Avallone, Antonio

After large earthquakes, parts of the fault continue to slip for days to months during the afterslip phase, a behaviour documented for many earthquakes. Yet, little is known about the early stage, i.e., from minutes to hours after the mainshock. Its detailed study requires continuous high-rate position time series close to the fault, and advanced signal processing to accurately extract the surface displacements. Here, we use refined kinematic precise point positioning processing to document the early postseismic deformation for three earthquakes along the South American subduction zone (2010 Mw8.8 Maule, Chile; 2015 Mw8.3 Illapel, Chile; 2016 Mw7.6 Pedernales, Ecuador). First, we show that early afterslip generates significant surface displacement as early as a few tens of minutes after the earthquake. Our analysis of the time series indicates that, over the first 36 hours, more than half of the displacement occurs within the first 12 hours, a time window often disregarded with daily positioning. Thus, estimates of coseismic offsets can be biased by more than 10% if early postseismic displacements are acknowledged as coseismic ones. Finally, these results highlight the difficulty to accurately evaluate the different contribution to the seismic cycle budget and thus the associated hazard on faults.

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.

When analyzing the rupture of a large earthquake, geodetic data are often critical. These data are generally characterized by either a good temporal or a good spatial resolution, but rarely both. As a consequence, many studies analyze the coseismic rupture with data that also include one or more days of early postseismic deformation. Here, we invert simultaneously for the coseismic and postseismic slip with the condition that the sum of the two models remains compatible with data covering the two slip episodes. We validate the benefits of this approach with a toy model and an application to the 2009 Mw6.3 L'Aquila earthquake, using a Bayesian approach and accounting for epistemic uncertainties. For the L'Aquila earthquake, we find that if early postseismic deformation is not an explicitly acknowledged coseismic signal, coseismic slip models may overestimate the peak amplitude while long‐term postseismic models may largely underestimate the total postseismic slip amplitude. This example illustrates how the proposed approach could improve our comprehension of the seismic cycle, fault frictional properties, and the spatial and temporal relationship between seismic rupture, afterslip, and aftershocks.

The 2015/11/17 Lefkada (Greece) earthquake ruptured a segment of the Cephalonia Transform Fault (CTF) where probably the penultimate major event was in 1948. Using near-source strong motion and high sampling rate GPS data and Sentinel-1A SAR images on two tracks, we performed the inversion for the geometry, slip distribution and rupture history of the causative fault with a three-step self-consistent procedure, in which every step provided input parameters for the next one. Our preferred model results in a ~70° ESE-dipping and ~13° N-striking fault plane, with a strike-slip mechanism (rake ~169°) in agreement with the CTF tectonic regime. This model shows a bilateral propagation spanning ~9 s with the activation of three main slip patches, characterized by rise time and peak slip velocity in the ranges 2.5-3.5 s and 1.4-2.4 m/s, respectively, corresponding to 1.2-1.8 m of slip which is mainly concentrated in the shallower (<10 km) southern half of the causative fault. The inferred slip distribution and the resulting seismic moment (M0 = 1.05 × 1019N m) suggest a magnitude of Mw6.6. Our best solution suggests that the occurrence of large (Mw > 6) earthquakes to the northern and to the southern boundaries of the 2015 causative fault cannot be excluded.

In actively deforming regions, crustal deformation is accommodated by earthquakes and through a variety of transient aseismic phenomena. Here, we study the 2010-2014 Pollino (Southern Italy) swarm sequence (main shock MW5.1) located within the Pollino seismic gap, by analysing the surface deformation derived from Global Positioning System and Synthetic Aperture Radar data. Inversions of geodetic time series show that a transient slip, with the same mechanism of the main shock, started about 3-4 months before the main shock and lasted almost one year, evolving through time with acceleration phases that correlate with the rate of seismicity. The moment released by the transient slip is equivalent to MW5.5, significantly larger than the seismic moment release revealing therefore that a significant fraction of the overall deformation is released aseismically. Our findings suggest that crustal deformation in the Pollino gap is accommodated by infrequent "large" earthquakes (MW ≥ 6.5) and by aseismic episodes releasing a significant fraction of the accrued strain. Lower strain rates, relative to the adjacent Southern Apennines, and a mixed seismic/aseismic strain release are in favour of a longer recurrence for large magnitude earthquakes in the Pollino gap.

We investigate a large geodetic data set of interferometric synthetic aperture radar (InSAR) and GPS measurements to determine the source parameters for the three main shocks of the 2016 Central Italy earthquake sequence on 24 August and 26 and 30 October (Mw 6.1, 5.9, and 6.5,respectively). Our preferred model is consistent with the activation of four main coseismic asperities belonging to the SW dipping normal fault system associated with the Mount Gorzano-Mount Vettore-Mount Bove alignment. Additional slip, equivalent to aMw~ 6.1–6.2 earthquake, on a secondary (1) NE dipping antithetic fault and/or (2) on a WNW dipping low-angle fault in the hanging wall of the main system is required to better reproduce the complex deformation pattern associated with the greatest seismic event (the Mw 6.5 earthquake). The recognition of ancillary faults involved in the sequence suggests a complex interaction in the activated crustal volume between the main normal faults and the secondary structures and a partitioning of strain release.

We present a full 3-D velocity field of the Earth’s surface in the Euro-Mediter ranean area obtained from a combination of three different velocity solutions computed at the Centro Nazionale Terremoti (CNT) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV). All the publicly available GPS data since 1993, have been fully reprocessed by three different software tools and the final velocity field is estimated combining three independent velocity solutions in a least squares sense. The input velocity solutions are treated as stochastic sam- ples of the true velocity field by loosening the reference frame constraints in the associated variance-covariance matrix. The proposed approach allows for a fast and efficient combination of multi velocity solutions, taking into account the full network covariance, if available. The velocity map for the Euro-Medi- terranean region will be updated and released regularly on the web portal of the National GPS Network (http://ring.gm.ingv.it) and made available to the scientific community. Here we show and discuss the data analysis and the combination schemes, and the results of the combined velocity field.

We used continuous Global Positioning System (GPS) measurements to infer the fault geometry and the amount of coseismic slip associated to the August 24, 2016 Mw 6 Amatrice earthquake. We realized a three-dimensional coseismic displacement field by combining different geodetic solutions generated by three independent analyses of the raw GPS observations. The coseismic deformation field described in this work aims at representing a “consensus” solution that minimizes the systematic biases potentially present in the individual geodetic solutions. Because of the limited number of stations available we modeled the measured coseismic displacements using a uniform slip model, deriving the geometry and kinematics of the causative fault, finding good agreement between our geodetically derived fault plane and other seismological and geological observations.

On November 17, 2015 07:10:07 UTC a strong, shallow Mw6.5 earthquake, occurred on the island of Lefkada along a strike-slip fault with right-lateral sense of slip. The event triggered widespread environmental effects at the south and western part of the island while, the intensity and severity of these earthquake-induced deformations is substantially decreased towards the eastern part of the island. Relocation of seismicity and inversion of geophysical (GPS, InSAR) data indicate that the seismic fault runs parallel to the west coast of Lefkada, along the Aegean – Apulia plate boundary. The fault plane strikes N20 ± 5°E and dips to east with an angle of about 70 ± 5°. Coseismic deformation was measured in the order of tens of centimeters of horizontal motion by continuous GPS stations of NOANET (the NOA GPS network) and by InSAR (Sentinel 1 A image pairs). A coseismic uniform-slip model was produced from inversion of InSAR data and permanent GPS stations. The earthquake measured Mw = 6.5 using both the geodetic moment produced by the slip model, as well as the PGD relation of Melgar et al. (2015, GRL). In the field we observed no significant vertical motion of the shoreline or surface expression of faulting, this is consistent with the predictions of the model. The interferograms show a large decorrelation area that extends almost along all the western coast of Lefkada. This area correlates well with the mapped landslides. The 2003–2015 pattern of seismicity in the Ionian Sea region indicates the existence of a 15-km seismic gap offshore NW Cephalonia.

We used High-Rate sampling Global Positioning System (HRGPS) data from 52 permanent stations to retrieve the coseismic dynamic displacements related to the 2016 August 24 Mw 6.0 Amatrice earthquake. The HRGPS position time series (named hereinafter "GPSgrams") were obtained with two different analysis strategies of the raw GPS measurements (Precise Point Positioning [PPP] and Double-Difference [DD] positioning approaches using the Gipsy-Oasis II and the TRACK (GAMIT/GLOBK) software, respectively). These GPSgrams show RMS accuracies mostly within 0.3 cm and, for each site, an agreement within 0.5 cm between the two solutions. By using cross-correlation technique, the GPSgrams are also compared to the doubly-integrated strong motion data at sites where the different instrumentations are co-located in order to recognize in the GPSgrams the seismic waves movements. The high values (mostly greater than 0.6) of the cross-correlation functions between these differently-generated waveforms (GPSgrams and the SM displacement time-histories) at the co-located sites confirm the ability of GPS in providing reliable waveforms for seismological applications.