Earthquake nucleation – viewpoint of dynamically growing asperities controlled by the fracture cohesion-zone and frictional shear

It has often been suggested that the frictional instability on small fault patches can lead to slow but accelerated creep and growth, which may explain the nucleation of earthquake rupture. Because of the small size of the asperity at depth, the nucleation process is difficult to verify by observations and its possible role for earthquake generation is still debated.

While most earthquake nucleation models are of complex geometry and assume that the asperity itself is static while its stress is increasing, we suggest a concept where the cohesion zone of a fault patch grows steadily and develops a self-induced high stress behind the crack tip. We show that a slip-weakening and velocity strengthening constitutive relation can generate the high stress cohesion zone. The aseismic growth of the asperity is accelerated, and the point to nucleate into a catastrophic rupture depends on the ambient stress on the fault and the stress drop in the centre of creeping segment. Interestingly the model predicts that earthquakes on faults with subcritical and small ambient stress will start with more energetic ruptures, as their Griffith energy is larger. This is unexpected and may question the common assumption that largest earthquake are triggered if the fault is critically stressed and the last earthquake occurred a long time before the average recurrence period.

Our fracture mechanical, theoretical asperity model is unconventional and questions established ideas on earthquake generation. We discuss the possible consequences and the postulated, testable predictions of the model to motivate laboratory and field experiments.