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Now showing items 1 - 15 of 15

  • Extraction of a target in sea clutter via signal decomposition

    Shao, Zhiyu   He, Jiangheng   Feng, Shunshan  

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  • Large fault slip peaking at trench in the 2011 Tohoku-oki earthquake

    Sun, Tianhaozhe   Wang, Kelin   Fujiwara, Toshiya   Kodaira, Shuichi   He, Jiangheng  

    During the 2011 magnitude 9 Tohoku-oki earthquake, very large slip occurred on the shallowest part of the subduction megathrust. Quantitative information on the shallow slip is of critical importance to distinguishing between different rupture mechanics and understanding the generation of the ensuing devastating tsunami. However, the magnitude and distribution of the shallow slip are essentially unknown due primarily to the lack of near-trench constraints, as demonstrated by a compilation of 45 rupture models derived from a large range of data sets. To quantify the shallow slip, here we model high-resolution bathymetry differences before and after the earthquake across the trench axis. The slip is determined to be about 62m over the most near-trench 40 km of the fault with a gentle increase towards the trench. This slip distribution indicates that dramatic net weakening or strengthening of the shallow fault did not occur during the Tohoku-oki earthquake.
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  • Thermal control of the seismogenic zone of southern central Chile

    Voelker, David   Grevemeyer, Ingo   Stipp, Michael   Wang, Kelin   He, Jiangheng  

    We developed thermal models for the Chile subduction zone along two profiles at 38.2 degrees S and 42 degrees S within the rupture area of the 1960 M = 9.5 Valdivia earthquake and south of the 2010 M = 8.8 Maule earthquake. The age difference of the subducting Nazca Plate has a major impact on the thermal regime, being much younger and hotter in the south. Seafloor heat flow observations confirm this difference but also indicate that in the southern area, heat advection at the outer rise cools the incoming plate. Heat flow values derived from the depth of gas hydrate bottom-simulating reflectors are in general agreement with probe and borehole measurements. The positions where the plate interface reaches temperatures of 100-150 degrees C and 350-450 degrees C differ between the two profiles. If these temperatures control the updip and downdip limits of the interplate seismogenic zone, the seismogenic zone widens and shifts landward to greater depths from south to north. Observed microseismicity, however, seems to fade at temperatures much lower than 350-450 degrees C. This discrepancy can be explained in three alternative ways: (1) deformation in a thick subduction channel controls the seismic/aseismic transition; (2) microseismicity recorded over a limited time period does not represent the rupture depth of large interface earthquakes; or (3) the serpentinized mantle wedge controls the downdip limit.
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  • Grain-size distribution in the mantle wedge of subduction zones

    Wada, Ikuko   Behn, Mark D.   He, Jiangheng  

    Mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones, including mantle flow and fluid migration. To investigate the grain-size distribution in the mantle wedge, we coupled a two-dimensional (2-D) steady state finite element thermal and mantle-flow model with a laboratory-derived grain-size evolution model. In our coupled model, the mantle wedge has a composite olivine rheology that incorporates grain-size-dependent diffusion creep and grain-size-independent dislocation creep. Our results show that all subduction settings lead to a characteristic grain-size distribution, in which grain size increases from 10 to 100 mu m at the most trenchward part of the creeping region to a few centimeters in the subarc mantle. Despite the large variation in grain size, its effect on the mantle rheology and flow is very small, as >90% of the deformation in the flowing part of the creeping region is accommodated by grain-size-independent dislocation creep. The predicted grain-size distribution leads to a downdip increase in permeability by similar to 5 orders of magnitude. This increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches similar to 100 km depth compared with shallower depths, potentially providing an explanation for the relatively uniform subarc slab depth. Seismic attenuation derived from the predicted grain-size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones, without requiring a significant contribution by the presence of melt.
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  • Identifying the rupture plane of the 2001 Nisqually, Washington, earthquake

    Kao, Honn   Wang, Kelin   Chen, Rong-Yuh   Wada, Ikuko   He, Jiangheng   Malone, Stephen D.  

    The 2001 Nisqually earthquake occurred within the subducting Juan de Fuca plate. Previous seismic and geodetic studies could not confidently identify its actual fault plane from the two nodal planes. In this study, we apply the recently developed source-scanning algorithm to local seismic waveforms and show unambiguously that the steeply east-dipping plane is the rupture plane. The rupture began near the bottom of the subducting crust and propagated downward into the subducting uppermost mantle. If intraslab earthquakes are assumed to be due to dehydration embrittlement, the source dimension is unlikely to grow any larger because the warm thermal state of the subducting Juan de Fuca plate limits dehydration to a shallow depth below the slab surface. Numerical modeling of the thermal structure indicates that dehydration embrittlement can only take place in the top 10 km of the subducting mantle, implying that the maximum size of an intraslab earthquake in northern Cascadia would be M(w) similar to 7 or less.
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  • A community benchmark for subduction zone modeling RID B-1220-2008 RID D-3364-2009

    van Keken, Peter E.   Currie, Claire   King, Scott D.   Behn, Mark D.   Cagnioncle, Amandine   He, Jiangheng   Katz, Richard F.   Lin, Shu-Chuan   Parmentier, E. Marc   Spiegelman, Marc   Wang, Kelin  

    We have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones. The benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically. We propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure. A comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers. A minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model. (C) 2008 Elsevier B.V. All rights reserved.
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  • A note on pore fluid pressure ratios in the Coulomb wedge theory

    Wang, Kelin   He, Jiangheng   Hu, Yan  

    The widely used exact Coulomb-wedge stress solution presented by F. A. Dahlen contains an approximation in dealing with pore fluid pressure ratios within the wedge and in the basal fault. We discuss the theoretical and practical ramifications of the approximation and propose simple modifications to make the solution exact. We illustrate that errors caused by the approximation are negligible for taper angles less than about 10 degrees but become significant for larger tapers.
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  • Diagenetic,metamorphic,and hydrogeologic consequences of hydrothermal circulation in subducting crust

    Spinelli, Glenn   Wada, Ikuko   Wang, Kelin   He, Jiangheng   Harris, Robert   Underwood, And Michael  

    The redistribution of heat by fluid circulation in subducting igneous crust generates thermal anomalies that can affect the alteration of material both within a subduction zone and in the incoming plate prior to subduction. This hydrothermal circulation mines heat from subducted crust and transports it seaward, resulting in anomalously high temperatures in material seaward of the trench and anomalously low temperatures in the subduction zone. Anomalously high temperatures on the incoming plate are spatially limited; for example, on the Nankai margin of southern Japan, a zone of high temperatures is within similar to 30 km of the accretionary prism deformation front. The incoming plate (Shikoku Basin) undergoes the high-temperature anomaly for less than 2 million years; so the alteration of clay minerals in Shikoku basin sediments advances only slightly because of the thermal anomaly. In contrast, subducted material is cooled by hydrothermal circulation, and therefore alteration of subducted sediment and igneous rock is shifted farther landward (i.e., delayed); in the Cascadia and Nankai margins, this includes the seismically inferred locations of the basalt-to-eclogite transition in the subducting crust. In very hot margins, hydrothermal circulation cools the subducting slab and affects where, and if, subducting material may melt. In southern Chile, this cooling helps explain the lack of a basaltic melt signature in arc lavas despite the young subducting lithosphere. Finally, the cooling of the subducting slab via hydrothermal circulation shifts fluid sources from dehydration reactions farther landward, delays metamorphic reactions that tend to reduce permeability, and increases fluid viscosity. The responses to hydrothermal circulation in subducting crust are most pronounced in the hottest subduction zones, where the lateral heat exchange in the subducting basement aquifer is greatest.
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  • Induced Seismicity in Western Canada Linked to Tectonic Strain Rate:Implications for Regional Seismic Hazard

    Kao, Honn   Hyndman, Roy   Jiang, Yan   Visser, Ryan   Smith, Brindley   Mahani, Alireza Babaie   Leonard, Lucinda   Ghofrani, Hadi   He, Jiangheng  

    A rapid increase of injection-induced earthquakes (IIE) is often linked to a higher level of seismic hazard. In this study, we compare the geodetically defined moment rate to seismicity distribution in western Canada where significant IIE are observed. The regional seismic pattern is dominated by IIE, both in number and moment, along a 150-km wide NW-SE band of moderate strain rate in the easternmost Cordillera and foothills. The observed rate of moment release from local earthquakes is much closer to the tectonic moment rate in the IIE-dominated areas. We conclude that, on a regional scale, tectonic strain rate is an important control on IIE. Injection in areas with moderate tectonic strain may temporarily increase the local seismic hazard, but widespread IIE over an extended period of time may deplete the available tectonic moment and could, under the right conditions, have a limited long-term effect of reducing regional seismic hazard. Plain Language Summary Fluids are commonly injected into oil and gas wells to shutter the reservoir formations of shale gas and tight oil and to increase production. However, not all injections cause induced earthquakes and some areas have more events than others. In this study, we examine the geological controls on where there is greatest susceptibility to induced earthquakes. In western Canada, we find that they are generally associated with the areas of moderate long-term geological/tectonic deformation rates, as mapped by the relative motion of high-resolution Global Navigation Satellite System stations. Most induced earthquakes are in a 150-km-wide band in the easternmost Canadian Cordillera and foothills. They are much less frequent farther to the east where the geological deformation rates are low. We conclude that the geological deformation rate is an important controlling factor for the occurrence of induced earthquakes. In some areas, the strain rate from induced earthquakes may temporarily exceed the natural geological strain rate. Widespread injections in areas with relatively high geological deformation rate may increase the regional seismic hazard in the short term. But if frequent occurrence of induced earthquakes persists over time, it could lead to a reduction of natural earthquake occurrence in the long term.
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  • Separation of Multicomponent Chirp Signals Using Morphological Component Analysis and Fractional Fourier Transform

    Shao, Zhiyu   He, Jiangheng   Feng, Shunshan  

    A chirp signal is a large-bandwidth signal which is widely used in engineering. In many applications, it is necessary to decompose a mixed chirp signal into its components. However, the traditional Fourier transform method cannot process a mixed chirp signal when its components intersect in the joint time-frequency domain. Combining the advantages of the morphological component analysis (MCA) with multicomponent signal processing and the fractional Fourier transform (FrFT) in chirp signal processing, this letter proposes the MCA-FrFT method to decompose a multicomponent chirp signal. First, the cost function is defined using the FrFT and optimized by the split augmented Lagrangian shrinkage algorithm (SALSA), and then, all the morphological components are obtained. The proposed method is verified by simulations, and simulation results show that the proposed method has good performance in separating the multicomponent chirp signals into components. Besides, the proposed method is evaluated experimentally in the sea target detection, and the experimental results confirm that the proposed method can not only extract the low observable targets from a heavy sea-clutter environment but also separate them from each other.
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  • Multistate storage nonvolatile memory device based on ferroelectricity and resistive switching effects of SrBi2Ta2O9 films

    Song, Zhiwei   Li, Gang   Xiong, Ying   Cheng, Chuanpin   Zhang, Wanli   Tang, Minghua   Li, Zheng   He, Jiangheng  

    A memory device with a Pt/SrBi2Ta2O9(SBT)/Pt(111) structure was shown to have excellent combined ferroelectricity and resistive switching properties, leading to higher multistate storage memory capacity in contrast to ferroelectric memory devices. In this device, SBT polycrystalline thin films with significant (115) orientation were fabricated on Pt(111)/Ti/SiO2/Si(100) substrates using CVD (chemical vapor deposition) method. Measurement results of the electric properties exhibit reproducible and reliable ferroelectricity switching behavior and bipolar resistive switching effects (BRS) without an electroforming process. The ON/OFF ratio of the resistive switching was found to be about 10(3). Switching mechanisms for the low resistance state (LRS) and high resistance state (HRS) currents are likely attributed to the Ohmic and space charge-limited current (SCLC) behavior, respectively. Moreover, the ferroelectricity and resistive switching effects were found to be mutually independent, and the four logic states were obtained by controlling the periodic sweeping voltage. This work holds great promise for nonvolatile multistate memory devices with high capacity and low cost.
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  • Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization

    Wada, Ikuko   Wang, Kelin   He, Jiangheng   Hyndman, Roy D.  

    The shallow part of the interface between the subducting slab and the overriding mantle wedge is evidently weakened by the presence of hydrous minerals and high fluid pressure. We use a two-dimensional finite element model, with a thin layer of uniform viscosity along the slab surface to represent the strength of the interface and a dislocation-creep rheology for the mantle wedge, to investigate the effect of this interface "decoupling.'' Decoupling occurs when the temperature-dependent viscous strength of the mantle wedge is greater than that of the interface layer. We find that the maximum depth of decoupling is the key to most primary thermal and petrological processes in subduction zone forearcs. The forearc mantle wedge above a weakened subduction interface always becomes stagnant (< 0.2% slab velocity), providing a stable thermal environment for the formation of serpentinite. The degree of mantle wedge serpentinization depends on the availability of aqueous fluids from slab dehydration. A very young and warm slab releases most of its bound H(2)O in the forearc, leading to a high degree of mantle wedge serpentinization. A very old and cold slab retains most of its H(2)O until farther landward, leading to a lower degree of serpentinization. Our preferred model for northern Cascadia has a maximum decoupling depth of about 70 - 80 km, which provides a good fit to surface heat flow data, predicts conditions for a high degree of serpentinization of the forearc mantle wedge, and is consistent with the observed shallow intraslab seismicity and low volume of arc volcanism.
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  • Earthquake Ground Motion and 3D Georgia Basin Amplification in Southwest British Columbia: Deep Juan de Fuca Plate Scenario Earthquakes

    Molnar, Sheri   Cassidy, John F.   Olsen, Kim B.   Dosso, Stan E.   He, Jiangheng  

    Finite-difference modeling of 3D long-period (> 2 s) ground motions for large (M-w 6.8) scenario earthquakes is conducted to investigate effects of the Georgia basin structure on ground shaking in Greater Vancouver, British Columbia, Canada. Scenario earthquakes include deep (> 40 km) subducting Juan de Fuca (JdF) plate earthquakes, simulated in locations congruent with known seismicity. Two sets of simulations are performed for a given scenario earthquake using models with and without Georgia basin sediments. The chosen peak motion metric is the geometric mean of the two orthogonal horizontal components of motion. The ratio between predicted peak ground velocity (PGV) for the two simulations is applied here as a quantitative measure of amplification due to 3D basin structure. A total of 10 deep subducting JdF plate earthquakes are simulated within 100 km of Greater Vancouver. Simulations are calibrated using records from the 2001 M-w 6.8 Nisqually earthquake. On average, the predicted level of average PGVat stiff soil sites across Greater Vancouver for anMw 6.8 JdF plate earthquake is 3: 2 cm= s (modified Mercalli intensity IV-V). The average increase in PGV due to basin structure across Greater Vancouver is 3.1. Focusing of northnortheast- propagating surface waves by shallow (< 1 km) basin structure increases ground motion in a localized region of south Greater Vancouver; hence, scenario JdF plate earthquakes located >= 80 km south-southwest of Vancouver are potentially the most hazardous.
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  • Stable Forearc Stressed by a Weak Megathrust:Mechanical and Geodynamic Implications of Stress Changes Caused by the M=3D9 Tohoku-Oki Earthquake

    Wang, Kelin   Brown, Lonn   Hu, Yan   Yoshida, Keisuke   He, Jiangheng   Sun, Tianhaozhe  

    Rupture-zone averaged static stress drop in the 2011 M=3D9 Tohoku-Oki earthquake was less than 5 MPa, but it caused a stress reversal in most of the offshore forearc, although the reversal is less well constrained far offshore by earthquake mechanisms because of 20- to 30-km errors in event depths. Using a finite element model of force balance, we demonstrate that the stress reversal unambiguously indicates (1) a very weak subduction megathrust and (2) very low differential stresses in the forearc. Prior to the reversal, the upper limit of megathrust strength could not be determined from forearc stresses. In the forearc, effects of megathrust friction and gravity are in a fragile balance, and stresses fluctuate around a neutral state in earthquake cycles. If most of the offshore forearc is to be compressive before but extensional after the earthquake, the effective friction coefficient of the megathrust must be similar to 0.032. Under low differential stresses associated with megathrust weakness, the forearc is generally well below yielding. Applying the concepts of dynamic Coulomb wedge, we show that the inner wedge, and by inference farther landward, stays stable throughout earthquake cycles. The outer wedge is stable most of the time but may occasionally enter a critical state during great earthquakes; its geometry suggests that complete stress drop of the underlying shallow megathrust is unlikely to have happened. We reason that the occurrence of earthquakes and active faulting under low stress in the stable forearc is due to heterogeneities in structure, stress, and/or pore fluid pressure.
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  • Thermally stimulated "runaway" downhole flow in a superhydrostatic ocean crustal borehole: Observations, simulations, and inferences regarding crustal permeability

    Davis, Earl E.   LaBonte, Alison   He, Jiangheng   Becker, Keir   Fisher, Andrew  

    An array of boreholes, drilled through a regionally continuous hydrologically confining layer of sediments into extrusive igneous basement rocks of the Juan de Fuca Ridge eastern flank, has been instrumented with CORK hydrologic observatories for long-term monitoring and fluid sampling. Omission of seals between nested casing strings reaching into basement at one site created a low-resistance connection between basement and the overlying water column, and despite the natural superhydrostatic state of basement water at that location, a "runaway" condition of cold seawater downhole flow into the crust was established, which persisted for more than 4 years. The existence of this condition, along with perturbations generated by it and by initial drilling operations observed at a properly sealed hole 2.4 km away, have been used with analytic and finite element model solutions to constrain formation permeability. The minimum threshold permeability allowing stable downhole flow is roughly 4 x 10(-13) m(2). A value of permeability similar to this (3-4 x 10(-13) m(2)) is estimated on the basis of the elapsed time for initial perturbations to propagate between the sites (similar to 2.5 days). The amplitude of the long-term flow perturbation observed at the sealed site (roughly 1.7 kPa) is smaller than that predicted by modeling (5-10 kPa). Models for flow in an anisotropically permeable layer show that this could be the consequence of low vertical permeability (e. g., arising from massive volcanic or sediment interlayering) or high permeability in the direction of the tectonic fabric generated at the ridge axis. Disagreement between the permeabilities estimated here with previous large-scale estimates appropriate for the cross-strike direction (the primary direction between the borehole sites) (10(-10)-10(-9) m(2)) is difficult to reconcile; it is possible that the holes are poorly connected to zones of high permeability that facilitate the large lateral fluid and heat fluxes previously inferred at this young crustal site.
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