Active tectonics and earthquake sources in the epicentral area of the 1857 Basilicata earthquake (southern Italy)

Abstract

The 1857 Basilicata earthquake (Imax=XI° MCS; Me=6.9) is one of the most destructive events that occurred in peninsular Italy; shaking effects and ground breaks were recorded over a large area extending from the Vallo di Diano (Campania) to the Val d'Agri (Basilicata) for a length of about 60 km and with a width of more than 10 km. Within this seismogenic belt, only another strong earthquake, with maximum intensities in the range of X° MCS (Me=6.4), occurred in 1561. In the epicentral area of the 1857 earthquake, two regional fault systems (i.e. the Val d'Agri and the Vallo di Diano fault systems) offset the main features of the southern Apennines fold and thrust belt; both systems show evidence of activity during Pleistocene times. The Vallo di Diano Fault System (DIFS) includes mostly NW–SE and WNW–ESE trending faults displaying long-term displacements of a few hundred meters; slip data from the latter faults record a kinematic transition from almost pure normal motion to dextral/oblique motion, whereas the NW–SE oriented faults are mostly dominated by normal/transtensional (sinistral) motion. The Val d'Agri Fault System (VAFS) is characterized by fault zones of different size; it is a kinematically coherent system including roughly N120° trending left-lateral strike-slip faults and N090°–N100° trending left-lateral transtensional faults. Inversion of fault slip data indicates that the stress field conditions responsible for the genesis and evolution of the two fault systems are quite different, with σ1 being: (1) sub-horizontal and WSW–ENE trending, in the case of the VAFS, and (2) sub-vertical, in the case of the DIFS. However, the two fault systems are characterized by a roughly N–S oriented extension, and by R-values indicating that σ1≌σ2>>σ3. This suggests the possibility that, in these areas, permutations between the principal maximum and intermediate axes of the stress ellipsoid may have frequently occurred during the faulting process. In this paper, we present new data for both the VAFS and DIFS and discuss the inferred modes of interaction between the two fault systems; this, in turn, suggests possible implications for seismic hazard analyses (SHA) in this sector of the southern Apennines.

Introduction

In the scientific literature concerning earthquake phenomena in southern Italy there is some controversy about the seismogenic structure responsible for the Me=6.9, 1857 Basilicata earthquake (Fig. 1a; Mallet, 1862, Baratta, 1901, Branno et al., 1983, Boschi et al., 1997). For example, Pantosti and Valensise (1990) discussed the possibility that the seismic source might be a NE-dipping normal fault buried beneath the coarse-grained deposits of the high Agri Valley, whereas Benedetti et al., 1998, Cello et al., 2000 suggested that the seismogenic structure responsible for the 1857 earthquake may be a SW-dipping fault zone which borders the northern rim of the high Agri Valley. The 1857 Basilicata earthquake is characterized by an epicentral area (Imax=X° MCS) which extends in a roughly NW–SE direction from the northern Vallo di Diano to the Val d'Agri area, for a total length of about 60 km and a width of more than 10 km (Fig. 1b).

Within the epicentral area, significant ground effects were recorded from Polla to Sala Consilina (in the Vallo di Diano) and from Marsico Nuovo to Montemurro, in Val d'Agri (Fig. 1b; Boschi et al., 1997). Similar effects were also observed in the area between Brienza and Pergola, in the Monti della Maddalena mountain range (Boschi et al., 1997). This latter evidence would seem to suggest that the seismogenic fault responsible for the 1857 earthquake extends with continuity from Polla to Montemurro. This possibility, however, is contradicted by the observation that, within the epicentral area, there are exposed two distinct active fault systems which are separated, at the surface, by the Monti della Maddalena ridge. Moreover, the study of Branno et al. (1983) emphasizes that the configuration of the isoseismal field of the 1857 earthquake might be due to the occurrence of two separate shocks separated by a time interval of a couple of minutes.

Given the earlier uncertainties, in this paper we focus on some basic fault properties that might be used to get insights on the possible modes of interaction between the two outcropping fault systems. Particularly interesting, for the topics discussed here, is also the dataset derived from a moderate earthquake (Imax=X° MCS; Me =6.4) that struck the areas around Polla in 1561 (Boschi et al., 1997).