Shear-wave splitting perspectives from the intense aftershock sequence of Damasi – Tyrnavos

The area of Damasi-Tyrnavos (Thessaloniki, Central Greece), in the vicinity of Larissa, was characterized  by low seismic activity during the last decades. Two strong earthquakes of Mw = 6.3 and Mw = 6.0 The area of Damasi – Tyrnavos (Thessaly, Central Greece), in the vicinity of Larissa, was characterized occurred in early March 2021, followed by an intense aftershock sequence, related to WNW-ESE to NW-SE oriented faulting. This sequence was recorded by a dense local seismological network that provided a rich dataset and a unique opportunity to investigate upper crust shear-wave splitting for the first time in the study area. A fully automated technique, employing the eigenvalues method and cluster analysis, was implemented to measure the fast shear-wave polarization direction and the time-delay between the two split-shear-waves. This procedure yielded 655 results of adequate quality grade at 9 stations, after analyzing 1602 events and applying strict selection criteria, including the shear-wave window. The measured directions revealed a complex upper crust anisotropic regime. WNW-ESE to NW-SE, in accordance both with the APE model, being parallel to the local Hmax direction, and the strike of the fault planes. On the other hand, stations at the central part exhibit NNW-SSE and NNE-SSW anisotropy directions. An interesting feature is that the two northern stations are characterized by larger normalized time-delay values, possibly related to the migration of seismicity to the north during the initial stage of the seismic sequence

Transnational Access to Research Facilities: an EPOS service to promote multi-domain Solid Earth Sciences in Europe

Transnational access (TNA) allows cross-border, short-term and frequently free-of-charge access to world-class research facilities, to foster collaborations and exchanges of experience. Specifically, TNA aims to encourage open science and innovation and to increase the efficient and effective use of scientific infrastructure. Within EPOS, the European Plate Observing System, the Volcano Observatories and Multi-scale Laboratories communities have offered TNA to their high-quality research facilities through national and European funding. This experience has allowed the definition, design, and testing of procedures and activities needed to provide transnational access inn the EPOS context. In this paper, the EPOS community describes the main objectives for the provision of transnational access in the EPOS framework, based on previous experiences. It includes practical procedures for managing transnational access from a legal, governance, and financial perspective, and proposes logistical and technical solutions to effectively execute transnational access activities. In addition, it provides an outlook on the inclusion of new thematic communities within the TNA framework, and addresses the challenges of providing market-driven access to industry.publishedVersio

Investigation of the Thiva 2020–2021 Earthquake Sequence Using Seismological Data and Space Techniques

We investigate an earthquake sequence involving an Mw = 4.6 mainshock on 2 December 2020, followed by a seismic swarm in July–October 2021 near Thiva, Central Greece, to identify the activated structures and understand its triggering mechanisms. For this purpose, we employ double-difference relocation to construct a high-resolution earthquake catalogue and examine in detail the distribution of hypocenters and the spatiotemporal evolution of the sequence. Furthermore, we apply instrumental and imaging geodesy to map the local deformation and identify long-term trends or anomalies that could have contributed to stress loading. The 2021 seismic swarm was hosted on a system of conjugate normal faults, including the eastward extension of the Yliki fault, with the main activated structures trending WNW–ESE and dipping south. No pre- or coseismic deformation could be associated with the 2021 swarm, while Coulomb stress transfer due to the Mw = 4.6 mainshock of December 2020 was found to be insufficient to trigger its nucleation. However, the evolution of the swarm is related to stress triggering by its major events and facilitated by pore-fluid pressure diffusion. The re-evaluated seismic history of the area reveals its potential to generate destructive Mw = 6.0 earthquakes; therefore, the continued monitoring of its microseismicity is considered important

Earthquake Diffusion Variations in the Western Gulf of Corinth (Greece)

International audienceEarthquake diffusion and the migration behaviour of seismic clusters are commonly studied to provide insight on the spatiotemporal evolution of seismicity and the interplaying driving mechanisms. Using a high-resolution relocated catalogue, we study the variations of the earthquake diffusion rates in the Western Gulf of Corinth during 2013–2014, a period with abundant local seismicity, including intense microseismic background, seismic swarms and mainshock-aftershock sequences. We treat earthquake occurrence as a point process in time and space and estimate the diffusion rates of the main seismic sequences and the background seismicity in terms of normalized spatial histograms and the evolutions of the mean squared distance of seismicity with time. The statistical analysis of the studied seismic sequences reveals that the mean squared distance of the hypocentres increases slowly with time, at a much lower rate than for a normal diffusion process. Such findings confirm previous results on weak earthquake diffusion, analogous to subdiffusion, in regional and clustered seismicity. In addition, seismic swarms associated with pore fluid pressure diffusion present considerably higher diffusion exponents compared to mainshock-aftershock-type sequences that are consistent with primary or secondary stress triggering effects and stress corrosion. The observed variations of the earthquake diffusion rates indicate the stochastic nature of the phenomenon and may provide novel constraints on the triggering mechanisms of clustered seismic activity in the Western Gulf of Corinth and in other seismically active regions

Spatiotemporal Properties of Seismicity and Variations of Shear-Wave Splitting Parameters in the Western Gulf of Corinth (Greece)

The Western Gulf of Corinth (WGoC) exhibits significant seismicity patterns, combining intense microseismic background activity with both seismic swarms and short-lived aftershock sequences. Herein, we present a catalogue of ~9000 events, derived by manual analysis and double-difference relocation, for the seismicity of the WGoC during 2013–2014. The high spatial resolution of the hypocentral distribution permitted the delineation of the activated structures and their relation to major mapped faults on the surface. The spatiotemporal analysis of seismicity revealed a 32-km-long earthquake migration pattern, related to pore-pressure diffusion, triggering moderate mainshock-aftershock sequences, as fluids propagated eastwards in the course of ~15 months. The anisotropic properties of the upper crust were examined through automatic shear-wave splitting (SWS) analysis, with over 2000 SWS measurements at local stations. An average fast shear-wave polarization direction of N98.8°E ± 2.8° was determined, consistent with the direction of the maximum horizontal regional stress. Temporal variations of normalized time-delays between fast and slow shear-waves imply alterations in the level of stress or microcrack fluid saturation during the long-lasting pore-pressure diffusion episode, particularly before major events. The present study provides novel insights regarding seismicity patterns, active fault structures, anisotropic properties of the upper crust and triggering mechanisms of seismicity in the WGoC

Probabilistic and Scenario-Based Seismic Hazard Assessment on the Western Gulf of Corinth (Central Greece)

The Gulf of Corinth (Central Greece) is one of the most rapidly extending rifts worldwide, with its western part being the most seismically active, hosting numerous strong (M ≥ 6.0) earthquakes that have caused significant damage. The main objective of this study was the evaluation of seismic hazard through a probabilistic and stochastic methodology. The implementation of three seismotectonic models in the form of area source zones via a logic tree framework revealed the expected level of peak ground acceleration and velocity for return periods of 475 and 950 years. Moreover, PGA values were obtained through the stochastic simulation of strong ground motion by adopting worst-case seismic scenarios of potential earthquake occurrences for known active faults in the area. Site-specific analysis of the most populated urban areas (Patras, Aigion, Nafpaktos) was performed by constructing uniform hazard spectra in terms of spectral acceleration. The relative contribution of each selected fault segment to the seismic hazard characterizing each site was evaluated through response spectra obtained for the adopted scenarios. Almost all parts of the study area were found to exceed the reference value proposed by the current Greek National Building Code; however, the three urban areas are covered by the Eurocode 8 regulations

Earthquake Diffusion Variations in the Western Gulf of Corinth (Greece)

Earthquake diffusion and the migration behaviour of seismic clusters are commonly studied to provide insight on the spatiotemporal evolution of seismicity and the interplaying driving mechanisms. Using a high-resolution relocated catalogue, we study the variations of the earthquake diffusion rates in the Western Gulf of Corinth during 2013-2014, a period with abundant local seismicity, including intense microseismic background, seismic swarms and mainshock-aftershock sequences. We treat earthquake occurrence as a point process in time and space and estimate the diffusion rates of the main seismic sequences and the background seismicity in terms of normalized spatial histograms and the evolutions of the mean squared distance of seismicity with time. The statistical analysis of the studied seismic sequences reveals that the mean squared distance of the hypocentres increases slowly with time, at a much lower rate than for a normal diffusion process. Such findings confirm previous results on weak earthquake diffusion, analogous to subdiffusion, in regional and clustered seismicity. In addition, seismic swarms associated with pore fluid pressure diffusion present considerably higher diffusion exponents compared to mainshock-aftershock-type sequences that are consistent with primary or secondary stress triggering effects and stress corrosion. The observed variations of the earthquake diffusion rates indicate the stochastic nature of the phenomenon and may provide novel constraints on the triggering mechanisms of clustered seismic activity in the Western Gulf of Corinth and in other seismically active regions

Investigation of the Factors Controlling the Duration and Productivity of Aftershocks Following Strong Earthquakes in Greece

Strong crustal earthquakes in Greece are typically followed by aftershocks, the properties of which are important factors in seismic hazard assessment. In order to examine the properties of earthquake sequences, we prepared an earthquake catalog comprising aftershock sequences with mainshocks of Mw ≥ 5.5 from 1995 to 2021. Regional aftershock parameters were estimated to highlight variations in aftershock decay and productivity among regions with similar seismotectonic characteristics. A statistically based method of estimating aftershock duration and a metric of relative aftershock productivity to examine the variations among the different cases were employed. From the detailed analysis of the selected seismic sequences, we attempt to unravel the physical mechanisms behind deviations in aftershock duration and productivity and resolve the relative contribution of background seismicity, the Omori–Utsu law parameters and the mainshock faulting properties. From our analysis, the duration of aftershock sequences depends upon the rupture process of the mainshock, independently of its magnitude. The same applies to aftershock productivity, however, other tectonic setting (e.g., seismic coupling) or source-related (e.g., focal depth, stress drop) parameters also contribute. The estimated regional parameters of the aftershock rate models could be utilized as initial ones to forecast the aftershock occurrence rates at the early stage following a mainshock

Investigation of the Factors Controlling the Duration and Productivity of Aftershocks Following Strong Earthquakes in Greece

Strong crustal earthquakes in Greece are typically followed by aftershocks, the properties of which are important factors in seismic hazard assessment. In order to examine the properties of earthquake sequences, we prepared an earthquake catalog comprising aftershock sequences with mainshocks of Mw ≥ 5.5 from 1995 to 2021. Regional aftershock parameters were estimated to highlight variations in aftershock decay and productivity among regions with similar seismotectonic characteristics. A statistically based method of estimating aftershock duration and a metric of relative aftershock productivity to examine the variations among the different cases were employed. From the detailed analysis of the selected seismic sequences, we attempt to unravel the physical mechanisms behind deviations in aftershock duration and productivity and resolve the relative contribution of background seismicity, the Omori–Utsu law parameters and the mainshock faulting properties. From our analysis, the duration of aftershock sequences depends upon the rupture process of the mainshock, independently of its magnitude. The same applies to aftershock productivity, however, other tectonic setting (e.g., seismic coupling) or source-related (e.g., focal depth, stress drop) parameters also contribute. The estimated regional parameters of the aftershock rate models could be utilized as initial ones to forecast the aftershock occurrence rates at the early stage following a mainshock