From Geologic Myth to Reality: Reactivated Faults

Unlocking the Mystery of Induced Earthquakes on Stable Faults

New research conducted by Utrecht University suggests that faults traditionally viewed as stable—those that strengthen as they slip—can still trigger earthquakes when disturbed by human activities. Published in Nature Communications, the study offers promising insights into why these events occur in tectonically quiet regions, potentially improving seismic hazard assessments for subsurface energy projects.

The Puzzle of Induced Seismicity in Stable Regions

Earthquakes induced by activities like gas extraction or geothermal energy production often strike in unexpected places. Unlike natural earthquakes, which cluster along tectonic plate boundaries at greater depths, induced events frequently happen in shallow, stable intraplate areas with little historical seismicity. Results from global catalogs indicate that these quakes are shallower and more hazardous due to their proximity to the surface, causing stronger ground shaking.

Consider the Groningen gas field in the Netherlands, where earthquakes up to magnitude 3.6 have been linked to reservoir depletion. Laboratory tests on local rock samples show velocity-strengthening (VS) friction—a property where faults gain resistance as slip accelerates, theoretically preventing instabilities. Yet, real-world events challenge this view. As the researchers from Utrecht University explain, this contradiction arises because conventional models overlook long-term fault evolution.

How Faults Heal Over Time

At the heart of the study is frictional healing, a time-dependent process where inactive faults regain strength. Much like how a scar tissue strengthens over years, faults "heal" through grain contact growth and compaction during periods of quiescence. In rate-and-state friction (RSF) models, this is captured by the parameter b, which governs how quickly friction evolves with slip history.

Laboratory slide-hold-slide experiments demonstrate that healing increases peak friction after inactivity, proportional to hold time. The team extended this to geological scales—thousands to millions of years—revealing strength gains up to 0.25 in friction coefficient. "Fault healing is important under long-term tectonic inactivity and thus could be key to explain why areas devoid of natural earthquakes may still be prone to seismicity in induced scenarios," the authors note in their analysis.

This healing allows VS faults to accumulate interface strength, setting the stage for instability when perturbed, such as by pore pressure changes from resource extraction.

Numerical Models Shed Light on Earthquake Nucleation

To resolve the paradox, the researchers developed quasi-dynamic models simulating fault behavior over vast timescales. Drawing from the Groningen setup, they modeled a 2-D normal fault crossing a depleting reservoir, incorporating RSF with an aging law for state evolution. A simplified 0-D version focused on critical fault points to explore parameters like healing rate (b) and duration.

Results indicated that after prolonged healing (e.g., 30 million years), VS faults can nucleate earthquakes with slip rates reaching 1 m/s and stress drops around 3 MPa, akin to observed events. However, these quakes happen only once; subsequent slips stabilize, reducing recurrence risk. In contrast, velocity-weakening (VW) segments may recur but with halved stress drops.

Nucleation on VS faults follows revised stages: an initial cohesive zone (length L_I), then immediate acceleration to rupture (L_III), bypassing a transitional phase seen in VW cases. Theoretical derivations adjust nucleation lengths to account for healed strength, enabling instabilities on otherwise stable faults. Encouragingly, the models match lab data and field observations, validating the approach.

Lead author Meng Li and colleagues emphasize that high healing rates and long inactivity are prerequisites, with VS rocks needing at least 25 million years for seismogenic potential in reservoir-like conditions.

Implications for Seismic Hazard and the Energy Transition

The findings have real-world relevance for managing induced seismicity amid the shift to sustainable energy. Healed VS segments can act as barriers, impeding rupture propagation and limiting earthquake magnitudes in fault networks. This suggests that the first induced event poses the greatest hazard, while follow-ups are less severe—stable on VS faults and diminished on VW ones.

For hazard assessment, integrating frictional properties, healing history, and operational strategies could guide safer subsurface use. Sites with slow-healing VS rocks or shorter inactivity periods appear less risky. The team envisions applications in forecasting for gas fields, geothermal projects, and carbon storage, fostering public confidence in these technologies.