The Decollement Depth of Active Thrust Faults in Italy: Implications on Potential Earthquake Magnitude

Abstract

Thrust fault ruptures during earthquakes do not often propagate down to the brittle‐ductile transition. Lithological variations control the behavior and depth of regional basal thrusts and decollement planes. Thrust fronts may be discontinuous along strike, limiting the dimension of single coseismic ruptures. These factors control the maximum expected magnitude in one region. This is the case of Italy where the convergence of few millimeter per year in the Apennines accretionary prism and along the retrobelt of the Alps generates compressional earthquakes with moderate to strong magnitudes. Here, using geological and geophysical data, we first compile a map of the undulated active basal thrust decollement for Italy that occurs from 1 to 17‐km depth. Then, we verify the relationship between the length of seismogenic ruptures in thrust faults (Lf) and the maximum depth of thrust faulting (zmax) of related earthquakes and find that their ratio (Lf/zmax) ranges between 2 and 4. Finally, we compute the potential seismogenic volume and estimate the maximum magnitude using an empirical relationship that multiplies the decollement depth and the Lf/zmax ratio. Maximum calculated magnitude is 6.7 ± 0.37 (depending on Lf/zmax and fault dip angle), consistent with the largest magnitude of thrust‐related earthquakes recorded in Italy (6.5–7.0). Lower magnitudes are predicted in the Ionian Seas at the external front of the Apennines where smaller crustal volumes are involved, whereas higher magnitudes are expected in the southern Po Basin, the western Adriatic Sea, Sicily offshore, and the Southern Alps where the decollement is deeper and the brittle volumes are far greater.

Late Quaternary Tectonics along the Peri-Adriatic Sector of the Apenninic Chain (Central-Southern Italy): Inspecting Active Shortening through Topographic Relief and Fluvial Network Analyses

“…Our findings of localized shortening-related uplift are in line with some of the recent literature which, exploiting seismological, geophysical, and/or morphotectonic analyses, conclude that shortening at the boundary between the central and southern Apennines can be possibly accommodated by seismogenic faults [48,51,168,169]. Differently, they are difficult to explain with long-wavelength regional uplift eventually related to dynamic processes (see [127] among the others; see Section 2.1 for more extensive literature) in the central-southern Apennines.…”

Turning the Orogenic Switch: Slab‐Reversal in the Eastern Alps Recorded by Low‐Temperature Thermochronology

“…These lines of thought are in agreement with present‐day seismic and kinematic observations. For example, present‐day seismic activity is predominantly centered to the south of the Alps (Figures 1b and 2c; Danesi et al., 2015; Petricca et al., 2019). GPS‐based kinematics of upper lithospheric deformation (Nocquet & Calais, 2004) suggest the regions of highest strain are at the southernmost extent of TRANSALP.…”

Active Fold‐Thrust Belt to Foreland Transition in Northern Adria, Italy, Tracked by Seismic Reflection Profiles and GPS Offshore Data

“…In this view, the outermost ramp of the northern Apennines belt in the studied marine region may be a potential seismic source, although other more internal thrust faults could be potentially seismic as well. Considering the detachment depth (Petricca et al, 2019), we cannot exclude, along these structures, seismic events, similar to the Mw 6.0 and Mw 5.8 2012 Emilia earthquakes (e.g., Govoni et al, 2014; Pezzo et al, 2013, 2018) and to those reported in the historical seismic catalog (e.g., Camassi et al, 1991 and references therein, Molin et al, 2008), even though seismic reflection profiles in our and previous studies (Di Bucci & Mazzoli, 2002 and references therein) show no evidence of active tectonic deformations. This lack of evidences in the recent Quaternary deposits could be related to the effect of the large increase of sediment supply in the foredeep during the Quaternary eustatic low stand, which eventually produced a smaller apparent fault throw or masked the internal tectonic deformation (Scrocca et al, 2007).…”