THE 1997 UMBRIA-MARCHE, ITALY, EARTHQUAKE SEQUENCE - A FIRST LOOK AT THE MAIN SHOCKS AND AFTERSHOCKS

A long sequence of earthquakes, six with magnitudes between 5 and 6, s truck Central Italy starting on September 26, 1997, causing severe dam ages and loss of human lives. The seismogenic structure consists of a NW-SE elongated fault zone extending for about 40 km. The focal mechan isms of the largest shocks reveal normal faulting with NE-SW extension perpendicular to the trend of the Apennines, consistently with the Qu aternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apen nines is accomplished by normal faults dipping at low angle (similar t o 40 degrees) to the southwest, and confined in the upper similar to 8 km of the crust. These normal faults might have reactivated thrust pl anes of the Pliocene compressional tectonics. The aftershock distribut ion and the damage patterns also suggest that the three main shocks ru ptured distinct 5 to 15 km-long fault segments, adjacent and slightly offset from one another.