HR: 1340h
AN: S43C-1013    [Abstracts]
TI: High resolution seismic imaging of an active normal fault in the Agri Valley, Southern Apennines, Italy
AU: * Improta, L
EM: improta@na.infn.it
AF: INGV - Roma 1, Via di Vigna Murata, 605, Roma, 00143 Italy
AU: Bruno, P
EM: bruno@ov.ingv.it
AF: INGV - Osservatorio Vesuviano, Via Diocleziano, 328, Napoli, 80124 Italy
AU: Di Fiore, V
EM: vdifiore@ov.ingv.it
AF: INGV - Osservatorio Vesuviano, Via Diocleziano, 328, Napoli, 80124 Italy
AU: Mariani, S
AF: INGV - Roma 1, Via di Vigna Murata, 605, Roma, 00143 Italy
AB: The Agri Valley is an intermontane basin located in the Southern Apennine seismic belt (Italy) whose formation in tied to large NW-trending trastensional and extensional faults active since Early Pleistocene. Recent faulting activity in the area is documented by faulted paleosoils and suggested by a M7 earthquake that struck the basin in 1857. On the contrary, present-day background seismicity in the area is extremely low. Despite intense geomorphic investigations, the identification of the source responsible for this historical event and of further large seismogenic faults in the area is still a matter of debate. A new NW trending normal faulting system has been recently recognized based on subtle geomorphic expressions on the ridge bounding the basin westward. Recent faulting activity along this structure is locally documented by a trench. Aimed at yielding new information about the shallow structure of the fault, we conducted a high resolution seismic experiment in a small lacustrine basin, located 4 km south of the trench, in which the presence of the fault is inferred by a linear surface warping but trench excavation is impractical. Both multi-fold wide-angle data and multichannel near vertical reflection data have been collected along a 220-m-long profile in order to obtain an accurate model of the basin combining seismic velocity and reflectivity images. About 3600 first arrival traveltimes picked on 36 wide-angle record sections have been inverted by a non-linear tomographic technique that is specially designed to image complex structures. The tomographic inversion provides a high-resolution velocity model of the basin down to 60 m depth. The model is strongly heterogeneous and displays sharp lateral velocity variations. Seismic reflection processing has been applied to both data sets. Data have been edited for trace quality and first (refracted and direct) arrivals have been muted. A following FK dip filtering on the shot gathers reduced the energy associated to ground roll. The traces were then gathered in CDP and corrected for statics. Note that both statics and the normal moveout correction have been applied using as background velocity the tomographic model. On multi-fold wide-angle data, a further iterative process, which consists of windowing the data using different offset ranges before CDP stacking, allowed to maximize the stack quality. The smooth velocity and stack sections confirm the fault location and delineate more fault splays, which are responsible for sharp lateral velocity changes and reflector truncations. The fault splays are located both beneath the surface warping and along the northern margin of the basin, thus indicating a quite complex faulting process. Moreover, the small amount (about 30 m) of total vertical slip separation imaged for the basin substratum across the fault zone allows explaining the subtle geomorphic expression of the faults.
DE: 0902 Computational methods, seismic
DE: 0910 Data processing
DE: 0935 Seismic methods (3025)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting