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Journal of geophysical research
American Geophysical Union
Journal of geophysical research

American Geophysical Union

2169-9313

Journal of geophysical research/Journal Journal of geophysical research
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    3D GNSS Velocity Field Sheds Light on the Deformation Mechanisms in Europe: Effects of the Vertical Crustal Motion on the Distribution of Seismicity

    Pina-Valdes, JesusSocquet, AnneBeauval, CelineDoin, Marie-Pierre...
    25页
    查看更多>>摘要：Crustal deformation and seismicity in Europe are still poorly understood. Seismic activity is classically ascribed to crustal strain rates generated by edge-driven tectonic forces. However, crustal deformation is not only due to tectonic loading, but can also be related to isostatic and mantellic processes that induce additional stress and strain on the crust by flexure. The influence that those processes have on seismic activity, as well their interaction, is still controversial. The main limitation to study it is because the deformation and its causal mechanisms are usually analyzed separately in small regions. We present here a 3D secular velocity field that covers Eurasia and its plate boundaries including 4,863 Global Navigation Satellite System stations obtained by combining 10 different datasets. We have developed a method based on spatial filtering to identify outliers and smooth the velocity field, and have computed both strain and uplift rate maps that are interpreted in the light of the different driving processes that contribute to the 3D deformation in Europe. The vertical and horizontal deformation features are compared with seismic rates obtained from the spatial and temporal distribution of the seismicity in Europe. Our results suggest that is not possible to explain the seismicity in Europe with edge-driven horizontal tectonic forces only. In some areas markers of the crustal flexure such as the vertical velocity field and its derivative, resulting from buoyancy-driven processes, may help to interpret earthquake distribution.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Deposit-Derived Block-and-Ash Flows: The Hazard Posed by Perched Temporary Tephra Accumulations on Volcanoes; 2018 Fuego Disaster, Guatemala

    Rosi, MauroPistolesi, MarcoSperanza, FabioRisica, Gilda...
    25页
    查看更多>>摘要：The impact of hazardous pyroclastic density currents (PDCs) increases with runout distance, which is strongly influenced by the mass flux. This article shows that the mass flux of a PDC may derive not only from vent discharge during the eruption, but also from partly hot, temporary stores (accumulations) of aerated pyroclastic material perched high on the volcano. The unforeseen PDC at Fuego volcano (Guatemala) on 3 June 2018 happened c.1.5 hr after the eruption climax. It overran the village of San Miguel Los Lotes causing an estimated 400+ fatalities. Analysis of the facies architecture of the deposit combined with video footage shows that a pulsatory block-and-ash flow flowed down the Las Lajas valley and rapidly waxed, the runout briefly increasing to 12.2 km as it filled and then spilled out of river channels, entered a second valley where it devastated the village and became increasingly erosive, prior to waning. Paleomagnetic analysis shows that the PDC contained only 6% very hot (>590 degrees C) clasts, 39% moderately hot (similar to 200 degrees C-500 degrees C) clasts, and 51% cool (<200 degrees C) clasts. This reveals that the block-and-ash flow mostly derived from collapse of loose and partly hot pyroclastic deposits, stored high on the volcano, gradually accumulated during the last 2-3 years. Progressive collapse of unstable deposits supplied the block-and-ash flow, causing a bulk-up process, waxing flow, channel overspill and unexpected runout. The study demonstrates that deposit-derived pyroclastic currents from perched temporary tephra stores pose a particular hazard that is easy to overlook and requires a new, different approach to hazard assessment and monitoring.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Element Partitioning and Li-O Isotope Fractionation Between Silicate Minerals and Crustal-Derived Carbonatites and Their Implications

    Su, Ben-XunWang, JingCui, Meng-MengWu, Yong...
    21页
    查看更多>>摘要：In order to investigate element partitioning and Li-O isotope fractionation between silicate minerals and carbonatite melts at variable levels from mantle to crust, we document elemental and Li-O isotopic data for major minerals from crust-derived carbonatites at Eppawala, Sri Lanka. Partition coefficients (D) of elements between olivine or clinopyroxene and carbonatite melts are consequently estimated. The estimated D values indicate that Li, Zn, Co, Cr, Mn, and Ni behave compatibly in olivine, while P and Sc are slightly compatible, and V and Al are mildly incompatible. Partition coefficients of elements between clinopyroxene and carbonatite melts are defined here, including highly compatible Li, Sc, Ti, V, Al, and Na, moderately compatible Zn, Co, Cr, and Ga, and incompatible Mn, Ni, P, and Cu. They are systematically higher than literature values obtained from mantle conditions, but their relative compatibilities at different systems are consistent. This indicates that element partitioning between silicates and carbonatite melts is highly temperature- and pressure-dependent and can be used to evaluate geochemical proxies of carbonatite metasomatism, and evolution and mineralization of carbonatite melts. Profile analyses on olivine grains reveal that Fe-loving elements in olivine could well preserve features of crystal growth and modal metasomatic interaction, while Li and O isotope fractionations are strongly controlled by element diffusion and Li isotopes are robust indicators of cryptic metasomatic interaction.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    The Subducting Slab as a Chromatographic Column: Regimes of Sub-Solidus Mass Transport as a Function of Lithospheric Hydration State, With Special Reference to the Fate of Carbonate

    Zhong, XinGalvez, Matthieu E.
    28页
    查看更多>>摘要：We investigate the chemical budget of subduction zones at sub-solidus conditions using a thermodynamic-numerical simulation in which all major rock components are treated as soluble and potentially mobile in aqueous fluids. This new strategy significantly improves the accuracy of predicted fluid-rock equilibrium compositions in open petrological systems. We show that all slabs release volatiles and nonvolatiles to the mantle wedge, contributing to its refertilization. But some mobile constituents, such as alkali and alumina, may be trapped along layer boundaries or traverse without interaction depending on chemical contrasts between adjacent lithologies. The accumulation of igneous alumina and silica in the limestones of the central-eastern Pacific slabs drives their decarbonation and is marked by metasomatic garnet growth. Those slabs are also predicted to lose much of their alkalis before sub-arc depth. Even when they are produced in the altered mafic and ultramafic layers, fluids reach the slab/mantle wedge interface with distinct compositional signatures that are typical of the sedimentary cover. We distinguished supply and transport limited regimes of element subduction by testing the sensitivity of our mass balance to changes in slab hydration state (HS). Transport limited slabs sensitive to HS include notably a hotspot of carbon release to the mantle wedge (e.g., Costa Rica). Finally, we show that the quantitative budgets do depend on the geometry of fluid flows, and on assuming that slabs are mechanically continuous structures, which is questionable. Taken together, these insights will help better constrain the long-term chemical evolution of the shallow planetary interior, and the thermomechanical behavior of the subduction interface.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    The High-Frequency Signature of Slow and Fast Laboratory Earthquakes

    Shreedharan, SrisharanMcLaskey, Gregory C.Riviere, JacquesShokouhi, Parisa...
    18页
    查看更多>>摘要：Tectonic faults fail through a spectrum of slip modes, ranging from slow aseismic creep to rapid slip during earthquakes. Understanding the seismic radiation emitted during these slip modes is key for advancing earthquake science and earthquake hazard assessment. In this work, we use laboratory friction experiments instrumented with ultrasonic sensors to document the seismic radiation properties of slow and fast laboratory earthquakes. Stick-slip experiments were conducted at a constant loading rate of 8 mu m/s and the normal stress was systematically increased from 7 to 15 MPa. We produced a full spectrum of slip modes by modulating the loading stiffness in tandem with the fault zone normal stress. Acoustic emission data were recorded continuously at 5 MHz. We demonstrate that the full continuum of slip modes radiate measurable high-frequency energy between 100 and 500 kHz, including the slowest events that have peak fault slip rates <100 mu m/s. The peak amplitude of the high-frequency time-domain signals scales systematically with fault slip velocity. Stable sliding experiments further support the connection between fault slip rate and high-frequency radiation. Experiments demonstrate that the origin of the high-frequency energy is fundamentally linked to changes in fault slip rate, shear strain, and breaking of contact junctions within the fault gouge. Our results suggest that having measurements close to the fault zone may be key for documenting seismic radiation properties and fully understanding the connection between different slip modes.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Deformed Vein Sets as a Record of Synmetamorphic Volume Change: Quantifying Solution Mass Transfer in Subduction-Type Metasediments of the Del Puerto Canyon Region, Franciscan Belt, USA

    Soejima, S.Wallis, S. R.
    24页
    查看更多>>摘要：The volume change component of deformation is often ignored or assumed to be zero in tectonic studies of metamorphic belts. However, when estimating the original geometries of deformed regions, the volume change is just as important as the other two components of deformation, finite strain, and rotation. Major permanent volume change in metamorphic rocks is accomplished by solution transfer facilitated by H2O fluids and estimates of volume change can be combined with solubilities to provide required time-integrated volumes of fluid flow. Previously applied methods for estimating rock volume change are based on absolute stretch or changes in whole-rock chemical compositions. Estimates based on absolute stretches generally indicates large volume reductions whereas geochemical measurements generally imply limited volume change. These contrasting results exist even when the two methods are applied to the same region. In this study, we develop a largely unexplored method for estimating volume change using the direction and deformation type of deformed mineral veins. The assumptions in this method are few and appropriate uncertainties can be estimated. Application of the new method to the metagreywacke in the Del Puerto Canyon of the Franciscan belt constrains the syn-metamorphic volume change to be greater than 7 vol.%, contrasting with previous proposals for large volume-loss in the same region. The results of previous studies can be modified by taking into account grain rigid body rotation and grain boundary sliding. The final result of our approach yields a volume change of 7-21vol.% and implies large amounts of water-rich fluid must have passed through the rock.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    An Experimental Model of Unconfined Bubbly Lava Flows: Importance of Localized Bubble Distribution

    Namiki, AtsukoBirnbaum, JanineBaur, JasperLev, Einat...
    19页
    查看更多>>摘要：Most lava flows carry bubbles and crystals in suspension. From earlier works, it is known that spherical bubbles increase the effective viscosity while bubbles deformed by rapid flow decrease it. Changes in the spatial distribution of bubbles can lead to variable rheology and flow localization and thus modify the resulting lava flow structure and morphology. To understand the roles of bubble and solid phase crystal distributions, we conducted a series of analog experiments of high bubble fraction suspensions. We poured the analog lava on an inclined slope, observed its shape, calculated the velocity field, and monitored its local thickness. A region of localized rapid flow and low vesicularity, whose thickness is thinner than the surrounding area, develops at the center of the bubbly flows. These features suggest that the locally higher liquid fraction decreases the effective viscosity, increases the fluid density, and accelerates the flow. We also found that a halted particle-bearing bubbly flow can resume flowing. We interpret this to result from the upward vertical separation of bubbles, which generates a liquid-rich layer at the bottom of the flow. In our experiment, bubbles are basically spherical and decrease the flow velocity, while our estimate suggests that bubbles in natural lava flows could increase or decrease flow velocity. Downstream decreases in flow velocity stops the bubble deformation and can cause a sudden increase of effective viscosity. The vertical segregation of the liquid phase at the slowed flow front may be a way to generate a cavernous shelly paho'eho'e.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Pn Anisotropic Tomography of Hainan Island and Surrounding Areas: New Insights Into the Hainan Mantle Plume

    Lu, HongbinLei, JiansheZhao, DapengXu, Yi-Gang...
    21页
    查看更多>>摘要：A high-resolution velocity and anisotropy model of the uppermost mantle under the Hainan Island and surrounding areas is obtained by a tomographic inversion of 8192 Pn arrival-time data hand-picked from high-quality seismograms recorded at a total of 137 seismic stations of our recently deployed HAVESArray portable seismic network and Chinese provincial stations. Our new model shows obvious velocity and anisotropic structural features related to the distribution of Quaternary volcanic rocks on the surface. Extensive low Pn velocity anomalies are detected under the Weizhou Island, northern Hainan Island, southern Leizhou Peninsula, southern South China, and around the Pearl River Mouth Basin (PRMB), whereas high Pn velocity anomalies are imaged under the PRMB and areas of 108 degrees-109 degrees E and between the Baise-Hepu fault and the Wuchuan-Sihui fault. The Pn fast propagation directions exhibit a radiated pattern around the PRMB. Integrated with previous global and regional tomographic models, our present results indicate that the Hainan plume is upwelling around the PRMB in the upper mantle, and then plume-related flow is split into several branches under the volcanoes in the northern Hainan Island, southern Leizhou Peninsula and Weizhou Island. This flow pattern could be associated with the westward deep subduction of the Philippine Sea and Pacific plates and eastward deep subduction of the Indian plate. Our results provide new insights into the formation mechanism of the Hainan-Leizhou and Weizhou volcanoes.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Estimation of Resolution and Covariance of Ambient Seismic Source Distributions: Full Waveform Inversion and Matched Field Processing

    Xu, ZongboMikesell, T. Dylan
    17页
    查看更多>>摘要：Both natural and anthropogenic seismic sources generate so-called ambient seismic waves. One in turn can use ambient seismic waves to estimate these source distributions and study source characteristics, for instance the source mechanism. A commonly used estimation method is called matched field processing (MFP), and the MFP results are inherently smeared by the array geometry. Another approach to estimate ambient seismic sources is to apply full waveform inversion (FWI) to the crosscorrelations of ambient seismic wave recordings. Both methods have pros and cons, but the model resolution and uncertainty in these two methods are important for the interpretation. Unfortunately, this topic has attracted little attention in the past. We propose to estimate both the model resolution matrix and model covariance matrix of the inversion using singular value decomposition. We demonstrate our estimates using two examples, one of which is an actual field array geometry. We quantitatively compare the model resolution of the two methods and discuss the model null space. We demonstrate that FWI has superior resolution with enough independent data and should be used when computational resources permit.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Crustal and Uppermost Mantle Heterogeneities Across the Ailaoshan Red River Shear Zone, SE Tibet: Implications for Cenozoic Magmatic Activity

    Xu, MijianYu, DayongHuang, ZhouchuanTong, Ping...
    14页
    查看更多>>摘要：The Ailaoshan Red River shear zone (ARSZ) was formed in the Mesozoic as a suture zone between the Indochina block and the Yangtze craton. Since the Cenozoic, block extrusion due to the Indo-Asian collision has reactivated the fault zone and caused large-scale shearing. Affected by the Cenozoic orogeny, a large volume of magmatic and metamorphic rocks developed in the ARSZ, forming many orogenic gold deposits. However, the source and the geodynamic process of these magmatic activities are still unclear. To gain a basic understanding of the subsurface magmatic activity, we deployed a dense array of 24 broadband seismic stations across the Daping and Chang'an gold deposits at the southern end of the ARSZ. Receiver function analysis, common conversion point stacking, and a joint inversion of receiver functions and surface wave dispersions are performed to image the detailed structure of the crust and uppermost mantle. Low-velocity zones in the mid-lower crust and thinned lithosphere (similar to 70 km) are imaged under the ARSZ. The observed subsurface structures are verified by 3D numerical modeling with the SEM-FK method. We speculate that the mantle upwelling caused by lithospheric delamination has provided the main source of the mantle component in the magmatic rocks since similar to 35 Ma; afterward, high temperatures produced partial melting in the lower crust, which was emplaced along active shear zones.
      原文链接:
    • NSTL
    • Amer Geophysical Union