Study of Algorithms for Fast Computation of Crack Expansion Problem

International audienceA problem of quasi-static growth of an arbitrary shaped-crack along an interface requires many times of iterations not only for finding a spatial distribution of discontinuity but also for determining the crack tip. This is crucial when refining model resolution and also when the phenomena progresses quickly from one step to another. We propose a mathematical reformu-lation of the problem as a nonlinear equation and adopt different numerical methods to solve it efficiently. Compared to a previous work of the authors, the resulting code shows a great improvement of performance. This gain is important for further application of aseismic slip process along the fault interface, in the context of plate convergence as well as the reactivation of fault systems in reservoirs

Using estimated risk to develop stimulation strategies for enhanced geothermal systems

Enhanced geothermal systems (EGS) are an attractive source of low-carbon electricity and heating. Consequently, a number of tests of this technology have been made during the past couple of decades, and various projects are being planned or under development. EGS work by the injection of fluid into deep boreholes to increase permeability and hence allow the circulation and heating of fluid through a geothermal reservoir. Permeability is irreversibly increased by the generation of microseismicity through the shearing of pre-existing fractures or fault segments. One aspect of this technology that can cause public concern and consequently could limit the widespread adoption of EGS within populated areas is the risk of generating earthquakes that are sufficiently large to be felt (or even to cause building damage). Therefore, there is a need to balance stimulation and exploitation of the geothermal reservoir through fluid injection against the pressing requirement to keep the earthquake risk below an acceptable level. Current strategies to balance these potentially conflicting requirements rely on a traffic light system based on the observed magnitudes of the triggered earthquakes and the measured peak ground velocities from these events. In this article we propose an alternative system that uses the actual risk of generating felt (or damaging) earthquake ground motions at a site of interest (e.g. a nearby town) to control the injection rate. This risk is computed by combining characteristics of the observed seismicity of the previous 6 h with a (potentially site-specific) ground motion prediction equation to obtain a real-time seismic hazard curve; this is then convolved with the derivative of a (potentially site-specific) fragility curve. Based on the relation between computed risk and pre-defined acceptable risk thresholds, the injection is increased if the risk is below the amber level, decreased if the risk is between the amber and red levels, or stopped completely if the risk is above the red level. Based on simulations using a recently developed model of induced seismicity in geothermal systems, which is checked here using observations from the Basel EGS, in this article it is shown that the proposed procedure could lead to both acceptable levels of risk and increased permeability

Multiscale heterogeneity of the 2011 Tohoku-oki earthquake by inversion

Earthquake fault heterogeneity is often studied on a set of subfaults in kinematic inversion, while it is sometimes described with spatially localized geometry. Aochi and Ide (EPS, 2011) and Ide and Aochi (submitted to Pageoph and AGU, 2012) apply a concept of multi-scale heterogeneity to simulate the dynamic rupture process of the 2011 Tohoku-oki earthquake, introducing circular patches of different dimension in fault fracture energy distribution. Previously the patches are given by the past moderate earthquakes in this region, and this seems to be consistent with the evolution process of this mega earthquake, although a few patches, in particular, the largest patch, had not been known previously. In this study, we try to identify patches by inversion. As demonstrated in several earthquakes including the 2010 Maule (M8.8) earthquake, it is possible to indentify two asperities of ellipse kinematically or dynamically (e.g. Ruiz and Madariaga, 2011, and so on). In the successful examples, different asperities are rather visible, separated in space. However the Tohoku-oki earthquake has hierarchical structure of heterogeneity. We apply the Genetic Algorithm to inverse the model parameters from the ground motions (K-net and Kik-net from NIED) and the high sampling GPS (GSI). Starting from low frequency ranges (> 50 seconds), we obtain an ellipse corresponding to M9 event located around the hypocenter, coherent with the previous result by Madariaga et al. (pers. comm.). However it is difficult to identify the second smaller with few constraints. This is mainly because the largest covers the entire rupture area and any smaller patch improves the fitting only for the closer stations. Again, this needs to introduce the multi-scale concept in inversion procedure. Instead of finding the largest one at first, we have to start to extract rather smaller moderate patches from the beginning of the record, following the rupture process

Induced seismicity along a fault due to fluid circulation: conception and application

It is believed that the some seismicity is driven by the fluid circulation within fault zone and different rheology models have been proposed principally based on the Darcy's law, fluid flow in porous medium. Although it is very difficult to quantify such feature in natural seismicity (some aftershocks of large earthquakes, or seismicity in subduction), the direct application is the induced seismicity at the geothermal sites where micro-fracturing (seismicity) is necessary to allow fluid circulation between two wells and thus the assessment of such seismicity becomes also important. In this study, we construct a conceptual model for the simulators, taking into account of elastic and plastic porosity change (e.g. Segall and Rice, 1995) and fault width evolution (e.g. Yamashita, 1999), supposing first that the seismicity (fluid flow) expands dominantly along a plane. In fact, for an injection of about a few 10 l/s, pore pressure increases immediately (about 1 min) up to more than 10 MPa. This is much faster than the fluid circulation in general. This requires that the fracturing co-seismic process should play a dominant role for bringing the fluid circulation

Assessing components of ground-motion variability from simulations for the Marmara Sea region (Turkey)

Recent studies have shown that repeatable travel-path terms make a high contribution to the overall variability in earthquake ground motions. Having maps of such terms available for a given recording site would, theoretically, allow removal of this component from the aleatory variability of ground-motion models. The assessment of such travel path terms for a given site, however, relies on having recorded a rich set of earthquakes at that site. Given the relative youth of strong-motion networks the assessment of such terms from observations is currently difficult for most parts of the world. Ground-motion simulations, however, provide an alternative method to assess such terms. In this article many dozens of earthquakes, distributed in a grid, are simulated for the Marmara Sea region (Turkey), which borders the megacity of Istanbul and is an area of high seismic hazard. Ground motions are simulated within a detailed 3D velocity structure model using a finite-difference method at 70 recording sites in the area (200 x 120km). Horizontal peak ground velocities from these simulations are regressed to derive a ground-motion model. Next, residuals from these GMPEs are computed to assess repeatable source, site and path terms and various components of ground-motion variability. These components are similar to those derived from real strong-motion data, thereby lending support to those estimates as well as showing the worth of simulations for this type of exercise