Abstract.We report that during the two devastating 1999 earthquakes in Turkey, rupture propagated over a large part of the nearly 200km long fault zone at supershear speed approaching 5km/s. We present observations and modeling which confirm the original inference of supershear rupture during the Izmit earthquake and we show that supershear rupture also occurred during the Diizce earthquake. We show that the rupture velocity measured -about v/• times the shear wave velocity -is the value predicted by theoretical studies in fracture dynamics. We look for clues to explain these observations.

International audienceThe deformation of northwestern Turkey is the result of the encounter of the westward extrusion of the Anatolian plate with the north-south extension of the Aegean domain. While the North Anatolian Fault localizes the former type of deformation and has been the site of more large earthquakes (9 events of magnitude ≥7) than any other continental fault over the past 100 years, the extension is diffuse and is characterized by spatial clusters of smaller earthquakes and near-continuous activity. We study the evolution of seismicity along the fault and in the clusters neighboring the fault before and after the two large earthquakes of 1999. We observe that the un-ruptured section of the fault and the extension clusters respond very differently to the earthquakes. While significant aftershock activity on the fault segments adjacent to the rupture only occurs at relatively short distance from the rupture (≤30 km), the clusters can be activated at much larger distances (300 km). Remarkably their triggering is not immediate after the earthquake but is delayed in time. Their peak seismic activation may occur weeks or months after the earthquake. This distant and delayed triggering, which is consistent with recent GPS observations, may help resolve some of the previously unexplained characteristics of the 1939-1999 sequence of large earthquakes along the fault

The complete Coulomb failure stress changes caused by the 1976 Ms7.3, Ms7.4 Yunnan Longling double main earthquakes are calculated. And dynamic and static stress triggering effects on the after earthquake sequence are studied. The results show that the second main shock is subjected to the triggering effect of dynamic and static Coulomb stresses induced by the first main shock. Among the thirteen strong aftershocks of the double main shocks, 90% of the aftershocks occurred inside the triangle block are subjected to the triggering effect of dynamic and static Coulomb stresses induced by the first or the second main shock. And 2/3 of the aftershocks occurred outside the triangle block are subjected to the triggering effect of dynamic Coulomb stresses induced by the second main earthquakes. The dynamic and static stress triggering effects of the second main earthquake are stronger than the first main earthquake whether in intensity or in extent. In near field the numbers of triggered aftershocks by dynamic and static Coulomb stress are about equal. However the range of triggering effect of dynamic Coulomb stress is larger than static Coulomb stress.

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