Coseismic and postseismic displacements related with the 1997 earthquake sequence in Umbria-Marche (central Italy)

We study the coseismic and postseismic displacements related with the 1997 Umbria-Marche earthquake sequence by means of leveling lines along a deform ed aqueduct located in the epicentral area. Comparing the 1960 and 10/1997 measurements we obtain 0.49 +/- 0.10 m of coseismic displacement distribute d along 3 km across the normal fault zone. Modeling of the coseismic surfac e dislocation is obtained from a combination of low angle (38 degrees) faul ts at depth and high angle (80 degrees) upper fault branches. The best fit model indicates that the upper branches stop at 0.4 km below the ground sur face and have 60% of slip with respect to the lower faults. The postseismic displacement measured during 1998 is 0.18 m and represents 36% of the appa rent coseismic deformation. Moderate earthquakes in the Apennines and relat ed surface deformation may thus result from curved faults that reflect the brittle-elastic properties of the uppermost crustal structures.