Lithological control on multiple surface ruptures during the 2016–2017 Amatrice-Norcia seismic sequence
Graphical abstract
Introduction
The spatio-temporal evolution of seismic sequence and surface ruptures are influenced by the complexity of the rock volume affected by earthquakes, such as lithological heterogeneities and tectonic settings (e.g., Yoshida et al., 1996; Stein et al., 1997; Lee et al., 2005; Valoroso et al., 2013; Valerio et al., 2017). Among these factors, the well-establish magnitude-displacement relationships show that an increase in earthquake magnitude is accompanied by an increase of surface displacement (Wells and Coppersmith, 1994; Ambraseys and Jackson, 1998). However, during the 2016–2017 Amatrice-Norcia seismic sequence in central Italy complex surface ruptures occurred along the fault system that generated the August 24th Mw 6.0 and the October 30th mainshocks (e.g., Pucci et al., 2017), implying complex interaction between magnitude and subsurface geology within the area affected by the seismic sequence. Along the fault scarp (nastrino) of the Mt. Vettore the main fault slipped 25 cm and even more than 100 cm during the two events. The October 30th Mw 6.5 earthquake was recorded by local GPS stations positioned at a distance of few meters both in the footwall and in the hangingwall, documenting that slip evolved in 2–3 seconds and that the peak ground acceleration occurred after this displacement. This proves that the displacement on the fault surface was not due to a surficial landslide induced by shaking, but to the upward propagation and emergence of the buried fault plane (Wilkinson et al., 2017). Conversely, minor or absent surface displacement occurred along the Mt. Gorzano fault (Fig. 1).
The wealth of good quality seismological (e.g., ; Chiaraluce et al., 2017a,b), geodetic (e.g., Cheloni et al., 2016; Cheloni et al., 2017; Huang et al., 2017; Valerio et al., 2018; Bignami et al., 2019), and geological data (e.g., Pucci et al., 2017) acquired for the 2016–2017 Amatrice-Norcia sequence provide an excellent background to better understand the dynamics of surface fault rapture in this case history in Italy and possibly in other extensional tectonic settings. We interpret the complex pattern of surface rupture along fault strike as the interaction of geological and seismological variables.
The August 24th 2016 Mw 6.0 and the on October 30th 2016 Mw 6.5 earthquakes were accompanied by more than 74,000 aftershocks during a one-year long seismic sequence, including a Mw 5.9 event on October 26th 2016 and four Mw > 5 events on January 18th, 2017 (Fig. 1, Fig. 2; ; Chiaraluce et al., 2017a; data from ISIDe working group, 2016; http://iside.rm.ingv.it/iside/standard/index.jsp). The seismic sequence activated the Mt. Vettore fault system (central and north-western areas) and the Mt. Gorzano fault (south-eastern area), involving a rock volume characterized, at the surface, by a length of about 80 km, a width of about 15 km, and extending down to a depth of about 8 km. However, this seismogenic volume was activated progressively. This segmentation has been explained by the strong interaction between the inherited compressional thrusts and the younger and active normal faults (Chiaraluce et al., 2017a,b). Available geophysical, geodetic and geological data allow us to propose an alternative/complementary explanation for the segmentation of the seismic sequence in the framework of the graviquake model (Doglioni et al., 2015).
In summary, the present work investigates the origin of the multistage evolution of the seismic sequence, with the progressive involvement of different rock volumes affected by thousands of small (1 < Mw<5) magnitude events. In addition, we analyse the geometry of fault rupture and propose a model involving the development of a normal fault propagation fold during the August 24th 2016, Mw 6.0 earthquake, later cross-cut by the upward fault propagation and slip during the October 30th Mw 6.5 earthquake.
Section snippets
Geological and geophysical backgrounds
In the area affected by the 2016–2017 seismic sequence, thrusting and folding juxtaposed, since Tortonian time, up to ∼4.5 km thick pre-orogenic passive continental margin (Late Triassic dolostones/anhydrites and Jurassic-Oligocene limestones with marly interbeds) and foreland basin (Aquitanian-Tortonian marls and claystones with limestones interbeds) deposits above up to ∼3.5 km thick syn-orogenic deposits (i.e., foredeep deposits; Messinian claystones, marls, and sandstones of the Laga Fm.)
Methods
In this work, the following methods were used:
- (1)
We processed SAR dataset (see Table 1) by means of classical differential SAR interferometry (Massonnet et al., 1993; Massonnet and Feigl, 1998). The phase topography component was removed thanks the SRTM Digital Elevation Model at 1 arcsec resolution (Farr et al., 2007). We reduced phase noise by applying adaptive filter (Goldstein and Werner, 1998), and we used the Minimum Cost Flow algorithm to unwrap the phase and obtain the deformation maps (
Seismological and InSAR data analysis
Fig. 5 shows surface deformation from Synthetic Aperture Radar Interferometry (InSAR) data and earthquake distribution for the Amatrice sequence, started with the 2016 August 24th Mw 6.0 event (left panel of Fig. 5), the Norcia-Visso seismic sequence associated with the 2016 October 26th Mw 5.9 and the 2016 October 30th Mw 6.5 events (central panel of Fig. 5), and the Campotosto Lake sequence, characterized by the four 2017 January 18th M > 5 events (right panel of Fig. 5).
From the combined
Discussion
Using geological, seismological, and geodetic data, we interpret the Amatrice-Norcia-Campotosto seismic sequence in the framework of the graviquakes model for normal fault-related earthquakes (Fig. 7), proposed for the 2009 L’Aquila seismic sequence (Doglioni et al., 2011). In this model, the main fault extends from the brittle upper crust down to the ductile lower crust, whereas an antithetic fault is confined within the brittle upper crust. During the interseismic phase, a dilated wedge
Conclusions
Analysing the Amatrice-Norcia-Campotosto seismic sequence we conclude that:
- 1)
the slip distribution at the surface can be interpreted in terms of the occurrence of a normal fault propagation fold below the Vettore Mt. area during the September 24th Mw 6.0 event and that the fold was cut by the propagating extensional fault during the October 30th Mw 6.5 event; on the contrary, faulting remained blind in the Campotosto area. This difference is likely explained by the occurrence of competent
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.
Acknowledgments
Federica Riguzzi, Emanuela Valerio, Pietro Tizzani and Patrizio Petricca are thanked for common work and for stimulating discussions. We also thank Lauro Chiaraluce for providing seismicity data. An anonymous reviewer and the Editor of Journal of Geodynamics are thanked for constructive criticism. Financial support from PRIN2015-Project 2015EC9PJ5_001, Progetti di Ateneo Sapienza 2016 (Carlo Doglioni and Luca Aldega), Progetto di Ateneo Sapienza 2017 (Eugenio Carminati) and CNR- Project:
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