We present the first high-quality catalog of early aftershocks of the three mainshocks of the 2016 central Italy Amatrice-Visso-Norcia normal faulting sequence. We located 10,574 manually picked ...aftershocks with a robust probabilistic, non-linear method achieving a significant improvement in the solution accuracy and magnitude completeness with respect to previous studies. Aftershock distribution and relocated mainshocks give insight into the complex architecture of major causative and subsidiary faults, thus providing crucial constraints on multi-segment rupture models. We document reactivation and kinematic inversion of a WNW-dipping listric structure, referable to the inherited Mts Sibillini Thrust (MST) that controlled segmentation of the causative normal faults. Spatial partitioning of aftershocks evidences that the MST lateral ramp had a dual control on rupture propagation, behaving as a barrier for the Amatrice and Visso mainshocks, and later as an asperity for the Norcia mainshock. We hypothesize that the Visso mainshock re-activated also the deep part of an optimally oriented preexisting thrust. Aftershock patterns reveal that the Amatrice Mw5.4 aftershock and the Norcia mainshock ruptured two distinct antithetic faults 3-4 km apart. Therefore, our results suggest to consider both the MST cross structure and the subsidiary antithetic fault in the finite-fault source modelling of the Norcia earthquake.
Starting from late May 2012, the Emilia region (Northern Italy) was severely shaken by an intense seismic sequence, originated from a ML 5.9 earthquake on May 20th, at a hypocentral depth of 6.3km, ...with thrust-type focal mechanism. In the following days, the seismic rate remained high, counting 50 ML≥2.0 earthquakes a day, on average. Seismicity spreads along a 30km east–west elongated area, in the Po river alluvial plain, in the nearby of the cities Ferrara and Modena. Nine days after the first shock, another destructive thrust-type earthquake (ML 5.8) hit the area to the west, causing further damage and fatalities. Aftershocks following this second destructive event extended along the same east-westerly trend for further 20km to the west, thus illuminating an area of about 50km in length, on the whole. After the first shock struck, on May 20th, a dense network of temporary seismic stations, in addition to the permanent ones, was deployed in the meizoseismal area, leading to a sensible improvement of the earthquake monitoring capability there. A combined dataset, including three-component seismic waveforms recorded by both permanent and temporary stations, has been analyzed in order to obtain an appropriate 1-D velocity model for earthquake location in the study area. Here we describe the main seismological characteristics of this seismic sequence and, relying on refined earthquakes location, we make inferences on the geometry of the thrust system responsible for the two strongest shocks.
•First complete analysis of the 2012 Emilia mainshocks–aftershocks seismic sequence.•New catalog of the seismic sequence including data from temporary stations•Sensible improvement in terms of the activated fault system geometry definition•The different dip of the activated fault segments is highlighted and discussed.•Basic information are provided for further specific studies and hazard scenarios.
SKS‐wave birefringence is likely caused by ongoing and/or fossil mantle flow. Splitting parameters in the southern Tyrrhenian subduction zone define a quite complex pattern of fast directions with ...large delay times up to 2.7 s. Fast directions are prevalently trench parallel below the slab and rotate to trench normal at the western edge of the slab depicting a ring around it. We interpret these anisotropic patterns, also observed in laboratory experiment, as toroidal mantle flow induced by rollback subduction.
The Calabrian subduction zone is one of the narrowest arcs on Earth and a key area to understand the geodynamic evolution of the Mediterranean and other marginal seas. Here in the Ionian Sea, the ...African plate subducts beneath Eurasia. Imaging the boundary between the downgoing slab and the upper plate along the Calabrian subduction zone is important for assessing the potential of the subduction zone to generate megathrust earthquakes and was the main objective of this study. Here we present and analyze the results from a 380‐km‐long, wide‐angle seismic profile spanning the complete subduction zone, from the deep Ionian Basin and the accretionary wedge to NE Sicily, with additional constraints offered by 3‐D gravity modeling and the analysis of earthquake hypocenters. The velocity model for the wide‐angle seismic profile images thin oceanic crust throughout the basin. The Calabrian backstop extends underneath the accretionary wedge to about 100 km SE of the coast. The seismic model was extended in depth using earthquake hypocenters. The combined results indicate that the slab dip increases abruptly from 2–3° to 60–70° over a distance of ≤50 km underneath the Calabrian backstop. This abrupt steepening is likely related to the rollback geodynamic evolution of the narrow Calabrian slab, which shows great similarity to the shallow and deep geometry of the Gibraltar slab.
Plain Language
We investigate the deep crustal structure of southern Italy and the central Mediterranean where some of the oldest oceanic crust on Earth is actively descending (subducting) into its interior (Speranza et al., 2012). This process causes much of the moderate seismicity observed in this region and may be responsible for strong historical earthquakes as well (Gutscher et al., 2006). Deep seismic data recorded during a marine geophysical expedition performed in 2014 allow us to reconstruct the 3‐D geometry of this subduction zone. Our data reveal a 1–4‐km‐thick evaporitic (salt bearing) layer in the 13‐km‐thick accretionary wedge. The thin underlying crust has characteristics of oceanic crust. The adjacent onshore domains (E Sicily and SW Calabria) are composed of 25–30‐km‐thick crust with velocities typical of continental crust. Together with earthquake travel‐time tomography (providing images of the subducting slab down to 300 km) and gravity modeling, we can for the first time image the abrupt steepening of the subducting slab, the “slab hinge,” where slab dip increases from ≤5° to >60° over a downdip distance of 50 km. This slab dip is steep compared to other subduction zones, for example, in Northern Honshu, Japan, or Sumatra, where the slab dip remains roughly 10°‐ down to 40‐km depth and therefore may have consequences on the seismicity of the region.
Key Points
Wide‐angle seismic and 3D gravity modeling image the deep structure of the Calabrian subduction zone
The gravity modeling implies that there is no mantle layer between the Calabrian backstop crust and the dipping slab
Extension of the model using earthquake hypocenters indicates that the slab dip increases abruptly at the Calabrian backstop
Seismic swarms frequently occur along continental fault systems and their relation with large earthquakes is often contradictory. Such a case is documented in the Pollino mountain range of southern ...Italy, a decoupling zone where the belt-normal stretching drastically rotates accommodating the differential SE-retreat of the Ionian slab. The paucity of historical large earthquakes has led to hypothesize the presence of a seismic gap. A long-lasting seismic swarm that climaxed with a M
L
= 5.2 earthquake in October 2012 was therefore thought as a possible signal of an impending large earthquake filling the gap. Seismicity data collected during a 4-years long monitoring are a powerful microscope to look through the seismic swarm. In this study, we present accurate relocations for 2385 earthquakes and high-resolution Vp and Vp/Vs models of the fault system. Seismicity occurred on two separate normal faults that were formerly part of a thrusts and back-thrusts system, originally formed as a pop-up at restraining bends of the Pollino fault, a wrench fault system that inverted the original left lateral sense of slip accommodating a differential motion induced by the southward retreat of the Ionian slab.
Andrea Argnani in his comment on Dellong et al. (2020, https://doi.org/10.1029/2019gc008586) (Geometry of the deep Calabrian subduction (Central Mediterranean Sea) from wide‐angle seismic data and ...3‐D gravity modeling) proposes an alternate interpretation of the wide‐angle seismic velocity models presented by Dellong et al. (2018, https://doi.org/10.1002/2017JB015312) and Dellong et al. (2020) and proposes a correction of the literature citations in these paper. In this reply, we discuss in detail all points raised by Andrea Argnani.
Key Points
This contribution is a reply on a comment submitted by A. Argnani
The alternate interpretation of the wide‐angle seismic model is discussed
The Alfeo Fault system is proposed to be the current location of STEP fault
On May 20, 2012 (02:03:53 UTC), an Mw 5.86 (Ml 5.9) earthquake struck the Pianura Padana Emiliana region (northern Italy), causing five deaths and damage to several villages and to the towns of ...Ferrara and Modena. The mainshock was preceded, three hours earlier, by a Mw 3.98 (Ml 4.1) foreshock, which almost co-located with the main event. After the main event, the seismic sequence included six earthquakes with magnitudes >5.0. The biggest aftershock was located about 12 km west of the first mainshock, and was a Mw 5.66 (Ml 5.8) earthquake that occurred on May 29, 2012 (07:00:03 UTC); this can be considered as a second mainshock. After this event, the official death toll of the seismic sequence was 17 people. Moreover, there had been severe damage to the economy of the region and there were 13,000 homeless. ...
Combining tectonics, with seismological and geochemical data, we reconstruct the deformation history of the presently narrow Calabrian slab and the path of mantle circulation during the last 10 Ma. ...We show that during the slab deformation the mantle laterally flowed inside the back arc region permitting its retrograde motion and giving a seismological and volcanological record after 1–2 myr.
Abstract Laser interferometry enables to remotely measure microscopical length changes of deployed telecommunication cables originating from earthquakes. Long reach and compatibility with data ...transmission make it attractive for the exploration of both remote regions and highly-populated areas where optical networks are pervasive. However, interpretation of its response still suffers from a limited number of available datasets. We systematically analyze 1.5 years of acquisitions on a land-based telecommunication cable in comparison to co-located seismometers, with successful detection of events in a broad magnitude range, including very weak ones. We determine relations between a cable’s detection probability and the events magnitude and distance, introducing spectral analysis of fiber data as a tool to investigate earthquake dynamics. Our results reveal that quantitative analysis is possible, confirming applicability of this technique both for the global monitoring of our planet and the daily seismicity monitoring of populated areas, in perspective exploitable for civilian protection.
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