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Transient Evolution of the Relative Size Distribution of Earthquakes as a Risk Indicator for Induced Seismicity

Description: 

Seismicity induced by human activity has gathered significant scientific attention in the past two decades, however the physical mechanisms driving induced earthquakes is yet to be fully understood. The injection of fluid in the subsurface in particular has been shown to cause changes in the stress field leading to the triggering of earthquakes. Events in engineered geothermal systems, like the ones recorded in Switzerland (Basel 2006, St-Gallen 2013) and Korea (Pohang 2017), have shown that such injection operations can have dramatic consequences. The hazard associated with these earthquakes needs to be managed to prevent infrastructure damages and protect both the population and economic viability of the project.

The b-value of the Gutenberg-Richter power law has been used as a proxy for the state of stress in the subsurface. We propose to use the temporal evolution of the b-value as a statistical tool to evaluate the changes in the stressing of the injection region. In the case of Basel, the b-value has been observed to drop a few days before the series of events leading to the shut-in and subsequent bleed-off of the geothermal well. We use a numerical model coupling a 3D fluid flow simulator and a stochastic geomechanical model to investigate temporal changes of the b-value and understand the observations made in Basel.

By comparing the observations in Basel and our numerical models, we establish a highly systematic behaviour of the b-value during an injection cycle. We show that the temporal evolution of the b-value is controlled by site specific conditions and pre-existing faults as well as the injection pattern. Our results open up new approaches to assess and mitigate seismic hazard and risk through careful site selection and adequate injection strategy, coupled to real-time monitoring and modelling during reservoir stimulation.

Session: De-risking Deep Geothermal Projects: Geophysical Monitoring and Forecast Modeling Advances

Type: Oral

Date: 4/18/2023

Presentation Time: 08:30 AM (local time)

Presenting Author: Vanille A. Ritz

Student Presenter: Yes

Invited Presentation: Yes

Authors