From fault creep to slow and fast earthquakes in carbonates

A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L'Aquila (Mw 6.1) in 2009 and Norcia (Mw 6.5) in 2016 in central Italy. Surprisingly, within this region, fast (≈3 km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite its relevance for seismic hazard studies, the transition from fault creep to slow and fast seismic rupture propagation is still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures were obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3-5 km depth (i.e., P = 100 MPa and T = 100 °C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of seismicity in central Italy. The transition from slow to fast rupture is explained by an increase in the ambient temperature, which enhances the elastic loading stiffness of the fault, i.e., the slip velocities during nucleation, allowing flash weakening and, in turn, the propagation of fast ruptures radiating intense high-frequency seismic waves.