PROGRESSIVE FAILURE ON THE NORTH ANATOLIAN FAULT SINCE 1939 BY EARTHQUAKE STRESS TRIGGERING

10 M greater than or equal to 6.7 earthquakes ruptured 1000 km of the North Anatolian fault (Turkey) during 1939-1992, providing an unsurpas sed 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 st ress throughout the sequence. Calculations of the change in Coulomb fa ilure stress reveal that nine out of 10 ruptures were brought closer t o failure by the preceding shocks, typically by 1-10 bar, equivalent t o 3-30 years of secular stressing. We translate the calculated stress changes into earthquake probability gains using an earthquake nucleati on constitutive relation, which includes both permanent and transient effects of the sudden stress changes. The transient effects of the str ess changes dominate during the mean 10 yr period between triggering a nd subsequent rupturing shocks in the Anatolia sequence. The stress ch anges result in an average three-fold gain in the net earthquake proba bility 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 greater than or equal to 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 we stern port city of Izmit. Such stress-based probability calculations m ay thus be useful to assess and update earthquake hazards elsewhere.