Analysis of local seismic response in the historical city centre of Nafplio (Greece)

Local seismic response analysis represents a fundamental tool for assessing the seismic risk of urban areas and Cultural Heritage (CH) sites, nevertheless several open questions remain when complex geological contexts are considered. This study has been carried out in the framework of the research project STABLE (H2020 RISE-Marie Curie Action) which has the aim of evaluating the seismic action on CH sites in the Mediterranean basin. One of them being the city of Nafplio (Greece), a site characterised by a peculiar geological setting and a medium-high regional seismic hazard. Geological and geophysical data have been collected to reconstruct both the subsoil model and the seismotectonic conditions. Several seismic ambient noise measurements were carried out to establish the site resonance frequency and to validate engineering-geological cross-sections for numerical modelling of the seismic wave propagation. Combined engineering-geological, geophysical and numerical modelling allowed evaluating the local seismic response of the urban area of Nafplio, providing a zonation map with homogeneous seismic response zones for which elastic response spectra will be obtained

High-precision relocation of seismic sequences above a dipping Moho: the case of the January–February 2014 seismic sequence on Cephalonia island (Greece)

Detailed velocity structure and Moho mapping is of crucial importance for a high precision relocation of seismicity occurring out of, or marginal to, the geometry of seismological networks. Usually the seismographic networks do not cover the boundaries of converging plates such as the Hellenic arc. The crustal thinning from the plate boundary towards the back-arc area creates significant errors in accurately locating the earthquake, especially when distant seismic phases are included in the analysis. The case of the Cephalonia (Ionian Sea, Greece) sequence of January–February 2014 provided an excellent example where the hypocentral precision was greatly affected by the crustal thinning from the plate boundary at the Ionian sea towards the Aegean sea. This effect was examined in detail by testing various velocity models of the region in order to determine an optimal model. Our tests resulted in the adoption of a velocity model that resembles the crustal thinning of the region. Then, a relocation procedure was performed in the Cephalonia sequence for the time period of 26 January to 15 May 2014 by applying probabilistic non-linear location algorithms. The high-precision relocation resulted in an improved spatial distribution of the seismicity with respect to the preliminary locations and provided a reliable basis to examine seismotectonic implications of the Cephalonia sequence

Observations on the stress related variations of soil radon concentration in the Gulf of Corinth, Greece

Our observations indicate a characteristic pattern in the long-term variation of soil radon concentrations, which seems to be consistent with the expected variation of regional stress in relation to seismicity. However, it seems that the major changes in radon level begin before the rock rapture, i.e. before the earthquake occurs. These conclusions have emerged after long-term observations with continuous and thorough real-time gamma-radiation monitoring in the seismically active area of the Gulf of Corinth, Greece. The recordings acquired close to a hot spring were of very high quality, implying that the deep hydraulic flow can possibly play a key role in the pre-earthquake variation of radon level. We were able to observe outstanding examples of radon level variations before significant seismic events in the Gulf of Corinth that cannot be attributed to other external factors such as atmospheric phenomena. © 2022, The Author(s)

Tsunami Potential of Moderate Earthquakes: The July 1, 2009 Earthquake (M w 6.45) and its Associated Local Tsunami in the Hellenic Arc

On July 1, 2009, a Mw 6.45 earthquake ruptured south of Crete Island (Greece) along the Hellenic Arc triggering a local tsunami. Eyewitness reported the tsunami from Myrtos and Arvi (south-eastern Crete) and from the north of Chrisi Islet, located to the southeast of Crete. The earthquake occurred offshore, about 80 km south of Crete, where routine earthquake locations are poor. The hypocentre is relocated using a 2-D velocity model and several local 1-D velocity models. Epicentral locations obtained by using the different velocity models show very minor variations. Instead, relocated hypocentres can be grouped into two sets of solutions: (1) those with a shallower depth (depth < 12 km) obtained with the 2-D velocity model and the 1-D velocity models having a shallower Moho at less than 30 km, and (2) those with a larger depth (depth of 28 and 40 km) obtained with the velocity models having a Moho at about 40 km. Shallower hypocentres are more consistent with the tsunamigenic nature of the earthquake as also supported by tsunami numerical simulations. In fact, shallow sources (depths < 12 km) are capable of generating tsunami waves, while it is not the case for deeper sources (depth > 25 km) either in the upper-plate or along the plate interface. Models accounting for either homogeneous or heterogeneous slip on the causative fault are tested, with the heterogeneous one better reproducing the observations in terms of number of tsunami waves reaching the shoreline and duration of the sea disturbance. The short travel time, about 10 min, of the first tsunami arrival at the southern coast of Crete represents a big challenge for tsunami early warning systems operating in the area. © 2019, Springer Nature Switzerland AG

Seismic hazard assessment for the protection of cultural heritage in Greece: Methodological approaches for national and local scale assessment (pilot areas of Aighio, Kalamata and Heraklion)

The rich Greek cultural heritage has been always threatened by the intense seismic activity in the broader Aegean region. The objective of the presented project is to develop an integrated tool for the engineers in order to protect the Greek monuments, museums and archaeological sites against strong earthquakes. In order to achieve this goal a GIS-based database was developed with a bidirectional purpose: to collect and combine all necessary data about the monuments and their regional geological and seismotectonic conditions and to assess seismic hazard for each and every monument using the most modern techniques. In this paper we present the structure and development of our database, we propose a methodological procedure for estimating seismic hazard in Greece which will be the basis for the structural assessment of historical structures. The preliminary results show that the estimated values of maximum ground acceleration are quite high for areas in close proximity with large faults, especially when combined with loose ground conditions. Therefore an update of the protection code would be necessary. Subsequently the estimated values of maximum ground acceleration have been applied to three cases of monuments for the determination of the most vulnerable parts of the structure and the verification of the observed pathology. © 2017 by Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved

Profiling of the recent deposits of Nafplio coastal plain (Greece) from engineering geological modelling and geophysical surveys

In the current paper, some preliminary results from the research performed in the EU funded project “STABLE: Structural Stability risk assessment”, which aims to develop a methodology and tools to determine the seismic vulnerability of buildings in historical centres, are presented. The area of the historical centre of Nafplio was selected as case-study since it is one of the most important historical centers in Greece and a not negligible local seismic response may be expected based on the geological and geomorphological setting. The geological - geotechnical conditions in the study area were collected and evaluated and a geological model was identified. For this study, seismic noise measurements have been performed in the coastal plain where the town is built and processed with output resonance varying from 1 Hz up to 5 Hz, moving from the coastline towards the Acronafplia ridge. The results will be used to perform a structural stability analysis of the historical buildings using a simplified approach for specific earthquake scenarios in the future