ALBERT

Description: We present a new GPS velocity field covering the peri-Adriatic tectonically-active belts and the entire Balkan Peninsula. From the velocities, we calculate consistent strain rate and interpolated velocity fields. Significant features of the crustal deformation include: (1) the eastward motion of the northern part of the Eastern Alps together with part the Alpine foreland and Bohemian Massif towards the Pannonian Basin, (2) shortening across the Dinarides, (3) a clock-wise rotation of the Albanides-Hellenides and (4) a southward motion South of 44 ∘ N of the inner Balkan lithosphere between the rigid Apulia and Black Sea, towards the Aegean domain. Using this new velocity-field, we derive the strain rate tensor to analyze the regional style of the deformation. Then, we devise a simple test based on the momentum balance equation, to investigate the role of horizontal gradients of gravitational potential energy in driving the deformation in the Peri-Adriatic tectonically-active mountain belts : the Eastern Alps, the Dinarides, the Albanides and the Apennines. We show that the strain rate fields observed in the Apennines and Albanides are consistent with a fluid, with viscosity η ∼ 3×10 21 Pas, deforming in response to horizontal gradients of gravitational potential energy. Conversely, both the Dinarides and Eastern Alps are probably deforming in response to the North and North-East oriented motion of the Adria-Apulia indenter respectively, and as a consequence of horizontal lithospheric heterogeneity.

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: The 6 April 2009 Mw 6.3 L'Aquila destructive earthquake was successfully recorded by closely spaced 10 Hz and 1 Hz recording GPS receivers and strong motion accelerometers located above or close to the 50° dipping activated fault. We retrieved both static and dynamic displacements from very high rate GPS (VHRGPS) recordings by using Precise Point Positioning kinematic analysis. We compared the GPS positions' time series with the closest displacement time series obtained by doubly integrating strong motion data, first, to assess the GPS capability to detect the first seismic arrivals (P waves) and, second, to evaluate the accelerometers' capability to detect coseismic offsets up to ∼45 s after the earthquake occurrence. By comparing seismic and VHRGPS frequency contents, we inferred that GPS sampling rates greater than 2.5 Hz (i.e., 5 or 10 Hz) are required in the near field of moderate-magnitude events to provide “alias-free” solutions of coseismic dynamic displacements. Finally, we assessed the consistency of the dynamic VHRGPS results as a constraint on the kinematic rupture history of the main shock. These results suggested that the high-rate sampling GPS sites in the near field can be as useful as strong motion stations for earthquake source studies.

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: [1]  Here I compare estimates of tectonic strain rates from dense Global Positioning System (GPS) measurements with the seismicity released in the last ~500 years in the Apennines (Italy). The rates of seismic moment accumulation from geodesy and of historical seismic release by earthquakes agree within the uncertainties, ruling out significant aseismic deformation. Within the considered 400 km-long section of the Apennines, this balance yields an average recurrence interval of 30-75 years for M W  ≥  6.5 events without requiring a future earthquake larger than those observed historically ( M W  ~ 7). A minimum estimate of unreleased strain allows M W  ≥  6.5 and M W  ≥  6.9 events to be released in ~35% and ~10% of the central-southern Apennines, respectively. The definition of the seismic potential for smaller events is more uncertain, and their occurrence remains a significant threat throughout the Apennines.