[1] We use GPS measurements and block modeling to investigate the present-day deformation of the Adriatic region, whose kinematics within the Nubia-Eurasia plate boundary zone is not well constrained and remains controversial. Block modeling allows us to compute rigid-plate angular velocities while accounting for elastic strain accumulation along block-bounding faults. Results suggest that the Adriatic is a microplate (Adria) and that the southern boundary with the Nubia plate and the Aegean domain may be located along the Apulia Escarpment and the Kefallinia fault. Geodetic data alone cannot discriminate between a single block (AP) or a two blocks (GDAP) description of Adria, but the GDAP model predicts boundary slip rates that are in better agreement with observations from previous studies.

The Adriatic region: an independent microplate within the Africa-Eurasia collision zone

Most of metropolitan France and conterminous Western Europe is currently located within the Eurasia intraplate domain, far from major plate boundaries (the Atlantic ridge and Nubia – Eurasia convergence zone). As in other intraplate regions, present-day deformation and seismicity rates are very slow, resulting in limited data and strong uncertainties on the ongoing seismotectonics and seismic hazards. In the last two decades, new geological, seismological and geodetic data and research have brought to light unexpected deformation patterns in metropolitan France, such as orogen-normal extension ca. 0.5 mm yr−1 in the Pyrenees and Western Alps that cannot be associated with their mountain-building history. Elsewhere, present-day deformation and seismicity data provide a partial picture that points to mostly extensive to strike-slip deformation regimes (except in the Western Alps foreland). A review of the numerous studies and observations shows that plate tectonics (plate motion, mantle convection) are not the sole, nor likely the primary driver of present-day deformation and seismicity and that additional processes must be considered, such as topography potential energy, erosion or glacial isostatic adjustment since the last glaciation. The exact role of each process probably varies from one region to another and remains to be characterized. In addition, structural inheritance (crust or mantle weakening from past tectonic events) can play a strong role in deformation localization and amplification up to factors of 5–20, which could explain some of the spatial variability in seismicity. On the basis of this review, we identify three research directions that should be developed to better characterize the seismicity, deformation rates and related processes in metropolitan France: macroseismic and historical seismicity, especially regarding moment magnitude estimations; geodetic deformation, including in regions of low seismicity where the ratio of seismic to aseismic deformation remains a key unknown; an integrated and consistent seismotectonic framework comprising numerical models, geological, seismological and geodetic data. The latter has the potential for significant improvements in the characterization of seismicity and seismic hazard in metropolitan France but also Western Europe.

/pdf/processes-and-deformation-rates-generating-seismicity-in-kxd6430xx1.pdf

Processes and deformation rates generating seismicity in metropolitan France and conterminous Western Europe

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JB017226

Fluid-Induced Swarms and Coseismic Stress Transfer: A Dual Process Highlighted in the Aftershock Sequence of the 7 April 2014 Earthquake (Ml 4.8, Ubaye, France)

Crustal deformation and seismicity in Europe are still poorly understood. Seismic activity is classically ascribed to crustal strain rates generated by edge-driven tectonic forces. However, crustal deformation is not only due to tectonic loading, but can also be related to isostatic and mantellic processes that induce additional stress and strain on the crust by flexure. The influence that those processes have on seismic activity, as well their interaction, is still controversial. The main limitation to study it is because the deformation and its causal mechanisms are usually analyzed separately in small regions. We present here a 3D secular velocity field that covers Eurasia and its plate boundaries including 4,863 Global Navigation Satellite System stations obtained by combining 10 different datasets. We have developed a method based on spatial filtering to identify outliers and smooth the velocity field, and have computed both strain and uplift rate maps that are interpreted in the light of the different driving processes that contribute to the 3D deformation in Europe. The vertical and horizontal deformation features are compared with seismic rates obtained from the spatial and temporal distribution of the seismicity in Europe. Our results suggest that is not possible to explain the seismicity in Europe with edge-driven horizontal tectonic forces only. In some areas markers of the crustal flexure such as the vertical velocity field and its derivative, resulting from buoyancy-driven processes, may help to interpret earthquake distribution. 

3D GNSS Velocity Field Sheds Light on the Deformation Mechanisms in Europe: Effects of the Vertical Crustal Motion on the Distribution of Seismicity

We present a new automated earthquake detection and location method based on beamforming (or back projection) and template matching, and apply it to study the seismicity of the Southwestern Alps. We use beamforming with prior knowledge of the 3D variations of seismic velocities as a first detection run to search for earthquakes that are used as templates in a subsequent matched-filter search. Template matching allows us to detect low signal to noise ratio events, and thus to obtain a high spatiotemporal resolution of the seismicity in the Southwestern Alps. We describe how we address the problem of false positives in energy-based earthquake detection with supervised machine learning, and how to best leverage template matching to iteratively refine the templates and the detection. We detected 18,754 earthquakes over one year (our catalog is available online), and observed temporal clustering of the earthquake occurrence in several regions. This statistical study of the collective behavior of earthquakes provides insights into the mechanisms of earthquake occurrence. Based on our observations, we infer the mechanisms responsible for the seismic activity in three regions of interest: the Ubaye valley, the Brianconnais and the Dora Maira massif. Our conclusions point to the importance of fault interactions to explain the earthquake occurrence in the Brianconnais and the Dora Maira massif, whereas fluids seem to be the major driving mechanism in the Ubaye valley.

/pdf/systematic-detection-of-clustered-seismicity-beneath-the-4hv2j257xq.pdf

Systematic detection of clustered seismicity beneath the Southwestern Alps

The availability of GPS survey data spanning 22 years, along with several independent velocity solutions including up to 16 years of permanent GPS data, presents a unique opportunity to search for persistent (and thus reliable) deformation patterns in the Western Alps, which in turn allow a reinterpretation of the active tectonics of this region While GPS velocities are still too uncertain to be interpreted on an individual basis, the analysis of range-perpendicular GPS velocity profiles clearly highlights zones of extension in the center of the belt (153 to 31 nanostrain/year from north to south), with shortening in the forelands The contrasting geodetic deformation pattern is coherent with earthquake focal mechanisms and related strain/stress patterns over the entire Western Alps The GPS results finally provide a reliable and robust quantification of the regional strain rates The observed vertical motions of 20 to 05 mm/year of uplift from north to south in the core of the Western Alps is interpreted to result from buoyancy forces related to postglacial rebound, erosional unloading, and/or viscosity anomalies in the crustal and lithospheric root Spatial decorrelation between vertical and horizontal (seismicity related) deformation calls for a combination of processes to explain the complex present-day dynamics of the Western Alps

https://hal.archives-ouvertes.fr/hal-01947724/document

Does Long-Term GPS in the Western Alps Finally Confirm Earthquake Mechanisms?

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Does Long-Term GPS in the Western Alps Finally Confirm Earthquake Mechanisms?