We apply a blind source separation algorithm to the ground displacement time series recorded at continuous Global Positioning System (GPS) stations in the European Eastern Alps and Northern Dinarides. As a result, we characterize the temporal and spatial features of several deformation signals. Seasonal displacements are well described by loading effects caused by Earth surface mass redistributions. More interestingly, we highlight a horizontal, nonseasonal, transient deformation signal, with spatially variable amplitudes and directions. The stations affected by this signal reverse the sense of movement with time, implying a sequence of dilatational and compressional deformation that is oriented normal to rock fractures in karst areas. The temporal evolution of this deformation signal is correlated with the history of cumulated precipitations at monthly time scales. This transient horizontal deformation can be explained by pressure changes associated with variable water levels within vertical fractures in the vadose zones of karst systems. The water level changes required to open or close these fractures are consistent with the fluctuations of precipitation and with the dynamics of karst systems.
In this work we present and discuss new geodetic velocity and strain-rate fields for the Euro-Mediterranean region obtained from the analysis of continuous GNSS stations. We describe the procedures and methods adopted to analyze raw GPS observations from >4000 stations operating in the Euro-Mediterranean, Eurasian and African regions. The goal of this massive analysis is the monitoring of Earth’s crust deformation in response to tectonic processes, including plate- and micro-plate kinematics, geodynamics, active tectonics, earthquake-cycle, but also the study of a wide range of geophysical processes, natural and anthropogenic subsidence, sea-level changes, and hydrology. We describe the computational infrastructure, the methods and procedures adopted to obtain a three-dimensional GPS velocity field, which is used to obtain spatial velocity gradients and horizontal strain-rates. We then focus on the Euro-Mediterranean region, where we discuss the horizontal and vertical velocities, and spatial velocity gradients, obtained from stations that have time-series lengths longer than 6 and 7 years, which are found to be the minimum spans to provide stable and reliable velocity estimates in the horizontal and vertical components, respectively. We compute the horizontal strain-rate field and discuss deformation patterns and kinematics along the major seismogenic belts of the Nubia-Eurasia plate boundary zone in the Mediterranean region. The distribution and density of continuous GNSS stations in our geodetic solution allow us to estimate the strain-rate field at a spatial scale of ∼27 km over a large part of southern Europe, with the exclusion of the Dinaric mountains and Balkans.
Constant redistribution of surface loads due to continental hydrology (van Dam et al., 2001) causes measurable deformation of the Earth's surface. In particular, seasonal hydrological mass movements turned out to influence tectonic deformation of the lithosphere and modulate seismicity rates in several tectonic environments (e.g.,
It is known that changes in continental water storage can produce vertical surface deformation, induce crustal stress perturbations and modulate seismicity rates. However, the degree to which local changes in terrestrial water content influence crustal stresses and the occurrence of earthquakes remains an open problem. We show how changes in terrestrial water storage, computed for a ~1000 km^2 basin, focus deformation in a narrow zone, causing horizontal, non-seasonal displacements. We present results from a karstic mountain range located at the edge of the Adria-Eurasia plate boundary system in northern Italy, where shortening is accommodatedacross an active fold-and-thrust belt. The presence of geological structures with highpermeabilities and of deeply rooted hydrologically-active fractures focus groundwater fluxes and pressure changes, generating transient horizontal deformation and perturbations of crustal stress up to 25 kPa, at seismogenic depths. The background seismicity rates appear correlated, without evident temporal delay, with the terrestrial water content in the hydrological basin. Being independent from hydraulic diffusivity, seismicity modulation is likely affected by direct stress changes on faults planes.
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