bluuebunny/crossref_metadata_2025 huggingface dataset:
Wrinkle‐like slip pulse on a fault between different materials
Authors: Andrews D. J. , Ben‐Zion Y.
Published: February 2004

Pulses of slip velocity can propagate on a planar interface governed by a constant coefficient of friction, where the interface separates different elastic materials. Such pulses have been found in two‐dimensional plane strain finite difference calculations of slip on a fault between elastic media with wave speeds differing by 20%. The self‐sustaining propagation of the slip pulse arises from interaction between normal and tangential deformation that exists only with a material contrast. These calculations confirm the prediction of Weertman [1980] that a dislocation propagating steadily along a material interface has a tensile change of normal traction with the same pulse shape as slip velocity. The self‐sustaining pulse is associated with a rapid transition from a head wave traveling along the interface with the S wave speed of the faster material, to an opposite polarity body wave traveling with the slower S speed. Slip occurs during the reversal of normal particle velocity. The pulse can propagate in a region with constant coefficient of friction and an initial stress state below the frictional criterion. Propagation occurs in only one direction, the direction of slip in the more compliant medium, with rupture velocity near the slower S wave speed. Displacement is larger in the softer medium, which is displaced away from the fault during the passage of the slip pulse. Motion is analogous to a propagating wrinkle in a carpet. The amplitude of slip remains approximately constant during propagation, but the pulse width decreases and the amplitudes of slip velocity and stress change increase. The tensile change of normal traction increases until absolute normal traction reaches zero. The pulse can be generated as a secondary effect of a drop of shear stress in an asperity. The pulse shape is unstable, and the initial slip pulse can change during propagation into a collection of sharper pulses. Such a pulse enables slip to occur with little loss of energy to friction, while at the same time increasing irregularity of stress and slip at the source.
Impact of blasting on groundwater composition in a fracture in Canada's Underground Research Laboratory
Authors: Gascoyne Mel , Thomas David A.
Published: February 2004

Groundwater composition in a discrete, water‐bearing fracture in granitic rock at the Underground Research Laboratory, Manitoba, was monitored during a period of underground excavation of adjacent rock to determine the impact of conventional blasting techniques and rock fracturing on the concentration of dissolved constituents. This work has relevance to the study of hydrogeochemical anomalies associated with seismic activity. Short‐lived anomalies such as decreases in dissolved anion (Cl, F, Br, SO4) and gas (He, Rn) concentrations and concurrent increases in NO3 and O2 concentrations were seen shortly after two blasts located opposite the groundwater sampling site. A third blast downstream of the site resulted in variability in dissolved gases concentrations but showed no impact on dissolved anion concentrations. The results are compared with various models used to account for hydrogeochemical fluctuations associated with earthquakes. The data best fit a general form of the aquifer breaching/fluid mixing model in which hydrochemical responses are caused by localized changes in hydraulic conductivity along the plane of a fracture which, in turn, cause changes in mixing ratio of groundwater at the monitoring site.
Paleofluids and Recent fluids in the upper continental crust: Results from the German Continental Deep Drilling Program (KTB)
Authors: Möller P. , Weise S. M. , Althaus E. , Bach W. , Behr H. J. , Borchardt R. , Bräuer K. , Drescher J. , Erzinger J. , Faber E. , Hansen B. T. , Horn E. E. , Huenges E. , Kämpf H. , Kessels W. , Kirsten T. , Landwehr D. , Lodemann M. , Machon L. , Pekdeger A. , Pielow H.‐U. , Reutel C. , Simon K. , Walther J. , Weinlich F. H. , Zimmer M.
Published: February 2004

The two German Continental Deep Drilling Program (KTB) boreholes provided samples for studies of fluid inclusions (paleofluids), “free fluids” of the crystalline basement, and their fingerprints on the chemical and isotopic composition of minerals and rocks to 9101 m depth, which allowed reconstruction of the evolution of fluids, their migration pathways, and their sources. Aqueous fluids were largely lost during the Devonian amphibolite facies metamorphism. Thereafter, radiogenic, nucleogenic, and fissiogenic gases, together with NH4‐fixed nitrogen, were released from host rocks and partly enclosed in secondary inclusions. During the Hercynian uplift, Na‐Cl fluids (formation water) infiltrated and dissolved noble gases and N2 largely originating from the host rocks. In the course of the Cretaceous denudation, high‐salinity Ca‐Na‐Cl brines, possibly derived from Permo‐Carboniferous sediments but altered by fluid/rock interaction, migrated into their present position. This fluid introduced low‐maturity hydrocarbons released together with nitrogen from early metamorphic organic‐rich sediments. The 4000‐m fluid from the KTB pilot hole pumping test, which was analyzed chemically and isotopically, seems to be a mixture of an ascending basement brine and a descending paleometeoric water, from which the late alteration minerals calcite and laumontite precipitated in fractures. The calcite is neither chemically nor isotopically in equilibrium with the recovered “free fluid.” Hydraulic tests indicate a communicating system of fractures between the boreholes with a distinct matrix and fracture porosity.
Estimation of the complete stress tensor to 8 km depth in the KTB scientific drill holes: Implications for crustal strength
Authors: Brudy M. , Zoback M. D. , Fuchs K. , Rummel F. , Baumgärtner J.
Published: February 2004

For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling‐induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well‐oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near‐hydrostatic pore pressure.
Identification and interpretation of azimuthally varying crustal reflectivity with an example from the southern Canadian Cordillera
Authors: Cook Frederick A. , Li Qing , Vasudevan Kris
Published: February 2004

A new approach to seismic interpretation, seismic skeletonization, is used to analyze crustal reflection data by identifying numbers of reflection events, their lengths, their energies, and their dips, with respect to azimuths of recorded profiles. Application of the method to crustal reflection data in the southern Canadian Cordillera demonstrates strong directional dependence of crustal reflectivity, with persistent trends at 025–040° (205–220°), 085–095° (265–275°), 115–125° (295–305°), and 140–160° (320–340°). These trends are visible in both the upper and lower crust and may be related to regional structural trends. Further applications of the technique, particularly in different tectonic regions, may allow categorization of seismic patterns and cataloguing of signatures for complex reflection data in a manner similar to seismic stratigraphy and structural patterns in stratified rocks.
Transform push, oblique subduction resistance, and intraplate stress of the Juan de Fuca Plate
Authors: Wang Kelin , He Jiangheng , Davis Earl E.
Published: February 2004

The Juan de Fuca plate is a small oceanic plate between the Pacific and North America plates. In the southernmost region, referred to as the Gorda deformation zone, the maximum compressive stress σ1 constrained by earthquake focal mechanisms is N‐S. Off Oregon, and possibly off Washington, NW trending left‐lateral faults cutting the Juan de Fuca plate indicate a σ1 in a NE‐SW to E‐W direction. The magnitude of differential stress increases from north to south; this is inferred from the plastic yielding and distribution of earthquakes throughout the Gorda deformation zone. To understand how tectonic forces determine the stress field of the Juan de Fuca plate, we have modeled the intraplate stress using both elastic and elastic‐perfectly plastic plane‐stress finite element models. We conclude that the right‐lateral shear motion of the Pacific and North America plates is primarily responsible for the stress pattern of the Juan de Fuca plate. The most important roles are played by a compressional force normal to the Mendocino transform fault, a result of the northward push by the Pacific plate and a horizontal resistance operating against the northward, or margin‐parallel, component of oblique subduction. Margin‐parallel subduction resistance results in large N‐S compression in the Gorda deformation zone because the force is integrated over the full length of the Cascadia subduction zone. The Mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike‐normal compressive stresses. Internal failure of the Gorda deformation zone potentially places limits on the magnitude of the fault‐normal stresses being transmitted and correspondingly on the magnitude of strike‐parallel subduction resistance. Transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner.
Double beam imaging: Mapping lower mantle heterogeneities using combinations of source and receiver arrays
Authors: Scherbaum Frank , Krüger Frank , Weber Michael
Published: February 2004

We present a new technique for imaging spatially distributed heterogeneities using information from combinations of source and receiver arrays. The method is based on the single scattering assumption and is closely related to the double beam method of Krüger et al. [1993, 1995, 1996] in that it exploits amplitude, delay time, slowness, and azimuth information in two arrays simultaneously. A crucial step in the method is the application of static time corrections for a chosen reference phase (here PcP). One type of image is obtained as a spatial likelihood distribution for prescribed source and receiver array slowness and azimuth values and delay times with respect to a reference phase. This allows the determination of the spatial origin of coherent phases in the P coda. In a complementary approach, we perform a double beam stack migration according to the theoretical slowness and azimuth values for candidate scatterers distributed over a three‐dimensional (3‐D) grid. The scattering strength is expressed as the resulting beam power, beam amplitude, or semblance in a time window determined from the theoretical delay time with respect to the reference phase. While likelihood mapping provides an image reflecting the probability of a region to explain observed kinematic phase properties by single scattering, double beam migration provides information about the scattering strength. The method is applied to nuclear explosion sources in eastern Kazakhstan recorded at the Yellowknife array in northern Canada. We identify several scattering volumes within the lower mantle below the arctic producing individual anomalous lower mantle phases in the P coda. These anomalies range in depth from the core‐mantle boundary (CMB) up to 500 km into the lowermost mantle and are mainly located under the Eurasian side of the arctic. The joint interpretation of these results with the results of previous studies suggests a connection of these anomalies to the Cenozoic and Mesozoic subduction of the Pacific plate and the Kula plate. The determination of the nature of such 3‐D anomalies necessitates further array studies with better azimuthal coverage
Internal thermal boundary layer stability in phase transition modulated convection
Authors: Butler Samuel , Peltier W. R.
Published: February 2004

The stability of a horizontal thermal boundary layer embedded within a very viscous fluid is investigated using the formalism of linear stability analysis. Thin thermal boundary layers in deep fluid regions and in the absence of phase transition and dynamical effects are thereby shown to be unstable at extremely long wavelengths. The stability of the internal thermal boundary layer which may exist at 660 km depth in the Earth's mantle as a consequence of the dynamical influence of the endothermic phase transition from γ spinel to a mixture of perovskite and magnesiowüstite, recently discussed in some detail by Solheim and Peltier [1994a], is investigated in order to better understand the “avalanche effect” observed in this and similar nonlinear, time dependent simulations of the mantle convection process. It is demonstrated that if the stability problem is treated as purely thermal, then the boundary layer is predicted to be extremely unstable and the presence of the 660‐km endothermic phase transition at middepth within the boundary layer is further destabilizing. When the kinematic effect of flow convergence onto the boundary layer and phase transition region is active, however, it is shown that the layer may be strongly stabilized. In the regime of physically realistic velocity convergence, the critical Rayleigh number is predicted to lie in the range suggested by the numerical simulations of Solheim and Peltier [1994a]. A threshold value of the magnitude of the Clapeyron slope of the endothermic phase transition for a given velocity convergence is also shown to exist, beyond which the fastest‐growing mode of instability changes from avalanche type to layered type.
Short‐period observation of the 520 km discontinuity in northern Eurasia
Authors: Ryberg T. , Wenzel F. , Egorkin A. V. , Solodilov L.
Published: February 2004

Short‐period three‐component recordings of the seismic wave field of peaceful nuclear explosions (PNEs) along several long‐range profiles in northern Eurasia are analyzed to study the structure of the upper mantle. We use the data to search for evidence of the disputed 520 km discontinuity in the mantle transition zone. Here we present short‐period evidence for the existence of this proposed mantle discontinuity. On three long‐range profiles in Siberia we observe in the distance range from 2400 to 2600 km a typical change of the apparent velocity of the first arrivals and the appearance of two characteristic travel time branches. We interpret these phenomena as being caused by the proposed upper mantle discontinuity. To determine the optimum one‐dimensional P wave velocity model, synthetic seismograms were calculated with the reflectivity method. By comparing them with the observed data, we estimate the P wave velocity contrast across the 520 km discontinuity to be 0.25±0.05 km/s, while the velocity gradient below the boundary is 0.001±0.0005 1/s. This result indicates that in the mantle transition zone both phase transitions and variations in chemical composition can occur.
Crust and upper mantle shear velocity structure beneath the Tibetan plateau and surrounding regions from interevent surface wave phase velocity inversion
Authors: Curtis Andrew , Woodhouse John H.
Published: February 2004

Average a priori shear velocity models are constructed for the Tien Shan, Tarim basin, Pamir‐Hindu Kush, Himalaya and NE India. These models are shown to account for most of the lateral velocity heterogeneity at wavelengths >1000 km. Interevent fundamental mode Rayleigh and Love wave phase velocities were measured in the period range 32–200 s and were inverted for path‐averaged shear velocity structures. These, in turn, were regionalized to estimate average, isotropic structures beneath the six regions listed above. Low upper mantle velocities are observed beneath the western and eastern Tien Shan. The Tarim basin structure is poorly constrained but exhibits very low upper crustal velocities (probably due to deep sedimentary accumulation), and either very high upper mantle velocities exist (>4.8 km/s) in a layer 70 km thick or the lithosphere is very thick (∼180 km). Low upper mantle velocities are observed beneath the Chang Thang region, the central, and the northeast plateau but not beneath the southern or southeastern plateau. The crustal velocity gradient with depth is zero in the east central plateau consistent with lower crustal basaltic intrusions there but is positive to the west and southeast of this zone. Upper mantle structures show that the Indian lithosphere could have subducted beneath the entire plateau only if either 1 mantle in the depth range 115–185 km has been altered (or removed) to reduce its average velocity or 2 the Indian lithosphere is only ∼85 km thick.
Mechanisms affecting ultrasonic wave propagation in fluid‐containing sandstones under high hydrostatic pressure
Authors: Wulff A.‐M. , Burkhardt H.
Published: February 2004

The elastic and anelastic parameters of sandstones at ultrasonic frequencies under low uniaxial pressure are strongly influenced by the fluid content. The two main mechanisms controlling the rock behavior are based on (1) local fluid flow and (2) a grain surface effect which results in a modulus reduction. Both mechanisms depend fundamentally on the microstructure, the critical factor controlling seismic wave velocity and attenuation in rocks under low pressure. Ultrasonic measurements have been performed on a fluid‐containing sandstone under hydrostatic pressure up to 200 MPa to investigate the mechanisms affecting rock behavior under in situ pressure conditions. At high hydrostatic pressure the measured P wave velocities indicate a change in the rock microstructure and in the influence of the fluid. By means of two models the effects of the mechanisms of local fluid flow [Murphy et al., 1986] and global fluid flow [Biot, 1956a, b] were calculated, and the loss mechanisms were compared in their capability of explaining the experimental data. The experimental and theoretical results show that for the investigated fluid‐containing sandstone both the local fluid flow mechanism and, in the case of polar fluids, the modulus reduction are still effective at high external pressure.
Degree one heterogeneity and hemispherical variation of anisotropy in the inner core from PKP(BC)–PKP(DF) times
Authors: Tanaka Satoru , Hamaguchi Hiroyuki
Published: February 2004

We have investigated the relationship between heterogeneity and anisotropy in the inner core. Heterogeneity was examined using PKP(DF) and PKP(BC) phases traveling in the east‐west direction (E‐W paths). The geographical pattern of the PKP differential travel time (BC‐DF) residuals was found to be a manifestation of degree one heterogeneity in the outermost 100 to 500 km of the inner core. Spherical harmonics analysis showed that the P velocities in the inner core are fast in the region from 43°E to 177°E (the quasi‐eastern hemisphere) with a peak value of +0.29±0.15% and slow in the region from 183°W to 43°E (the quasi‐western hemisphere) with a peak value of −0.66±0.20%. Regional variation of anisotropy in the inner core was examined using PKP phases traveling nearly parallel to the Earth's spin axis (polar paths). The averages of BC–DF time residuals were 2.4±0.8 s in the quasi‐western hemisphere and 0.5±0.4 s in the quasi‐eastern hemisphere. We found that the inner core anisotropy predominates only in the quasi‐western hemisphere in which the 0.66% slow P velocity is observed for the E‐W paths. Geophysical implications of the degree one pattern in the inner core were briefly discussed. We propose that the hemispherical structure of the inner core reflects ancient core dynamics immediately after core formation.
Influence of change in physical state on elastic nonlinear response in rock: Significance of effective pressure and water saturation
Authors: Zinszner Bernard , Johnson Paul A. , Rasolofosaon Patrick N. J.
Published: February 2004

We describe Young's mode resonant bar results obtained under effective pressure at two saturation states: dry and water saturated. We monitor primary manifestations of nonlinear response in these experiments: the harmonic content, the source extinction intensity, and fundamental resonant frequency shift. In addition, we describe the hysteretic behavior of the static pressure response, the linear modulus, and Q. Because we currently lack a complete theoretical description of nonlinear behavior under resonance at pressure, we provide relative measures of nonlinear response rather than absolute values. The rocks include Fontainebleau and Meule sandstones and Lavoux limestone. Dynamic strain levels range from 10−8 to 10−5 and frequencies range from 1 to 10 kHz. The elastic nonlinear response of each of the rocks is markedly different over the range of physical property states explored. The different responses are related to differences in mechanical response resulting from rock type, grain cement type, etc. In all of the samples studied, the change in resonant frequency as a function of excitation intensity is not measurable above approximately 10 MPa; however, harmonics are observed at larger effective pressure levels. Hysteresis in velocity and Q versus pressure vary considerably between the rocks. The effect of Q on the experiments is marked. When Q is low (<10) as for some saturated samples, relative excitations must be large in order to induce equivalent dry sample strains.
Mantle convection and stability of depleted and undepleted continental lithosphere
Authors: Doin Marie‐Pierre , Fleitout Luce , Christensen Ulrich
Published: February 2004

We address the question of how convective processes control the thicknesses of oceanic and continental lithospheres. The numerical convection model involves a Newtonian rheology which depends on temperature and pressure. A repeated plate tectonic cycle is modeled by imposing a time‐dependent surface velocity. One part of the surface, representing a continent, never subducts. The asymptotic equilibrium thickness of the lithosphere varies with the viscosity at the base of the lithosphere, but is not directly sensitive to the pressure dependence of the viscosity law and to the plate velocity. For small activation volumes, and average upper mantle viscosities deduced from postglacial rebound, the equilibrium plate thickness is more than 400 km (regimes 1 and 2). The equilibrium thickness of the oceanic lithosphere (around 100 km) implies that the viscosity in the asthenosphere is less than 7×1018 Pa s. Only models with strongly pressure‐dependent viscosity laws (activation volumes greater than 9×10−6 m3/mol) are able to reconcile this value with the average upper mantle viscosity (5×1020 Pa s). For these models, there are two lithospheric thicknesses such that the heat supplied by convection at the base of the lithosphere equals the surface conductive heat flow (regime 3). They could be that of an aged oceanic lithosphere and that of a shield lithospheric root. They indeed appear as points of prefered thickness in our numerical models. However, convection triggered by the lateral density jumps at the boundaries between the root and the thinner lithosphere slowly destabilizes the thick lithosphere. A plausible degree of chemical buoyancy in a depleted lithospheric root does not prevent convective erosion. In our simulations, long‐term stability of a cratonic lithospheric root is best achieved when its material is both buoyant and more viscous than the surrounding mantle. Extensive devolatilization of the refractory rocks forming the root is invoked to explain this viscosity increase.
The transition from brittle faulting to cataclastic flow: Permeability evolution
Authors: Zhu Wenlu , Wong Teng‐fong
Published: February 2004

Triaxial compression experiments were conducted to investigate influences of stress and failure mode on axial permeability of five sandstones with porosities ranging from 15% to 35%. In the cataclastic flow regime, permeability and porosity changes closely track one another. A drastic decrease in permeability was triggered by the onset of shear‐enhanced compaction caused by grain crushing and pore collapse. The compactive yield stress C* maps out a boundary in stress space separating two different types of permeability evolution. Before C* is attained, permeability and porosity both decrease with increasing effective mean stress, but they are independent of deviatoric stresses. However, with loading beyond C*, both permeability and porosity changes are strongly dependent on the deviatoric and effective mean stresses. In the brittle faulting regime, permeability and porosity changes are more complex. Before the onset of shear‐induced dilation C', both permeability and porosity decrease with increasing effective mean stress. Beyond C', permeability may actually decrease in a dilating rock prior to brittle failure. After the peak stress has been attained, the development of a relatively impermeable shear band causes an accelerated decrease of permeability. Permeability evolution in porous sandstones is compared with that in low‐porosity crystalline rocks. A conceptual model for the coupling of deformation and fluid transport is proposed in the form of a deformation‐permeability map.
Relationship between anisotropy of P and S wave velocities and anisotropy of attenuation in serpentinite and amphibolite
Authors: Kern Hartmut , Liu Bin , Popp Till
Published: February 2004

Velocities and Q values of P and S waves as functions of pressure and temperature (at 100 and 600 MPa) are presented for a serpentinite and an amphibolite. Both rocks exhibit a strong lattice preferred orientation (LPO) of the major mineral phases antigorite and hornblende, respectively. Velocities and Q values increase with pressure; the rate of increase is different in the three orthogonal directions (normal and parallel to foliation and lineation) and closely related to progressive closure of microcracks. Increasing temperature decreases velocities and Q values only slightly as long as thermal cracking is prevented by the applied confining pressure. Substantial anisotropy of velocities and Q in P and S waves is observed in both rocks but is found to be different in origin. Anisotropy of P and S wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the LPO of major minerals. Increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate. The residual anisotropy of P and S wave velocities (shear wave splitting) at high confining pressure is mainly a result of preferred mineral orientation. By contrast, anisotropy of Q is very low at low confining pressure and markedly enhanced as pressure is increased. At high confining pressure, substantial anisotropy of Q in P waves is apparent but reversed from that of P wave velocities: Qp is highest in the direction normal to the foliation plane whereas Vp (and VS) is lowest in this direction. The generation of a pronounced anisotropy of Qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation. The increase of Q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain. The reverse is true for the X and Y directions (serpentinite) and X direction (amphibolite) parallel to the foliation plane.
Formation of ore minerals in metamorphic rocks of the German continental deep drilling site (KTB)
Authors: Kontny A. , Friedrich G. , Behr H. J. , de Wall H. , Horn E. E. , Möller P. , Zulauf G.
Published: February 2004

The German Continental Deep Drilling Program (KTB) drilled a 9.1 km deep profile through amphibolite facies metamorphic rocks and reached in situ temperatures of 265°C. Each lithologic unit is characterized by typical ore mineral assemblages related to the regional metamorphic conditions. Paragneisses contain pyrrhotite + rutile + ilmenite ± graphite, metabasic units bear ilmenite + rutile + pyrrhotite ± pyrite, and additionally, the so‐called variegated units yield pyrrhotite + titanite assemblages. In the latter unit, magnetite + ilmenite + rutile + titanite assemblages related to the lower amphibolite facies breakdown of ilmenite‐hematite solid solution also occur locally. Retrograde hydrothermal mineralization which commenced during Upper Carboniferous times is characterized by the following geochemical conditions: (1) low saline Na‐K‐Mg‐Cl fluids with sulfur and oxygen fugacities at the pyrite‐pyrrhotite buffer and temperatures of 400–500°C, (2) fluids with CO2, CH4±N2, andpH, Eh, sulfur, and oxygen fugacity in the stability field of graphite + pyrite at temperatures of 280–350° and (3) moderate to high saline Ca‐Na‐Cl fluids with CH4+ N2; sulfur and oxygen fugacity are in the stability field of pyrrhotite at temperatures <300°C. The latter environment is confirmed by in situ conditions found at the bottom of the deep drilling. Monoclinic, ferrimagnetic pyrrhotite is the main carrier of magnetization which disappears below about 8.6 km, corresponding to in situ temperatures of about 250°C. Below this depth, hexagonal antiferromagnetic pyrrhotite with a Curie temperature of 260°C is the stable phase. Temperature‐dependent transformation of pyrrhotite and the reaching of its Curie isotherm within the Earth crust are one of the striking results of the KTB deep drilling project.
Petrophysical properties of the 9‐km‐deep crustal section at KTB
Authors: Berckhemer H. , Rauen A. , Winter H. , Kern H. , Kontny A. , Lienert M. , Nover G. , Pohl J. , Popp T. , Schult A. , Zinke J. , Soffel H. C.
Published: February 2004

Petrophysical properties of drill core and drill cuttings samples from both bore holes of the German Continental Deep Drilling Program (KTB) measured at atmospheric pressure and room temperature in the field laboratory are presented, along with data of core samples measured at simulated in situ conditions by other laboratories. Most of the petrophysical properties show a bimodal frequency distribution corresponding to the two main lithologies (gneiss and metabasite), except electrical resitivity and Th/U ratio which are lithology independent (monomodal distribution). Low resistivities are mainly associated with fractures zones enriched in fluids and graphite. The most abundant ferrimagnetic mineral is monoclinic pyrrhotite. Below 8600 m, hexagonal pyrrhotite with a Curie temperature of 260°C is the stable phase. Thus the Curie isotherm of the predominant pyrrhotite was reached (bottom hole temperature about 265°C). The highest values of magnetic susceptibility are linked with magnetite. Microcracks grow due to pressure and temperature release during core uplift. This process continues after recovery and is documented by the anelastic strain relaxation and acoustic emissions. The crystalline rocks exhibit marked reversible hydration swelling. Anisotropy of electrical resistivity, permeability, P and S wave velocity is reduced significantly by applying confining pressure, due to closing of microcracks. Fluids within the microcracks also reduce the P wave velocity anisotropy and P wave attenuation. Anisotropy and shear wave splitting observed in the field seismic experiments is caused by the foliation of rocks, as confirmed by laboratory measurements under simulated in situ conditions. The petrophysical studies provide evidence that microfracturing has an important influence on many physical rock properties in situ.
Upper mantle compressional velocity structure beneath the West Mediterranean Basin
Authors: Pino N. Alessandro , Helmberger Donald V.
Published: February 2004

P waveforms of regional crustal earthquakes have been modeled to obtain an upper mantle compressional velocity model for the West Mediterranean Basin. Data come from long‐period stations of the World‐Wide Standardized Seismograph Network and broadband stations located in the Iberian Peninsula. Synthetic waveforms have first been computed for published velocity models corresponding to different tectonic provinces and obtained with analogous techniques. A model that strongly improves the fits to the data is then derived. The proposed model is characterized by a 100‐km‐thick lid overlaying a not very pronounced low‐velocity zone and a 3% discontinuity at 368 km where an increase of the velocity gradient also occurs. These features could be explained either as a deflection of the olivine‐to‐spinel phase transition, regionally detected at about 395 km, resulting from the lower temperature produced by the subduction of the African plate, or as being due to the presence below 300 km depth of a layer of silicate melt, producing a strong reflection from its bottom, and a more usual depth for the olivine‐spinel transition. In both cases the occurrence of relatively low velocities beneath 300 km is interpreted as being caused by the presence of melt associated with the northward dipping subduction of the African plate.
Empirical Green's function analysis: Taking the next step
Authors: Hough S. E.
Published: February 2004

An extension of the empirical Green's function (EGF) method is presented that involves determination of source parameters using standard EGF deconvolution, followed by inversion for a common attenuation parameter for a set of colocated events. Recordings of three or more colocated events can thus be used to constrain a single path attenuation estimate. I apply this method to recordings from the 1995–1996 Ridgecrest, California, earthquake sequence; I analyze four clusters consisting of 13 total events with magnitudes between 2.6 and 4.9. I first obtain corner frequencies, which are used to infer Brune stress drop estimates. I obtain stress drop values of 0.3–53 MPa (with all but one between 0.3 and 11 MPa), with no resolved increase of stress drop with moment. With the corner frequencies constrained, the inferred attenuation parameters are very consistent; they imply an average shear wave quality factor of approximately 20–25 for alluvial sediments within the Indian Wells Valley. Although the resultant spectral fitting (using corner frequency and κ) is good, the residuals are consistent among the clusters analyzed. Their spectral shape is similar to the the theoretical one‐dimensional response of a layered low‐velocity structure in the valley (an absolute site response cannot be determined by this method, because of an ambiguity between absolute response and source spectral amplitudes). I show that even this subtle site response can significantly bias estimates of corner frequency and κ, if it is ignored in an inversion for only source and path effects. The multiple‐EGF method presented in this paper is analogous to a joint inversion for source, path, and site effects; the use of colocated sets of earthquakes appears to offer significant advantages in improving resolution of all three estimates, especially if data are from a single site or sites with similar site response.
Deep azimuthal seismic anisotropy in the southern Kurile and Japan subduction zones
Authors: Sandvol Eric , Ni James
Published: February 2004

Shear wave splitting parameters of local and teleseismic S waves from intermediate and deep earthquakes in the southern Kurile and Japan subduction zones are combined with splitting parameters obtained from SKS and SKKS waves to determine depth variation in shear wave splitting both above and below the earthquake. Local S wave splitting results measured at station MAJO (Matsushiro, Japan) show fast directions, from NNE to NE, parallel to the extension directions measured from Quaternary fault and geodetic data. The inferred finite strain field from shear wave splitting is consistent with the closing of the Sea of Japan. At YSS (Yuzhno Sakhalinsk, Russia) the shear wave splitting parameters obtained from local S waves are approximately oriented N‐S and appear to be controlled by deformation of the upper plate rather than the subducted slab. SKS phases recorded at YSS, located on the southern tip of Sakhalin Island, show an approximately N‐S oriented fast direction and lag time of 1.3±0.3 s. Local S phases recorded at station YSS yield fast directions similar to the SKS results and the magnitude of splitting varies systematically with depth. These results combine to indicate that very little splitting occurs in the asthenosphere below the southern Kurile slab at about 450 km depth. S waves from deep earthquakes beneath MDJ (Mudanjiang, Heilongjiang Province, China) show ∼1.0 s of lag time, while SKKS results show ∼1.6 s of lag time. One SKS phase, however, shows similar lag times as those observed by the split local S waves. This difference in shear wave splitting lag times indicates significant variation in azimuthal anisotropy beneath a minimum depth of 350 km. This inference is consistent with the source‐side splitting of ∼0.8 s lag time observed from deep teleseismic S waves which traverse the lower parts of the upper mantle and the upper mantle/lower mantle boundary. A plausible explanation for the presence of deep seismic anisotropy is that shear wave splitting is occurring in the metastable olivine in the flattened and broadened southern Japan slab. Another explanation for these observations is the presence of an anisotropic layer composed primarily of highly anisotropic β‐spinel at the base of the 410‐km discontinuity.
Temporal variations of the coda decay rate on Kamchatka: Are they real and precursory?
Authors: Gusev A. A.
Published: February 2004

Temporal variations of the decay rate of the coda of local earthquakes are studied for four Kamchatka stations. To suppress any bias caused by the coda Q versus lapse time dependence, I use the deviation of an individual coda decay function from the empirical reference decay function, and the analysis is done over a fixed lapse time window. For each individual coda record, a decay rate parameter “α” is determined by the following procedure. First, the reference log coda decay function is subtracted from an individual one giving a residual function; second, the slope of this residual versus lapse time is determined over the fixed lapse time window giving an α estimate. The data used are records of 150–250 Kamchatka earthquakes of 1967–1990 (ML=3.5–5) obtained by four stations employing three‐component 1‐s seismographs. The processed data show apparent temporal variation of α with high statistical significance. Several possibilities are then investigated of mimicking the genuine temporal variation of α by systematic variation of other parameters, first of all epicenter wandering and nodal plane rotation. This analysis does not reveal any strong bias and thus suggests that the observed variation is genuine. Then I show the significance of five apparently precursory anomalies identified retroactively. Also, I describe briefly the real‐time prediction experiment conducted on Kamchatka for 1982–1990 using coda decay, aimed at the intermediate‐term forecasting of large Benioff zone earthquakes. This experiment resulted in the successful forecast of the August 17, 1983, Mw=7.0 event with an accuracy of 2 months in time and 100–200 km in location; magnitude was overestimated by 0.5. Later, a false alarm was also issued. Thus the experiment confirms the reality of the coda decay rate precursor but also shows that it needs further improvement to become reliable. Physically, the variations of the coda decay rate are associated with time‐dependent local variations of scatterer density in the lithosphere.
Seismicity and structure of the Kamchatka Subduction Zone
Authors: Gorbatov Alexei , Kostoglodov Vladimir , Suárez Gerardo , Gordeev Evgeni
Published: February 2004

The configuration of the Pacific plate subducted beneath the Kamchatka peninsula and the stress distribution in the Kamchatka subduction zone (KSZ) were studied using the catalog of the Kamchatka regional seismic network, focal mechanism solutions estimated from P wave first motions, the formal inversion of long‐period waveforms, and centroid moment tensor solutions. To the south of ∼55°N, the slab shows an approximately constant dip angle of ∼55°. To the north of ∼55°N, the dip of the slab becomes shallower reaching ∼35°. The maximum depth of seismicity, Dm, varies from ∼500 km depth near 50°N to ∼300 km depth at ∼55°N. The volcanic front is almost linear along the main part of the KSZ whereas it is sharply shifted landward to the north of ∼55°N. The variation of Dm is apparently consistent with the standard empirical relation Dm=ƒ(ϕ), where ϕ is the thermal parameter of the subducted slab. To the north of ∼55°N, the slab is offset toward the northwest, and it is sharply deformed in a narrow contorted zone which is ∼30 km wide (∼56°N, ∼161°E). To the north of this contortion, Dm decreases to ∼100 km. The landward shift of the northern part of the slab is reflected by a sharp deviation of the volcanic front to the northwest which follows the ∼90–160 km isodepth range of the subducted slab. The observed value of Dm in the northern segment significantly diverges from the global relation Dm=ƒ(ϕ). We interpret this as an effective decrease of the thermal thickness of the subducted lithosphere.
Continental subduction and three‐dimensional crustal structure: The northern South Island, New Zealand
Authors: Eberhart‐Phillips Donna , Reyners Martin
Published: February 2004

The three‐dimensional Vp and Vp/Vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment, and 3146 earthquakes have been relocated. Between 40 km and 100 km depth, the subducted plate is imaged as a relatively low‐velocity feature in the uppermost mantle, reflecting the continental nature of the subducted crust in this region. An increase in amplitude of this low‐velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction. Velocity variations within the subducted and overlying plates show some spatial correlation. This suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath Marlborough is currently not accommodating much active subduction. In the overlying plate, the Awatere fault is a major structural feature, associated with a low‐velocity zone extending to 23 km depth. There is a marked change in structure near this fault, with seismic velocities being lower to the southeast. A relatively high level of seismicity occurs in this region of lower seismic velocities, suggesting a relationship between the two. A possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust. The low‐velocity region in the overlying plate coincides with the region of most intense active deformation, suggesting it is relatively weak.
Frictional coefficient on faults in a seismogenic region inferred from earthquake mechanism solutions
Authors: Iio Yoshihisa
Published: February 2004

The coefficient of friction associated with faults in a seismogenic region is inferred from numerous focal mechanism solutions. Earthquake slip can occur on a favorably oriented fault plane when preexisting fault planes are oriented in various directions. The fault plane is thought to be selected when the Coulomb failure stress given by σf=τ − μ(σn − p) − S0 has a maximum value along a slip direction on the plane. Here, τ is the shear stress, μ is the coefficient of friction, σn is the normal stress, p is the pore pressure, and S0 is the cohesion. Consequently, the geometry of the plane and the slip direction include information concerning the coefficient of friction for the faults. In the present paper the coefficient of friction is inferred from the distribution of P axis orientations, assuming hydrostatic pore pressure. The method is very simple and has the advantage that the results obtained do not depend on pore pressure, when pore pressure is not proportional to the normal stress. Observed distributions of P axis azimuths and plunges display a very narrow peak having a width of about 20°. The coefficient of friction on the faults can then be estimated from the width to be less than or equal to μ = 0.2. When the stress is balanced by friction, the shear stress can be calculated. Here, the shear stress (in megapascals) has a value of 4.0z, where z is the depth (in kilometers), under the assumption of hydrostatic pore pressure and a coefficient of friction of 0.2. The value obtained for the coefficient of friction is very small compared with those derived from laboratory frictional experiments. It is thought that this small value of μ is attributed to the friction itself, when pore pressure is not proportional to the normal stress.
How stratified is mantle convection?
Authors: Puster Peter , Jordan Thomas H.
Published: February 2004

We quantify the flow stratification in the Earth's mid‐mantle (600–1500 km) in terms of a stratification index for the vertical mass flux, Sƒ (z) = 1 − ƒ(z) / ƒref (z), in which the reference value ƒref(z) approximates the local flux at depth z expected for unstratified convection (Sƒ=0). Although this flux stratification index cannot be directly constrained by observations, we show from a series of two‐dimensional convection simulations that its value can be related to a thermal stratification index ST(Z) defined in terms of the radial correlation length of the temperature‐perturbation field δT(z, Ω). ST is a good proxy for Sƒ at low stratifications (Sƒ<0.2), where it rises with stratification strength much more rapidly than Sƒ. Assuming that the shear‐speed variations δβ(z, Ω) imaged by seismic tomography are primarily due to convective temperature fluctuations, we can approximate ST by Sβ, the analogous index for the radial correlation length of δβ, and thereby construct bounds on Sƒ. We discuss several key issues regarding the implementation of this strategy, including finite resolution of the seismic data, biases due to the parameterization of the tomographic models, and the bias and variance due to noise. From the comparison of the numerical simulations with recent tomographic structures, we conclude that it is unlikely that convection in the Earth's mantle has Sƒ≳0.15. We consider the possibility that this estimate is biased because mantle convection is intermittent and therefore that the present‐day tomographic snapshot may differ from its time average. Although this possibility cannot be dismissed completely, we argue that values of Sƒ≳0.2 can be discounted under a weak version of the Uniformitarian Principle. The bound obtained here from global tomography is consistent with local seismological evidence for slab flux into the lower mantle; however, the total material flux has to be significantly greater (by a factor of 2–3) than that due to slabs alone. A stratification index, Sƒ≲0.2, is sufficient to exclude many stratified convection models still under active consideration, including most forms of chemical layering between the upper and lower mantle, as well as the more extreme versions of avalanching convection governed by a strong endothermic phase change.