Although several investigators mention the existence of scarps or fault breaks in young (post-Middle Pleistocene) deposits (e.g. Ascione et al., 1998, at an unspecified location at the edges of the Boiano Plain; Basili et al., 1999, along Mt. Patalecchia; Blumetti et al., 1999, across the Guardiaregia fan and at several other locations), no conclusive evidence exists to date for genuine surface faulting associated with the main active structures of this area.
Trenching across one of these features (Blumetti et al., 2000) gave ambiguous results about the fault trace location and consequently about the recognition and dating of paleoearthquakes. The geometry of the main driving faults is therefore inferred from indirect geological observations. In particular, consensus on the assumed NE dip of the main fault has been gathered only in the past few years, and some investigators still contend that SW-dipping faults bounding the basin to the E also play a major role in the seismogenic process.
Given the expected size of the fault that controls the Boiano Basin and of the earthquakes generated by it (e.g., the 1805 event), the lack of unambiguous surface breaks could be interpreted as simply due to the youthfulness of the present tectonic regime, a condition observed elsewhere along the central and southern Apennines.
The extent of the Boiano Basin fault segment and of the 1805 coseismic rupture is discussed by Di Bucci et al. (2001), who point out that the Boiano extensional system is fully part of the much longer system that runs along this stretch of the Apennines. Normal faulting continues in the NW-SE direction well beyond the northwestern end of the Boiano Plain proper for at least 15 km (based on data from Corrado et al., 1998). Faults in this northern sector dip both toward the SW and the NE (although the master fault appears to be NE-dipping), favour the development of small Pleistocene basins such as the Carpino Basin, control the local drainage and generate a series of small horst and graben along this reach of the Matese-Frosolone structure. This section is separated from the Boiano Basin along an alignment that runs between Carpinone and Mt. Patalecchia, which corresponds to an important drainage divide and marks the boundary between areas with predominant E-W (to the SE of it) or N-S (to the NW of it) inherited strike-slip faults. Is this also an important segment boundary marking the northwestern end of the 1805 rupture? If the answer is yes, the section of fault to the north of this boundary would be unruptured in 1805 or in any other large earthquake of the past 4-5 centuries.
Di Bucci et al. (2001) also locate the southeastern boundary of the Boiano Basin extensional system between Guardiaregia and the Vinchiaturo Pass, just to the west of the Sepino Plain. They also suggest that the Morcone and Sepino Plains may be controlled by additional though secondary elements of the broader Matese extensional system, but contend that the 1805 was not large enough to have ruptured through these areas.
The total length of the Boiano segment proposed by Di Bucci et al. (2001) is 25 km. This estimate compares well with the ca. 20 km geological model fault of Cucci et al. (1996) and with the ca. 25 km long source and 6.5 equivalent magnitude inferred for the 1805 earthquake by Gasperini et al. (1999). The three solutions also share a similar strike (within 5¡) and general fault location (within about 2 km).
In contrast with the previous interpretation and based on contemporary reports for surface breaks and geochemical phenomena, Blumetti et al. (1999) (based on evidence first put forward and discussed by Esposito et al., 1987) propose for the 1805 earthquake a coseismic rupture covering the entire distance from Isernia to Morcone (about 50 km). This interpretation effectively reduces the hazard associated with the Isernia-Carpino Plain and with the Sepino and Morcone Plains (respectively to the NW and SE of the Boiano Plain) because the relevant sections of the Apennines extensional system would have ruptured less than 200 years ago in earthquakes that were large enough to produce sizeable surface effects (>6.0?).
1) Is the 25 km-long Boiano segment alone responsible for the 1805 earthquake, or did the rupture extend further towards the NW and SE?
2) What is the potential of adjacent fault portions of same extensional trend?
3) Does any of the reports of 1805 earthquake truly describe genuine surface breaks that could be used for characterising the activity of this segment?
4) How much is the total slip accumulated along this fault? Is it another example of geologically youthful that would confirm the trend suggested by other large seismogenic sources of the southern Apennines?
5) Are the disturbances exposed in the only trench opened across a possible 1805 earthquake scarp unambiguous enough to be used as actual evidence for paleoearthquakes?
Brancaccio et al. (1979)
They are the first investigators to describe the Boiano Basin as having formed during a tectonic phase that has operated since the Middle Pleistocene. They also point out that the marginal portions of the basin are characterised by hanging deposits associated with sub-planar relict surfaces, such as in the case of SW-dipping (that is, dipping towards the Matese Mts.) lacustrine deposits cropping out in the S. Massimo area at about 800 m elevation (nearly 300 m above the alluvial plain). Thanks to the presence of pyroclastic deposits within the lacustrine sequence, they date this complex at 1.0-1.5 Ma.
Esposito et al. (1987)
They describe in detail a number of historical reports of surface effects of the 1805 earthquake including changes in spring discharge, sulfur exhalations, and degassation phenomena that were observed over a distance of about 50 km between Isernia and the Morcone Plain.
Russo and Terribile (1995)
They describe the geomorphology of the Boiano Basin and contend that it is filled by a sequence of fluvial-lacustrine Quaternary sediments ranging in thickness between a minimum of 160 m to a few hundred meters. According to these investigators the Boiano Basin is a Lower to Middle Pleistocene, relatively undeformed, feature ÒcarvedÓ into an older depression generated by a different tectonic regime.
Cucci et al. (1996)
These workers investigate the drainage pattern of the area to detect anomalies that could be interpreted as the effect of repeated slip on a large deep-seated normal fault. The existence of a fault with these characteristics is required by the seismicity of the area and particularly by the occurrence of the large 26 July 1805 earthquake. Their analysis points out that the centripetal pattern of drainage inside the basin, as well as the mere presence of the basin itself, is consistent with sustained slip along a ca. 20 km-long, NW-SE striking, NE-dipping, blind normal fault having the basin on its hanging-wall. Their model fault projects near the southwestern side of the basin, along the foothills of the Matese Massif.
Corrado et al. (1997) and Ferranti (1997)
These investigators describe a drastic Middle Pleistocene change in the regional stress field after which SW-NE extension becomes the dominant tectonic regime. According to Corrado et al. (1998), the new stress configuration is responsible for the formation of an important system of NW-SE-trending normal faults, but also for the reactivation of existing strike-slip faults with opposite sense of shear and with a dip-slip component.
Ascione and Cinque (1997) and Cinque (1998)
These papers describe many Òscarps on faultsÓ (which, according to the definition of Ascione and Cinque (1997) include Òfaults scarpsÓ and Òfault-line scarpsÓ) bounding the Boiano Basin; they interpret most of them as fault line scarps due to selective exhumation of tectonic contacts derived from pre-extensional tectonic activity.
Ascione et al. (1998)
They suggest the existence of remnants of terraces located around the Boiano Plain at slightly higher elevation with respect to it, and contend they were originated by unspecified tectonic activity.
They also describe NW-SE striking faults displacing the fluvial- lacustrine basin filling deposits as much as a few tens of meters at unspecified locations near the NE and SW margins of the basin. They also report that the northeastern side of the basin is affected by NW-SE and SW-NE trending rectilinear scarps uplifting an erosional glacis cut in soft flysch deposits.
Basili et al. (1999)
These investigators analyse the elevation relationships between strath terraces (at lower elevations) and relict surfaces (at higher elevations) that exist all around the basin. They find that correlation is possible for the lower terraces, which are seen at roughly the same elevation regardless of their location, but not for the relict surfaces, which form a separate sequence in the southwestern part of the basin. They conclude that this last sequence must be separated from the others by a primary normal fault running along the SW margin of the basin and dipping towards the NE.
Blumetti et al. (1999)
They describe about 50 contemporary reports of surface breaks and other surface effects related with the occurrence of the 1805 earthquake. In particular they refer to contemporary sources describing the inception of new springs, sulfur exhalations, and degassation phenomena leading to the death of vegetation along sub-linear features (these occurrences are described more extensively in Esposito et al.
(1987)). According to these investigators, the surface phenomena suggest that the 1805 earthquake sequence was generated by a set of faults rupturing for a total length of nearly 50 km from Isernia to the Morcone basin.
Gasperini et al. (1999)
They model the well-documented damage pattern of the 1805 earthquake through an analytical approach. Their modelling returns an equivalent magnitude Me of 6.5, a rupture length of about 25 km, an orientation of the source area of 124¡+/-27¡, and a location of the surface projection of the seismic source almost coincident with the basin itself.
Blumetti et al. (2000)
They opened a trench at locality ÒIl LagoÓ, about 1 km NE of Pettoranello del Molise and near the northern end of the region affected by the large 1805 earthquake. Two faulting events are identified in the sedimentary sequence, the most recent of which generated 10-15 cm of surface displacement. The two events are interpreted as being compatible with the presumable height of a scarp generated by the 1805 earthquake, but no age constraints can be derived for them from the paleoseismological analysis.
Di Bucci et al. (2001)
These investigators contend that the development of the Boiano Basin is controlled by the simultaneous generation of new faults and reactivation of pre-existing ones under roughly NE-SW regional extension. They also point out that no unambiguous surface evidence of the seismogenic source has been identified so far. The extensional system is formed by two principal fault families. Faults of the first and main family strike NW-SE, show dip- slip features and are interpreted as the direct response to extension acting in the region since the Middle Pleistocene. Faults of the second family strike E-W and have high-angle or vertical dips; two clearly distinct generations of slickensides record both pure strike- slip, mainly left-lateral (older) and normal-oblique (newer) movement. According to these investigators, this second family of faults operated over a much longer time span than the first family, suggesting that they play only a passive role under the present tectonic conditions. They also propose a geological cross- section based on the joint interpretation of a variety of observations including oil industry reflection seismology data. The section shows a complex pattern of synthetic and antithetic extensional faults related to master fault dipping towards the NE at an angle of 50¡ to 60¡ and extending to a depth of at least 8 km (inferred 10 km: corresponding fault width is 13 km). The hanging-wall of the master fault hosts the Boiano Plain and the Quaternary sediments that partially fill the depression beneath it. The section shows that the architecture of these deposits is asymmetric as their thickness increases towards the SW (that is, towards the main fault). Total displacement along the master fault is reported to be limited (no more than a few hundred meters).In contrast with Esposito et al. (1987) and Blumetti et al. (1999), Di Bucci et al. (2001) propose that the 1805 rupture terminated against two significant segment boundaries located between Carpinone and Mt. Patalecchia and between Guardiaregia and the Vinchiaturo Pass, respectively to the NW and SE. The 1805 earthquake would thus have been a Mw 6.6- 6.8 event caused by a 25 km-long, 13 km-wide normal fault rupture.