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 5 km/s. We present observations and modeling which confirm the original inference of supers-hear rupture during the Izmit earthquake and we show that supershear rupture also occurred during the D¿zce earthquake. We show that the rupture velocity measured-about √2 times the shear wave velocity-is the value predicted by theoretical studies in fracture dynamics. We look for clues to explain these observations. ¿ 2001 American Geophysical Union