Geodetically constrained models of viscoelastic stress transfer and earthquake triggering along the North Anatolian Fault

Abstract:

Over the past 80 years, 8 M_W>6.7 strike-slip earthquakes west of 40º longitude have ruptured the North Anatolian fault (NAF), largely from east to west. The series began with the 1939 Erzincan earthquake in eastern Turkey, and the most recent 1999 M_W=7.4 Izmit earthquake extended the pattern of ruptures into the Sea of Marmara in western Turkey. The mean time between seismic events in this westward progression is 8.5±11 years (67% confidence interval), much greater than the timescale of seismic wave propagation (seconds to minutes). The delayed triggering of these earthquakes may be explained by the propagation of earthquake-generated diffusive viscoelastic fronts within the upper mantle that slowly increase the Coulomb failure stress change (CFS) at adjacent hypocenters. Here we develop three-dimensional stress transfer models with an elastic upper crust coupled to a viscoelastic Burgers rheology mantle. Both the Maxwell (η_M=10^18.6-19.0 Pa∙s) and Kelvin (η_K=10^18.0-19.0 Pa∙s) viscosities are constrained by viscoelastic block models that simultaneously explain geodetic observations of deformation before and after the 1999 Izmit earthquake. We combine this geodetically constrained rheological model with the observed sequence of large earthquakes since 1939 to calculate the time-evolution of CFS changes along the North Anatolian Fault due to viscoelastic stress transfer. Critical values of mean CFS at which the earthquakes in the eight decade sequence occur between -0.007 to 2.946 MPa and may exceed the magnitude of static CFS values by as much as 180%. The variability of four orders of magnitude in critical triggering stress may reflect along strike variations in NAF strength. Based on the median and mean of these critical stress values, we infer that the NAF strand in the northern Marmara Sea near Istanbul, which previously ruptured in 1509, may reach a critical stress level between 2015 and 2032.