Record of seismic slip in carbonates: Insights from the Venere Fault during the 1915 Avezzano earthquake (Mw 7.0), Central Italy

The Mw 7.0 Avezzano earthquake in the Abruzzo region of Italy claimed similar to 33,000 lives on January 13, 1915 making it one of the worst disasters in modern Italian history. The main rupture occurred along the Venere Fault, characterized by a polished, locally shiny, or powdery fault mirror showing extensive downdip striations, slickensides, and local reddish iron-oxide/hydroxide stains. The layer immediately below the mirror is a carbonate ultracataclasite that locally grades into an unconsolidated carbonate gouge. This type of carbonate fault mirror typically forms through two distinct synkinematic processes: i) intense frictional heating causing decarbonation, or ii) progressive grain-size reduction during slip at seismic velocities. In either case, friction drops substantially after initial displacement. The first process also results in intense fault pressurization followed by subsequent drastic drop in normal stress. Despite recent advances, the switch from high-friction/low slip velocity to low-friction/high slip velocity conditions in carbonate is still not fully understood. The Venere Fault, characterized by proven friction at seismic slip velocity, provides an ideal setting to investigate the nature and extent of dynamic weakening processes in carbonate faults. We use the high temperature sensitivity of iron oxide/hydroxide assemblages, and their magnetic remanence, to estimate frictional heat. Evidence for seismic slip in iron oxides and temperature uniformity along the fault surface have been tested through demagnetization experiments and 1D heat conduction modeling. Our data shows that the fault mirror underwent frictional heating during the 0.8 m slip event, but that this displacement was insufficient to reach pervasive decarbonation. We constrain the peak coseismic temperature along the fault plane to <400 degrees C through demagnetization experiments and 1D heat conduction modeling. Our results emphasize that coseismic deformation along natural faults is complex and therefore requires complementary field observations at multiple scales in order to encompass a broad range of faulting processes.