Stephen A. Miller

High-pressure fluid at hypocentral depths in the L'Aquila region inferred from earthquake focal mechanisms

We apply a new analysis technique using earthquake focal mechanisms to infer the 3-D fluid pressure field at depth in the source region of the A.D. 2009 L9Aquila earthquake/aftershock sequence. The technique, termed focal mechanism tomography, inverts for fluid pressure by examining the fault orientation relative to the regional tectonic stress pattern. We identify three large-scale pockets of high fluid pressure (up to 50 MPa above hydrostatic pressure) at depths of 7–10 km that strongly correlates with an independent data set of well-located foreshocks and aftershocks. The shape of overpressured regions and the evolution of seismicity indicate a plausible scenario that this sequence is being driven in part by the poro-elastic response of trapped reservoirs of high-pressure fluid, presumably CO 2 , and postseismic fluid flow initiated by the main shock.

A three-dimensional fluid-controlled earthquake model: Behavior and implications

We describe the behavior of a three-dimensional, fluid-controlled fault model that couples the dominant mechanical effects of fluid within a cellular fault zone with shear stress accumulation from constant plate motion applied at the downward continuation of the fault. Improvements from a previous model include long-term plate motion loading and porosity creation through dilatant slip, which allow the model to evolve to its steady state dynamic equilibrium. The examined results include slip and slip-deficit accumulation, pore pressure buildup and release, stress states, the emergence of seismic scaling relationships, and frequency-size statistics of model earthquakes. We find that asperities develop naturally within the model, reflecting the disorganization of the evolving stress state in Mohr space. The dynamical interaction of shear stress and effective normal stress perturbs the initial uniform stress state to a complex state that produces transient asperity development along the fault. These “Mohr-space” asperities spontaneously evolve, disintegrate, reemerge, and migrate along the fault plane. The general model behavior is independent of the state of the fluid pressure. In four examined cases, which span the range of possible fault zone overpressures, the equilibrium condition is that which occupies all of the available Mohr space. Maximum slip deficits along the fault depend on the degree of fault weakness, ranging from about 3 m for a weak fault to over 30 m for a strong fault after 4000 years of model evolution. For events that breach the surface the seismic moment scales with the cube of the source dimension Mo ∼L3, which reflects the slipped area times the depth extent of the rupture. This scaling crosses isolines of stress drop. For confined events, Mo ∼L2 along isolines of stress drop, but no general scaling emerges. Clusters emerge between stress drop versus seismic moment and stress drop versus source dimension, with large events converging to average stress drops of about 8 MPa for a weak fault and about 20 MPa for a strong fault.

A forward model for earthquake generation on interacting faults including tectonics, fluids, and stress transfer

We present a forward model of interacting faults for systems of any geometry. The model generalizes that of Miller et al. [1996, 1999] to a fully three-dimensional model where faults of any strike and geometry interact through an elastic matrix using the general solutions of Okada [1992]. The model includes large-scale plate motion loading and increasing pore pressures from a source term, undrained poroelastic effects, large coseismic hydraulic property changes, and porosity creation through dilatant slip. To illustrate the basic behavior and utility of the model, results are presented of the long-term evolution (≈9300 years) for a generic case of a blind, dipping fault and a subvertical strike-slip fault in a transpressional environment. We show the stress state evolution along both faults, seismicity time lines, quasi-static rupture propagation including rake angle changes, local and regional stress buildup and rotations, static and dynamic fault interactions, and ΔCFS (changes in Coulomb Failure Stress) within the fault system. Large compartments of varying overpressure result on both faults from coseismic pore pressure changes and contribute to the complexity of the stress state. For the considered case, we find that the poroelastic effects on the receiver fault are about twice the change in the shear stress, providing a significant contribution to the ΔCFS. Regional stress rotations in response to the model seismicity indicate that further model developments must include dynamic generation of new faults in response to the evolving tectonic regime.

The Role of Fluids in Tectonic and Earthquake Processes

Abstract Fluids play an integral role in the geodynamical system, from consumption through serpentinization at mid-ocean ridges and outer rises, to release through dehydration and decarbonization within subduction zones and beyond. Fluids affect a number of critical elements of the tectonic cycle, including weakening plate boundaries and catalyzing mantle wedge melting for feeding volcanic arcs. This review paper summarizes the vast topic of the hydrogeological cycle of the solid earth, and how fluids affect, and are affected by, tectonic processes. Ultimately these fluids must either remain trapped in the mantle or return to the surface at high pressure via ductile processes or fracture networks. High pressure fluids returning to the surface may get trapped at the base of the brittle crust, where they can contribute to earthquake nucleation and genesis. Evidence suggests that high pressure fluids are active participants in tectonic earthquakes, and the relatively recent discovery of slow slip earthquakes and non-volcanic tremor phenomena all point to trapped, over-pressured fluids as an underlying mechanical cause. Fluids play an integral role in lithospheric geodynamics, which provides for some speculations about fluids and earthquakes in a general sense. One such speculation is that spatial aftershock patterns reflect fluid pathways taken by the release of high pressure fluids triggered by the earthquake mainshock. Some of these patterns are shown, and I introduce the term “Zen Trees” to describe them because of their aesthetic form and their resemblance to Eastern calligraphy. I hypothesize that earthquakes that do not spawn significant aftershock sequences indicate little if any trapped high pressure fluids at depth, while earthquakes producing long-lived aftershock sequences point to large reservoirs of trapped high pressure fluids. Although the viscous mantle is the ultimate geophysical fluid, the focus in this paper is limited to fluids in the lithosphere because this boundary, typically treated as a thermal boundary layer, is controlled by complex dynamical interactions between fracture, deformation, dissolution/precipitation, and fluid flow.

Buoyancy induced mobility in two-phase debris flow

This paper shows that buoyancy enhances mobility in two-phase debris flow with an analysis based on the generalized two-phase debris flow model proposed by Pudasaini [1]. The model (the most generalized two-phase flow model to date) incorporates many essential physical phenomena, including solid-volume-fraction-gradient-enhanced non-Newtonian viscous stress, buoyancy, virtual mass and a generalized drag force. We find a strong coupling between the solid- and the fluid-momentum transfer, where the solid normal stress is reduced by buoyancy, which in turn diminishes the frictional resistance, enhances the pressure gradient, and reduces the drag on the solid component. This leads to higher flow mobility. Numerical results show that the model can adequately describe the dynamics of buoyancy induced mobility in two-phase debris flows, and produces observable geometry of flowing mass in the run-out zone. The results presented here are consistent with the physics of the flow.

Remotely triggered nonvolcanic tremor in Sumbawa, Indonesia

We present, for the first time, evidence for triggered tremor beneath the island of Sumbawa, Indonesia. We show triggered tremor in response to three teleseismic earthquakes: the Mw9.0 2011 Tohoku earthquake and two oceanic strike-slip earthquakes (Mw 8.6 and Mw8.2) offshore of Sumatra in 2012. We constrain an apparent triggering threshold of 1 mm/s ground velocity that corresponds to about 8 kPa dynamic stress. Peak tremor amplitudes of about 180 nm/s are observed, and scale with the ground velocity induced by the remote earthquakes. Triggered tremor responds to 45–65 s period surface waves and predominantly correlates with Rayleigh waves, even though the 2012 oceanic events have stronger Love wave amplitudes. We could not locate the tremor because of minimal station coverage, but data indicate several potential source volumes including the Flores Thrust, the Java subduction zone, or Tambora volcano.

A Real Two-Phase Submarine Debris Flow and Tsunami

The general two-phase debris flow model proposed by Pudasaini [1] is employed to study subaerial and submarine debris flows, and the tsunami generated by the debris impact at lakes and oceans. The model, which includes three fundamentally new and dominant physical aspects such as enhanced viscous stress, virtual mass, and generalized drag (in addition to buoyancy), constitutes the most generalized two-phase flow model to date. The advantage of this two-phase debris flow model over classical single-phase, or quasi-two-phase models, is that the initial mass can be divided into several parts by appropriately considering the solid volume fraction. These parts include a dry (landslide or rock slide), a fluid (water or muddy water; e.g., dams, rivers), and a general debris mixture material as needed in real flow simulations. This innovative formulation provides an opportunity, within a single framework, to simultaneously simulate the sliding debris (or landslide), the water lake or ocean, the debris impact at the lake or ocean, the tsunami generation and propagation, the mixing and separation between the solid and fluid phases, and the sediment transport and deposition process in the bathymetric surface. Applications of this model include (a) sediment transport on hill slopes, river streams, hydraulic channels (e.g., hydropower dams and plants); lakes, fjords, coastal lines, and aquatic ecology; and (b) submarine debris impact and the rupture of fiber optic, submarine cables and pipelines along the ocean floor, and damage to offshore drilling platforms. Numerical simulations reveal that the dynamics of debris impact induced tsunamis in mountain lakes or oceans are fundamentally different than the tsunami generated by pure rock avalanches and landslides. The analysis includes the generation, amplification and propagation of super tsunami waves and run-ups along coastlines, debris slide and deposition at the bottom floor, and debris shock waves. It is observed that the submarine debris speed can be faster than the tsunami speed. This information can be useful for early warning strategies in the coastal regions. These findings substantially increase our understanding of complex multi-phase systems and multi-physics and flows, and allows for the proper modeling of landslide and debris induced tsunami, the dynamics of turbidity currents and sediment transport, and the associated applications to hazard mitigation, geomorphology and sedimentology.

A Full GPU Simulation of Evolving Fracture Networks in a Heterogeneous Poro-Elasto-Plastic Medium with Effective-Stress-Dependent Permeability

The wide range of timescales and underlying physics associated with simulating poro-elasto-plastic media present significant computational challenges. GPU technology is particularly advantageous to overcome these problems because even though the physics are the same, computational times are orders of magnitude faster. Poro-elasticity could be implemented in GPU, however GPU implementation of plastic stresses pose problems because branching is introduced into the program and thus introduces efficiency penalties. In general any element by element evaluation to deal with branching in GPU is very inefficient. In this paper, we describe fracture evolution in a poro-elasto-plastic medium and use a switch-on/switch-off function to avoid branching, allowing efficient computation of plasticity in GPU. We benchmark for the elasto-plastic part by investigating the angles of developed shear bands, and benchmark the non-linear diffusion part of the code using the method of manufactured solutions. Model results are presented for fluid pressure propagation through an elasto-plastic matrix subjected to compression, and another for extension. The results demonstrate how fluid flow is restricted in the compression case because of the load-induced low permeability, while fluid flow is encouraged in the extensional case because of the extension-induced high permeability. Code performance is excellent in GPU, and we are able to runs months of simulation using time steps of a few seconds within a few hours. With this new algorithm, many problems of couple fluid flow and the mechanical response can be efficiently simulated at very high resolution.

On the Role of Thermal Stresses during Hydraulic Stimulation of Geothermal Reservoirs

Massive quantities of fluid are injected into the subsurface during the creation of an engineered geothermal system (EGS) to induce shear fracture for enhanced reservoir permeability. In this numerical thermoelasticity study, we analyze the effect of cold fluid injection on the reservoir and the resulting thermal stress change on potential shear failure in the reservoir. We developed an efficient methodology for the coupled simulation of fluid flow, heat transport, and thermoelastic stress changes in a fractured reservoir. We performed a series of numerical experiments to investigate the effects of fracture and matrix permeability and fracture orientation on thermal stress changes and failure potential. Finally, we analyzed thermal stress propagation in a hypothetical reservoir for the spatial and temporal evolution of possible thermohydraulic induced shear failure. We observe a strong influence of the hydraulic reservoir properties on thermal stress propagation. Further, we find that thermal stress change can lead to induced shear failure on nonoptimally oriented fractures. Our results suggest that thermal stress changes should be taken into account in all models for long-term fluid injections in fractured reservoirs.

HULK Simple and fast generation of structured hexahedral meshes for improved subsurface simulations

Short for Hexahedra from Unique Location in (K)convex Polyhedra HULK is a simple and efficient algorithm to generate hexahedral meshes from generic STL files describing a geological model to be used in simulation tools based on the finite element, finite volume or finite difference methods. Using binary space partitioning of the input geometry and octree refinement on the grid, a successive increase in accuracy of the mesh is achieved. We present the theoretical basis as well as the implementation procedure with three geological models with varying complexity, providing the basis on which the algorithm is evaluated. HULK generates high accuracy discretizations with cell counts suitable for state-of-the-art subsurface simulators and provides a new method for hexahedral mesh generation in geological settings. HighlightsA simple and efficient method for structured hexahedral mesh generation is proposed.Allows direct transfer of structural geological information to the numerical simulator.Binary space partitioning significantly improves octree based mesh generation.