Connecting seismically active normal faults with Quaternary geological structures in a complex extensional environment: The Colfiorito 1997 case history (northern Apennines, Italy)

Abstract

The northern Apennines of Italy are characterized by a complex Tertiary tectonic history, where superposed compressional and extensional deformation occurred. In such regions, characterized by low active extensional strain rate, the coseismic surface ruptures are rare and often a matter of much debate resulting in a difficult connection between “geological” faults (i.e., faults which can be mapped at the surface) and “seismological” faults (i.e., faults which actually generate earthquakes). The availability of detailed geological mapping and high‐resolution seismological data for the Colfiorito area, struck in 1997 by a sequence of six 5 < Mw ≤ 6 normal faulting earthquakes, allow us to compare and verify the existence of geometric and kinematic correspondence between the mapped geological Quaternary faults and the activated structures. In map view, the earthquakes distribution reflects the fault pattern mapped at the surface, the length of activated and mapped faults is quite similar (7–10 km), the coseismic subsided region imaged by interferometric synthetic aperture radar data, is located in the hanging wall of the mapped normal faults that bound the Quaternary basins. In section view, there is a geometric connection between mapped normal faults and the aftershock alignments used to image fault geometry at depth. Comparison of striated fault planes and aftershock focal mechanism solutions show a strong kinematic consistency. This study points out that the Quaternary tectonosedimentary evolution and the present‐day geological and geomorphologic setting of the Colfiorito area can be interpreted as the result of repeated, extensional earthquakes, similar to the 1997 events, occurring on NW‐SE trending normal faults. Our data also show that the main shocks of the Colfiorito sequence nucleated close to the intersections between the normal faults and the preexisting compressional/transpressional structures, which in 1997 acted as lateral barriers to rupture propagation and consequently constrained the fault size.

Comparison between reprocessed seismic profiles: Seismologic and geologic data — A case study of the Colfiorito earthquake area

“…Despite the high-resolution locations of the seismic sequence, a direct connection between the activated faults at depth and the outcropping normal faults ( Figure 3a) is still a matter of debate, mainly because surface ruptures during the 1997-1998 seismic crisis cannot be well placed and surface breaks have been associated with only a few fault segments (Cinti et al, 2000). However, the available data indicate a strong relationship between ground deformation, the trace of the Quaternary normal faults at the surface, the subsided area, and the earthquake locations (Chiaraluce et al, 2005). This relationship suggests a connection between the surface structures and the faults activated between 1997 and 1998.…”

“…With regard to the deep attitude of the main active faults in the Colfiorito area, no definitive data are yet available to fully define their geometry at depth. However, on the basis of the gathered data (Barchi, 2002;Mirabella and Pucci, 2002;Collettini et al, 2003;Chiaraluce et al, 2005), it seems reasonable to consider a variation of dip with depth from about 55…”

“…We selected this depth because in this part of the sequence the aftershocks shallower than 3 km are related to a secondary shallow strike-slip event (4.5 Mw) that occurred on October 16, 1997, at about 1 km depth located very near CMP 1600 Chiaraluce et al, 2005). We used 158 earthquake locations (Selvaggi et al, 2002) obtained by the Hypoinverse location code (Klein, 1989) using the 1D velocity model derived from the minimization of P-and S-wave residuals on the whole data set (Kissling et al, 1994;see Table 3 for the velocity model).…”

“…Detail of the CF-1 profile (∼15 km long), showing the reprocessing results for the line close to Colfiorito (a) before and (b) after reprocessing. structures with the activated fault segments (Chiaraluce et al, 2005).…”

“…• -150 • ), inferred by both focal solutions (e.g., Chiaraluce et al, 2003) and surface geologic data (e.g., Barba and Basili, 2000;Mirabella and Pucci, 2002;Chiaraluce et al, 2005), the earthquakes were projected following the mean strike of the active normal faults (i.e., along a N145…”