Possible Multiple Sources of the Strong 1117 Po Plain Earthquake, Inferred from the Plio-Quaternary Evolution of the Northern Adriatic Area

The strongest documented seismic disaster ever occurred in the Po Plain area (January 3, 1117, M = 6.5) involved significant damage over a large zone. The genetic mechanism of such an event, most probably caused by more than one earthquake, is still an object of debate. Above all, the sources so far proposed cannot account for significant features of the observed macroseismic field. In this work, we suggest that the damage in the Verona zone was caused by the activation of a fault in the Lessini tectonic district, while damage in the central Po Plain may be related to a thrust fault located beneath the Giudicarie belt. The effects felt in northern Tuscany might derive from the seismic activation of the presumed SW-ward buried prolongation of the Giudicarie fault. The presence of such transpressional lithospheric discontinuity in the Adriatic domain since the upper Miocene and its reactivation (Pliocene-Pleistocene) as a thrust zone is mainly suggested by an accurate analysis of the observed deformation pattern in the central Mediterranean region. The proposed Giudicarie source may also help to explain the damage observed in the central Po Plain on December 25, 1222, which is not compatible with the seismic sources so far proposed.


Introduction
On January 3, 1117, considerable damage occurred in northern Italy (Figure 1 [6] reported on the Structural map of Italy [13]. The stars indicate the epicentral locations of the Padania and Tuscany earthquakes suggested by the seismic catalogue ( [4] [7]). (B) Locations of the proposed seismic sources of the January, 3, 1117 earthquakes [20].
Alternative hypotheses have been advanced to account for the damage observed in the Verona zone ( Figure 2(B)). Some authors [2] [18] [19] [21] suggest the possible role of reverse faults aligned with the Bassano-Montello compressional front (Figure 1), respectively located west (Lessini) and east (Thiene-Bassano) of the Schio-Vicenza fault system. Other epicentral locations, spanning from Lake Garda to the Verona zone, are suggested by other authors (e.g., [3]). [15] and [20] suggest two faults in the Lessini zone (Nogara and S. Ambrogio). [22] proposes a source in the Lessini district, although they consider that the January 3, 1117 event remains one of the most problematic cases, both for the location and estimate of the effects. These authors suggest that the Lessini structure has undergone an articulated anticlinal uparching, with the formation of small blocks affected by differential uplift and tilting [23] [24]. A roughly N-S direction of maximum compressional stress is inferred from focal mechanisms by [25]. The presence of seismogenetic faults in the Lessini district is also suggested by other authors (e.g., [15] [20] [26] [27]).
To explain the damages observed in the central Po Plain [2] has proposed a second source involving a thrust fault located along the Emilian buried folds (Piadena anticline, near the town of Cremona, Figure 2(B)). This author tentatively correlates the anomalies of the Po and Oglio rivers and the migration toward the south of the Po river during historical time with the recent uplift of the Piadena anticline. However, it must be considered that during the Middle and Upper Pleistocene, tectonic activity in the Piadena anticline has been characterized by a general decreasing trend [28]. This is shown by the strong decrease of the uplift rates from 1.20 ± 0.12 mm/yr (in the 1.50 -1. 25 Myr interval) to 0.06 ± 0.05 mm/yr, in the last 0. 45 Myr, but it is also well documented on a more regional scale ( [29] and reference therein  [28]). It must be underlined that [2] admits that the source he proposes cannot account for the damages observed in the Piacenza, Parma and Modena zones.
As regards the event in northern Tuscany, only a few observed intensity points are available (Figure 2(A)), which implies a considerable uncertainty in the location of the epicentre and focal depth (e.g., [3] [5]). The CFTI catalogue [4] indicates an epicenter in the Garfagnana zone, which is north of the observed maximum damage (Figure 2(A)), while the CPTI catalogue ( [7]) provides a generic epicenter in the Monti Pisani zone. Most authors (e.g., [4]) claim that the distance (about 180 km) between the worst damage that occurred in Tuscany and the epicentral zone of the concomitant earthquake in the Po Plain, combined with the lack of information on damage between the two areas, can only be explained by the occurrence of an earthquake distinct from the one that caused the effects in the Po Plain.
This work describes an attempt to identify alternative seismic sources able to better account for the macroseismic field observed in the Po Valley and Tuscan zones.

Activation of Major Fault Zones in the Northern Adriatic Domain since the Upper Miocene
In the period considered, two major decoupling zones developed in the northern Adriatic domain (e.g., [30] [31] [32]). The first is the Giudicarie discontinuity ( Figure 3), an old thrust fault that was reactivated as a sinistral transpressional shear zone around the Tortonian (e.g., [14] [33] [34]). The Neogene Giudicarie fault system has been interpreted as a lateral ramp (e.g., [35] [36]), but this hypothesis cannot easily be reconciled with major evidence on the Neogenic deformation pattern in the central Mediterranean region, as extensively argued in previous papers (e.g., [17] [31] [32]). Furthermore, lateral ramps alone cannot explain the complex features and the kinematics of the South Giudicarie belt.
The other major decoupling zone is the Schio-Vicenza fault system (SV in The geodynamic context and the tectonic evolution that led to the development of such discontinuity ( Figure 3) have been discussed in a number of papers (e.g., [30]  In the Middle Miocene ( Figure 3(A)), buoyancy forces in the Adria-Eurasia collision zone (Alps) are shown to have strongly counteracted to any further crustal shortening, due to the large amount of light upper crustal material accumulated during the long phase of plate convergence (e.g., [46]  line (PG in Figure 3(A)). In the wake of such extrusion extensional deformation developed, with the exhumation of the Tauern window complex (TW in Figure   3(B), e.g., [37] [51] [52] [38]). Other information about the timing of this major decoupling mechanism may be inferred from the exhumation history of the Adamello intrusion, in particular the fact that such process was characterized by a phase of maximum activity around the Tortonian (8 -9 My, [57]).
The displacement of the northern Adriatic domain caused lowering of its subducted margin (buried below the Apennine belt), as tentatively reconstructed in Figure 4.
The subsequent subsidence of this structure with respect to the domain lying west of the Giudicarie discontinuity caused the onset of a pronounced vertical throw ( Figure 5(B)).
The presence of a greater dip in the eastern Po Plain Adriatic monocline, with respect to the western domain (underlying the central Po Plain) is testified by cross sections (Mesozoic units in Figure 6) and is suggested by some authors (e.g., [8] [58] and references therein). A different structural setting in the eastern and western Po Plain is evidenced by other authors (e.g., [59] [60]), which indicate the Giudicarie discontinuity as a possible boundary between the two regions.
The other decoupling discontinuity in the northern Adriatic area, the Schio-Vicenza (SV) fault system, developed around the Messinian (e.g., [39] due to the   reactivation of an old weakness zone in the Adriatic foreland (e.g., [61]).
This event developed in the framework of a major tectonic reorganization in the whole central Mediterranean region [30] [31] [32] [41], which involved a decoupling of the Adria plate from Africa, by the Sicily Channel-Medina-Victor-Hensen fault zone (Figure 3(C)), and from the western Po Plain domain, by activating the SV fault system which was more suitably oriented for the NNWward motion trend of the newly independent Adria microplate (Figure 3(C)).
The peculiar location of the SV decoupling fault was mainly influenced by the fact that in the Miocene (Figure 3(B)) the resistance against the northward motion of the Adriatic plate was considerably lower in the eastern Southern Alps, facing the Tauern window (Figure 3(B)) than in the central Alps, where more rigid structures were located, such as the Adamello intrusion and the Trento Horst ( Figure 5(B)) well connected with the underlying basement (Figure 3(C) and Figure 5(C)). The shear stress induced by such different resistances in front of the Adriatic indenter led to the activation of the SV fault system (e.g., [14] [31] [34]). The hypothesis that the Adamello intrusion acted as an undeformable body has been suggested by some authors (e.g., [63]). The important role that    International Journal of Geosciences dence may also affect (even if with lower rates) the western Po Plain, the almost null vertical motion indicated by GPS data (Figure 8(B)) may imply that such zone is undergoing a contemporaneous uplift of a few mm/y. [69] suggests that vertical motions in this area are mainly influenced by the retreat of the Adriatic subduction. However, the implications of that driving force can hardly be reconciled with the major Neogene tectonic processes recognized in the central Mediterranean region (e.g., [17] [30] [31] [32]).

Present Geodynamic/Tectonic Setting and Possible Sources of the 1117 Earthquakes
The evolutionary history of the study area suggests that tectonic activity in the northern Adriatic region is driven by the northward motion of the Adriatic plate with respect to Europe (e.g., [30]  On the basis of the above considerations, we propose that the 1117 destruction was produced by three almost simultaneous earthquakes, one located in the Lessini district, another in the thrust fault presumably lying below the Giudicarie belt and a third one in the southward buried prolongation of the Giudicarie lithospheric discontinuity, underlying northern Tuscany. To tentatively identify the location and hypocentral parameters of the 3 sources that best account for the observed macroseismic field (Figure 2), we have adopted the following strategy. For the source in the Lessini zone (L in Figure 10) we have assumed the location and hypocentral parameters provided by the seismic catalogue [7]. In the central Po Plain, we have taken into account two possible segments (G1 or G2 in Figure 10) of the thrust fault lying below the Giudicarie belt. For the Tuscany event, the possible sources are located along the buried sector of the proposed lithospheric discontinuity (T1 or T2 in Figure 10).   [7] and the attenuations laws suggested by Decanini and Mollaioli (1997) [74].
The results obtained ( Figure 11) indicate that, among the solutions considered, the best agreement between the computed intensities and the observed ones is obtained by taking into account the three sources identified by L, G2 and T2 (Figure 11(D)). It can be noted that such solution could even account for the relatively high intensity values felt in the Modena zone, since such zone is roughly located where the combined effects of the three sources considered (mainly the VII and VII-VIII intensity values) may produce a local high in the macroseismic field.
The hypothesis that the January 3, 1117 disaster was caused by the activation Figure 11. Each picture shows the intensity isolines related to three sources (Figure 10), one located in the Lessini zone, one in the Giudicarie belt (G1 or G2) and one under Northern Tuscany (T1 or T2). The colors of the computed isolines correspond to those of the small colored points at the bottom of each picture, related to the observed intensities (Figure 2). fairly close to the sources here proposed ( Figure 12). This example may be interesting since it involved a subcrustal earthquake (depth = 72 km) beneath northern Tuscany, which is not a very frequent event.
The presence of an additional seismogenic fault located under the Giudicarie belt (for instance the one indicated by G2 in Figure 10) might also help to ex-   were felt at distances greater than 150 km from the most intense effects ( Figure   13) would suggest a non-shallow focal depth for the sources involved.
The tectonic context here proposed implies that the accelerations of the main Adriatic plate, triggered by major earthquakes at the right-lateral (Northern Dinarides), left-lateral (Northern Apennines) and frontal (eastern Southern Alps) boundaries of such plate, may lead to an increase of stress in the zone where the eastern Po Plain Adriatic domain (a portion of the main Adria plate) is bounded by the Lessini structure and the Giudicarie belt ( Figure 14). This tectonic connection might involve some correspondences between the most intense seismic phases in the Lessini -Giudicarie zone (1693, 1781-1810, 1866-1904, 1932-1951, 1968-1976, 2003 evidenced by blue bands in Figure 14) and the main seismic periods at the boundaries of the northern Adria domain (1684-1700, 1775-1794, 1873-1895, 1917-1936, 1963-1976, 1998-2004, Figure 14).

Conclusions
In literature, the genetic mechanism of the most intense documented earthquake