changes and induced seismicity in tectonically active regions
After 2001, the central and eastern United States has experienced an unprecedented acceleration in seismicity close to hydrocarbon reservoirs. In Europe, the potential for injection-induced seismicity as a result of high-rate (and high-pressure) stimulations has also been recognized many years ago. While much progress has been made in understanding and modeling near-field induced-seismicity, there are still many outstanding issues. I investigate the controls on the occurrence of injection-induced seismicity in geothermal and fluid disposal cases, focusing on regions in California, Oklahoma and geothermal reservoirs in Germany and France. I perform in-depth analysis of geologic setting, operational parameters and stress state with the aim to detect similarities between induced seismicity cases and regions without induced earthquakes. I examine the extent and distance fall-off of induced seismicity associated with injection in geothermal and water disposal reservoirs and compare the observed seismicity decay functions with observations from spatial aftershock decay.
Stress heterogeneity and spectral analysis of microseismic events in Southern California
The seismically active slip along tectonic faults generally results in a decrease in fault stresses which may vary spatially depending on fault geometry and local crustal characteristics. The mechanisms that influence variations in earthquake stress-drops, as well as regional deviations from self-similar source parameter scaling, are insufficiently understood.
I investigate spatial variations in stress-drops of small and large seismic events in Southern California. To this aim, I compute stress drops based on event spectra determined by decomposing the recorded waveforms in the log-frequency domain into source, site, and event contributions.The stress drop values are influenced by distance to the nearest, major, late-quaternary fault, hypocentral depths, and the type of faulting. Spatial variations in stress-drops are sensitive to the interplay between local variations in crustal characteristics and fault properties and can provide important insights into regional seismic hazard.
micro-seismicity in laboratory stick-slip experiments
Fault zone structure varies as function of depths, lithology, and distance along strike. The complexity of strain accumulation along fault zones poses a large challenge for the predictability of earthquakes. To reduce the inherent complexity of the natural faulting process, I study fault formation and evolution in laboratory earthquake-analog experiments. These experiments are novel in that they enable me to monitor fault properties and seismic energy release in form of acoustic emissions at high strains while controlling loading stresses, fault orientations and roughness. Few studies have attempted a quantitative comparison between fault structure and seismicity distributions. In previous studies, I studied aftershock characteristics of stick-slips on smooth and structurally complex faults and extended the link between laboratory stick-slip experiments and natural earthquakes. Our laboratory-created faults exhibit hallmarks similar to crustal faults, including zones of localized slip, transitional damage zones and central gouge layers. I investigated changes in scaling relations of frequency-size distributions of laboratory micro-seismic events which follow a powerlaw similar to the Gutenberg-Richter relation. The powerlaw exponent of these distributions, i.e., the b-value, varies systematically with loading stresses and structural heterogeneity. Our results support a connection between b value variations and stress, but also highlight the influence of fault zone complexity which can mask stress-driven b value variations. Furthermore, I tested the connection between predefined fault roughness and micro-seismic event distributions during laboratory stick-slip experiments. I show-ed that the roughness of saw-cut surfaces can be inferred from the distribution of micro-seismic off-fault activity during laboratory stick-slip experiments. The seismic activity at increasing fault-normal distance can be described by a powerlaw with an exponent that varies systematically for different roughness. The enhancement of off-fault micro-cracking for rougher faults can be explained by the spatial extent of stress perturbations due to increased asperity sizes. (Goebel et al., 2012; Goebel et al. 2013; Goebel et al., 2017)