The Athens earthquake of 7 September 1999

Based on detailed aftershock monitoring, the first model of the Athens eart hquake is formulated, which is consistent with global, regional, and focal strong motion data of the mainshock, and fits with geological setting. The 30-station temporary network located 450 aftershocks. During the first 20 o bservation days the aftershocks identified the mainshock fault plane dippin g 52 degrees and striking 117 degrees, consistently with the teleseismic fa ult-plane solution. A formal upward continuation of the fault plane interse cts the surface close to the Pill fault. Numerical modeling of the broadban d regional data at 10 stations (epicentral distances 140 to 370 km) estimat es the centroidal source depth of 10 km and yields an average source durati on of 5 to 6 sec. The interstation variability of the apparent duration ind icates source directivity. The empirical Green's function modeling at the c losest broadband station suggests a fault length of 20 or 10 km. Both the n umerical and empirical modelings give a very short rise time of 0.1 to 0.3 sec. The short rise time seems to favor the nearly complete stress release of an asperity. A 10 km asperity (stress drop of 2.7 M Pa) is in agreement with a gap, identified during the first 12 observation days between two aft ershock clusters. The strong-motion accelerograms in Athens also indicate a short apparent duration due to directivity (about 3 sec), and confirm an a brupt rupture beginning. There is no evidence for an abrupt stopping. The s hort rise time and short apparent source duration were two principal factor s determining the damaging ground motions in Athens.