How fast is rupture during an earthquake? New insights from the 1999 Turkey earthquakes

We report that during the two devastating 1999 earthquakes in Turkey, ruptu re propagated over a large part of the nearly 200km long fault zone at supe rshear speed approaching 5km/s. We present observations and modeling which confirm the original inference of supershear rupture during the Izmit earth quake and we show that supershear rupture also occurred during the Duzce ea rthquake. We show that the rupture velocity measured - about root2 times th e shear wave velocity - is the value predicted by theoretical studies in fr acture dynamics. We look for clues to explain these observations.