Fault Aperture Laws Reshape Induced Seismicity Forecasts at Belgian Geothermal Site

Induced seismicity remains one of the biggest obstacles facing geothermal reservoir stimulation, and a new modeling study from the VITO geothermal site in Belgium suggests that a seemingly small modeling choice—how faults open and close during fluid injection—can substantially change seismic hazard forecasts. Operations at the site have already induced seismic events with moment magnitudes up to MW 2.0 within the fractured Dinantian limestone reservoir, making reliable seismicity models a practical necessity for operators.

Numerical models are widely used to forecast and reassess seismic hazard during fluid injection, helping to shape operational strategies that reduce the likelihood of larger seismic events. The new study tested how much this simplification matters by comparing three aperture formulations: a constant aperture, an aperture that varies with effective normal stress, and an aperture that depends on both effective normal stress and slip displacement along the fault.

The results show that the formulation matters for what the models predict. The effective normal stress dependent aperture law best reproduced the observed b-value at the VITO site.

However, matching the b-value was not the whole story. The model in which fracture stiffness characteristics depend on both effective normal stress and slip displacement provided a more conservative estimate of the maximum moment magnitude, meaning it was less likely to underestimate the occurrence of larger seismic events.

There was also a practical trade-off in computational expense. The combined stress- and slip-dependent model maintained computational costs comparable to the constant aperture model and roughly half those of the model that depended solely on effective normal stress.

The authors argue that their findings highlight the importance of incorporating dynamic aperture variations to improve the reliability of seismicity models used for deterministic seismic hazard assessment. In other words, treating a fault’s aperture as fixed, even as a computational convenience, can bias both the statistical character of simulated seismicity and the estimated magnitude of the largest events a stimulation might trigger.

Subject of Research: How fault aperture variation laws affect numerical forecasts of injection-induced seismicity at the VITO geothermal site in Belgium.

Article Title: Impact of fault aperture variation on induced seismicity at the VITO geothermal site

Article References: Lindner, N., Hofmann, H., Blöcher, G., Broothaers, M., Cacace, M., Hernandez, E., Pogacnik, J., & Stefanou, I. (2026). Impact of fault aperture variation on induced seismicity at the VITO geothermal site. Geothermal Energy. https://doi.org/10.1186/s40517-026-00412-6

Image Credits: AI Generated

DOI: 10.1186/s40517-026-00412-6

Keywords: geothermal energy, induced seismicity, fault aperture, VITO geothermal site, Dinantian limestone, numerical modeling, seismic hazard assessment, b-value, moment magnitude, fluid injection, effective normal stress, Belgium