Abstract
On 18 January 2017, the 2016–2017 central Italy seismic sequence reached the Campotosto area with four events with magnitude larger than 5 in three hours (major event MW 5.5). To study the slip behavior on the causative fault/faults we followed two different methodologies: (1) we use Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1 satellites) and Global Positioning System (GPS) coseismic displacements to constrain the fault geometry and the cumulative slip distribution; (2) we invert near-source strong-motion, high-sampling-rate GPS waveforms, and high-rate GPS-derived static offsets to retrieve the rupture history of the two largest events. The geodetic inversion shows that the earthquake sequence occurred along the southern segment of the SW-dipping Mts. Laga normal fault system with an average slip of about 40 cm and an estimated cumulative geodetic moment of 9.29 × 1017 Nm (equivalent to a MW~6). This latter estimate is larger than the cumulative seismic moment of all the events, with MW > 4 which occurred in the corresponding time interval, suggesting that a fraction (~35%) of the overall deformation imaged by InSAR and GPS may have been released aseismically. Geodetic and seismological data agree with the geological information pointing out the Campotosto fault segment as the causative structure of the main shocks. The position of the hypocenters supports the evidence of an up-dip and northwestward rupture directivity during the major shocks of the sequence for both static and kinematic inferred slip models. The activated two main slip patches are characterized by rise time and peak slip velocity in the ranges 0.7–1.1 s and 2.3–3.2 km/s, respectively, and by ~35–50 cm of slip mainly concentrated in the shallower northern part of causative fault. Our results show that shallow slip (depth < 5 km) is required by the geodetic and seismological observations and that the inferred slip distribution is complementary with respect to the previous April 2009 seismic sequence affecting the southern half of the Campotosto fault. The recent moderate strain-release episodes (multiple M~5–5.5 earthquakes) and the paleoseismological evidence of surface-rupturing events (M~6.5) suggests therefore a heterogeneous behavior of the Campotosto fault.
Highlights
In recent decades, the increasing availability of geodetic measurements such as short repeat-time Interferometric Synthetic Aperture Radar (InSAR) measurements and continuous Global Positioning System (GPS) observations in combination with seismological data has revealed that slip-on tectonic faults can occur in a continuous spectrum of modes, including seismic and aseismic events of different extent, duration, and temporal evolution (e.g., [1,2,3,4,5,6,7])
Laga normal fault system with an average slip of about 40 cm and an estimated cumulative geodetic moment of 9.29 × 1017 Nm. This latter estimate is larger than the cumulative seismic moment of all the events, with MW > 4 which occurred in the corresponding time interval, suggesting that a fraction (~35%) of the overall deformation imaged by InSAR and GPS may have been released aseismically
The best-fit uniform slip fault parameters are used as a priori information for estimating the cumulative geodetic slip distribution using both InSAR and GPS datasets
Summary
The increasing availability of geodetic measurements such as short repeat-time Interferometric Synthetic Aperture Radar (InSAR) measurements and continuous Global Positioning System (GPS) observations in combination with seismological data has revealed that slip-on tectonic faults can occur in a continuous spectrum of modes, including seismic (earthquakes) and aseismic (e.g., creep events, afterslip, slow slips) events of different extent, duration, and temporal evolution (e.g., [1,2,3,4,5,6,7]). We use geodetic and seismological observations to document the coseismic and aseismic deformation associated with the 18 January 2017 multiple moderate seismic events (main event MW 5.5) that struck the Campotosto area in central Italy. This swarm of moderate-size events is part of the 2016–2017 central Italy seismic sequence that affected the central Apennines starting on 24 August 2016 with a MW 6.0 event in the Amatrice area (Figure 1), and culminated with the 30 October 2016 MW 6.5 largest event [11]. While some authors define the Laga Mts. fault structure as a single continuous segment (e.g., [24,25]), other studies propose instead a segmentation of the fault system into two independent seismogenic sources—the minor (~10 km long) Amatrice fault northern segment, bounding the Amatrice basin, and the ~20 km long Campotosto fault southern segment (e.g., [21,26,27]), bounding the Campotosto basin
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