bluuebunny/crossref_metadata_2025 huggingface dataset:
A transient subduction zone slip episode in southwest Japan observed by the nationwide GPS array
Authors: Miyazaki Shin'ichi , McGuire Jeffrey J. , Segall Paul
Published: May 2003

We investigate the spatiotemporal variation of fault slip on the subduction interface in southwest Japan following the two 1996 Hyuganada earthquakes (M 6.7) using continuous GPS observations. A silent thrust slip event has been reported by Hirose et al. [1999], following these earthquakes approximately 100 km to the north beneath the Bungo channel. We employ the Network Inversion Filter to rigorously invert for the fault slip distribution in space and time. The GPS time series is modeled as a sum of: (1) secular deformation; (2) coseismic station displacements; (3) transient fault slip; (4) random benchmark motion; (5) reference frame shifts; and (6) observational errors. Nonnegativity constraints effectively prohibit normal faulting on the subduction interface. Spatial and temporal smoothing parameters are estimated by the extended Kalman filter. The estimated fault slip‐rate images postseismic slip following the two Hyuganada earthquakes. Slip associated with the silent thrust event began roughly 1 month after the second earthquake but initiated ∼100 km to the northeast of the Hyuganada earthquakes. The slow event propagated southwest and downdip beneath the Bungo channel, lasting approximately 1 year with maximum slip‐rates of 0.6 m/yr. The equivalent seismic moment was MW ∼ 7.2. We can rule out the possibility that slip propagated from the source region of the Hyuganada earthquakes to the location of the silent earthquake. However, the timing of the events suggests a causal connection, perhaps due to static stress transfer. This kind of transient event, or silent earthquake, may reflect different fault zone characteristics and may be responsible for accommodating strain that would otherwise be released in large earthquakes.
Atmospheric, hydrological, and ocean current contributions to Earth's annual wobble and length‐of‐day signals based on output from a climate model
Authors: Zhong Min , Naito Isao , Kitoh Akio
Published: May 2003

The atmospheric, hydrologic, and ocean current contributions to the annual wobble and length‐of‐day change are investigated, based upon 10 years of monthly values simulated by a coupled ocean‐atmosphere general circulation model (COAGCM). Also used are time series of geodetic excitation inferred from the observed wobble in the “SPACE97” data and, for reference, atmospheric angular momentum (AAM) functions from operational objective analysis data of Japan Meteorological Agency (JMA) for the period 1988–1997. The simulated annual variation in equatorial AAM function, which includes pressure and wind contributions, agrees well with that from the JMA operational analysis for 90°E component, but disagrees for 0° component due to differences in both pressure and wind contributions over ocean hemisphere. In the seasonal cycle the simulated hydrologic and ocean current excitation functions plus the AAM function from the JMA operational analysis tend to be in better agreement with the geodetic excitation function than those from the COAGCM output themselves. Because of the small magnitude of the simulated ocean current effects, the simulated hydrological cycle is found to be an important contributor to the nonatmospheric annual variations in polar motion. This confirms the importance of hydrology as a source of the annual wobble.
Multiparticle simulation of collapsing volcanic columns and pyroclastic flow
Authors: Neri Augusto , Esposti Ongaro Tomaso , Macedonio Giovanni , Gidaspow Dimitri
Published: May 2003

A multiparticle thermofluid dynamic model was developed to assess the effect of a range of particle size on the transient two‐dimensional behavior of collapsing columns and associated pyroclastic flows. The model accounts for full mechanical and thermal nonequilibrium conditions between a continuous gas phase and N solid particulate phases, each characterized by specific physical parameters and properties. The dynamics of the process were simulated by adopting a large eddy simulation approach able to resolve the large‐scale features of the flow and by parametrizing the subgrid gas turbulence. Viscous and interphase effects were expressed in terms of Newtonian stress tensors and gas‐particle and particle‐particle coefficients, respectively. Numerical simulations were carried out by using different grain‐size distributions of the mixture at the vent, constitutive equations, and numerical resolutions. Dispersal dynamics describe the formation of the vertical jet, the column collapse and the building of the pyroclastic fountain, the generation of radially spreading pyroclastic flows, and the development of thermal convective instabilities from the fountain and the flow. The results highlight the importance of the multiparticle formulation of the model and describe several mechanical and thermal nonequilibrium effects. Finer particles tend to follow the hot ascending gas, mainly in the phoenix column and, secondarily, in the convective plume above the fountain. Coarser particles tend to segregate mainly along the ground both in the proximal area close to the crater rim because of the recycling of material from the fountain and in the distal area, because of the loss of radial momentum. As a result, pyroclastic flows were described as formed by a dilute fine‐rich suspension current overlying a dense underflow rich in coarse particles from the proximal region of the flow. Nonequilibrium effects between particles of different sizes appear to be controlled by particle‐particle collisions in the basal layer of the flow, whereas particle dispersal in the suspension current and ascending plumes is determined by the gas‐particle drag. Simulations performed with a different grain‐size distribution at the vent indicate that a fine‐grained mixture produces a thicker and more mobile current, a larger runout distance, and a greater elutriated mass than the coarse‐grained mixture.
Fractal pore structure of sedimentary rocks: Simulation by ballistic deposition
Authors: Dutta Tapati , Tarafdar S.
Published: May 2003

Experimental studies show that porous sedimentary rocks have a fractal pore‐grain interface. The bidisperse ballistic deposition model (BBDM) generates a porous structure by computer simulation. The model is able to reproduce the behavior of several macroscopic properties of sedimentary rocks quite well. In the present paper we show that the microstructure of the pore space created in BBDM is also realistic. Two‐point correlations of the pore space agree qualitatively with results for real rocks, and the sample spanning pore has a fractal dimension in the range 2.29–2.99 for different porosities. We also study diffusion in the pore space by a random walk and show that it is anomalous at low porosities.
Late Pleistocene to Holocene slip rates for the Gurvan Bulag thrust fault (Gobi‐Altay, Mongolia) estimated with 10Be dates
Authors: Ritz J.‐F. , Bourlès D. , Brown E. T. , Carretier S. , Chéry J. , Enhtuvshin B. , Galsan P. , Finkel R. C. , Hanks T. C. , Kendrick K. J. , Philip H. , Raisbeck G. , Schlupp A. , Schwartz D. P. , Yiou F.
Published: May 2003

We surveyed morphotectonic markers along the central part of the Gurvan Bulag thrust, a fault that ruptured with the Bogd fault during the Gobi‐Altay earthquake (1957, M 8.3), to document climatic and tectonic processes along the fault for the late Pleistocene‐Holocene period. The markers were dated using 10Be produced in situ. Two major periods of alluviation ended at 131 ± 20 and 16 ± 4.8 ka. These appear to be contemporaneous with global climatic changes at the terminations of marine isotope stages (MIS) 6 and 2. The vertical slip rates, determined from offset measurements and surfaces ages, are 0.14 ± 0.03 mm/yr over the late Pleistocene‐Holocene and between 0.44 ± 0.11 and 1.05 ± 0.25 mm/yr since the end of the late Pleistocene. The higher of these slip rates for the last ∼16 kyr is consistent with paleoseismic investigations along the fault [Prentice et al., 2002], and suggests that, at the end of late Pleistocene, the fault evolved from quiescence to having recurrence intervals of 4.0 ± 1.2 kyr for surface ruptures with ∼4 m vertical offset (similar to that of 1957). The inferred recurrence interval is comparable to that of the Bogd fault (3.7 ± 1.3 kyr) suggesting that the two faults may have ruptured together also earlier during the last ∼16 kyr.
Role of element solubility on the kinetics of element partitioning: In situ observations and a thermodynamic kinetic model
Authors: Dohmen Ralf , Chakraborty Sumit , Palme Herbert , Rammensee Werner
Published: May 2003

To elucidate the nature of element exchange between crystals via the mediation of a fluid phase, the mechanism and kinetics of the exchange reaction: 2Fe(Metal) + Mg2SiO4(Ol) = 2 Mg(Gas) + Fe2SiO4(Ol) were studied through a set of Knudsen cell mass spectrometry experiments. The reaction progress was monitored in situ by measuring vapor pressures of individual species (mainly Mg, SiO, and Fe) during experiments as well as by measurement of compositional profiles in crystals (Fe‐rich rims) after the anneals. The experimental observations were reproduced by a numerical model which accounted for compositionally dependent diffusion in olivine, surface‐controlled evaporation/condensation reactions (moving boundaries) and transport out of the system (open system behavior). The main finding of the study is that the composition (Fe/Mg ratio) at the surface of olivine varies with time at constant temperature, i.e., the final surface composition was not achieved instantaneously compared to the diffusion time scale, even during exchange with a vapor phase at high temperatures. The low concentration of Mg in the gas phase explains this behavior. On the basis of these observations a simple general model for element exchange between a solid and a fluid phase could be formulated and analytically solved. A single parameter may be defined combining thermodynamic and kinetic factors. The value of this parameter demarcates three domains: (1) disequilibrium at the surface, (2) equilibrium at the surface, and (3) no observable reaction, i.e., frozen system. It is shown that all three classes of behavior may be observed in natural fluids such as an ideal gas, aqueous solutions, and silicate melts.
Crustal magnetization and accretion at the Southwest Indian Ridge near the Atlantis II fracture zone, 0–25 Ma
Authors: Hosford Allegra , Tivey Maurice , Matsumoto Takeshi , Dick Henry , Schouten Hans , Kinoshita Hajimu
Published: May 2003

We analyze geophysical data that extend from 0 to 25‐Myr‐old seafloor on both flanks of the Southwest Indian Ridge (SWIR). Lineated marine magnetic anomalies are consistent and identifiable within the study area, even over seafloor lacking a basaltic upper crust. The full spreading rate of 14 km/Myr has remained nearly constant since at least 20 Ma, but crustal accretion has been highly asymmetric, with half rates of 8.5 and 5.5 km/Myr on the Antarctic and African flanks, respectively. This asymmetry may be unique to a ∼400 km wide corridor between large‐offset fracture zones of the SWIR. In contrast to the Mid‐Atlantic Ridge, crustal magnetization amplitudes correlate directly with seafloor topography along the present‐day rift valleys. This pattern appears to be primarily a function of along‐axis variations in crustal thickness, rather than magnetic mineralogy. Off‐axis, magnetization amplitudes at paleo‐segment ends are more positive than at paleo‐segment midpoints, suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle. Alteration of the magnetic source layer at paleo‐segment midpoints reduces magnetization amplitudes by 70–80% within 20 Myr of accretion. Magnetic and Ocean Drilling Program (ODP) Hole 735B data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust. Thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization. As the crust ages, the lower crust becomes increasingly important in preserving marine magnetic stripes.
Earthquake clusters resulting from delayed rupture propagation in finite fault segments
Authors: Hainzl Sebastian , Zöller Gert , Scherbaum Frank
Published: May 2003

The question whether the preparation process of foreshocks and main shocks is different from other earthquakes is of great interest with regard to earthquake predictability. We show that the most conspicuous properties of earthquake clustering can be explained without assuming any differences in the initiation processes. In particular, the Gutenberg‐Richter law as well as the Omori law for foreshock and aftershock sequences can be reproduced by model simulations with the simple assumption that all subsequent events are initiated in the same manner at the edges of the recently ruptured area. In this way, the empirically observed b and p values are reproduced naturally without any parameter tuning as well as their differences with regard to foreshock and aftershock activity. These properties are shown to result from the shrinking of the loaded fault region with time. In the model, foreshocks occur in extended and almost compact fault segments, whereas aftershocks are mostly restricted to recesses left unruptured by the main shock. Our investigations lead to the conclusion that the spatial effects rather than the temporal effects of the initiation mechanism are decisive for earthquake clustering.
Analysis of continental midcrustal strain localization induced by microfracturing and reaction‐softening
Authors: Gueydan Frédéric , Leroy Yves M. , Jolivet Laurent , Agard Philippe
Published: May 2003

Low‐angle extensional shear zones, which often characterize the brittle–ductile transition of the continental crust, are seen here to result from strain localization. The potentially destabilizing deformation mechanism is assumed to be the progressive transformation of fractured coarse feldspar grains into white mica as observed in the East Tenda Shear Zone, Alpine Corsica. The coupling between microfracturing and feldspar‐to‐mica reaction is coeval with strain localization that occurred in that field case at a depth close to 15 km. This reaction is proposed as the main destabilizing factor responsible for the onset of localization, with feldspar having a stationary dislocation creep flow stress larger than mica. To test this hypothesis, a rheological model is constructed based on the field observations for a mixture of three phases—mica, quartz and feldspar—deforming at a common strain rate. The phase concentrations change with time according to the feldspar‐to‐mica reaction, which takes place only if feldspar grains are fractured, a condition detected with the Mohr–Coulomb criterion. The tendency for the strain to localize is assessed by numerical means for the structure composed of an upper crust gliding rigidly over the lower crust, which sustains an overall simple shear. The onset of strain localization is defined by an increase of at least two orders of magnitude in strain rate over part of the lower crust. The upper crust gliding velocity has to be increased by at least a factor of 5 for localization to occur. The time lapse for this velocity change determines the depth of the shear zone (15–17 km). The kinetics of the metamorphic reaction and the final amount of white mica control its width (1–4 km). The time of the shear zone formation is less than half a million years.
Strain distribution and fabric development modeled in active and ancient transpressive zones
Authors: Schulmann Karel , Thompson Alan Bruce , Lexa Ondrej , Ježek Josef
Published: May 2003

A model based on kinematics of transpression allows the measurable internal parameters (simultaneous strain, fabric, and vertical elevation) to be simulated in relation to the macroscopically determined external parameters of transpressive zones (i.e., zone width, velocity, angle of convergence, and zone depth). We present a diagram of convergence angle against time for homogeneous transpression, where isolines of strain intensity D, strain symmetry K, and vertical elevation of rock samples are superposed. However, the distribution of internal strain parameters is sensitive to three types of strain partitioning: (1) Discrete partitioning results in general decrease of finite strain accumulations and in increase of pure shear component, (2) ductile partitioning splits the transpressional domain into a pure shear zone where strain accumulations decreases and in a wrench‐dominated zone where strain accumulations increases, and (3) viscosity partitioning is marked by different strain rates in zones of different viscosity and therefore by different strain parameters.
Azimuthal variation in seismic anisotropy of the southern California uppermost mantle
Authors: Davis P. M.
Published: May 2003

The Backus [1965] theory of seismic anisotropy is used to obtain expressions for the velocities and polarizations of quasi‐P and S waves for rays making small angles to the symmetry axes of an orthorhombic elastic tensor. Expressions are found for P wave polarization and S wave splitting parameters as a function of incidence angle θ and azimuth z. For small θ, splitting and P wave velocity exhibit predominantly a two z variation. P wave velocity squared is given by vp2 = v02 + c1θ2 + c2cos(2z)θ2, where the ci are combinations of coefficients of the elasticity tensor. The eigenvalues corresponding to the fast and slow S waves are given by similar expressions. Variation in P wave horizontal polarization exhibits a 2z variation; δzp = −Δz sin(2z) with little dependence on θ. SKKS and SKS splitting parameters exhibit an azimuthal variation given by ϕ = ϕ0 + d1 sin(2z) θ2 and δt = δt0 + e1θ2 + e2 cos(2z)θ2, where ϕ is the fast direction and δt is the time delay. We report 2z variation in splitting parameters at the Global Seismic Network stations in southern California. We use other estimates of seismic anisotropy to invert for depth‐averaged values of the elasticity tensor. The resulting tensor adequately describes azimuthal variation in P wave horizontal polarization [Schulte‐Pelkum et al., 2001], Pn, Rayleigh and Love wave velocities, as well as SKKS and SKS splitting, suggesting they all arise from a common source of anisotropy in the upper 200 km of the mantle.
Reservoir depletion at The Geysers geothermal area, California, shown by four‐dimensional seismic tomography
Authors: Gunasekera Rashmin C. , Foulger G. R. , Julian B. R.
Published: May 2003

Intensive geothermal exploitation at The Geysers geothermal area, California, induces myriads of small‐magnitude earthquakes that are monitored by a dense, permanent, local seismometer network. Using this network, tomographic inversions were performed for the three‐dimensional Vp and Vp/Vs structure of the reservoir for April 1991, February 1993, December 1994, October 1996, and August 1998. The extensive low‐Vp/Vs anomaly that occupies the reservoir grew in strength from a maximum of 9% to a maximum of 13.4% during the 7‐year study period. This is attributed to depletion of pore liquid water in the reservoir and replacement with steam. This decreases Vp by increasing compressibility, and increases Vs because of reduction in pore pressure and the drying of argillaceous minerals, e.g., illite, which increase the shear modulus. These effects serendipitously combine to lower Vp/Vs, resulting in a strong overall effect that provides a convenient tool for monitoring reservoir depletion. Variations in the Vp and Vs fields indicate that water depletion is the dominant process in the central part of the exploited reservoir, and pressure reduction and mineral drying in the northwest and southeast parts of the reservoir. The rate at which the Vp/Vs anomaly grew in strength in the period 1991–1998 suggests most of the original anomaly was caused by exploitation. Continuous monitoring of Vp, Vs, and Vp/Vs is an effective geothermal reservoir depletion monitoring tool and can potentially provide information about depletion in parts of the reservoir that have not been drilled.
Assessing pyroclastic fall hazard through field data and numerical simulations: Example from Vesuvius
Authors: Cioni Raffaello , Longo Antonella , Macedonio Giovanni , Santacroce Roberto , Sbrana Alessandro , Sulpizio Roberto , Andronico Daniele
Published: May 2003

A general methodology of pyroclastic fall hazard assessment is proposed on the basis of integrated results of field studies and numerical simulations. These approaches result in two different methods of assessing hazard: (1) the “field frequency,” based on the thickness and distribution of past deposits and (2) the “simulated probability,” based on the numerical modeling of tephra transport and fallout. The proposed methodology mostly applies to volcanoes that, by showing a clear correlation between the repose time and the magnitude of the following eruptions, allows the definition of a reference “maximum expected event” (MEE). The application to Vesuvius is shown in detail. Using the field frequency method, stratigraphic data of 24 explosive events in the 3–6 volcanic explosivity index range in the last 18,000 years of activity are extrapolated to a regular grid in order to obtain the frequency of exceedance in the past of a certain threshold value of mass loading (100, 200, 300, and 400 kg/m2). Using the simulated probability method, the mass loading related to the MEE is calculated based on the expected erupted mass (5 × 1011 kg), the wind velocity profiles recorded during 14 years, and various column heights and grain‐size populations. The role of these factors was parametrically studied performing ∼160,000 simulations, and the probability that mass loading exceeded the chosen threshold at each node was evaluated. As a general rule, the field frequency method results are more reliable in proximal regions, provided that an accurate database of field measurements is available. On the other hand, the simulated probability method better describes events in middle distal areas, provided that the MEE magnitude can be reliably assumed. In the Vesuvius case, the integration of the two methods results in a new fallout hazard map, here presented for a mass loading value of 200 kg/m2.
New paleomagnetic and geochronologic results from Ethiopian Afar: Block rotations linked to rift overlap and propagation and determination of a ∼2 Ma reference pole for stable Africa
Authors: Kidane Tesfaye , Courtillot Vincent , Manighetti Isabelle , Audin Laurence , Lahitte Pierre , Quidelleur Xavier , Gillot Pierre‐Yves , Gallet Yves , Carlut Julie , Haile Tigistu
Published: May 2003

Joint French–Ethiopian field trips in 1995–1996 yield new geochronologic and paleomagnetic data, which significantly expand our knowledge of the recent magmatic and tectonic history of the Afar depression. Twenty‐four new K‐Ar ages range from 0.6 to 3.3 Ma. There is quite good agreement between magnetic polarities and Geomagnetic Polarity Timescale (GPTS). Eight age determinations with uncertainty less than 50 kyr can be used in future reassessments of the GPTS (upper and lower Olduvai/Matuyama reversals and Reunion and Mammoth subchrons). Paleomagnetic analysis of 865 cores from 133 sites confirms that low‐Ti magnetites are the main carrier of the Characteristic Remanent Magnetization (ChRM). A positive tilt test (based on two subgroups with 63 and 23 sites, respectively) confirms that this ChRM is likely the primary magnetization. The main paleomagnetic results can be summarized as follows. A ∼2 Ma reference pole for stable Africa is determined based on 26 sites located on either side of the northern termination of the East African rift. It is located at λ = 87.2°N, ϕ = 217.1°E (A95 = 4°). A 4.6 ± 1.8° (2σ) inclination shallowing is identified within a population of 231 stratoid lava flows, consistent with a global axial quadrupole of 6 ± 2% of the axial dipole. Combined with earlier data of Acton et al. [2000], our new data allow mean paleomagnetic field directions to be determined for five individual, fault‐bounded blocks previously identified by tectonic analysis within central Afar. These all have suffered negligible rotations about vertical axes since emplacement of the lava. This contrasts with the significant rotations previously uncovered to the east in Djiboutian Afar for three major individual blocks. Taken altogether, the declination differences with respect to reference directions are 2 ± 4° for central Afar and 13 ± 4° for eastern Afar, consistent with the model of Manighetti et al. [2001a]. It appears that in the last ∼3 Ma the Afar depression was extensively floored by trap‐like basalts, which were deformed by a single but complex physical (tectonic) process, combining diffuse extension, rift localization, propagation, jumps and overlap, and bookshelf faulting.
Coupled magma chamber inflation and sector collapse slip observed with synthetic aperture radar interferometry on Mt. Etna volcano
Authors: Lundgren P. , Berardino P. , Coltelli M. , Fornaro G. , Lanari R. , Puglisi G. , Sansosti E. , Tesauro M.
Published: May 2003

Volcanoes deform dynamically due to changes in both their magmatic system and instability of their edifice. Mt. Etna features vigorous and almost continuous eruptive activity from its summit craters and periodic flank eruptions. Even though its shape is that of a large stratovolcano, its structure features two rift systems and a flank collapse structure similar to Hawaiian shield volcanoes. We analyze European remote sensing (ERS) satellite differential interferometric synthetic aperture radar (InSAR) data (1993–1996) for Mt. Etna spanning its quiescence from 1993 through the initiation of renewed eruptive activity in late 1995. We use synthetic aperture radar (SAR) data from both ascending and descending ERS satellite tracks. Comparison of independent interferograms covering the first 2 years of the inflationary period shows a pattern consistent with inflation of the volcano. Calculation of the tropospheric path delay based on meteorological data does not change this interpretation. Interferograms from late summer 1995–1996 show no significant deformation. Joint inversion of interferograms from ascending and descending satellite tracks require both inflation from a spheroidal magmatic source located beneath the summit at 5 km below sea level, and displacement of the east flank of Etna along a basal decollement. Both sources of deformation were contemporaneous within the resolution of our data and suggest that inflation of the central magma chamber acted to trigger slip of Etna's eastern flank. These results demonstrate that flank instability and recharge of a volcano's magma system must both be considered toward understanding how volcanoes work and in their hazard evaluation.
A strong correlation between induced peak dynamic Coulomb stress change from the 1992 M7.3 Landers, California, earthquake and the hypocenter of the 1999 M7.1 Hector Mine, California, earthquake
Authors: Kilb Debi
Published: May 2003

The 1992 M7.3 Landers earthquake may have played a role in triggering the 1999 M7.1 Hector Mine earthquake as suggested by their close spatial (∼20 km) proximity. Current investigations of triggering by static stress changes produce differing conclusions when small variations in parameter values are employed. Here I test the hypothesis that large‐amplitude dynamic stress changes, induced by the Landers rupture, acted to promote the Hector Mine earthquake. I use a flat layer reflectivity method to model the Landers earthquake displacement seismograms. By requiring agreement between the model seismograms and data, I can constrain the Landers main shock parameters and velocity model. A similar reflectivity method is used to compute the evolution of stress changes. I find a strong positive correlation between the Hector Mine hypocenter and regions of large (>4 MPa) dynamic Coulomb stress changes (peak Δσf(t)) induced by the Landers main shock. A positive correlation is also found with large dynamic normal and shear stress changes. Uncertainties in peak Δσf(t) (1.3 MPa) are only 28% of the median value (4.6 MPa) determined from an extensive set (160) of model parameters. Therefore the correlation with dynamic stresses is robust to a range of Hector Mine main shock parameters, as well as to variations in the friction and Skempton's coefficients used in the calculations. These results imply dynamic stress changes may be an important part of earthquake trigging, such that large‐amplitude stress changes alter the properties of an existing fault in a way that promotes fault failure.
Hydroxide in olivine: A quantitative determination of the absolute amount and calibration of the IR spectrum
Authors: Bell David R. , Rossman George R. , Maldener Joachim , Endisch Denis , Rauch Friedel
Published: May 2003

Olivine is an important host of hydrogen in the Earth's upper mantle, and the OH abundance in this mineral determines many important physical properties of the planet's interior. To date, natural and experimentally hydrated olivines have been analyzed by uncalibrated spectroscopic methods with large (±100%) uncertainties in accuracy. We determined the hydrogen contents of three natural olivines by 15N nuclear reaction analysis and used the results to calibrate the common infrared (IR) spectroscopic method for quantitative hydrogen analysis of olivine. OH content (expressed as parts per million H2O by weight) is 0.188 times the total integrated absorbance of the fundamental OH stretching bands in the 3750–3100 cm−1 region. The results indicate that an upward revision of some previous determinations by factors of between 2 and 4 is necessary. The most hydrous naturally occurring mantle‐derived olivine analyzed to date contains 240 ppm wt. H2O. Retrospective application of this calibration to experimentally hydrated olivines may be limited by spectral differences in some cases and by the previous use of nonpolarized IR spectra.
Evidence for a Battle Mountain‐Eureka crustal fault zone, north‐central Nevada, and its relation to Neoproterozoic‐Early Paleozoic continental breakup
Authors: Grauch V. J. S. , Rodriguez Brian D. , Bankey Viki , Wooden Joseph L.
Published: May 2003

Combined evidence from gravity, radiogenic isotope, and magnetotelluric (MT) data indicates a crustal fault zone that coincides with the northwest‐trending Battle Mountain‐Eureka (BME) mineral trend in north‐central Nevada, USA. The BME crustal fault zone likely originated during Neoproterozoic‐Early Paleozoic rifting of the continent and had a large influence on subsequent tectonic events, such as emplacement of allochthons and episodic deformation, magmatism, and mineralization throughout the Phanerozoic. MT models show the fault zone is about 10 km wide, 130‐km long, and extends from 1 to 5 km below the surface to deep crustal levels. Isotope data and gravity models imply the fault zone separates crust of fundamentally different character. Geophysical evidence for such a long‐lived structure, likely inherited from continental breakup, defies conventional wisdom that structures this old have been destroyed by Cenozoic extensional processes. Moreover, the coincidence with the alignment of mineral deposits supports the assertion by many economic geologists that these alignments are indicators of buried regional structures.
Comparison of geodetic and geologic data from the Wasatch region, Utah, and implications for the spectral character of Earth deformation at periods of 10 to 10 million years
Authors: Friedrich Anke M. , Wernicke Brian P. , Niemi Nathan A. , Bennett Richard A. , Davis James L.
Published: May 2003

The Wasatch fault and adjacent fault zones provide an opportunity to compare present‐day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows, ranging from the last millennium up to 10 Myr. The three easternmost GPS sites of the Basin and Range Geodetic Network (BARGEN) at this latitude define a ∼130‐km‐wide region spanning three major normal faults extending east‐west at a total rate of 2.7 ± 0.4 mm/yr, with an average regional strain rate estimated to be 21 ± 4 nstrain/yr, about twice the Basin and Range average. On the Wasatch fault, the vertical component of the geologic displacement rate is 1.7 ± 0.5 mm/yr since 6 ka, <0.6 mm/yr since 130 ka, and 0.5–0.7 mm/yr since 10 Ma. However, it appears likely that at the longest timescale, rates slowed over time, from 1.0 to 1.4 mm/yr between 10 and 6 Ma to 0.2 to 0.3 mm/yr since 6 Ma. The cumulative vertical displacement record across all three faults also shows time‐variable strain release ranging from 2 to 4 mm/yr since 10 ka to <1 mm/yr averaged over the past 130 kyr. Conventional earthquake recurrence models (“Reid‐type” behavior) would require an accordingly large variation in strain accumulation or loading rate on a 10‐kyr timescale, for which there appears to be no obvious geophysical explanation. Alternatively, seismic strain release, given a wide range of plausible constitutive behaviors for frictional sliding, may be clustered on the 10‐kyr timescale, resulting in the high Holocene rates, with comparatively low, uniform strain accumulation rates on the 100‐kyr timescale (“Wallace‐type” behavior). The latter alternative, combined with observations at the million‐year timescale and the likelihood of a significant contribution of postseismic transients, implies maxima of spectral amplitude in the velocity field at periods of ∼10 Myr (variations in tectonic loading), ∼10 kyr (clustered strain release), and of 100 years (postseismic transients). If so, measurements of strain accumulation and strain release may be strongly timescale‐dependent for any given fault system.
A model for enhanced fluid percolation in porous media by application of low‐frequency elastic waves
Authors: Iassonov Pavel P. , Beresnev Igor A.
Published: May 2003

Numerous observations and laboratory experiments suggest that elastic vibrations can significantly enhance transport of nonaqueous phase liquids (NAPLs) in porous media. Our analyses suggest that in the low‐frequency range, capillary forces and nonlinear rheology of the fluid may be predominant mechanisms of vibratory stimulation. Consequently, a model of these mechanisms is built to explain the effect of sonic waves on fluid percolation. The model shows that the low‐frequency elastic waves of relatively low intensity can significantly enhance the flow rate of a yield stress fluid under small external pressure gradients and aid in the mobilization of entrapped NAPL blobs by reducing the value of the threshold gradient needed to displace the fluid. We estimate the intensity of a sonic field to be used in the possible field implementation of this method to be in the range of 0.2–125 W/m2.
Analysis of similar event clusters in aftershocks of the 1994 Northridge, California, earthquake
Authors: Shearer Peter M. , Hardebeck Jeanne L. , Astiz Luciana , Richards‐Dinger Keith B.
Published: May 2003

We perform waveform cross‐correlation on over 14,000 aftershocks of the 1994 Northridge MW 6.7 earthquake in southern California as recorded by short‐period stations of the Southern California Seismic Network (SCSN). Approximately 10–30% of the events belong to similar event clusters, depending upon the similarity criteria that are applied. We relocate events within 218 of these clusters to a relative location accuracy of about 30 m using the differential times obtained from the cross‐correlation. These relocated event clusters often show planar features suggestive of faults at depth, and we apply principal parameter analysis to characterize the shape of each cluster and to compute best fitting planes. In several cases, these planes are parallel to the main shock fault plane; however, more generally, the seismicity planes exhibit a wide range of orientations, suggesting complexity in the aftershock faulting. Composite focal mechanisms can be obtained for each cluster by combining the P polarity data from individual events. A comparison of polarity measurement differences within similar event clusters provides constraints on the error rate in the individual focal mechanisms. For some clusters, we are able to resolve the primary versus auxiliary fault plane ambiguity by comparing the computed focal mechanisms with the best fitting seismicity planes. Individual event focal mechanisms are in general agreement with the composite focal mechanisms for the similar event clusters. Events occurring along the main shock rupture plane are mainly thrust, whereas events in the hanging wall are predominately strike‐slip.
Spatiospectral localization of isostatic coherence anisotropy in Australia and its relation to seismic anisotropy: Implications for lithospheric deformation
Authors: Simons Frederik J. , van der Hilst Rob D. , Zuber Maria T.
Published: May 2003

We investigate the two‐dimensional (2‐D) nature of the coherence between Bouguer gravity anomalies and topography on the Australian continent. The coherence function or isostatic response is commonly assumed to be isotropic. However, the fossilized strain field recorded by gravity anomalies and their relation to topography is manifest in a degree of isostatic compensation or coherence which does depend on direction. We have developed a method that enables a robust and unbiased estimation of spatially, directionally, and wavelength‐dependent coherence functions between two 2‐D fields in a computationally efficient way. Our new multispectrogram method uses orthonormalized Hermite functions as data tapers, which are optimal for spectral localization of nonstationary, spatially dependent processes, and do not require solving an eigenvalue problem. We discuss the properties and advantages of this method with respect to other techniques. We identify regions on the continent marked by preferential directions of isostatic compensation in two wavelength regimes. With few exceptions, the short‐wavelength coherence anisotropy is nearly perpendicular to the major trends of the suture zones between stable continental domains, supporting the geological observation that such zones are mechanically weak. Mechanical anisotropy reflects lithospheric strain accumulation, and its presence must be related to the deformational processes affecting the lithosphere integrated over time. Three‐dimensional models of seismic anisotropy obtained from surface wave inversions provide an independent estimate of the lithospheric fossil strain field, and simple models have been proposed to relate seismic anisotropy to continental deformation. We compare our measurements of mechanical anisotropy with our own model of the azimuthally anisotropic seismic wave speed structure of the Australian lithosphere. The correlation of isostatic anisotropy with directions of fast wave propagation gleaned from the azimuthal anisotropy of surface waves decays with depth. This may support claims that above ∼200 km, internally coherent deformation of the entire lithosphere is responsible for the anisotropy present in surface wave speeds or split shear waves.
Tsunamis generated by subaerial mass flows
Authors: Walder Joseph S. , Watts Philip , Sorensen Oscar E. , Janssen Kenneth
Published: May 2003

Tsunamis generated in lakes and reservoirs by subaerial mass flows pose distinctive problems for hazards assessment because the domain of interest is commonly the “near field,” beyond the zone of complex splashing but close enough to the source that wave propagation effects are not predominant. Scaling analysis of the equations governing water wave propagation shows that near‐field wave amplitude and wavelength should depend on certain measures of mass flow dynamics and volume. The scaling analysis motivates a successful collapse (in dimensionless space) of data from two distinct sets of experiments with solid block “wave makers.” To first order, wave amplitude/water depth is a simple function of the ratio of dimensionless wave maker travel time to dimensionless wave maker volume per unit width. Wave amplitude data from previous laboratory investigations with both rigid and deformable wave makers follow the same trend in dimensionless parameter space as our own data. The characteristic wavelength/water depth for all our experiments is simply proportional to dimensionless wave maker travel time, which is itself given approximately by a simple function of wave maker length/water depth. Wave maker shape and rigidity do not otherwise influence wave features. Application of the amplitude scaling relation to several historical events yields “predicted” near‐field wave amplitudes in reasonable agreement with measurements and observations. Together, the scaling relations for near‐field amplitude, wavelength, and submerged travel time provide key inputs necessary for computational wave propagation and hazards assessment.
Consequences of viscous drag beneath a transform fault
Authors: Savage J. C. , Lachenbruch A. H.
Published: May 2003

A transform fault is modeled as a vertical cut through an elastic layer (schizosphere) of thickness H overlying a viscous substrate (plastosphere). We consider a steady transform motion accommodated in the schizosphere wholly by slip on the fault and in the plastosphere, insofar as possible, by viscous flow. For the case where the viscosity in the plastosphere is strain rate dependent but independent of temperature, the velocity solution in the plastosphere is Vθ/π, where V is the slip rate on the fault in the schizosphere and r and θ are the cylindrical coordinates with the origin at the bottom of the fault. The viscous stress is singular at the bottom of the fault (r = 0) and exceeds the brittle (frictional) strength for r < r0, where r = r0(θ) defines the brittle–ductile boundary in the plastosphere. Then the transform motion Vθ/π in the plastosphere is accommodated by pervasive Coulomb failure (granular flow) for r < r0 and by viscous flow for r > r0. Equating the brittle strength to the viscous stress defines the brittle–ductile boundary r0 in the plastosphere as a function of H and viscosity. The additional condition that r0 must be small allows the viscosity to be estimated from H. For small r, the temperature‐independent solution is a valid approximation to the temperature‐dependent solution, and the relation between viscosity and H should remain valid. From the temperature‐independent model, we estimate that self‐heating due to dissipation in the plastosphere for reasonable Earth parameters is less than ∼20°C.
The evolution of regional seismicity between large earthquakes
Authors: King Geoffrey C. P. , Bowman David D.
Published: May 2003

We describe a simple model that links static stress (Coulomb) modeling to the regional seismicity around a major fault. Unlike conventional Coulomb stress techniques, which calculate stress changes, we model the evolution of the stress field relative to the failure stress. Background seismicity is attributed to inhomogeneities in the stress field which are created by adding a random field that creates local regions above the failure stress. The inhomogeneous field is chosen such that when these patches fail, the resulting earthquake size distribution follows a Gutenburg‐Richter law. Immediately following a large event, the model produces regions of increased seismicity (aftershocks) where the overall stress field has been elevated and regions of reduced seismicity where the stress field has been reduced (stress shadows). The high stress levels in the aftershock regions decrease due to loading following the main event. Combined with the stress shadow from the main event, this results in a broad seismically quiet region of lowered stress around the epicenter. Pre‐event seismicity appears as the original stress shadows finally fill as a result of loading. The increase in seismicity initially occurs several fault lengths away from the main fault and moves inward as the event approaches. As a result of this effect, the seismic moment release in the region around the future epicenter increases as the event approaches. Synthetic catalogues generated by this model are virtually indistinguishable from real earthquake sequences in California and Washington.
Constitutive properties for the shear failure of intact granite in seismogenic environments
Authors: Kato Aitaro , Ohnaka Mitiyasu , Mochizuki Hiromine
Published: May 2003

To investigate how the parameters prescribing the slip‐dependent constitutive law are affected by seismogenic environments, we conducted a series of triaxial fracture experiments using intact Tsukuba‐granite samples, which were deformed at wet, temperature‐pressure conditions that correspond to a crustal depth range down to 15 km at a strain rate of 10−5/s. It is conclusively demonstrated that the critical slip displacement Dc is virtually constant at the conditions of depths shallower than 10 km, and that Dc increases with increasing depth at depths greater than 10 km, because of the effect of temperature in the brittle‐plastic transition regime. The breakdown stress drop for intact rock is found to be roughly constant (100 MPa) in a depth range down to 15 km. The apparent fracture energy increases with increasing depth at depths greater than 10 km. These results consistently lead to the conclusion that the stability of the breakdown process is enhanced at depths greater than 10 km.
Survey of precursors to P′P′: Fine structure of mantle discontinuities
Authors: Xu Fei , Vidale John E. , Earle Paul S.
Published: May 2003

The jumps in seismic velocity and density near 410‐ and 660‐km depths have been postulated to mark phase changes in a locally, chemically homogeneous mantle. Seismic estimates of upper mantle discontinuity thickness and lateral variability have varied, posing difficulties for assessing models with chemical and phase transition layering. Observations of underside high‐frequency (∼1 Hz) P wave reflections from the 410‐ and 660‐km discontinuity that precede the P′P′ phase provide unique constraints on the physical properties of these dynamically critical features. We have collected high‐quality recordings of P′P′ and its precursors from three sources: the California Seismic Network, the Large Aperture Seismic Array in Montana, and the Global Seismic Network. The array and regional network data have great sensitivity to weak P′P′ precursors, whereas the global network recordings have good geographical distribution of bounce points. The only consistently observed reflections originate from depths near 410 km and 660 km. The reflector observed near 520‐km depth with longer period waves, which has been interpreted as the olivine β–γ phase transition, is too small in impedance contrast or too thick to reflect 1‐Hz P waves. Data from all three networks indicate that the 1‐Hz P waves from the “660” are bigger than those from the “410.” Although large variability in P′P′ and its precursors' amplitudes prevents strong conclusions, we show that the P′P′ precursor amplitudes, travel time dispersion, and frequency content are consistent with a sharp (≤2 km) 660 and a more diffuse 410, consistent with mineral physics predictions.