pan Yongxin , Zhu Rixiang , Shaw John , Liu Qingsong , Guo Bin
Published: February 2004

Although the wind‐blown loess and interbedded paleosol sequence in the Loess Plateau of China has been extensively studied and regarded as one of the most complete terrestrial records of both the geomagnetic field and climatic changes during the Quaternary, little attention has been paid to it as a source of relative paleointensity. In this study, we examine the Malan loess (L1), the last glacial sediments, in the Lingtai section to determine whether loess is able to reliably record the relative paleointensity. Toward that end both the conventional normalizing method as well as the pseudo‐Thellier method [Tauxe et al., 1995] were used in conjunction with the examination of the rock magnetic properties and natural remanent magnetization (NRM). Rock magnetic properties of L1 show the uniformity in terms of magnetic mineralogy and grain size, suggesting that they may be suitable for relative paleointensity studies. Normalized remanences derived from L1 show highs between both 72–66 ka and 62–51 ka and lows near 63 ka, 42 ka and between 20 and 10 ka. Comparable results using different normalization parameters suggest that large‐amplitude variation of normalized magnetization may reflect the intensity variation of the geomagnetic field. However, comparisons of the normalized magnetization with contemporaneous marine records show some dissimilarities. In particular, the intensity low at 20–10 ka, corresponding to the last glacial maximum, is not compatible with the Sint‐800 composite record. On the other hand, spectral coherence analyses of middle‐ and high‐frequency components of the normalized magnetizations suggest some climatic influence. Thus this apparent discrepancy may be explained by climatic changes in the Loess Plateau and the resulting effect on the NRM.
An integrated mechanical model of the San Andreas Fault in central and northern California
Authors: Chéry Jean , Zoback Mark D. , Hassani Riad
Published: February 2004

Several lines of evidence support the general view of the San Andreas fault system (SAFS) as a major lithospheric weakness in a generally transpressive plate margin setting. However, the influence of the weakness of the SAFS on the observed stress and deformation fields is not straightforward because factors such as interactions between the brittle upper crust and ductile lower crust, lateral fault strength variation, and the amount of convergence may all be important. The goal of this study is to model steady state deformation, relative fault‐parallel velocity, and crustal stress orientations in central and northern California using realistic rheologies and boundary conditions. Using a simplified three‐dimensional (3‐D) finite element analysis in large strain, we model the SAFS in 2‐D cross sections with no stress or strain variations along strike. We investigate the influence of different parameters such as the frictional properties of the fault zone and the adjacent crust, the viscous properties of the lower crust as determined by its thermal structure, and the thermal structure of the lithosphere. Our model appears to provide a good conceptual framework for some first‐order aspects of actively deforming plate margins such as the SAFS. The following findings emerge from a variety of numerical experiments: (1) The difference in the manner of transpressive strain partitioning in central and northern California can be explained by different fault strengths and the manner in which heat flow varies with distance from the fault. (2) Only the combination of a weak fault (with an effective friction coefficient of ≈0.1) and a strong lateral heat flow variation predicts approximately correct stress directions in the crust adjacent to the SAFS.
A three‐dimensional crustal velocity model of the southwestern Alps from local earthquake tomography
Authors: Paul Anne , Cattaneo Marco , Thouvenot François , Spallarossa Daniele , Béthoux Nicole , Fréchet Julien
Published: February 2004

A temporary network of 65 short‐period seismological stations was installed in the southwestern Alps during the second half of 1996. It complemented the permanent monitoring networks, obtaining an average interstation distance of ∼10 km. Travel time data from 446 local earthquakes and 104 quarry blasts were inverted simultaneously for hypocenter parameters and three‐dimensional velocity structure. The P wave velocity model displays strong lateral contrasts both at shallow and deeper levels. A low‐velocity anomaly stands out at shallow depths beneath the Digne and Castellane nappes in the southwestern part of the investigated area. Farther east, the Monviso ophiolitic massif appears to have a much larger extension at depth than previously assumed. The largest and strongest anomaly is located under the Dora Maira massif and the westernmost Po plain. It correlates with the well‐known Ivrea body, which is classically interpreted as a wedge of Adriatic upper mantle. At the best resolved depths (10 and 15 km) it appears as a rather thin (10 to 15 km), north‐south elongated, high‐velocity (7.4 to 7.7 km s−1) anomaly with very sharp edges, extending to the south as far as 10 km north of the surface trace of the Frontal Penninic Thrust. Special care was taken with regard to the quantitative estimation of the resolution for the main anomalies using the inversion of synthetic travel time data.
Effect of seafloor temperature and pressure variations on methane flux from a gas hydrate layer: Comparison between current and late Paleocene climate conditions
Authors: Xu Wenyue , Lowell Robert P. , Peltzer Edward T.
Published: February 2004

We investigate the response of a methane hydrate layer in marine sediments to cyclic seafloor perturbations of temperature and pressure in order to determine the change in seafloor methane flux resulting from gas hydrate dissociation or accumulation. By using a one‐dimensional model describing mass, energy, and methane transport through porous sediments we show that seafloor pressure changes have negligible effect on methane transport to the seafloor. The effect of seafloor temperature perturbations is more pronounced than that of pressure. With an initial seafloor temperature of 3°C, which corresponds to current conditions on Earth, a +4°C seafloor temperature perturbation occurring over 104 years does not significantly effect methane transport. Thus such a perturbation is not likely to have a significant impact on the current global climate or to give rise to an event such as the δ13C excursion during the late Paleocene thermal maximum (LPTM). If the initial seafloor temperature is assumed to be 11°C, which corresponds to the conditions of the late Paleocene, a +4°C temperature perturbation over a period of 104 years could result in complete dissociation of methane hydrate layers situated at water depths around 1200 m. In this case, the calculations show that the change in methane flux might be able to explain the δ13C excursion of marine carbonate fossils during the LPTM. This result is weakened because the simplifications in the model tend to yield overestimates of the change of methane flux. The principal point is that strong coupling between methane transport and seafloor temperature occurs because significant hydrate accumulation and dissociation take place near the seafloor only when the seafloor temperature is relatively high. This was the case during the late Paleocene, but it is not the case at present.
Normal fault interaction caused by coseismic and postseismic stress changes
Authors: Nostro Concetta , Piersanti Antonio , Cocco Massimo
Published: February 2004

We study coseismic and postseismic stress fields caused by a normal faulting earthquake in a self‐gravitating, stratified, viscoelastic spherical Earth over distances from a few to hundreds of kilometers. We investigate the contribution of postseismic relaxation on the induced Coulomb stress for extensional tectonic settings accounting for the effects of the Earth stratification. We use a numerical code based on the spherical self‐gravitating Earth model developed by Piersanti et al. [1995, 1997]. We study how postseismic relaxation can modify the state of stress at the base of the seismogenic layer where large earthquakes are believed to nucleate. We compare our results with those obtained by means of a three‐dimensional dislocation model in an elastic half‐space, which does not account for the time‐dependent postseismic stress transfer. The viscoelastic relaxation process modifies the coseismic stress changes during time periods from several decades to centuries. The postseismic stress is generally greater than the coseismic stress change. Postseismic relaxation increases the Coulomb stress near the causative faults and tends to reduce the stress shadow areas. The temporal evolution of Coulomb stress reveals that in addition to the viscosity value, the thickness of the elastic layer controls the time at which the relaxation process is completed. A larger thickness of the elastic layer yields a faster relaxation in the first few decades after the seismic event but smaller postseismic stress amplitudes at longer timescales. One of the most significant results of this study is the extreme sensitivity of the timescales of the viscoelastic relaxation to small changes in the thickness and depth of the shallowest viscoelastic layer as well as in variation of the viscosity. Such a result suggests that the interpretation of the time evolution of the postseismic signals only in terms of viscosity values could lead to misleading conclusions.
Elastohydrodynamic lubrication of faults
Authors: Brodsky Emily E. , Kanamori Hiroo
Published: February 2004

The heat flow paradox provides evidence that a dynamic weakening mechanism may be important in understanding fault friction and rupture. We present here a specific model for dynamic velocity weakening that uses the mechanics of well‐studied industrial bearings to explain fault zone processes. An elevated fluid pressure is generated in a thin film of viscous fluid that is sheared between nearly parallel surface. This lubrication pressure supports part of the load, therefore reducing the normal stress and associated friction across the gap. The pressure also elastically deforms the wall rock. The model is parameterized using the Sommerfeld number, which is a measure of the lubrication pressure normalized by the lithostatic load. For typical values of the material properties, slip distance and velocity, the Sommerfeld number suggests that lubrication is an important process. If the lubrication length scales as the slip distance in an earthquake, the frictional stress during dynamically lubricated large earthquakes is 30% less than the friction with only hydrostatic pore pressure. Elastohydrodynamic lubrication also predicts a decrease in high‐frequency (>1 Hz) radiation above a critical slip distance of a few meters. This prediction is well matched by the strong motion data from the 1999 Taiwan earthquake. The observed 2 orders of magnitude variation in scaled radiated energy between small (Mw < 4) and large earthquakes (Mw > 6) is also predicted by the lubrication model.
Erosion and rheology controls on synrift and postrift evolution: Verifying old and new ideas using a fully coupled numerical model
Authors: Burov E. , Poliakov A.
Published: February 2004

We use a fully coupled asymmetric dynamic finite element model to study interactions between thermomechanical behavior and surface processes during continental rift evolution. The model accounts for (1) nonlinear brittle‐elastic‐ductile rheology, layered lithological structure, and faulting; (2) heat transport and thermal buoyancy forces; and (3) “true” erosion and sedimentation (grid elements are eliminated and recreated). Faults are not predefined but are self‐localized; their distributions and geometry are model outputs, which provides new geologically sensible constraints on its validity. We test previous ideas on rift evolution based on numerical and analytical component theories (or individual parts) of our model. After demonstrating that our coupled model reproduces classic rift features, we then demonstrate that synrift surface processes result in enhanced lithospheric thinning and widening of the basin, so that the apparent stretching factors increase by a factor of 1.5–2. Sedimentation results not only in thermal but also in localized flexural weakening of the lithosphere, which locally compensates strengthening due to cooling. Erosion on the uplifted flanks produces local strengthening and rebound. Surface processes produce pressure gradients, which drive a ductile crustal flow that (1) provides a fast feedback with tectonic processes and controls subsidence rates and flank stability and (2) drives a secondary extension or compression and uplift on the late synrift/early postrift phase. Our results indicate that kinematic and dynamic rift models that ignore erosion may produce misleading results in many rifts. We reproduced and explained a number of enigmatic synrift phenomena, such as (1) polyphase subsidence provoked by switching of the level of necking between different competent lithological layers and (2) synrift and postrift stagnation and vertical accelerations unassociated with tectonic stress inversion or phase changes.
Direct inversion of earthquake first motions for both the stress tensor and focal mechanisms and application to southern California
Authors: Abers Geoffrey A. , Gephart John W.
Published: February 2004

In inverting populations of focal mechanisms to estimate stress, misfits may arise owing to (1) errors in determining the fault plane solutions and (2) real variations in the state of stress. Among a number of techniques that invert for stress, most effectively regard only the latter and only indirectly describe the former through statistical means so make it difficult to differentiate real stress variations from noisy data. Ambiguity arises because realistic methods for determining focal mechanisms for small earthquakes depend on P wave first motions in a highly nonlinear manner, which is difficult to propagate into stress inversions. To address these issues, we present a new method for constraining stress tensors, in which first‐motion observations are directly inverted for stress without assuming that focal mechanisms are known. The technique produces estimates of four stress parameters, a suite of focal mechanisms consistent with both the stress tensor and the first motions, and estimates of the uncertainty in stress. It also provides a natural test of the stress homogeneity hypothesis and a means to identify those earthquakes whose first motions are not consistent with a homogeneous stress tensor. When tested against artificial data, the method correctly recovers input stresses and yields uncertainties that depend upon both data distribution and reliability of first motion picks, as expected. Utilizing first motions from the southern California catalog, the method is applied to several clusters and aftershock sets, from 1981 to 1992. Typical uncertainties in stress orientation exceed 20–30° at the 95% confidence level, resulting from ∼5% mispick rates typical of even the cleanest data. These uncertainties exceed some previous estimates for similar populations, presumably because they account for focal mechanism un‐certainties, and perhaps indicating that some previously reported stress variations are not statistically significant. Most populations are found to be internally consistent with a homogeneous stress field, the most significant exceptions being aftershock sequences of some major earthquakes. In at least one case, the 1992 Landers sequence, aftershocks at the north end of the rupture zone appear to reflect exceedingly heterogeneous stresses. Overall, the method provides a useful and robust approach for evaluating stress constraints provided by P wave first motions.
Correction to “A geomagnetic record over the last 3.5 million years from deep‐tow magnetic anomaly profiles across the Central Indian Ridge” by Gaud Pouliquen, Yves Gallet, Jérôme Dyment, Philippe Patriat, and Chiori Tamura
Authors: Pouliquen Gaud , Gallet Yves , Dyment Jérôme , Patriat Philippe , Tamura Chiori
Published: February 2004

High‐resolution records of the geomagnetic field intensity over the last 3.5 Myr provided by paleomagnetic analyses of marine sediments and volcanics have shown the occurrence of short‐lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep‐tow (∼500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well‐dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short‐wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific‐Antarctic Ridge. Our results suggest that in addition to polarity reversals, geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small‐scale magnetic anomaly, likely generated by several closely spaced excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near‐bottom results support the existence of three geomagnetic features between the Gauss‐Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A. ln subchron within the Gauss chron. This study emphasizes the potential of deep‐tow magnetic surveys in detecting fluctuations in geomagnetic field intensity and, in particular, short‐lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum.
Mass balance equations for open magmatic systems: Trace element behavior and its application to open system melting in the upper mantle
Authors: Ozawa Kazuhito
Published: February 2004

A general mass conservation equation for either stable or radioactive chemical components in an open magmatic system is formulated. The equation is applied to trace elements by assuming melt‐mineral equilibrium defined by a partition coefficient. A derived analytic expression has very wide applicability, and most of the mass conservation equations in the literature for stable trace elements are obtained by choosing appropriate values for the controlling model parameters, such as material influx or separation rate and melt fraction in the system. The general equation is also modified to facilitate an application to multistage problems. It is demonstrated that the general open system mass conservation equation for melting processes expressed without melt fraction is mathematically identical to that for one‐dimensional steady state two‐phase flow with sink and source terms. This one‐dimensional steady state model is applied to depleted abyssal peridotites to constrain their melting processes in an ascending mantle beneath ridges. The rare earth element (REE) patterns of clinopyroxene are fitted by optimizing melting parameters, and the results indicate a continuous influx of light REE (LREE)‐enriched melt up to 3–11% in degree of melting, which amounts in total to less than 1% of the initial peridotite mass. The open system melting equation for trace elements is also applied to peridotites from the Hayachine‐Miyamori ophiolite, northeastern Japan. The REE contents in clinopyroxene and the modal abundances in four peridotite groups are fitted all together to estimate melting reaction stoichiometry and REE contents in the influxing material. The estimated REE pattern is characterized by a strong LREE enrichment, which is comparable to that of melt in equilibrium with hornblende in the most refractory harzburgite from the ophiolite.
The giant Ruatoria debris avalanche on the northern Hikurangi margin, New Zealand: Result of oblique seamount subduction
Authors: Collot Jean‐Yves , Lewis Keith , Lamarche Geoffroy , Lallemand Serge
Published: February 2004

Despite convergent margins being unstable systems, most reports of huge submarine slope failure have come from oceanic volcanoes and passive margins. Swath bathymetry and seismic profiles of the northern Hikurangi subduction system, New Zealand, show a tapering 65–30 km wide by 65 km deep margin indentation, with a giant, 3150±630 km3, blocky, debris avalanche deposit projecting 40 km out across horizontal trench fill, and a debris flow deposit projecting over 100 km. Slide blocks are well‐bedded, up to 18 km across and 1.2 km high, the largest being at the avalanche deposit's leading edge. Samples dredged from them are mainly Miocene shelf calc‐mudstones similar to those outcropping around the indentation. Cores from cover beds suggest that failure occurred ∼170±40 ka, possibly synchronously with a major extension collapse in the upper indentation. However, the northern part of the indentation is much older. The steep, straight northern wall is close to the direction of plate convergence and probably formed around 2.0–0.16 Ma as a large seamount subducted, leaving in its wake a deep groove obliquely across the margin and an unstable triangle of fractured rock in the 60° angle between groove and oversteepened margin front. The triangle collapsed as a blocky avalanche, leaving a scalloped southern wall and probably causing a large tsunami. Tentative calculations of compacted volumes suggest that the indentation is over 600 km3 larger than the avalanche, supporting a two‐stage origin that includes subduction erosion. Since failure, convergence has carried the deposits ∼9 km back toward the margin, causing internal compression. The eventual subduction/accretion of the Ruatoria avalanche explains the scarcity of such features on active margins and perhaps the nature of olistostromes in fold belts.