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Low-deforming regions are characterized by long earthquake recurrence intervals; thus, it is fundamental to extend back as much as possible the record of past events. Evidence from single sites or proxies may be not compelling, whereas a more substantial picture may be obtained from the integration of paleo- and archaeo-seismic evidence at multiple sites, eventually supplemented with historical chronicles. Here, we document deformations observed in a stratigraphic sequence (i.e., Via Manzoni Site) and in an archaeological site (Roman Baths) in the city of Como (N. Italy). We perform stratigraphic and sedimentological analyses on the sedimentary sequences at via Manzoni and we document Earthquake Archaeological Effects at the Roman Baths by means of Structure from Motion and field surveys. We interpret the observed deformations as due to earthquake ground shaking and provide constraints on the lower threshold for the triggering of such evidence. Radiocarbon datings and chronological constraints from the archaeological site allow to bracket the time of occurrence of the deformations at the VI century AD. We move toward a more regional view to infer possible seismogenic sources by exploiting a dataset of published paleoseismic evidence in Swiss and N. Italy lakes. We perform an inverse grid search to identify magnitude and location of an earthquake that can explain all the positive and negative evidence consistent with the time interval of the event dated at Como. Our results show that a so far undocumented earthquake (minimum Mw 6.32) with epicenter located at the border between Italy and Switzerland may account for all the observed effects. Our study calls for the need to refine the characterization of the local seismic hazard, especially considering that this region seems unprepared to face the effects of a potential earthquake similar to the VI century AD one.

P>The shallow subsurface structure of the 2009 April 6 M-w 6.3 L'Aquila earthquake surface rupture at Paganica has been investigated with ground penetrating radar to study how the surface rupture relates spatially to previous surface displacements during the Holocene and Pleistocene. The discontinuous surface rupture stepped between en-echelon/parallel faults within the overall fault zone that show clear Holocene/Pleistocene offsets in the top 10 m of the subsurface. Some portions of the fault zone that show clear Holocene offsets were not ruptured in 2009, having been bypassed as the rupture stepped across a relay zone onto a fault across strike. The slip vectors, defined by opening directions across surface cracks, indicate dip-slip normal movement, whose azimuth remained constant between 210 degrees and 228 degrees across the zone where the rupture stepped between faults. We interpret maximum vertical offsets of the base of the Holocene summed across strike to be 4.5 m, which if averaged over 15 kyr, gives a maximum throw-rate of 0.23-0.30 mm yr-1, consistent with throw-rates implied by vertical offsets of a layer whose age we assume to be similar to 33 ka. This compares with published values of 0.4 mm yr-1 for a minimum slip rate implied by offsets of Middle Pleistocene tephras, and 0.24 mm yr-1 since 24.8 kyr from palaeoseismology. The Paganica Fault, although clearly an important active structure, is not slipping fast enough to accommodate all of the 3-5 mm yr-1 of extension across this sector of the Apennines; other neighbouring range-bounding active normal faults also have a role to play in the seismic hazard.

Surface faulting earthquakes are known to cluster in time from historical and palaeoseismic studies, but the mechanism(s) responsible for clustering, such as fault interaction, strain-storage, and evolving dynamic topography, are poorly quantified, and hence not well understood. We present a quantified replication of observed earthquake clustering in central Italy. Six active normal faults are studied using 36Cl cosmogenic dating, revealing out-of-phase periods of high or low surface slip-rate on neighboring structures that we interpret as earthquake clusters and anticlusters. Our calculations link stress transfer caused by slip averaged over clusters and anti-clusters on coupled fault/shear-zone structures to viscous flow laws. We show that (1) differential stress fluctuates during fault/shear-zone interactions, and (2) these fluctuations are of sufficient magnitude to produce changes in strain-rate on viscous shear zones that explain slip-rate changes on their overlying brittle faults. These results suggest that fault/shear-zone interactions are a plausible explanation for clustering, opening the path towards process-led seismic hazard assessments.

Lake Garda, the largest lake in Italy at the southern front of the central Alps, provides a unique opportunity to reconstruct Quaternary geological processes related to the interplay between deglaciation driven sedimentation and tectonic activity. In fact, the topographic depression presently occupied by the lake was site of a major glacial tongue during the LGM, and is located close to the epicentral areas of two of the largest magnitude historical earthquakes recorded in the Po Plain, the Verona (Jan. 13, 1117) and Brescia, (Dec. 25, 1222) seismic events, characterized by Io IX-X MCS, respectively. Despite this peculiarity, geophysical surveys of the lake are scant. We present results of a waterborne geophysical survey of Lake Garda, leading to the acquisition of a densely-spaced grid of high-resolution seismic reflection profiles which image the lake floor and uppermost sedimentary sequence. These data enabled us to compile thematic maps which provide insights on active geologic processes in different sectors of the basin. We focused our work on a prominent seismic reflector (H1), correlated over the entire lake surface, to estimate the sediment deposition rate at the scale of the last 10 ka, and to evaluate the variability of seismic facies and geological processes in different sectors of the basin. Based on available stratigraphic data, we discuss whether H1 could mark the diachronic glacial/fluvio-glacial transition after the Last Glacial Maximum, when the ice tongues south of the Alps started to melt and collapse. We observed that sediments of the southern Lake Garda are affected by incipient tectonic deformation, and two major seismoturbidite-homogenite beds might represent over 50% of the Holocene sedimentary record in the lake depocenter, making the study of the Lake Garda stratigraphy an interesting tool for assessing earthquake hazards at the southern front of the Alpine chain.