Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering

10 M ≥ 6.7 earthquakes ruptured 1000 km of the North Anatolian fault (Turkey) during 1939–1992, providing an unsurpassed opportunity to study how one large shock sets up the next. We use the mapped surface slip and fault geometry to infer the transfer of stress throughout the sequence. Calculations of the change in Coulomb failure stress reveal that nine out of 10 ruptures were brought closer to failure by the preceding shocks, typically by 1–10 bar, equivalent to 3–30 years of secular stressing. We translate the calculated stress changes into earthquake probability gains using an earthquake-nucleation constitutive relation, which includes both permanent and transient effects of the sudden stress changes. The transient effects of the stress changes dominate during the mean 10 yr period between triggering and subsequent rupturing shocks in the Anatolia sequence. The stress changes result in an average three-fold gain in the net earthquake probability during the decade after each event. Stress is calculated to be high today at several isolated sites along the fault. During the next 30 years, we estimate a 15 per cent probability of a M ≥ 6.7 earthquake east of the major eastern centre of Ercinzan, and a 12 per cent probability for a large event south of the major western port city of Izmit. Such stress-based probability calculations may thus be useful to assess and update earthquake hazards elsewhere.     

Frictional changes induced by seismic slip on a non-planar fault

We propose a model which describes the formation of a strong asperity on a fault. We consider a fault surface which differs slightly from a plane due to a jog-like topographic variation. The fault is placed in an elastic space and is subject to a uniform stress field. The orientation of the fault is such that the normal traction (which is compressive) is greater on the topographic variation, determining a higher static friction and hence an asperity. The value of friction on this asperity depends on the magnitude of shear stress. For times of seismological interest, the increase in shear stress, at rates typical of tectonic processes, does not produce a sensible increase in friction with respect to the adjacent fault segments. A considerable increase in friction and the formation of a strong asperity (or even a barrier) can occur due to repeated seismic-slip episodes on the fault. Slip results in an elastic medium deformation, causing an increase in normal traction on the asperity and hence in friction. This process is described with the aid of a tensile Somigliana dislocation. Regions with high friction undergo partial fracturing of the fault-face material, which can produce fault gouge. The tensile dislocation introduces a small non-double-couple component in the seismic moment of the seismic event, the magnitude of this component depending mainly on the relative size of the asperity.