Earthquake Science Center Seminars

We will have three short 10-15 minute talks from ESC and VSC researchers.

Proper descriptions of heterogeneity are essential for understanding and modeling single phase (e.g. contaminant transport, saltwater intrusion) and multiphase (e.g. geologic carbon storage, enhanced oil recovery) transport problems from the sub-core scale to reservoir scale. Application of medical imaging to experimentally quantify these processes has led to significant progress in measurement and understanding of material and fluid transport behavior across laboratory scales. While widely utilized in cancer diagnosis and management, cardiology, and neurology, Positron Emission Tomography (PET) has had relatively limited applications in earth science. PET, and in particular small-bore micro-PET, is ideally suited for 3D time-lapse measurement of radiotracer advection, dispersion, and diffusion in geologic media. This seminar will focus on recent studies utilizing micro-PET to quantify heterogeneous pore water velocity, permeability, and relative permeability in sandstone cores and multimodal imaging strategies for characterizing flow in fractured basalts.

We have produced a large set of broadband (0-10 Hz) synthetic seismograms for Mw 9.0 earthquakes on the Cascadia megathrust by combining synthetic seismograms derived from 3D finite-difference simulations ( 1 Hz) with finite-source, stochastic synthetics ( 1 Hz). We used a compound rupture model consisting of high stress drop M8 sub-events superimposed on large, shallower slip with long slip duration, informed by observations of the M9.0 Tohoku, Japan and M8.8 Maule, Chile earthquakes. Fifty 3D simulations were run, considering a variety of rupture parameters, to determine the range of expected ground motions. For sites not in sedimentary basins, the spectral accelerations of the synthetics are similar to the BC Hydro ground-motion prediction equations for periods of 0.1-6 s, but exceed them at periods greater than 6 s. Response spectra from the synthetics show large basin amplification factors of 2-5 at periods of 1-10 s for sites in the Seattle and Tacoma sedimentary basins.

Injection-induced seismicity is frequently characterized by swarms of low magnitude earthquakes. The identification of these small magnitude earthquakes can greatly aid our understanding of induced seismicity, but traditional earthquake detection approaches struggle to identify these earthquakes with low signal-to-noise ratios. The Repeating Signal Detector (RSD) algorithm was created to increase the detection sensitivity of earthquakes swarms on both the local and regional scale. RSD can quickly identify low magnitude, repeating earthquakes buried within years of seismic records by grouping signals based on spectral and time domain characteristics. The identification of a characteristic repeating waveform, the primary limitation of seismic waveform template matching, is also addressed through the use of RSD. By using RSD and multi-station template matching to improve the completeness of earthquake catalogs in the United States, we are able to better characterize injection-induced earthquake sequences. We find that the likelihood of injection-induced earthquakes is primarily controlled by the proximity of injection to the Precambrian basement, and seismicity induced by hydraulic fracturing is much more common than previously recognized.

Conventional principles of the design and operation of geologic carbon storage (GCS) require injecting CO2 below the caprock fracturing pressure to ensure the integrity of the storage complex. In non-ideal storage reservoirs with relatively low permeability, modest injection rates can lead to pressure buildup and hydraulic fracturing of the reservoir and caprock. While the GCS community has generally viewed hydraulic fractures as a key risk to storage integrity, a carefully-designed stimulation treatment could provide improved injectivity while maintaining overall seal integrity. A vertically-contained hydraulic fracture, either in the reservoir rock or extending a limited height into the caprock, provides an effective means to access reservoir volume far from the injection well. Employing a fully-coupled numerical model of hydraulic fracturing, solid deformation, and fluid flow, we study the likelihood, processes, and consequences of hydraulic fracturing during CO2 injection. A hydraulic fracture's pressure-limiting behavior dictates that the near-well fluid pressure is only slightly higher than the fracturing pressure of the rock and is insensitive to injection rate and mechanical properties of the formation. Although a fracture contained solely within the reservoir rock, with no caprock penetration, would be an ideal scenario, poroelastic principles dictate that sustaining such a fracture would require continuously increasing pressure until the caprock is fractured. We also investigate the propagation behavior and injection pressure responses of a hydraulic fracture propagating in a caprock subjected to heterogeneous in situ stress. The results could have important implications for the use of geomechanically-contained hydraulic fracturing as a tool to manage storage performance.

The simulations in this study were performed on GEOS, a massively-parallel, fracture-centric, code developed at the Lawrence Livermore National Laboratory for computational geosciences applications. We will provide a brief overview of this code at the seminar.

Determining earthquake source parameters is a challenging task, in part due to the biases and artifacts introduced when transforming the data from the time- to the frequency-domain. In addition, separating the effects of source and attenuation is difficult, especially for small earthquakes. In this study, a new approach for determining the source spectral parameters using the signals in the time domain is presented. Theoretical equations relating source spectral parameters to the root-mean-square of the ground motions (displacement, velocity and acceleration) are derived using the omega squared model of Brune. These relations constitute a set of three independent equations with three unknowns: the displacement low frequency spectral plateau, the source corner frequency and the attenuation parameter. The spectral parameters are obtained in the time domain, and the trade-off between the effects of the corner frequency and the attenuation parameter is quantified and addressed. The results of the new method are compared with those of the standard frequency-domain approach. It is concluded that the new scheme is more stable than the frequency-domain method, resulting in considerably reduced intra-event source parameter variability.

We present a Thermo-Hydro-Mechanical (THM) simulation framework for coupled fluid and heat flow with geomechanical deformation in naturally fractured three-dimensional subsurface formations.

The multi-component, multiphase flow and transport processes of both mass and heat are modeled at the macroscopic scale. The mechanical behavior of the natural fractures is modeled as a contact problem between two computational planes. The system of nonlinear conservation equations and associated constitutive relations is solved using finite-volume discretization schemes for the flow and transport of mass and heat, and a Galerkin finite-element method is used for the geomechanics. We employ fully unstructured grids, whereby a Discrete Fracture Model (DFM) is used to represent the natural fractures as lower dimensional objects. The mechanics and the flow (mass and energy) problems share the same conformal DFM-based unstructured grid. The THM modeling capabilities have been integrated into the Automatic Differentiation based General Purpose Research Simulator (AD-GPRS) developed by the reservoir simulation research group (SUPRI-B) at Stanford.

We demonstrate the simulation framework for a cold-water injection problem into a warm three-dimensional heterogeneous fractured carbonate reservoir. The computational domain reflects main geological features of the reservoir and contains three regions: the first region represents the oil and gas reservoir, the second region is a salt dome, and the third region represents the bedrock. The reservoir part contains explicitly defined fractures of various lengths and heights. Primarily, we focus on the simulation of shear deformation of natural fractures and we study the changes of the fracture network transmissivity as a function of the waterflooding operations.

We discuss the open challenges associated with the DFM-based modeling approach and focus on issues related to the contact problem, the particular THM coupling strategy, and the nonlinear solver.

Unusually large, rare, and unexpected earthquakes overwhelm mitigation measures and the societal capacity to respond, leading to a cascade of disastrous effects. In order to assess the potential for such rare events, an approach is required to calibrate how effectively geometrical complexities, such as stepovers and restraining bends along strike-slip faults, impede earthquake propagation. I define the term 'earthquake gate' to describe areas of fault complexity that halt earthquake ruptures conditionally as a result of proximal fault geometry, rupture direction, and prior earthquake history. To test and develop the earthquake gate concept I summarize collaborative efforts to document rupture history and slip rate gradients through major geometric barriers of the Altyn Tagh fault in western China. We compare numerical model results with these geologic observations in order to assess how prior earthquake history and fault geometry control whether an earthquake gate is open or closed to rupture propagation. Along the Aksay restraining double bend of the Altyn Tagh fault, opposing slip rate gradients on two parallel fault strands result from distributed deformation that occurs as earthquake ruptures terminate within the bend. From this pattern of fault activity we calibrate a two-dimensional, multi-cycle numerical rupture simulation with visco-elastic interseismic deformation, from which we predict that less than 10% of events are able to breach this earthquake gate. Paleoseismic results validate model results that (1) earthquake histories of the two strands are largely dissimilar, (2) frequency of rupture events decreases within the bend, and (3) events within the bend are age-correlative to sites outside of the bend on the same fault strand. We conclude that the earthquake gate concept provides a useful framework for synthesizing earthquake geology and numerical rupture modeling to improve estimates of large earthquake rupture probabilities.

The Pacific-North America (PA-NA) plate boundary between Vancouver Island and Alaska is similar to the PA-NA boundary in California in its kinematic history and the rate and azimuth of current relative motion, yet their deformation styles are distinct. The California plate boundary shows a broad zone of parallel strike-slip and thrust faults and folds, whereas the 49 mm/y PA-NA relative plate motion in Canada and Alaska is centered on a single, narrow, continuous ~900-km-long fault, the Queen Charlotte Fault (QCF). Using gravity analysis, we propose that this plate boundary is centered on the ocean-continent boundary (OCB), an unusual location for continental transform faults because plate boundaries typically localize within the continental lithosphere, which is weaker. Because the OCB is a boundary between materials of contrasting elastic properties, once a fault is established there, it will probably remain stable. We propose that deformation progressively shifted to the OCB in the wake of Yakutat terrane's northward motion along the margin. Minor convergence across the plate boundary is probably accommodated by normal faults reactivation on Pacific crust and by an eastward dipping QCF. Underthrusting of Pacific slab under Haida Gwaii occurs at convergence angles >14°-15° and may have been responsible for the emergence of the archipelago. The calculated slab entry dip (5°-8°) suggests that the slab probably does not extend into the asthenosphere. The PA-NA plate boundary at the QCF can serve as a structurally-simple site to investigate the impact of rheology and composition on crustal deformation and the initiation of slab underthrusting.