From Finite Fault Slip to Seismic Moment Tensor: Simulating Induced Earthquakes in Groningen
J. Ruan (TU Delft - Applied Geophysics and Petrophysics)
R. Ghose – Promotor (TU Delft - Applied Geophysics and Petrophysics)
W.A. Mulder – Promotor (TU Delft - Applied Geophysics and Petrophysics)
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Abstract
This thesis investigates the mechanism of an induced earthquake associated with subsurface reservoir depletion, focusing on numerical simulations that incorporate the real-world reservoir geometry of the Groningen gas field in the Netherlands. It begins with a review of the poroelastic theory and its relevance to stress changes induced by reservoir depletion. The study then examines how fault offset and three-dimensional structural complexities, particularly fault intersections and horst formations, influence stress localisation and fault reactivation. While earlier studies typically consider a single fault in simplified reservoir settings, this work demonstrates that accurate modelling of the full 3-D fault system is critical for capturing realistic rupture behaviour of induced earthquakes.
To quantify these effects, we perform 3-D geomechanical simulations incorporating a faulted reservoir model based on the Groningen field, including two intersecting faults and the resulting horst structure. The study specifically focuses on the 2018 ML Zeerijp earthquake, using numerical simulations to calculate the stress evolution over the reservoir’s production history and the fault slip during the induced earthquake. Synthetic seismic data are generated and benchmarked against field observations, including event magnitude, depletion level at reactivation, waveforms and the inverted focal mechanism.
The results demonstrate that the fault intersection angle influences not only the depletion level required for reactivation, but also the location of the slip initiation and the resulting rupture pattern. In the subsequent simulation of the 2018 Zeerijp earthquake, we observed a rupture pattern consistent with that seen in the sensitivity study for a similar intersection angle. The model also reproduces similar depletion levels, local magnitude, waveform characteristics, and focal mechanisms. These results demonstrate that current poroelastic models, when combined with realistic geological and structural representations, are capable of capturing key features of induced seismicity.
We also investigate the relationship between the inferred hypocentre location and the frequency content of the input waveforms used in inversion. This analysis is based on the simulated rupture of the 2018 Zeerijp earthquake, using both synthetic and field-observed waveforms. We observe that the estimated hypocentre shifts from the centre of the slip patch to the initial slip area when higher frequency components are included. This shift is attributed to the fact that faster slip during the rupture generates higher frequency seismic waves, a behaviour previously observed in large tectonic earthquakes. Our results show that this effect is also detectable in moderate-magnitude induced events, suggesting the potential of frequency-dependent waveform analysis to resolve rupture histories and source dynamics of reservoir-depletion-induced earthquakes.