Seismo-Hydraulic Pressure Mapping (SHPM. Different approaches were proposed in the scientific literature to relate induced earthquake occurrence to hydraulic reservoir properties. For example, Xxxxxxx and Xxxxx (1985) and later Xxxxxxx et al. (1997) relate the spatio-temporal evolution of induced seismicity to hydraulic diffusivity and reservoir permeability (Xxxxxxx et al., 1999). Terakawa et al. (2012) and Terakawa (2014) infer hydraulic pressure from focal mechanisms of induced earthquakes (focal mechanism tomography, FMT). By calculating stress changes induced by precursory events, Xxxxxx et al. (2012) estimate the pore pressure increase required to trigger swarm earthquakes occurring in the Vogtland area, Czech Republic. All of these approaches, however, are strongly depending on a priori assumptions regarding in situ stress conditions, namely on the level of hydraulic overpressure required to cause seismicity. To reduce the dependency on assumptions, we have developed a novel approach as part of this deliverable which is based on fracture patches slipping repeatedly during fluid injection. Densely spaced clusters of induced earthquakes are commonly observed during fluid injection experiments (x.x. Xxxxxx et al., 2002; Deichmann et al., 2014). Frequently, the inter-event spacing is much smaller than the source dimensions, indicating repeated slip of the same fracture patch. Based on waveform similarity, Xxxxxx et al. (2008) identified a sequence of repeated slip induced by high pressure fluid injection at the German deep drilling site (Kontinentale Tiefbohrung, KTB). Conceptually, this can be explained by the model of Xxxxxx and Xxxxxx (2003), where repeated activation of the same fracture patch is a consequence of coseismic stress drop being small compared to the total shear stress acting on the fracture. Once activated, the fracture remains close to stress criticality due to a small stress drop.
