ALBERT

Description: Here we present the results of the inversion of a new geodetic dataset covering the 2012 Emilia seismic sequence and the following one year of post-seismic deformation. Modeling of the geodetic data together with the use of a catalog of 3-D relocated aftershocks, allows us to constrain the rupture geometries and the coseismic and post-seismic slip distributions for the two main events ( M W 6.1 and 6.0) of the sequence, and to explore how these thrust events have interacted with each other. Dislocation modeling reveals that the first event ruptured a slip patch located in the center of the Middle Ferrara thrust with up to 1 m of reverse slip. The modeling of the second event, located about 15 km to the southwest, indicates a main patch with up to 60 cm of slip initiated in the deeper and flatter portion of the Mirandola thrust, and progressively propagated post-seismically towards the top section of the rupture plane, where most of the aftershocks and afterslip occurred. Our results also indicate that between the two main events, a third thrust segment was activated releasing a pulse of aseismic slip equivalent to a M W 5.8 event. Coulomb stress changes suggest that the aseismic event was likely triggered by the preceding mainshock and that the aseismic slip event probably brought the second fault closer to failure. Our findings show significant correlations between static stress changes and seismicity and suggest that stress interaction between earthquakes plays a significant role among continental en echelon thrusts.

Description: SUMMARY In this study, we revisit the mechanism of the 1976 Friuli (NE Italy) earthquake sequence (main shocks M w 6.4, 5.9 and 6.0). We present a new source model that simultaneously fits all the available geodetic measurements of the observed deformation. We integrate triangulation measurements, which have never been previously used in the source modelling of this sequence, with high-precision levelling that covers the epicentral area. We adopt a mixed linear/non-linear optimization scheme, in which we iteratively search for the best-fitting solution by performing several linear slip inversions while varying fault location using a grid search method. Our preferred solution consists of a shallow north-dipping fault plane with assumed azimuth of 282° and accommodating a reverse dextral slip of about 1 m. The estimated geodetic moment is 6.6 × 10 18 Nm ( M w 6.5), in agreement with seismological estimates. Yet, our preferred model shows that the geodetic solution is consistent with the activation of a single fault system during the entire sequence, the surface expression of which could be associated with the Buia blind thrust, supporting the hypothesis that the main activity of the Eastern Alps occurs close to the relief margin, as observed in other mountain belts. The retrieved slip pattern consists of a main coseismic patch located 3–5 km depth, in good agreement with the distribution of the main shocks. Additional slip is required in the shallower portions of the fault to reproduce the local uplift observed in the region characterized by Quaternary active folding. We tentatively interpret this patch as postseismic deformation (afterslip) occurring at the edge of the main coseismic patch. Finally, our rupture plane spatially correlates with the area of the locked fault determined from interseismic measurements, supporting the hypothesis that interseismic slip on the creeping dislocation causes strain to accumulate on the shallow (above ∼10 km depth) locked section. Assuming that all the long-term accommodation between Adria and Eurasia is seismically released, a time span of 500–700 years of strain-accumulating plate motion would result in a 1976-like earthquake.

Description: The inversion of multitemporal DInSAR and GPS measurements unravels the coseismic and postseismic (afterslip) slip distributions associated with the 2009 MW 6.3 L'Aquila earthquake and provides insights into the rheological properties and long-term behavior of the responsible structure, the Paganica fault. Well-resolved patches of high postseismic slip (10–20 cm) appear to surround the main coseismic patch (maximum slip ≈1 m) through the entire seismogenic layer above the hypocenter without any obvious depth-dependent control. Time series of postseismic displacement are well reproduced by an exponential function with best-fit decay constants in the range of 20–40 days. A sudden discontinuity in the evolution of released postseismic moment at ≈130 days after the main shock does not correlate with independent seismological and geodetic data and is attributed to residual noise in the InSAR time series. The data are unable to resolve migration of afterslip along the fault probably because of the time interval (six days) between the main shock and the first radar acquisition. Surface fractures observed along the Paganica fault follow the steepest gradients of postseismic line-of-sight satellite displacements and are consistent with a sudden and delayed failure of the shallow layer in response to upward tapering of slip. The occurrence of afterslip at various levels through the entire seismogenic layer argues against exclusive depth-dependent variations of frictional properties on the fault, supporting the hypothesis of significant horizontal frictional heterogeneities and/or geometrical complexities. We support the hypothesis that such heterogeneities and complexities may be at the origin of the long-term variable behavior suggested by the paleoseismological studies. Rupture of fault patches with dimensions similar to that activated in 2009 appears to have a ≈500 year recurrence time interval documented by paleoseismic and historical studies. In addition to that, paleoseismological evidence of large (〉0.5 m) coseismic offsets seems to require seismic events, recurring every 1000–2000 years, characterized by (1) multisegment linkage, (2) surface ruptures larger than in 2009, and (3) complete failure of the 2009 coseismic and postseismic patches.

Description: Here we use continuous GPS observations to document the geodetic strain accumulation across the South-Eastern Alps (NE Italy). We estimate the interseismic coupling on the intra-continental collision thrust fault and discuss the seismic potential and earthquake recurrence. We invert the GPS velocities using the back-slip approach to simultaneously estimate the relative angular velocity and the degree of interseismic coupling on the thrust fault that separates the Eastern Alps and the Venetian-Friulian plain. Comparison between the rigid-rotation predicted motion and the shortening observed across the area indicates that the South-Eastern Alpine thrust front absorbs about 80% of the total convergence between the Adria and Eurasia plates. The coupling is computed on a north-dipping fault following the continuous external seismogenic thrust front of the South-Eastern Alps. The modelled thrust fault is currently locked from the surface to a depth of ≈ 10 km. The transition zone between locked and creeping portions of the fault roughly corresponds with the belt of microseismicity parallel and to the north of the mountain front. The estimated moment deficit rate is 1.3 ± 0.4 × 10 17  Nm/yr. The comparison between the estimated moment deficit and that released historically by the earthquakes suggests that to account for the moment deficit the following two factors or their combination should be considered: (1) a significant part of the observed interseismic coupling is released aseismically; (2) infrequent “large" events with long return period (〉 1000 years) and with magnitudes larger than the value assigned to the largest historical events ( M W  ≈ 6.7).

Description: We investigate a large geodetic dataset of InSAR and GPS measurements to determine the source parameters for the three mainshocks of the 2016 Central Italy earthquake sequence on 24th August, 26th and 30th October (M W 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 Mt. Gorzano-Mt. Vettore-Mt. Bove (MGVB) alignment. Additional slip, equivalent to a M W ~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 M W 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.

Description: Here we inverted the GPS data to infer the coseismic slip of the Tohoku-Oki earthquake and the time-dependent afterslip distribution in the 4 months following the main shock. The Tohoku-Oki earthquake showed an unexpected magnitude and a characteristic depth-dependent differentiation of seismic energy radiation. In this context the estimation and comparison of the distribution of the fault portions that slip coseismically and post-seismically contribute to a better understanding of the variation of frictional characteristics of the plate interface. The inferred coseismic slip extends in a relatively compact region located updip from the hypocentre and reaches its highest value (about 60 m) near the trench. Afterslip occurs mostly outside the coseismic rupture and is distributed in two main modal centres. It reaches its largest values in an area located downdip of the coseismic slip and extends to a depth of 80 km. In the depth range between 30 and 50 km afterslip overlaps the portion of the fault that experienced historical moderate earthquakes, high-frequency seismic radiation and thrust-type aftershocks. The behaviour of this area can be explained by a rheologically heterogeneous region made of a ductile fault matrix interspersed with compact brittle asperities. On the contrary, the region beneath 50–60 km depth is probably characterized by a fully velocity strengthening behaviour. Southern afterslip, located off-Chiba Prefecture, is probably related to the M w 7.9 Ibaraki-Oki aftershock. The northward extension of the afterslip stops at a latitude of about 40°N, just south of the off-Aomori region. This may be related to three large events occurred in this area during the last century and the consequent strong coupling or complete depletion of the accumulated strain that characterize this region.

Description: Existing models for the rupture geometry and slip distribution associated with the 30 October MW 6.6 Mt. Vettore-Mt. Bove earthquake in central Italy show significant dissimilarities. Indeed, due to the quite complicated observed deformation pattern, the activation of a complex multifault structure during a single seismic event was invoked. In this study, we explore different rupture scenarios and we develop a robust model of the rupture process of the 30 October earthquake, designed from new field observations, aftershock distribution, and static coseismic offsets including new near-field survey-mode global position system measurements, regional Global Positioning System observations, Interferometric Synthetic Aperture Radar interferograms, and static displacements derived from strong-motion stations. Our preferred best fit model involves the simultaneous rupture of the master Mt. Vettore-Mt. Bove normal fault and of at least two secondary antithetic faults (as they significantly contributed to the total deformation field), which overall describe a “simple conceptual” half-graben normal fault system and whose arrangement fits the geological, seismological, and coseismic evidence of surface faulting. Notably, our model fits the geometry of seismogenic structures defined prior to the 2016–2017 seismic sequence by field Quaternary geological observations. In addition, no significant coseismic slip on faults alternative to the master and antithetic faults is necessary to explain the observed surface displacements during the 30 October Mt. Vettore-Mt. Bove earthquake.

Description: 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 (M w 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 a M w  ~ 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 M w  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. ©2017. The Authors.