Investigating the transition between the 1967 Mudurnu Valley and 1999 Izmit earthquakes along the North Anatolian fault with static stress changes

Abstract

The last major rupture along the North Anatolian fault (NAF) prior to the Izmit and Düzce earthquakes in 1999 was the Ms = 7.1 1967 July 22 Mudurnu Valley Earthquake, which ruptured approximately 80 km along the surface of the west-central NAF. The geographic continuity of the Mudurnu Valley and Izmit ruptures corresponds to the historical westward progression of major earthquakes along the NAF and encourages us to test the stress-triggering effects of the 1967 earthquake on the 1999 earthquake. Although the triggered earthquake sequence from 1939 to 1999 has been investigated by earlier workers, this study addresses several outstanding problems related to this most recent earthquake interaction in light of more detailed fault configurations and a posteriori knowledge of the 1999 events. The questions we hope to answer are: (1) did the stress perturbation caused by the 1967 Mudurnu Valley earthquake promote failure at the 1999 Izmit earthquake hypocentre and along Izmit fault segments? (2) Why did the 1999 Izmit earthquake rupture along faults in the centre of the Izmit-Akyazi depression rather than along the western continuation of the Mudurnu Valley fault that ruptured in 1967? (3) What is the rupture configuration at the western end of the 1967 earthquake fault that would most favour slip on the 1999 August 17 Izmit earthquake fault? Using a 3-D boundary element method, we test three possible rupture configurations for the 1967 Mudurnu Valley earthquake. Coulomb stress changes are calculated on major faults within the region due to non-uniform slip associated with the 1967 Mudurnu Valley earthquake. We find that for all three potential 1967 Mudurnu Valley rupture geometries there are positive Coulomb stress changes at the hypocentre of the 1999 Izmit earthquake. We also find, however, that only when subsurface rupture of the 1967 Mudurnu Valley earthquake deviates from the mountain-front fault and extends towards Lake Sapanca does an Izmit rupture segment (Sakarya segment) receive greater Coulomb stress changes than the mountain-front fault west of the Mudurnu rupture.