Coulomb stress changes over a 660-year period in central Italy: Implications for understanding fault interactions and earthquake occurrence

Abstract:

A record of 27 historical earthquakes in central Italy extending back to 1349 AD on faults with known geological slip-rates enables us to investigate the effect of Coulomb stress interactions between faults over long time scales. Modeling the effect of these interactions between active faults in low strain-rate regions is challenging due to a paucity of long earthquake records and poorly constrained long-term strain rates, yet this is key if we are to understand the effect of these interactions on earthquake occurrence. The central Apennines, Italy, with 27 well constrained historical earthquakes over 660 years and over 100 measurements of fault slip rate, provides a natural laboratory for testing models of fault interaction and determining how Coulomb stress interactions between faults affect the timing and location of future earthquakes.

The central Apennines has parallel sets of NW-SE striking active normal faults. Since 1349, earthquakes have clustered along the northeast side of the fault system whereas Holocene averaged strain-rates are more evenly distributed across strike. We model the Coulomb stress changes caused by each of the 27 events and resolve stresses on all faults in the region. Our modeling includes interseismic loading over this period, with stress accumulating on shear zones beneath the seismogenic portion of each fault constrained by the measurements of fault slip-rate, and measurements of fault kinematics from frictional wear striae on bedrock fault scarps.

We show that earthquakes occurred on faults where the net accumulation of stress was positive over the timescale modeled. Co-seismic Coulomb stress increases on the order of 0.01-0.1 MPa along strike appear to occasionally trigger large earthquakes yet are more often eclipsed by interseismic loading stresses on the order of 10^-3 MPa/yr. Importantly, the effect of across strike co-seismic Coulomb stress decreases is more pervasive and can cause changes in earthquake recurrence of 10²-10³ years.