95 research outputs found

    Elasto-thermo-visco-plastic numerical modelling from a laboratory to geodynamic scale: implications for convergence-driven experiments

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    The development of a subduction zone, whether spontaneous or induced, encompasses a stage of strain localization and is epitomized by the growth of lithospheric-scale shear bands. Our aim in this paper, using a solid-mechanical constitutive description relevant for oceanic lithosphere, is to investigate factors that promote or inhibit localization of deformation in brittle and ductile regimes in convergence-driven numerical experiments. We used the Drucker-Prager yield criterion and a non-associative flow rule, allowing viscoplastic deformation to take directions independent of the preferred direction of yield. We present a step-by-step description of the constitutive law and the consistent algorithmic tangent modulus. The model domain contains an initial weak-zone on which localization can potentially nucleate. In solving the energy conservation problem, we incorporate a heat source term from the mechanical deformations which embodies the irreversible plastic work done. This work term couples the energy equation to the constitutive description, and hence hence the stress balance, via the evolving temperature field. On a sample-scale, we first conduct a series of isothermal benchmark tests. We then explore behavior including shear heating and volumetric work both separately and in concert. and thereby address the (in)significance of the latter, and hence assess their potential importance. We find that dilatational effects mostly enhance both shear band development and shear heating. We also observe that high temperature promotes shear band development whereas high confining pressure inhibits it, and infer that the competition between these factors is likely to be the major influence on the position within the lithosphere where shear bands nucleate

    A Laboratory Earthquakeā€Based Stochastic Seismic Source Generation Algorithm for Strikeā€Slip Faults and its Application to the Southern San Andreas Fault

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    There is a sparse number of credible source models available from largeā€magnitude past earthquakes. A stochastic sourceā€modelā€generation algorithm thus becomes necessary for robust risk quantification using scenario earthquakes. We present an algorithm that combines the physics of fault ruptures as imaged in laboratory earthquakes with stress estimates on the fault constrained by field observations to generate stochastic source models for largeā€magnitude (M_w 6.0ā€“8.0) strikeā€slip earthquakes. The algorithm is validated through a statistical comparison of synthetic groundā€motion histories from a stochastically generated source model for a magnitude 7.90 earthquake and a kinematic finiteā€source inversion of an equivalent magnitude past earthquake on a geometrically similar fault. The synthetic dataset comprises threeā€component groundā€motion waveforms, computed at 636 sites in southern California, for 10 hypothetical rupture scenarios (five hypocenters, each with two rupture directions) on the southern San Andreas fault. A similar validation exercise is conducted for a magnitude 6.0 earthquake, the lower magnitude limit for the algorithm. Additionally, ground motions from the M_w 7.9 earthquake simulations are compared against predictions by the Campbellā€“Bozorgnia Next Generation Attenuation relation, as well as the ShakeOut scenario earthquake. The algorithm is then applied to generate 50 source models for a hypothetical magnitude 7.9 earthquake originating at Parkfield, California, with rupture propagating from north to south (toward Wrightwood), similar to the 1857 Fort Tejon earthquake. Using the spectral element method, threeā€component groundā€motion waveforms are computed in the Los Angeles basin for each scenario earthquake and the sensitivity of groundā€shaking intensity to seismic source parameters (such as the percentage of asperity area relative to the fault area, rupture speed, and rise time) is studied

    On the importance of 3D stress state in 2D earthquake rupture simulations with off-fault deformation

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    During the last decades, many numerical models have been developed to investigate the conditions for seismic and aseismic slip. Those models explore the behavior of frictional faults, embedded in either elastic or inelastic mediums, and submitted to a far field loading (seismic cycle models), or initial stresses (single dynamic rupture models). Those initial conditions impact both on-fault and off-fault dynamics. Because of the sparsity of direct measurements of fault stresses, modelers have to make assumptions about these initial conditions. To this day, Anderson's theory is the only framework that can be used to link fault generation and reactivation to the three-dimensional stress field. In this work we look at the role of the three dimensional stress field in modelling a 2D strike-slip fault under plane-strain conditions. We show that setting up the incorrect initial stress field, based on Anderson's theory, can lead to underestimation of the damage zone width by up to a factor of six, for the studied cases. Moreover, because of the interactions between fault slip and off-fault deformation, initial stress field influences the rupture propagation. Our study emphasizes the need to set up the correct initial 3D stress field, even in 2D numerical simulations

    A Micromechanics Based Constitutive Model for Brittle Failure at High Strain Rates

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    The micromechanical damage mechanics formulated by Ashby and Sammis, 1990, ā€œThe Damage Mechanics of Brittle Solids in Compression,ā€ Pure Appl. Geophys., 133(3), pp. 489ā€“521, and generalized by Deshpande and Evans 2008, ā€œInelastic Deformation and Energy Dissipation in Ceramics: A Mechanism-Based Constitutive Model,ā€ J. Mech. Phys. Solids, 56(10), pp. 3077ā€“3100. has been extended to allow for a more generalized stress state and to incorporate an experimentally motivated new crack growth (damage evolution) law that is valid over a wide range of loading rates. This law is sensitive to both the crack tip stress field and its time derivative. Incorporating this feature produces additional strain-rate sensitivity in the constitutive response. The model is also experimentally verified by predicting the failure strength of Dionysus-Pentelicon marble over strain rates ranging from ~10^(āˆ’6) to 10^3s^(āˆ’1). Model parameters determined from quasi-static experiments were used to predict the failure strength at higher loading rates. Agreement with experimental results was excellent
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