INsights into rupture DIrectivity patterns from microseismicity to weak events: implications on induced seismicity by fluid injection processes (INDI)

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Understanding the process of nucleation and rupture growth of natural and induced earthquakes is important for the assessment and mitigation of the corresponding time-dependent hazards to people and infrastructure. Advanced seismic source analysis, including point source models but also finite source inversions, provide information on rupture geometry, rupture size, rupture velocity, slip amplitude and the configuration of main slip patches in an earthquake rupture. In addition, rupture directivity, describing a potential asymmetry in rupture propagation is one of the most relevant aspects to emerge from such inversions. Understanding what controls the dominant rupture direction is key to forecast how the rupture process of large earthquakes may evolve, their final rupture size and thus their consequences. The main target of this project is to assess the physical processes which drive nucleation and growth of seismic sources, stimulated by fluid injection operations, in order to understand what controls the direction the rupture front propagates. In particular, the project aims to understand why uni- or bilateral rupture modes are observed and why earthquake ruptures propagate dominantly along a certain direction, discussing finite source inversion results in relation to the pre-seismic stress conditions. Technically, the research proposal aims to develop efficient tools for extended fault inversion and rupture directivity at different spatial and magnitude scales, aiming to provide detailed and robust information about the seismic source properties of induced seismicity. We will discriminate and investigate cases of induced seismicity related to fracturing and fault reactivation processes at different magnitude scales: i) the largest induced earthquakes related to wastewater disposal activities (e.g Oklahoma earthquakes); ii) acoustic emission (AE) events with magnitude less than zero recorded in a dedicated near field network for hydraulic fracturing experiments that took place in the Äspö Hard Rock Laboratory (Sweden). In summary, this project aims to extend the current state-of-the-art by: 1) Resolve robustly point and finite source parameters (rupture duration, rupture directivity and rupture size) on different scales from moderate/large induced earthquakes to hydraulic fractures. 2) Understand whether human operations and stress perturbation control the rupture directivity and growth, and thus the earthquake magnitude. 3) Bridge the link between induced and natural seismicity: induced seismicity, occurring in environments where triggering factors and stress histories are better known, can be used to learn about rupture nucleation and propagation for tectonic earthquakes.