bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic blocking temperatures of magnetite calculated with a three‐dimensional micromagnetic model
Authors: Winklhofer Michael , Fabian Karl , Heider Franz
Published: February 2004

We present an analysis of thermal stability of magnetic remanence in fine grains of magnetite (grain size d = 15–120 nm). In order to model incoherent transitions between single‐domain (SD) and pseudo‐single‐domain (PSD) magnetization configurations, we employ a three‐dimensional constrained minimization method proposed by Enkin and Williams [1994]. Using this approach, one can track in detail the transition from one local energy minimum state into another by constraining the magnetization vectors of appropriate cells in a discrete model. For each particle, we obtain the energy barriers EB(T) from 25° to 578°C. Magnetic blocking temperatures (TB) are calculated by integrating EB(T) for two extreme cooling schedules representing laboratory and geological timescales. The computed blocking temperatures for laboratory timescales are in excellent agreement with the experimentally determined blocking temperatures for magnetite by Dunlop [1973b]. The results of our computations are summarized as relaxation time versus blocking temperature curves, which deviate from the curves of Pullaiah et al. [1975] for particles with grain sizes in the SD‐PSD transition region. A consequence of the dependence of TB on timescale is that some PSD size particles are blocked in vortex states on geologic timescales but are blocked in the SD state on laboratory timescales. Paleointensity determinations with the Thellier method on such samples can therefore underestimate the paleofield. The superparamagnetic to SD threshold size dS is determined as 50 nm for cubic grains, whereas a small aspect ratio of q=1.1 is sufficient to depress dS to 27 nm. SD particles of magnetite with small shape anisotropy and cubic grains with 58 nm≤d≤72 nm are reliable carriers of paleomagnetic information.
The roles of time and displacement in velocity‐dependent volumetric strain of fault zones
Authors: Beeler N. M. , Tullis T. E.
Published: February 2004

The relationship between measured friction μA and volumetric strain during frictional sliding was determined using a rate and state variable dependent friction constitutive equation, a common work balance relating friction and volume change, and two types of experimental faults: initially bare surfaces of Westerly granite and rock surfaces separated by a 1 mm layer of <90 μm Westerly granite gouge. The constitutive equation is the sum of a constant term representing the nominal resistance to sliding and two smaller terms: a rate dependent term representing the shear viscosity of the fault surface (direct effect), and a term which represents variations in the area of contact (evolution effect). The work balance relationship requires that μA differs from the frictional resistance that leads to shear heating by the derivative of fault normal displacement with respect shear displacement, dδn/dδs. An implication of this relationship is that the rate dependence of dδn/dδs contributes to the rate dependence of μA. Experiments show changes in sliding velocity lead to changes in both fault strength and volume. Analysis of data with the rate and state equations combined with the work balance relationship preclude the conventional interpretation of the direct effect in the rate and state variable constitutive equations. Consideration of a model bare surface fault consisting of an undeformable indentor sliding on a deformable surface reveals a serious flaw in the work balance relationship if volume change is time‐dependent. For the model, at zero slip rate indentation creep under the normal load leads to time‐dependent strengthening of the fault surface but, according to the work balance relationship, no work is done because compaction or dilatancy can only be induced by shearing. Additional tests on initially bare surfaces and gouges show that fault normal strain in experiments is time‐dependent, consistent with the model. This time‐dependent fault normal strain, which is not accounted for in the work balance relationship, explains the inconsistency between the constitutive equations and the work balance. For initially bare surface faults, all rate dependence of volume change is due to time dependence. Similar results are found for gouge. We conclude that μA reflects the frictional resistance that results in shear heating, and no correction needs to be made for the volume changes. The result that time‐dependent volume changes do not contribute to μA is a general result and extends beyond these experiments, the simple indentor model and particular constitutive equations used to illustrate the principle.
Joint hypocenter‐velocity inversion for the eastern Tennessee seismic zone
Authors: Vlahovic Gordana , Powell Christine A. , Chapman Martin C. , Sibol Matthew S.
Published: February 2004

A joint hypocenter‐velocity inversion for the eastern Tennessee seismic zone (ETSZ) has resolved velocity features in basement rock below detached Appalachian thrust sheets. P and S wave arrival times from 492 earthquakes have been inverted for one‐(1‐D) and three‐dimensional (3‐D) velocity models to midcrustal depths. The 3‐D P and S wave velocity solutions are computed independly and are very similar. In relation to the 1‐D model, velocity anomalies range from −8% to +16% in the first layer (upper 5 km) and between ±7% in deeper layers. Prominent velocity anomalies parallel the seismic zone and are consistent from layer to layer. The most persistent anomaly is a low‐velocity region that borders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity. The New York‐Alabama (NY‐AL) magnetic lineament coincides with or lies close to the southeast boundary of the prominent velocity low in both the P and S wave velocity images. The spatial coincidence between velocity, gravity, and magnetic gradients suggests that major discontinuities are present in the basement. Relocation in the 3‐D velocity model reduced the number of very deep earthquakes (below 20 km) and further accentuated differences in seismogenic properties on either side of the NY‐AL lineament. After relocation, most earthquakes occur in a vertically bounded region roughly 30 km wide extending from 4 to 22 km in depth. Most earthquakes occur in regions characterized by either average velocity or small velocity anomalies.
Distributed damage, faulting, and friction
Authors: Lyakhovsky Vladimir , Ben‐Zion Yehuda , Agnon Amotz
Published: February 2004

We present a formulation for mechanical modeling of geological processes in the seismogenic crust using damage rheology. The seismogenic layer is treated as an elastic medium where distributed damage, modifying the elastic stiffness, evolves as a function of the deformation history. The model damage rheology is based on thermodynamic principles and fundamental observations of rock deformation. The theoretical analysis leads to a kinetic equation for damage evolution having two principal coefficients. The first is a criterion for the transition between strength degradation and recovering (healing), and is related to friction. The second is a rate coefficient of damage evolution which can have different values or functional forms for positive (degradation) and negative (healing) evolution. We constrain these coefficients by fitting model predictions to laboratory data, including coefficient of friction in sawcut setting, intact strength in fracture experiments, first yielding in faulting experiments under three‐dimensional strain, onset and evolution of acoustic emission, and dynamic instability. The model damage rheology accounts for many realistic features of three‐dimensional deformation fields associated with an earthquake cycle. These include aseismic deformation, gradual strength degradation, development of process zones and branching faults around high‐damage areas, strain localization, brittle failure, and state dependent friction. Some properties of the model damage rheology (e.g., cyclic stick‐slip behavior with possible accompanying creep) are illustrated with simplified analytical results. The developments of the paper provide an internally consistent framework for simulating long histories of crustal deformation, and studying the coupled evolution of regional earthquakes and faults. This is done in a follow up work.
Spectral analysis of vector magnetic field profiles
Authors: Parker Robert L. , O'Brien Michael S.
Published: February 2004

We investigate the power spectra and cross spectra derived from the three components of the vector magnetic field measured on a straight horizontal path above a statistically stationary source. All of these spectra, which can be estimated from the recorded time series, are related to a single two‐dimensional power spectral density via integrals that run in the across‐track direction in the wavenumber domain. Thus the measured spectra must obey a number of strong constraints: for example, the sum of the two power spectral densities of the two horizontal field components equals the power spectral density of the vertical component at every wavenumber and the phase spectrum between the vertical and along‐track components is always π/2. These constraints provide powerful checks on the quality of the measured data; if they are violated, measurement or environmental noise should be suspected. The noise due to errors of orientation has a clear characteristic; both the power and phase spectra of the components differ from those of crustal signals, which makes orientation noise easy to detect and to quantify. The spectra of the crustal signals can be inverted to obtain information about the cross‐track structure of the field. We illustrate these ideas using a high‐altitude Project Magnet profile flown in the southeastern Pacific Ocean.
Fluid venting in the eastern Aleutian Subduction Zone
Authors: Suess Erwin , Bohrmann Gerhard , von Huene Roland , Linke Peter , Wallmann Klaus , Lammers Stephan , Sahling Heiko , Winckler Gisela , Lutz Richard A. , Orange Daniel
Published: February 2004

Fluid venting has been observed along 800 km of the Alaska convergent margin. The fluid venting sites are located near the deformation front, are controlled by subsurface structures, and exhibit the characteristics of cold seeps seen in other convergent margins. The more important characteristics include (1) methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges; (2) prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps, over‐steepened folds or outcrops of bedding planes; (3) calcium carbonate and barite precipitates at the surface and subsurface of vents; and (4) carbon isotope evidence from tissue and skeletal hard parts of biota, as well as from carbonate precipitates, that vents expel either methane‐ or sulfide‐dominated fluids. A biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5.5±0.7 L m−2 d−1 for tectonics‐induced water flow [Wallmann et al., 1997b]. A geophysical estimate of dewatering from the same area [von Huene et al., 1997] based on sediment porosity reduction shows a fluid loss of 0.02 L m−2 d−1 for a 5.5 km wide converged segment near the deformation front. Our video‐guided surveys have documented vent biota across a minimum of 0.1% of the area of the convergent segment off Kodiak Island; hence an average rate of 0.006 L m−2 d−1 is estimated from the biogeochemical approach. The two estimates for tectonics‐induced water flow from the accretionary prism are in surprisingly good agreement.
Crustal structure of the East African Plateau from receiver functions and Rayleigh wave phase velocities
Authors: Last Robert J. , Nyblade Andrew A. , Langston Charles A. , Owens Thomas J.
Published: February 2004

The origin of the East African Plateau and rift valleys is poorly understood largely because little is known about the crust and upper mantle beneath East Africa. The center of the plateau is composed of the Archean Tanzania Craton and is flanked by the Proterozoic Kibaran, Ubendian and Mozambique Belts to west, southwest, and east, respectively. Cenozoic faults of the East African rift system lie primarily within the mobile belts. New estimates of Moho depths, mean shear velocity
, and Poisson's ratio for the crust of the East African Plateau are presented and used to address questions related to the tectonic development of the plateau and rift valleys. The new constraints on crustal structure are obtained by separately modeling receiver functions and Rayleigh wave phase velocities from teleseismic earthquakes recorded by a deployment of 20 broadband seismic stations spread across Tanzania in 1994 and 1995 and then by combining the results of the separate analyses to obtain estimates of mean crustal structure that satisfy both sets of observations. For the Tanzania Craton,
is 3.79 km/s, Moho depths lie between 37 and 42 km, and estimates of Poisson's ratio are between 0.24 and 0.26. For the Mozambique Belt,
is 3.74 km/s, Moho depths range between 36 and 39 km, and estimates of Poisson's ratio are between 0.24 and 0.27. Parameter uncertainties are ±0.10 km/s for
, ±4 km for Moho depth, and ±0.02 for Poisson's ratio. Results from stations in the Ubendian Belt indicate a
of ∼3.74 km/s and Moho depths between 40 and 45 km. Based on a comparison of these results to global averages for Precambrian crust, it can be concluded that there are no regional scale anomalies in the crustal structure that can easily explain the isostatic uplift of the East African Plateau and that Archean and Proterozoic crust in East Africa may be slightly more felsic than Precambrian crust elsewhere. In addition, patterns of crustal thinning beneath rifted areas in East Africa appear to be consistent with amounts of extension deduced from surface structures.
Effective elastic thickness of the lithosphere along the Easter Seamount Chain
Authors: Kruse Sarah E. , Liu Zhengrong J. , Naar David F. , Duncan Robert A.
Published: February 2004

Bathymetry and gravity data collected during Legs 5, 6, and 7 of the 1993 GLORIA Expedition and the recently released 2‐min altimetry‐derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the Easter Seamount Chain (ESC). Forward modeling, admittance, and coherence methods yield consistent results. With the exception of the eastern and western ends of the ESC the effective elastic thickness along the chain is ∼1–4 km. The thin elastic thickness for the majority of the ESC seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the Nazca plate seafloor age. The elastic thickness southeast of the Nazca fracture zone is ∼6 km, apparently because of the seafloor age discontinuity across the fracture zone. The elastic thickness near the San Felix Island, at the eastern end of the ESC, is even greater (∼11 km), which is compatible with the estimated seafloor age at the time of loading. A slight increase in the effective elastic thickness of the far western part of the ESC suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center. These findings combined with published geochemistry support a hotspot origin for the ESC, complicated by large‐scale plate boundary reorganizations and channeling of plume material to the East Pacific Rise.
Branching sheeted dikes and seafloor spreading within an Early Proterozoic intra‐arc basin
Authors: Dann Jesse C.
Published: February 2004

The Payson ophiolite (∼ 1.73 Ga) is exposed as a shallow‐dipping, pseudostratigraphic sequence of gabbro, sheeted dikes, and submarine volcanic rocks, with screens and roof pendants of older arc crust. The sheeted dike complex is particularly well exposed, and analysis of measured sections reveals a threefold increase in the average dike width with depth in the complex. Magmatic temperatures and slow cooling at the base of the sheeted dike complex, indicated by gabbro‐dike mingling and mutual intrusion and coarse textures, may have facilitated the growth of wide dikes. In contrast, extreme alteration of the top of the sheeted dike complex suggests that hydrothermal circulation dominated and caused rapid cooling as thin dikes. The connection between wide dikes at depth with thinner dikes toward the top requires a new model, where dikes branch upward in response to the strong vertical gradient in the rate of cooling and solidification. The branching dike model has important implications for the timing and spatial distribution of seafloor phenomena in the neovolcanic zone of spreading centers.
Comparison of P and S station corrections and their relationship to upper mantle structure
Authors: Robertson G. S. , Woodhouse J. H.
Published: February 2004

We use International Seismological Centre data between the years 1964 and 1994 to derive a data set of P and S station corrections. By restricting the raw P and S data to those which share the same source to receiver paths, we minimize any bias to the station corrections from differential P and S data distributions. We find that both static and azimuth dependent terms are highly correlated. In addition, we perform synthetic tests to determine the sensitivity of our data to mantle structure and show that static terms are sensitive to structure, at least throughout the upper mantle. In determining the regression slope of the static terms, we allow for regional offsets and find a best‐fit global slope of S to P correction a=2.85±0.19. This is lower than that obtained in previous station correction studies but is in agreement with results in the upper mantle using differential travel times and with values derived for the upper parts of the lower mantle. Regionalizing the data by tectonics does not give statistically distinct ratios of S to P station correction. The result for shield areas, however, is significantly smaller than previously believed, probably because of our ability to derive station corrections from similar data sets. Variations in the second azimuthal terms are similar in both fast direction and relative magnitude. In another synthetic experiment we find that their general trends can be predicted well by isotropic models of heterogeneous velocity structure. From this result and also because they do not correlate with SKS splitting data, we conclude that station correction second azimuthal terms should not be interpreted in terms of anisotropy.
Paleomagnetism and paleothermometry of the Sydney Basin 2. Origin of anomalously high unblocking temperatures
Authors: Dunlop D. J. , Özdemir Ö. , Schmidt P. W.
Published: February 2004

The Milton Monzonite of southeastern Australia was thermoviscously remagnetized as a result of Cretaceous burial and uplift. Thermal demagnetization separates the low unblocking temperature (LT) overprint from the high unblocking temperature (HT) primary remanence, with a relatively sharp junction between LT and HT components in vector projections. For single‐domain grains, the junction temperature TL between two such vectors corresponds to the maximum blocking temperature Tr reactivated in nature, apart from a correction for the difference between natural and laboratory timescales. However, measured TL values are distributed over an implausibly wide range (>250°C) for burial remagnetization of an untilted intrusion like the Milton Monzonite. Furthermore, many TL values are anomalously high compared to the predictions of single‐domain theory. Multidomain grains are the cause of these anomalies. Samples pretreated before thermal demagnetization by zero‐field cycling to liquid nitrogen temperature, so as to erase multidomain remanence and isolate single‐domain remanence, do have the theoretically expected TL values. In these samples, realistic remagnetization time and temperature (tr, Tr) conditions in nature are predicted using the t‐T contours of Pullaiah et al. [1975]. The anomalously high TL values before low‐temperature treatment are due to multidomain grains, which carry ≥50% of the LT overprint. The LT thermal demagnetization curve in samples dominated by multidomain grains is quasi‐exponential in shape with a high‐temperature tail extending almost to the Curie point, as predicted by multidomain theory. These high LT unblocking temperatures, which are much greater than plausible remagnetization temperatures reached in nature, overlap and mask the lower part of the HT unblocking temperature spectrum, driving up TL values and leading to inflated estimates of Tr. Although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the Milton Monzonite, chemical overprinting may be a factor in other lithologies and tectonic settings.
Paleomagnetism and paleothermometry of the Sydney Basin 1. Thermoviscous and chemical overprinting of the Milton Monzonite
Authors: Dunlop D. J. , Schmidt P. W. , Özdemir Ö. , Clark D. A.
Published: February 2004

The Early Triassic (∼245 Ma) Milton Monzonite of the Sydney Basin, Australia, has four distinct components of natural remanent magnetization (NRM) with only slightly overlapping ranges of unblocking temperatures. The low‐temperature (LT) component, the first to be thermally demagnetized, is thought to be a Late Cretaceous (≈100 Ma) thermoviscous overprint acquired in slow cooling during uplift. The high‐temperature (HT) component, the second to be demagnetized, is probably the primary thermoremanent magnetization (TRM) of the Milton intrusion but could possibly be a Jurassic overprint. LT and HT are usually carried by magnetite and occasionally by pyrrhotite. Samples from nine sites have a further NRM component which unblocks at higher temperatures than HT but below the magnetite Curie temperature of 580°C. This component is argued to be a chemical remanent magnetization (CRM) because of its discrete range of high unblocking temperatures, above those of the thermal components HT and LT, and is called CRM1. CRM1 has almost the same direction as LT and is likely carried by authigenic magnetite produced during uplift ∼100 Ma. Samples from five sites have a fourth NRM component, with a direction resembling that of HT but carried by hematite. This fourth component could be a primary TRM but is more likely a CRM and is therefore called CRM2. The HT‐CRM2 mean direction is D=50°, I=75.5°, defining a paleopole at 16°S, 172°E. The HT‐CRM2 paleopole falls near 150 Ma on the Australian apparent polar wander path but is a considerable distance from paleopoles of Permian and Early Triassic age. There is no known tectonic or other remagnetizing event in the Sydney Basin around 150 Ma. For this reason, we propose that the HT‐CRM2 paleopole defines a new Triassic segment of the Australian polar wander path. The LT‐CRM1 mean direction is D=348°, I=−79°, with a paleopole falling at 56°S, 1580°E, near 100 Ma on the polar wander path. This age is consistent with uplift and cooling related to initial rifting of the Tasman Sea.
GPS and geologic estimates of the tectonic activity in the Marmara Sea region, NW Anatolia
Authors: Straub Christian , Kahle Hans‐Gert , Schindler Conrad
Published: February 2004

A dense network of 52 Global Positioning System (GPS) sites is used to determine the velocity field and strain rate pattern at the western end of the North Anatolian Fault Zone (NAFZ) in the Marmara Sea region, NW Anatolia. Based on four GPS campaigns carried out biannually between 1990 and 1996, the detailed kinematic field of crustal motion is calculated. The southernmost station shows an average rate of 22±3 mm/yr oriented westward relative to Istanbul (Eurasia). This value is indicative of the dextral strike‐slip motion of NW Anatolia relative to the Black Sea. The GPS velocity field reveals that the sites located on the Thrace‐Black Sea block show negligible motion relative to Istanbul. The GPS‐based deformation pattern of three sections crossing the westward prolongation of the NAFZ is compared with neotectonic data. Both the GPS and the neotectonic data give clear evidence that most of the deformation occurs along a relatively narrow E‐W oriented zone. This belt extends from the single fault trace of the NAFZ through the Gulf of Izmit, the Marmara Sea, the Şarköy region, and the Gulf of Saros into the North Aegean Trough (NAT). In addition, the cross sections show the increasing influence of the N‐S oriented extension in the Aegean Sea region. From the comparison of the directions and rates of shear and normal strain deduced from GPS data with geologic and seismic observations we conclude that the most active zone of dextral shear follows a line along Mudurnu Valley, the northern Marmara Sea and the Saros Trough where it joins the NAT.
Upper mantle structure of the southwest Pacific from regional waveform inversion
Authors: Xu Yingbiao , Wiens Douglas A.
Published: February 2004

We determine the upper mantle seismological structure of the southwest Pacific by inverting complete regional seismograms recorded by the Southwest Pacific Seismic Experiment, a deployment of 12 broadband seismographs in the Fiji, Tonga, and Niue islands. We first present a method to invert entire seismic waveforms for a laterally homogeneous velocity structure. Partial derivatives are computed using an efficient reflectivity code for differential seismograms [Randall, 1994], and a conjugate gradient method is used to perform the nonlinear inversion. Band pass‐filtered (10–55 mHz) vertical and radial component seismograms, extending from the P arrival to the surface waves, are used to constrain the structure. Waveforms are selected to provide regional distance (400–1500 km) propagation paths predominantly within one of the tectonic regions of the southwest Pacific. Source depths ranging from 10 to 240 km allow the deeper regions of the model to be resolved with relatively short path lengths. Results show extremely large lateral variability in the upper mantle structure of the southwest Pacific. The inactive South Fiji Basin shows upper mantle velocities much lower than those previously observed for the Pacific lithosphere of a similar age. The North Fiji and Lau Basins, containing active back arc spreading systems, show upper mantle shear velocities as slow as 3.8 km s−1. The exceptionally low seismic velocities in the active back arc regions may represent the slowest upper mantle velocities in the world. The velocity difference between the active back arc basins and the old Pacific lithosphere immediately east of the Tonga Trench is maximum at 30–90 km depth, with a variation of ∼18%, indicating partial melting in this depth range. Velocity heterogeneity of >4% extends to a depth of 170 km. Velocity structure below 220 km depth is poorly resolved, but there is some evidence of a 2% difference extending to depths of 350 km.
Tomographic systems of equations with irregular cells
Authors: Sambridge Malcolm , Guđmundsson Ólafur
Published: February 2004

How to implement seismic or any other type of ray‐based tomography in a new class of cellular parameterization built from any chosen distribution of nodes in two or three dimensions is shown. The complete flexibility of this novel approach allows one to introduce detail in a tomographic model only where desired, thereby reducing memory and computation time, or to impose complex a priori constraints on the inversion. Full details of powerful new algorithms are given to generate unequally sized tetrahedral or polyhedral cells and to calculate the necessary Frechet derivatives required in linearized tomography. These algorithms are efficient enough in three dimensions, to allow the parameterization to be refined during an inversion. The methods are illustrated with numerical examples. It is concluded that in linear or nonlinear inversion the computational cost of the new algorithms will not be significantly higher than that incurred by using a regular Cartesian grid.
Eurasian surface wave tomography: Group velocities
Authors: Ritzwoller Michael H. , Levshin Anatoli L.
Published: February 2004

This paper presents the results of a study of the dispersion characteristics of broadband fundamental surface waves propagating across Eurasia. The study is broader band, displays denser and more uniform data coverage, and demonstrates higher resolution than previous studies of Eurasia performed on this scale. In addition, the estimated group velocity maps reveal the signatures of geological and tectonic features never before displayed in similar surface wave studies. We present group velocity maps from 20 s to 200 s period for Rayleigh waves and from 20 s to 125 s for Love waves. Broadband waveform data from about 600 events from 1988 through 1995 recorded at 83 individual stations across Eurasia have produced about 9000 paths for which individual dispersion curves have been estimated. Dispersion curves from similar paths are clustered to reduce redundancy, to identify outliers for rejection, and to assign uncertainty estimates. On average, measurement uncertainty is about 0.030–0.040 km/s and is not a strong function of frequency. Resolution is estimated from “checker‐board” tests, and we show that average resolutions across Eurasia range from 5° to 7.5° but degrade at periods above about 100 s and near the periphery of the maps. The estimated maps produce a variance reduction relative to the Preliminary Reference Earth Model (PREM) of more than 90% for Rayleigh waves below 60 s period but reduce to about 70% between 80 and 200 s period. For Love waves, variance reductions are similar, being above 90% for most periods below 100 s and falling to 70% at 150 s. Synthetic experiments are presented to estimate the biases that theoretical approximations should impart to the group velocity maps, in particular source group time shifts, azimuthal anisotropy, and systematic event mislocations near subducting slabs. The most significant problems are probably caused by azimuthal anisotropy, but above 100 s the effect of source group time shifts may also be appreciable. These effects are probably below the signal levels that we interpret here, however. Many known geological and tectonic structures are observed in the group velocity maps. Of particular note are the signatures of sedimentary basins, continental flood basalts, variations in crustal thickness, backarc spreading, downgoing slabs, and continental roots. Comparison of the estimated group velocity maps with those predicted by CRUST5.1/S16B30 is qualitatively good, but there are significant differences in detail which provide new information that should help to calibrate future crustal and upper mantle models of Eurasia.
Determination of constitutive relations of fault slip based on seismic wave analysis
Authors: Ide Satoshi , Takeo Minoru
Published: February 2004

Constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure. Although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields, no actual relation during a natural earthquake has been determined. The 1995 Kobe earthquake is suitable for detailed kinematic analysis, and this enables the first evaluation of constitutive relations for a natural earthquake. In this study, we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 Kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane. An inversion method based on Bayes theorem is employed to obtain a spatiotemporal slip distribution, and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints. This slip distribution is then used as part of the boundary condition in the finite difference calculation. The time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane. They show slip weakening relations almost everywhere on the fault plane, while slip rate dependency is not clear. The slip weakening behavior has a clear depth dependency indicating that the slip weakening rate (dτ/du) is smaller in the shallow crust than that in the deep crust. This may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems.
Seismogram envelope inversion for the spatial distribution of high‐frequency energy radiation from the earthquake fault: Application to the 1994 far east off Sanriku earthquake, Japan
Authors: Nakahara Hisashi , Nishimura Takeshi , Sato Haruo , Ohtake Masakazu
Published: February 2004

Incoherency of high‐frequency seismic waves leads to the idea to analyze seismogram envelopes disregarding phase information for the source study. We develop an inversion method for estimating high‐frequency (above 1 Hz) energy radiation from the earthquake fault and site amplification factors from observed mean square S‐wave seismogram envelopes, The inversion is executed by using the s‐wave envelope Green function from the onset to coda for a point shear dislocation source in a scattering medium, which is formulated based on an extended version of the radiative transfer theory. The use of the envelope Green function enables us to estimate the spatial distribution of high‐frequency energy radiation on a fault plane. This has been difficult by conventional waveform inversion methods. The rupture velocity and the duration time of energy radiation on each subfault are also estimated by grid search method. We apply this new method to the 1994 far east off Sanriku, northeastern Japan, earthquake (Mw 7.7), which ruptured a 90‐km segment of the plate boundary along the Japan Trench. Inverting observed seismogram envelopes at 10 strong‐motion stations in Japan, it is concluded that above 90% of the high‐frequency energy was radiated from the western half of the fault with the largest energy radiation near the deep‐side edge. The rupture velocity is estimated to be 2.7 km/s. The estimated site amplification factors, ranging between 0.3 and 15, are consistent with those independently estimated by the coda normalization method.
A geometrical approach to the elastic complex screen
Authors: Wild Andrew J. , Hudson J. A.
Published: February 2004

The elastic complex screen, or “phase screen” approach to the forward modeling of elastic waves promises significant advantages over other computational schemes, because it is potentially much less intensive numerically than finite difference methods, and yet it models essentially the whole useful wave field. In this paper it is shown how the equations that describe the technique can be derived through a geometrical construction. The new formulation is presented in conjunction with an overview of the earlier derivation of Wu [1994] in order to emphasize the similarities and differences between the two approaches. The geometrical derivation provides for a formulation in terms of interfaces (steps) rather than slabs. It also leads quite naturally to the incorporation of backscattered energy in the model. An algorithm to implement this important capability is developed in some detail and includes a backpropagation technique, which improves storage efficiency by removing the need to store the backscattered field at each screen independently. Numerical results are presented for some simple backscattering models, first of all to validate the theory, but then more specifically to illustrate the frequency response of the model and the implications it has for the choice of a suitable source spectrum.
Electroseismic waves from point sources in layered media
Authors: Haartsen Matthijs W. , Pride Steven R.
Published: February 2004

In a porous material saturated by a fluid electrolyte, mechanical and electromagnetic (EM) disturbances are coupled. The coupling is electrokinetic in nature. The seismic waves generate relative fluid‐solid motion that induces an electrical streaming current. When a seismic pulse traverses contrasts in elastic and/or fluid‐chemistry properties, the streaming‐current imbalance creates dipolar and multipolar charge separations across the interface that, in turn, produce EM disturbances that are measurable at the earth's surface. This paper numerically determines a full‐waveform electroseismic point‐source response in a stratified porous medium. It is shown that the macroscopic governing equations controlling the coupled electromagnetics and acoustics of porous media decouple into two sets corresponding to vertical or horizontal polarization of the transverse wave fields. The frequency content of the converted EM field has the same frequency content (at the generating interface) as the incident seismic pulse. Snapshots in time and converted EM amplitudes versus source to antenna offset are calculated for contrasts in mechanical and/or electrical medium properties. The converted EM radiation pattern away from the interface is similar to having an effective vertical‐electric dipole centered right beneath the source on the contrast. The transverse magnetic mode amplitudes fall off rapidly with distance, from the generating interface thus suggesting the importance of a vertical electroseismic profiling geometry to record the converted EM signal at antennas close to an interface of interest.
Magnetic susceptibility patterns in a Cordilleran granitoid: The Las Tazas Complex, northern Chile
Authors: Wilson Jeff
Published: February 2004

The Las Tazas Complex is a ferromagnetic I‐type granitoid that crops out in the Coastal Cordillera of northern Chile. Magnetic susceptibility patterns across the complex are controlled by modal magnetite contents, rather than overall mineralogy, resulting in an ambiguous correlation between susceptibility magnitude variation and lithological zoning. There is a good correlation between plagioclase magmatic fabric orientations and magnetic fabric orientation, demonstrating that anisotropy of magnetic susceptibility (AMS) measurements can accurately characterize magmatic fabric patterns in Cordilleran I‐type granitoids. However, there is no simple quantitative relationship between magmatic and magnetic fabric intensities. The magmatic and magnetic fabrics share a similar character, and it is clear that AMS fabrics are strongly influenced by the dominant plagioclase fabric across the Las Tazas Complex but cannot be directly equated in quantitative strain analysis.
The rise and fall of the Coaxial hydrothermal site, 1993–1996
Authors: Baker Edward T. , Massoth Gary J. , Feely Richard A. , Cannon Glenn A. , Thomson Richard E.
Published: February 2004

In June 1993, a seafloor dike intrusion along the CoAxial segment of the Juan de Fuca Ridge was acoustically detected. A near‐immediate field response and repeated plume mapping and sampling surveys during eight cruises over the next 3 years have provided a unique opportunity to estimate heat and mass fluxes from both event and chronic discharge during the life cycle of a newly created hydrothermal system. The intrusion triggered the release of at least three event plumes followed by chronic discharge focused at two sites: Flow, the site of a lava eruption at the distal end of the intrusion; and Floe, 30 km back along the trend of the dike. We have combined measurements of plume temperature anomalies, plume areal extent, and year‐long averages of current flow at both sites to estimate the chronic hydrothermal heat flux Hu. Initial values of Hu at both sites were of order 104 MW, declining over time t as Hu = at−k, with k ≈ 1. Significant plumes were no longer detectable at Flow by June 1995, or at Floe by June 1996. Elemental fluxes from the CoAxial system have been derived from Hu and measurements of the ratios Mn/heat, Fe/heat, and particulate S/heat in chronic plumes. While Mn and Fe fluxes mirrored the power curve decline of heat, the combined regional particulate S (PS) flux experienced a second pronounced maximum some months after the eruption owing to a sharp increase in the S/heat ratio at Floc. Integrated inventories from chronic discharge were ∼4×1017 J for heat, ∼3×108 mol for Mn, ∼2×108 mol for Fe, and ∼1×108 mol for PS. Realistic uncertainties for all species are roughly a factor of 2. The three event plumes accounted for <5% of the chronic plume inventories.
Critical wedges in three dimensions: Analytical expressions from Mohr‐Coulomb constrained perturbation analysis
Authors: Enlow R. L. , Koons P. O.
Published: February 2004

We present simple analytical expressions for evaluating three‐dimensional stress fields in critical wedges (Coulomb failure throughout) that are deforming within an oblique convergent zone. We assume that the load and geometry variations in the lateral direction, and the topographic slope, are relatively small. The stress state of a two dimensional critical wedge is perturbed by admitting a small lateral shear, which is accommodated (to maintain criticality) by a reduction in the dominant normal compressive stress. Cohesionless cases are further simplified with Taylor series expansions in the topographic slope. The analytical expressions distinguish the behavior of strong base wedges associated with steep slopes from weak base wedges with more modest slopes, and are reliable for lateral shear values as large as half the lithostatic load. In the weak base case where the vertical shear is small, the orientations of the principal stresses are very sensitive to the perturbation. As the lateral shear increases through relatively small values, the roles of the lateral and vertical coordinate axes are interchanged, with a resultant transition from predominantly thrust failure to strike slip failure. Transition to strike slip in the strong base case is more gradual. The particular value of horizontal shear for which transition occurs also depends on the difference of the minor normal stresses. We produce an analytical expression predicting displacement patterns compatible with those observed in natural and analog orogens. Displacement partitioning is predicted for weak base wedges due to minor changes in lateral boundary conditions associated with variable orogen geometry.
Global mapping of topography on transition zone velocity discontinuities by stacking SS precursors
Authors: Flanagan Megan P. , Shearer Peter M.
Published: February 2004

We stack long‐period, transverse‐component seismograms recorded by the Global Digital Seismograph Network (GDSN) (1976–1996), Incorporated Research Institutions for Seismology‐International Deployment of Accelerometers (IRIS‐IDA) (1988–1996), and Geoscope (1988–1996) networks to map large‐scale topography on the 410‐ and 660‐km seismic velocity discontinuities. Underside reflections from these discontinuities arrive as precursors to the SS phase, and their timing can be used to obtain global variations of the depth to the reflectors. We analyze over 13,000 records from events mb>5.5, focal depth <75 km, and range 110° to 180° by picking and aligning on SS, then stacking the records along the theoretical travel time curves for the discontinuity reflections. Separate stacks are obtained for 416 equally spaced caps of 10° radius; clear 410‐ and 660‐km reflections are visible for almost all of the caps while 520‐km reflections are seen in about half of the caps. The differential travel times between the precursors and the SS arrival are measured on each stack, with uncertainty estimates obtained using a bootstrap resampling method. We then compute discontinuity depths relative to the isotropic Preliminary Reference Earth Model (PREM) at 40‐s period, correcting for surface topography and crustal thickness variations using the CRUST5.0 model of Mooney et al. [1995], and for upper mantle S velocity heterogeneity using model S16B30 of Masters et al. [1996]. The resulting maps of discontinuity topography have more complete coverage than previous studies; observed depths are highly correlated between adjacent caps and appear dominated by large‐scale topography variations. The 660‐km discontinuity exhibits peak‐to‐peak topography of about 38 km, with regional depressions that correlate with areas of current and past subduction around the Pacific Ocean. Large‐scale topography on the 410‐km discontinuity is lower in amplitude and largely uncorrelated with the topography on the 660‐km interface. The width of the transition zone, WTZ, as measured by the separation between the 410‐ and 660‐km discontinuities, appears thickest in areas of active subduction (e.g., Kurils, Philippines, and Tonga) and thins beneath Antarctica and much of the central Pacific Ocean. Spatial variations in WTZ appear unrelated to ocean‐continent differences but do roughly correlate with the S16B30 velocities in the transition zone, consistent with a common thermal origin for both patterns. The lower‐amplitude 520‐km reflector is more difficult to resolve but appears to be a global feature as it is observed preferentially for those bounce point caps with the most data.
Experimental investigation of silicate‐carbonate system at high pressure and high temperature
Authors: Martinez Isabelle , Chamorro Peréz Eva Maria , Matas Jan , Gillet Philippe , Vidal Gérard
Published: February 2004

Melting and subsolidus relations in the (Mg,Fe)SiO3‐(Mg,Fe)CO3, (Mg,Fe) 2SiO4‐(Mg,Fe)CO3, and (Mg,Fe)O‐(Mg,Fe)CO3 systems have been investigated at 14, 15, 16 and 25 GPa, 1973 K and 2173 K, using a 1000 t uniaxial multi anvil split sphere apparatus. The iron‐magnesium partition coefficients between magnesite and silicates or oxides have been measured in subsolidus assemblages. Iron is always partitioned preferentially in the silicate and oxide phases, the order of increasing partitioning being pyroxene, olivine, silicate perovskite, wadsleyite and magnesiowüstite. A thermodynamic model of iron‐magnesium distribution between magnesite and all these phases, based on Gibbs free energy minimization, is established. Melting of pyroxene‐magnesite and olivine‐magnesite pseudo binary systems is eutectic, with eutectic points close to 1973 K and 60 mol % carbonate at 15 GPa in both systems. In the more complex mantle system, it is likely that such melts would form in the transition zone by heating and homogenization of deep subducted carbonates. The melts formed in the olivine‐carbonate system are characterized by high Mg+Fe/Si ratios and thus unlikely to be primary kimberlitic magmas, a conclusion in agreement with previous studies in the peridotite‐CO2 system. On the other hand, the observed pyroxene‐magnesite melts formed at transition zone conditions have Mg+Fe/Si ratios that are comparable to those of natural kimberlites, suggesting that melting of carbonated pyroxenites at high pressures could be a source of kimberlitic magmas.