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An earthquake swarm occurred during 2020-2021 at the Perachora peninsula, in the eastern Gulf of Corinth (Greece), one of the most seismically active areas in Europe. In this work, we analyzed the swarm using a high-resolution earthquake catalog and advanced processing techniques. We employed both single-station and multi-channel template matching to produce additional detections of over 7,600 and 24,000 events, respectively. The resulting catalogs have different completeness magnitudes and location uncertainties for the added events. The frequency-magnitude distributions were characterized using the Gutenberg-Richter scaling relation, examining possible b-value temporal variations. We found that the seismic bursts associated with the swarm were short-lived and dominated the catalogs. Spatiotemporal clustering methods were used to analyse the swarm's evolution, showing that the seismicity rate significantly increased, likely due to a pore-pressure transient, without stress transfer caused by a major earthquake. Migration velocities, high b-values and clustering effects of multiplet families, observed at all time scales, suggest a pore-pressure triggering mechanism. Some anomalies in the frequency-magnitude distribution of the enhanced catalogs were observed, mainly attributed to the poor network coverage of the area of interest. Despite these limitations, the enhanced catalogs can be quite useful in studying the spatiotemporal properties of microseismicity in the region. This study highlights the importance of high-resolution data and advanced analytical techniques in understanding earthquake swarms in tectonically active regions.AcknowledgementsThe research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers” (Project Number: 00256)., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

In this work, seismic hazard is evaluated for the Administrative Region of Attica (Greece) and its islands. The assessment is performed by computing Peak Ground Acceleration (PGA) and Peak Ground Velocity (PGV) values for return periods of 475 and 950 years for five separate groups to cover the entirety of the study area in detail. In addition, PGA hazard curves and Uniform Hazard Spectra (UHS), in terms of Spectral acceleration (Sa), are constructed for Athens and the capital of each island of Attica. The computational scheme contains two crustal seismotectonic models, i.e., the Euro–Mediterrenean Seismic Hazard Model 2013 (ESHM13) and its update ESHM20, and one slab seismotectonic model. Ground Motion Prediction Equations (GMPEs) proposed for the Greek territory are adopted and ranked for the crustal seismotectonic models. Each GMPE is re–constructed as a weight–specific model to account for both normal and non–normal fault plane solutions for each area source. The final results are obtained by implementing a logic tree, whose major branches are the three seismotectonic models and the minor branches the GMPEs. The latter reduces epistemic uncertainties due to the implementation of multiple logic tree paths per area source. The results indicated that seismic hazard is higher in groups that are close either to the Gulf of Corinth or the slab interface. The results could aid in the update of the building code in Attica.Acknowledgements:The present study is co-funded by the Special Account for Research Grants (S.A.R.G.) of the National and Kapodistrian University of Athens., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Messinia is located in SW Peloponnese (Greece), in the vicinity of the Hellenic Arc which is one of the most seismically active areas of Europe. The arc is dominated by reverse faulting, whereas normal faults are mapped onshore, mainly striking N-S. Large earthquakes have occurred in the study area, both in the historical and instrumental periods. In the present study, a Probabilistic Seismic Hazard Assessment (PSHA) is applied to estimate the Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV), Peak Ground Rotational Acceleration (PGRA) and Peak Ground Rotational Velocity (PGRV) for the broader Messinia region. PGRA and PGRV are not often examined in detail in the literature, even though they are useful for the evaluation of the possible damages in structures. The widely used approach proposed by Cornell and McGuire is implemented taking into account: (a) the seismotectonic model proposed by Seismic Hazard Harmonization in Europe (SHARE), (b) an earthquake catalogue for the instrumental period and (c) Ground Motion Prediction Equations (GMPEs) proposed for the Greek territory. The computational grid spacing for Messinia was set to 1 km, in order to accurately calculate the hazard parameters and to reduce the bias of the results through interpolation processes. For PGA and PGV, a logic tree approach is considered, where every branch is a hybrid version of each GMPE considering the percentage of the type (normal or non-normal) of focal mechanisms for all zones of the seismotectonic model. The results show an increase of the hazard values in the NW part of the study area, where the Greek Building Code provides lower PGA values. In addition, hazard curves in terms of PGA for multiple probabilities of exceedance in 50 years are determined for five major towns of Messinia, i.e. Kalamata, Messini, Filiatra, Kyparissia and Pylos. Kyparissia has the higher hazard in all probabilities of exceedance, which is in full agreement with the PGA and PGV results. Pylos and Filiatra have intermediate to high hazard, whereas Kalamata and Messini show intermediate hazard values. Furthermore, the Uniform Hazard Spectrum (UHS) in terms of Spectral Acceleration for the same towns is computed. The latter provides essential information about the design parameters. Lastly, the results are compared to the seismic histories of the five major towns in terms of European Macroseismic Scale (EMS98) intensity, plotted for the last 200 years.

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.

Earthquakes, large or even moderate, are often followed by secondary phenomena, such as landslides, tsunamis, fires and technological disasters, leading to cascading effects that may, in turn, cause severe repercussions. Before, during and after the occurrence of these events, risk communication, currently evolved to codified legislation, is a crucial factor. Policy selection in the present study is approached by the application of the risk game tree and its formation. The events studied here in view of policy making have occurred both in the historical and the instrumental era, to account for different level of exposure and anthropogenic hazards, in Greece (1894 Atlanti, 1953 Kefallinia series, 2003 and 2015 Lefkas), Italy (1976 Friuli), Japan (2011 Tohoku) and Slovenia (1917 Brezice). In all case studies the whole disaster management cycle is examined, i.e. mitigation, preparedness, response and recovery. Disaggregation of earthquake-related direct and cascading effects, as well as risk communication are taken into account and ethical challenges are posed both to scientists and policy makers.