Elsevier

Geomorphology

Volume 374, 1 February 2021, 107486
Geomorphology

Geomorphological and paleoseismological evidence of capable faulting in the Northern Apennines (Italy): Insights into active tectonics and seismic hazard of the Lunigiana basin

https://doi.org/10.1016/j.geomorph.2020.107486Get rights and content

Highlights

  • Western Lunigiana landscape records distinct tectonic events.

  • Multidisciplinary approach allowed identifying the first evidence of capable faults.

  • Geomorphological marker offsets allow estimating throw rates since Middle Pleistocene.

  • Historical slip events along the LANF were revealed by paleoseismic analysis.

  • Our results call for seismic hazard revision in this understudied area.

Abstract

The Lunigiana basin is a NW trending tectonic depression located in the Northern Apennines of Italy. The basin is bounded by active normal faults, characterized by moderate to strong seismicity, with historical earthquakes reaching up to magnitude 6 (February 14, 1834) and instrumental events with magnitude up to 5.4 (June 21, 2013).

Using the classical methodologies of active tectonic studies (i.e. remote sensing and digital topography analysis, field mapping, structural geology, and tectonic geomorphology), the main active faults have been mapped for their geometries, kinematics and evidence of activity. Both fault systems bounding the basin to the northeast and southwest have revealed clear evidence of fault capability, i.e., tectonic displacement of the latest Pleistocene-Holocene sediments and geomorphic markers. Then, the study focused on the southwestern Arzelato-Mulazzo-Tresana fault system (AMT), depicting its Late Quaternary tectono-sedimentary evolution and linkage relationship between important fault segments. The throw measurement of key geomorphic markers on the central fault of the AMT allowed to derive medium- (last 350–250 ka) and short-term (18–13 ka) vertical slip rates: 0.44–0.62 and 0.45–0.68 mm/yr, respectively.

Lastly, an exploratory trench for paleoseismological analysis was dug across the fault scarp where geomorphic indicators of active surface faulting were most pronounced, just south of the village of Mulazzo.

Its paleoseismological analysis revealed at least two events of surface faulting, with a cumulative throw in excess of 70 cm, occurred in historical times. Radiocarbon dating of the exposed alluvial and slope deposits allowed to bracket the surface rupture events to 14th to 19th century CE and after the 18th century CE. Typically, in the active extensional belt of the Apennines, the observed offsets correspond to earthquakes of magnitude >6.3, also confirmed by the empirical fault-scaling laws. This magnitude is, therefore, higher than that currently inferred for this seismic source (Mw 5.7÷6.0 in the DISS catalog) and is comparable to that experienced in the adjoining Garfagnana basin (Mw 6.5 on September 7, 1920). The findings of this research suggests a need to revise the seismic hazard of the Lunigiana basin, including greater consideration of the surface faulting hazard in the seismic microzonation studies.

More generally, our findings provide new insight into the comprehension of a) the still unclear deep geometry (listric or not?) and the hierarchy of bounding faults, and b) provide new elements to decipher the puzzling seismotectonic relationships of Lunigiana with the adjoining basins on both its ends, characterized by quite different structural settings.

Introduction

This paper presents a geomorphic and paleoseismological analysis of faults bounding the Lunigiana basin (Central Italy), a crucial area among the many seismogenic intermontane depressions distributed along the Apennines orogenic belt, being the northernmost major basin with “apenninic” trend (i.e., NW-SE). Just north of it, the similarly seismogenic Val di Taro depression displays an opposite trend (ENE-WSW) and the full interpretation of this sharp structural change is still missing or controversial (see Molli et al., 2018a and references therein). Some earthquakes with damaging effects (Rovida et al., 2019) are known to have occurred in Lunigiana in 1481 (Mw = 5.6), 1834 (Mw = 6.0) and 1837 (Mw = 5.9). Moreover, the 1920 (Mw = 6.5) Garfagnana earthquake took place close to its southern termination, where the two basins merge (Fig. 1).

In the Lunigiana depression, although the Quaternary age of the basin bounding faults has been recognized decades ago (Bartolini et al., 1982; Bernini and Papani, 2002), the faulting activity has not been further constrained and no direct relationship has been established so far with the historical seismicity.

In areas of crustal extension seismicity, coseismic surface faulting commonly occurs starting from magnitudes close to 6 (e.g., Wells and Coppersmith, 1994; Thingbaijam et al., 2017). The faults that can produce a significant and permanent deformation at or near the ground surface are named capable faults (e.g., IAEA, 2010). The investigation and dating of the stratigraphic and geomorphic evidence of slip episodes along these faults (paleoseismology) allow inferring the occurrence of prehistoric or historic seismic events whose magnitude is proportional to the amount of slip (e.g., IAEA International Atomic Energy Agency, 2015). Thus, paleoseismological analysis is a powerful tool to assess the actual capability of a fault, its seismogenic potential and the hazard posed by surface displacement. Because of its historical record of earthquakes, not exceeding M 6 inside the basin, the Lunigiana seismogenic sources have not been considered capable so far (e.g., DISS Working Group, 2018). In many slopes, the dense vegetation cover and the moderate to high erodibility of the faulted rocks and sediments (Di Naccio et al., 2013) has hindered the identification of fault scarps, so that, until now, specific research has never been devoted to seek for evidence of paleoseismicity imprinted on the local faults.

This study improves our insight into active faulting, surface rupturing and seismic hazard of this still poorly characterized basin, providing new data derived from structural, geomorphological and geophysical studies coupled with a paleoseismological analysis on the Mulazzo fault. The latter pertains to the main fault system bounding the basin to the southwest. Furthermore, there are several tectonic aspects of more general interest that can benefit from the ongoing study of the Lunigiana basin. Among them, the shape of the basin, apparently bounded by major faults on both sides, and the geometry at depth of the fault system including the Mulazzo fault. The latter is assumed to have a low seismic potential also because interpreted by some authors to be a LANF (low angle normal fault) flattening at shallow depth (Argnani et al., 2003; Molli et al., 2018a).

Section snippets

Geological and seismotectonic setting

The architecture of the Northern Apennines results from the post-Oligocene involvement of the Adria continental crust in the tectonic deformation that followed the consumption of the Ligurian Tethys ocean (Late Cretaceous-Middle Eocene) and its consequent collision with the European plate. This led to the formation of a stack of tectonic units, with the ocean-derived units (Ligurian Units) overriding the transitional (Subligurian Units) and continental margin units (Tuscan and Umbrian-Marchean

Evidence for active and capable faulting on the western border of the Lunigiana basin

We performed an integrated analysis of all of the available information in the geological literature possibly dealing with the presence of active structures in the Lunigiana basin. The relevant data and maps were compiled and analyzed in a GIS environment, including processing of the very high resolution (1 m grid) digital terrain model (DTM) from airborne LiDAR provided by the Tuscany Regional Authority and MATTM – Environmental Ministry of Italy (available at //www502.regione.toscana.it/geoscopio/cartoteca.html

Depositional and faulting events in the trenched Mulazzo fault

The stratigraphic units recognized in the trench logs document the close interaction between surface processes and activity of the Mulazzo fault, depicted through the following stages:

1) local activation and advancing of a colluvial apron resting in contact with the bedrock on the fault and recorded by units 1–2 (Fig. 10);

2) Infill, by successive depositional events, of a small channel attached to the slope (unit 3), indicated by the intervening debris-flow deposits (Fig. 10). The proximity to

Conclusions

The desk and field investigations described before have yielded the key results summarized below:

  • The Mulazzo and Arzelato faults, two segments of the AMT fault system bounding to the west the Lunigiana basin, are hard linked totaling a length of 17 km.

  • The downthrow movement of the Mulazzo fault generated clear effects on the morphology of the mountain front and influenced the deposition of Late Pleistocene and Holocene alluvial deposits. The development of a gentle rollover anticline affected

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Regione Toscana [Delibera: Regione Toscana - Decreto Dirigenziale n.9272 del 29/05/2018 - ai sensi della Del. GRT n. 237 del 13/03/2018]. A. M. Michetti and two anonymous reviewers are thanked for providing supportive comments on the first version of the manuscript. M. Galeotti and S. Nandotti (Mulazzo Municipality) are thanked for providing important help for the paleoseismological trench organization and execution.

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