# Physics and Interpretation of Fluid Induced Seismicity

#### Bounds of Magnitude Probability for Seismicity Induced by Stimulations of Reservoirs

Induced large-magnitude events at geothermal and hydrocarbon reservoirs are frequently underrepresented in comparison with the Gutenberg-Richter statistics. This discrepancy is the more significant the shorter is the time elapsed after beginning a stimulation of rocks. This indicates that a rupture of a fluid-induced earthquake is only probable along a surface located mainly inside a stimulated rock volume. We analyze influences of the finiteness of a perturbed volume on the frequency of large-magnitude events. Previously we accepted an approximation that events can occur on rupture surfaces contained completely within the stimulated volume. Here we consider both, rupture surfaces located completely within or only intersecting the stimulated volume, and we derive lower and upper probability bounds of a given-magnitude induced event.

#### Characterisation of the Seismotectonic State of Reservoir Locations Using the Magnitude Distribution of Earthquakes

Identifying the parameters which define the earthquake magnitude and its frequency is a key point to assess and to mitigate possible seismic hazard due to fluid injections in geothermal and hydrocarbon reservoirs. Recently, we found that the injected fluid mass on the one hand, and site-specific seismotectonic parameters on the other hand are controlling factors of the magnitude distribution of fluid-induced events. In order to characterize the seismotectonic state of a reservoir location we introduced the seismogenic index. It can be computed from the induced seismicity observed during injection and from the injected fluid mass. We comparatively analyzed the seismotectonic state of several geothermal as well as non-geothermal reservoir locations. The seismogenic indices of the considered locations are in the range of -10 to 0.5. Although the number of reservoirs under examination is limited, we see a clear separation between geothermal and hydrocarbon reservoirs with respect to the seismotectonic state. We found tendencies in the correlation between the seismogenic index with other injection-, reservoir- and seismicity-related parameters. Such correlations contribute to build a data base which aims to evaluate the seismotectonic conditions of so-far undeveloped reservoirs. We also addressed the question whether natural tectonic seismic activity within an area where a fluid injection is planned can be used for such an evaluation. We reformulated the theoretical framework of the SBRC approach to describe naturally occurring seismicity and introduced the tectonic seismogenic index. The derived algorithms can potentially be used to avoid the occurrence of perceptible (or even hazardous) fluid-induced earthquakes by properly selecting and developing reservoir locations.

#### Inter Event Times of Fluid Induced Earthquakes Suggest their Poisson Nature

We finalized our analysis of the inter event time distribution of fluid-injection-induced earthquakes. We used six catalogs collected at geothermal injection sites at Soultz-sous-ForĂȘts and Basel. We found that the distribution of waiting times during phases of constant seismicity rate coincides with the exponential distribution of the homogeneous Poisson process (HPP). We analyzed the waiting times for the complete event catalogs and found that, as for naturally occurring earthquakes, injection induced earthquakes are distributed according to a non homogeneous Poisson process in time. Moreover, the process of event occurrence as a function of injection volumes is a HPP. These results strongly indicate that fluid-injection-induced earthquakes are directly triggered by the loading resulting from the fluid injection.

We also consider the spatial distance between events and perform a nearest neighbor analysis in the time-space-magnitude domain. Our analysis including a comparison to a synthetic catalog created according to the ETAS model reveals no signs of causal relationships between events. Therefore, coupling effects between events are very weak. The Poisson model seems to be a very good approximation of fluid induced seismicity.