Vassilios Karastathis

National Observatory of Athens | NOA

This comment is intended to provide clarifications on the tsunami warning operational procedures followed by the Hellenic National Tsunami Warning Centre (NOA-HLNTWC), one of the Tsunami Service Providers of the North-Eastern Atlantic, Mediterranean and connected seas Tsunami Warning System (NEAMTWS), after the May 2nd, 2020 South Crete earthquake,...

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) whic...

Local seismic response analysis and microzonation studies represent fundamental tools for assessment and mitigation of the seismic risk in urban areas and cultural heritage sites. In this work, in the framework of the H2020-RISE “STABLE - Structural stABLity risk assEsment” project, local seismic response analysis by an integrated approach was perf...

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...

The Arkalochori village in central Crete was hit by a large earthquake (Mw = 6.0) on 27 September 2021, causing casualties, injuries, and severe damage to the infrastructure. Due to the absence of apparent surface rupture and the initial focal mechanism solution of the seismic event, we initiated complementary, multi-disciplinary research by combin...

Thessaly lies on the Aegean (micro-)plate, undergoing internal crustal deformation due to the plate relative motion against the adjacent Anatolian and Nubian plates. As a result, the whole Thessalian Basin was supposed to be under an extensional tectonic regime of N-S direction. However, the recent earthquake sequence of March 2021 which occurred c...

The July 20, 2017, MW 6.6 Kos–Gökova Gulf earthquake occurred offshore, near Bodrum of Turkey and Kos of Greece. It was one of the strongest in the broader area during the last many decades causing two deaths, many injuries and extensive damages. We investigated the evolution of the seismic sequence using seismological and geological tools. The aft...

Elastic 3D wave-field simulations were performed in the seismically active region of Eastern Gulf of Corinth, in the area of Loutraki city. A new methodology was tested with the aim of performing multiple simulations for a large variety of realistic sources located around the study area, by employing 3D finite-difference modeling using matrix opera...

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...

We present an interdisciplinary study on Earth-geospheres-interaction and human health by integrating space and ground-data observations. Following the initial studies of the deep origin of geogas (e.g., Rn, CH4, CO2, He) during geodynamic activities (Gold and Soter, 1985), we are exploring the detection feasibility of their concentration variation...

We investigated the strong July 20, 2017 earthquake and its aftershock sequence occurred in SE Aegean Sea. Several secondary ground effects were observed (mainly liquefaction and tsunami), but no primary ground ruptures. We performed a hypocentral relocation of the sequence in order to detect the fault pattern that revealed the existence of three m...

We present seismic and geodetic data analysis of the shallow, normal-faulting earthquake sequence offshore Lesvos (Aegean Sea, Greece) that was initiated by the June 12, 2017 M6.3 earthquake. We use seismological data (relo-cated events and Moment Tensor solutions from NOA and KOERI catalogues) to identify the ESE-WNW striking seismic fault and to...

In this paper we defined the local complex fault system of the Cephalonia three strongest earthquakes (Mw6.1, Mw5.3 and Mw6.0) of the January-February 2014 sequence and the Lefkada mainshock (Mw6.4) on November 17, 2015, in order to investigate the fault interaction i) within the 2014 sequence, ii) between the 2014 sequence and the fault that produ...

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-...

The historical earthquake of the 1894 in Atalanti, Greece, produced widespread liquefaction effects on the broader coastal area. Although this earthquake has been extensively studied in the past, its magnitude still remains controversial. In the present study, we propose an indirect method for assessing its magnitude by simulating the ground motion...

Considerably improved hypocentral locations of the 274 earthquakes, with magnitudes between 1.5 to 4.1 Ml recorded during the period from 2009 to 2010 by the Hellenic Unified Seismographic Network (HUSN), have been obtained for the area of North Euboean Gulf after the implementation of a 3D non-linear location algorithm and a previously calculated...

The high vulnerability of a small Byzantine Church of St. Nikolaos (Platani Village in North Peloponnese, Greece) constructed in 12th century, in certain geological conditions and tectonically active regime, led to a thorough study of the foundation soil with an integrated geophysical investigation (GPR, ERT and seismic surveys). The geophysical me...

The modern tools for time frequency analysis have been proved to be able to achieve phase separation of the body waves and clearly detect the P- and S-wave arrivals. We developed software based on a combination of various time frequency analysis methods that performs automatic detection of the body wave phases with very high accuracy. After compari...

In this paper we defined the local complex fault system of the Cephalonia three strongest earthquakes (M w 6.1, M w 5.3 and M w 6.0) of the January-February 2014 sequence and the Lefkada mainshock (M w 6.4) on November 17, 2015, in order to investigate the fault interaction i) within the 2014 sequence, ii) between the 2014 sequence and the fault th...

In this paper we defined the local complex fault system of the Cephalonia three strongest earthquakes (M w 6.1, M w 5.3 and M w 6.0) of the January-February 2014 sequence and the Lefkada mainshock (M w 6.4) on November 17, 2015, in order to investigate the fault interaction i) within the 2014 sequence, ii) between the 2014 sequence and the fault th...

The use of local velocity model in the analysis of seismicity recorded by the Hellenic Unified Seismological Network (HUSN), provides the opportunity to determine accurate hypocentral solutions using the weighted P- and S- wave arrival times for the November 2013, November 2014 and June 2015 North Gulf of Evia (Euboea) sequences. The hypocentral lo...

On 17 November 2015 a Mw6.5 earthquake ruptured offshore Lefkada Isl. in Ionian Sea, Greece, causing two victims, damage and ground failures particularly in the SW part of the island, which is consistent with the ground deformation pattern shown by InSAR analysis. Fault plane solutions released by CMT, NOA and other institutes are consistent indica...

This work presents the results of relocation and stress inversion analysis for two recent earthquake sequences in the northern Gulf of Evia, central Greece. On 12 November 2013 (18:09, UTC) a moderate earthquake, ML=4.8 occurred onshore northern Evia, near the village Taxiarxis. The epicentral area of this event was manually located: ϕ=38.9133 • ,...

The identification of the structure of seismicity associated with active faults is of great significance particularly for the densely populated areas of Greece, such as Corinth Gulf, SW Peloponnese and central Crete. Manual analysis of the seismicity that has been recorded by the Hellenic Unified Seismological Network (HUSN) for the recent years pr...

Crete lies in the forearc basin of the collision zone between the Eurasian and African plates-one of the highest seismicity regions in the world. The purpose of this study is to map ground deformation using time series analysis. Up until now, most geodetic observations used only GPS data. The key difference between space-based measurements and GPS...

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 towar...

No standard methodologies regarding the short-term (hours, days, few weeks) forecasting of earthquakes have been widely adopted so far. However, promising approaches from ground-based (e.g. foreshocks) and space-based (e.g. thermal anomalies) observations have been described. We propose to apply a multidisciplinary strategy by performing real-time...

Online Material: Tables of moment tensor elements; SAR image pairs and coseismic displacement gradients; figures of InSAR data, model parameter tradeoffs, and observed and modeled displacements. On 26 January 2014 at 13:55 UTC, an M w 6.0 earthquake struck the island of Cephalonia, Greece, followed five hours later by an M w 5.3 aftershock and by...

A clear picture of the fault that facilitates the hydrothermal flux in one the most important geothermal fields of the Greek mainland, in the North Euboean Gulf, has been acquired through the implementation of a combined onshore and offshore seismic survey. The results of the seismic investigations accurately map the structural characteristics of t...

We perform numerical simulation of the ground motion and liquefaction at several sites located at the coastal area of Atalanti Gulf (Greece). To incorporate region-specific geologic effects in the ground motion and liquefaction hazard estimates, subsurface information, particularly Vs and dynamic properties, are compiled and evaluated with the aid...

BEYOND Center of Excellence primarily builds upon state-of-the-art optical remote sensing technologies, differential interferometry techniques and Persistent Scatterer methods. The resulting products are integrated with in-situ observations from the National Seismological Network, the NOANET GPS network, and the ENIGMA magnetometer network establis...

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 towar...

On 7 August 2013 two moderate earthquakes (09:02:45 and 09:06:51 UTC), with moment magnitude M w =4.3 and M w =5.4 respectively, triggered a seismic sequence in Central Greece. The epicentral area was in Kallidromo Mountain to the east of Amfikleia town and to the west of Regginio village. The M w =5.4 epicentre was manually located: φ=38.701°, λ=2...

On 8 January 2013 (14:16 UTC) a moderate earthquake, M L =5.8 occurred off the coast of Lemnos Island, Northern Aegean Sea. The earthquake's geographical coordinates were calculated by NOA (National Observatory of Athens) as φ=39.666°Ν, λ=25.562°Ε, depth d=31 km (http:. This event was felt by the population of the surrounding Greek islands, as well...

The origin of tsunamis in the Mediterranean region and its connected seas, including the Marmara Sea, the Black Sea and the SW Iberian Margin in the NE Atlantic Ocean, is reviewed within the geological and seismotectonic settings of the region. A variety of historical documentary sources combined with evidence from onshore and offshore geological s...

On 26.1.2014 and 3.2.2014 two strong earth-quakes of M w 6.0 and M w 5.9 ruptured the west-ern Cephalonia Isl., Ionian Sea (Greece), at the SSW-wards continuation of the Lefkada segment of the Cephalonia Transform Fault Zone (CTFZ), causing considerable damage and a variety of ground failures. High-preci-sion relocation of the aftershocks implies t...

On 28 December 2013 (15:21 GMT) a strong mainshock occurred in the northwestern termination of the Cyprus Arc in the east basin of the Mediterranean Sea. The earthquake was strongly felt in several localities of Cyprus as well as of SW Turkey. According to preliminary determinations (e.g. GFZ, NEIC, Harvard etc.) the earthquake moment magnitude was...

On 26 January 2014 a strong (Mw6) shallow earthquake ruptured the western side of Cephalonia island in the Ionian Sea, Greece. The main shock was followed by abundant aftershocks, one of them of Mw5.4. The earthquake caused damage but no human victims mainly in the area of Lixouri town in the western part of the island. However, the rest part of th...

On August 7, 2013 a moderate earthquake (NOA ML = 5.1, NOA Mw = 5.4) occurred in central Kallidromon Mountain, in the Pthiotis region of central Greece. 2270 aftershocks were relocated using a modified 1-D velocity model for this area. The b-value of the aftershock sequence was b = 0.85 for a completeness magnitude of Mc = 1.7. The rate of aftersho...

We investigate mainshock slip distribution and aftershock activity of the 8 January 2013 Mw=5.7 Lemnos earthquake, north Aegean Sea. We analyse the seismic waveforms to better understand the spatiotemporal characteristics of earthquake rupture within the seismogenic layer of the crust. Peak slip values range from 50 to 64 cm and mean slip values ra...

On August 7, 2013 a moderate earthquake (NOA ML= 5.1, NOA Mw= 5.4) occurred in central Kallidromon Mountain, in Pthiotis region of central Greece. 2270 aftershocks were relocated using a modified 1-D velocity model for this area. The b-value of the aftershock sequence was b=0.85 for a completeness magnitude of Mc=1.7. The rate of aftershock decay w...

A combined application of the Multichannel Analysis of Surface Waves (MASW), electrical resistivity tomography (ERT) and crosshole seismic tomography between crest and tunnels successfully provided crucial information about seepage and differential settlement effects in the Mornos dam, central Greece. The Mornos dam is an 816 m long and 139 m high...

Ground granulated blast-furnace slag (GGBS) is a secondary raw material that can be used as an alternative low energy binder. Hydraulic properties can be occurred through its alkali activation. GGBS is characterized by the glassy to crystalline ratio and by its chemical and mineralogical composition. Acidic slag cannot easily get crystallized in op...

Non-destructive resistivity imaging techniques have been successfully applied at the area of the ancient Temple of Olympian Zeus in Athens aiming to detect subsurface voids responsible for ground failures observed during constructive works in the ground surface. Synthetic modelling and inversion based imaging resistivity techniques identified a num...

We examine the possible non-linear behaviour of potentially liquefiable layers at selected sites located within the expansion area of the town of Nafplion, East Peloponnese, Greece. Input motion is computed for three scenario earthquakes, selected on the basis of historical seismicity data, using a stochastic strong ground motion simulation techniq...

We present an example of how geophysical methodologies can considerably contribute to seismic and liquefaction risk assessment in an area where urban development is planned. The inspection for possible hidden faults by geophysical methods is particularly critical, since such a possibility could practically hinder the town planning in this area. How...

The 2008 mainshock (Mw=6.4) was the first modern, strong strike–slip earthquake in the Greek mainland. The fault strikes NE–SW, dips ∼85°NW while the motion was right-lateral with small reverse component. Historical seismicity showed no evidence that the fault ruptured in the last 300years. For rectangular planar fault we estimated fault dimensions...

New findings about the deep origin of the geothermal fields and volcanic centres of the North Evian Gulf area, Central Greece, were arisen by combining a three-dimensional traveltime inversion study of a microseismic dataset recorded of 37 portable stations together with a Curie depth point analysis based on aeromagnetic data. A possible magma cham...

Along the coast of the North Evian Gulf, Central Greece, there are significant geothermal sites, thermal springs as Aedipsos, Yaltra, Lichades, Ilia, Kamena Vourla, Thermopylae etc. but also volcanoes of the Quaternary - Pleistocene age as Lichades and Vromolimni. Since for these local volcanoes and geothermal fields, their deep origin and their re...

An unusual cluster of four M w > 6.2 earthquakes ruptured the Hellenic Arc and Trench (HA-T) from 6 January to 15 July 2008 with a variety of focal depths and mechanisms (Figure 1 and Table 1). Common earthquake clusters incorporate a sequence of dependent events: a main shock, numerous aftershocks, and sometimes foreshocks preceding the main shock...

A combined onshore -offshore network of 30 portable seismographs (23 land-stations and 7 OBS) was deployed in N. Evoikos Gulf area, Central Greece, to record the microseismic activity for a 4-month period, in the summer of 2003. We located more than 2000 earthquakes ranging from 0.7 to 4.5 ML (with a 6 records per event limit).

In the frame of the FP6 EC SEAHELLARC project, we deployed, in Fall 2006, a combined on/offshore seismic array consisting of 17 4C ocean bottom seismographs and 15 3C land-stations in the Kyparissiakos Gulf and surrounding area, southwestern Hellenic arc. More than 3500 micro-earthquakes were located with a minimum of 6 records per event and by usi...

The 2008 mainshock (M w = 6.4) was the first modern, strong strike–slip earthquake in the Greek mainland. The fault strikes NE–SW, dips ∼85°NW while the motion was right-lateral with small reverse component. Historical seismicity showed no evidence that the fault ruptured in the last 300 years. For rectangular planar fault we estimated fault dimens...

A combined onshore - offshore network of 30 portable seismographs (23 land-stations and 7 OBS) was deployed in N. Evoikos Gulf area and recorded the microseismic activity for a 4 months period. More than 2000 earthquakes ranging from 0.7 to 4.5 ML (with a 6 records per event limit) were located. From the initial dataset we selected the most reliabl...

A combined on-offshore seismic network consisting of 17 4C ocean bottom seismographs and 15 3C land stations deployed in the surrounding area of Kyparissiakos gulf (western Peloponesse) and rec-orded the microseismic activity for a 2 months period. More than 3500 earthquakes ranging from 0.3 to 4.0 Md) were located by using a limit of 6 arrivals pe...

An on/offshore seismic network consisting of 31 three-component stand-alone digital stations was deployed in the north Evoikos gulf and surrounding region, central Greece, operating from 30 June to 24 October 2003. More than 2000 microearthquakes, ranging from 0.7 to 4.5 ML were located, using six stations as minimum to define foci parameters. Hypo...

Numerical modeling of a landslide process at the continental slope by taking into account slope parameters is performed for the tsunami event of February 7, 1963, reported in the Corinth Gulf, central Greece. A layered sediment structure was considered, and an initial external dynamic action at the landslide process was introduced. The results obta...

Shallow seismic surveys can constitute a useful tool in actively deforming regions by yielding precise information on the location of active faults, their geometry and cumulative displacement. Since the structures in these regions are rarely flat but always with sharp dips, 3D effects and side reflections can influence the results, thus the interpr...

In the present paper we analyze the effect of local geology on ground motion by means of numerical calculations (numerical models) using total (TS) and effective stress (ES) methods. These numerical calculations have been applied to the site of Itea, Corinth Gulf, which was chosen based on liquefaction susceptibility criteria and field inspection....

Post-event field surveys were conducted and measurements were taken in Sri Lanka and Maldives about two weeks after the catastrophic Indian Ocean tsunami of 26 December 2004. The measurements taken were cross-checked after interviewing with local people. In the southwest, south and east coastal zones of Sri Lanka maximum water levels ranging from h...

An onshore/offshore seismic array consisting of 15 three-component seismic digital stations was deployed in the area of Rhodos, southeastern Hellenic arc in Fall 2003. The geometry and location of the network was designed to observe the offshore activity and correlate active faults onshore with those offshore. Within a period of 70 days, more than...

The extensive damages and human life loss related to the September 7, 1999 earthquake in the Athens area (Greece) initiated an effort to re-evaluate seismic hazard in various regions around the capital. One of the target areas selected within the framework of the specially designed research project ESTIA was the industrial area of Aspropirgos, wher...

An on-/offshore seismic network consisting of 36 three-component stand-alone digital stations was deployed in the area of the Saronikos Gulf, in the vicinity of Athens (Greece), in the fall of 2001. In the present study, from an initial set of more than 1000 micro-earthquakes, 374 were selected and 6666 P-and S-wave arrivals were inverted, based on...

In this paper, we apply current geological knowledge on faulting processes to digital processing of Digital Elevation Models (DEM) in order to pinpoint locations of active faults. The analysis is based on semiautomatic interpretation of 20- and 60-m DEM and their products (slope, shaded relief). In Northern–Eastern Attica, five normal fault segment...

This paper uses the seismological data provided by analysis of the July 26, 2001 Skyros earthquake (Mw = 6.4) aftershock sequence to discuss various models for Quaternary deformation of the central Aegean region. In particular, it is suggested that the left-lateral Skyros fault is a primary strike-slip fault formed in Early Quaternary times and str...

The western Hellenic Arc is one of the most seismically active regions in the western Eurasia. Large, shallow and intermediate depth earthquakes take place there with maximum magnitudes up to 7.5 occurring around Kythira isl. between Peloponnese to the north and Crete isl. to the south. This is also supported by historical seismicity data which ind...

The static displacement field of the Athens 1999 earthquake has been numerically modeled by a BEM method and analysed from SAR interferometry images with compatible results: (a) for a fault that reaches the surface the subsidence field coincides with the hangingwall domain of the Fili neotectonic normal fault with maximum amplitude, d max, 5.5–7 cm...

HEAD 1. O: A Unified HEllenic Accelerogram Database N. Theodulidis I I Kalogeras 2 C Papazachos 3 V. Karastathis 2 B Margaris , Ch. Papaioannou I , and A. Skarlatoudis 3 During the past thirty years various accelerogram data sets recorded by "flee field" strong-motion stations in Greece have been compiled by two institutes that are in charge of ru...

During the past thirty years various accelerogram data sets recorded by “free field” strong-motion stations in Greece have been compiled by two institutes that are in charge of running national strong-motion networks. These data sets have been of limited potential use for research because of nonstandard processing, nonhomogeneous earthquake paramet...

The earthquake (Mw 6.2, Ms 6.4) of 14 August 2003 which occurred in the Lefkada segment of the Cephalonia Transform Fault, Ionian Sea (Greece), was associated with dextral strike-slip faulting striking NNE-SSW. Reevaluation of instrumental and documentary sources show that the 1914, 1948 and 2003 earthquakes ruptured the same fault segment and that...

This paper summarizes the results of the earlier phase (1991–8) of geoarchaeological investigations at the Canal of Xerxes in northern Greece and then presents the findings of recent work. Through the combination of geophysical survey and analysis of sediments obtained from bore holes drilled along the supposed course of the canal it was establishe...

Shallow reflection and refraction seismic studies were carried out in Greece in the eastern leg of the Chalkidiki peninsula, in order to test the validity of reports in history books which describe a legendary canal built by the engineers of King Xerxes during the major Persian invasion of Europe through Greece in the 5th century B.C. In the narrow...

3-D velocity model was constructed for the neotectonic basin of Saronikos Gulf, near Attica, Greece. 3-D linearised tomography algorithm inverted 3487 P-wave arrivals from 374 located earthquakes in the area. The data were acquired after the installation of a local network of 10 ocean bottom seismographs (OBS) and 29 stand-alone land-stations for a...

Foreshocks are characterized by some distinct properties: (1) power-law trend of increase of the number of foreshocks with time, (2) the b-value drops with respect to that of the background seismicity and aftershocks, (3) their incidence is favoured by the geometric and compositional heterogeneity in the crust, (4) the larger the heterogeneity the...

In Fall 2000 we observed two seismic profiles, each of 160Km length, along the eastern Corinthiakos Saronikos gulf, and across the northern part of the Cycladic mass. Along each profile we deployed 35 ocean bottom seismographs (OBS) and 5 stand-alone seismic stations. The seismic energy was generated by a tuned 52lt airgun array. Shots were placed...

On July 26, 2001 a strong earthquake of magnitude M_w = 6.5 hit the central Aegean Sea at 00:21:39 GMT. The event took place off shore of Skyros Island in a distance of 135 km NNE of Athens. A sequence of many aftershocks has followed with the magnitude of the largest of them reaching to Ms = 5.4. The aftershock sequence temporal and spatial charac...

3D and 2D seismic tomography surveys were carried out by the National Observatory of Athens, on behalf of the British School of Athens, in December 2000 in order to study the buried ancient canal of Xerxes on the Athos peninsula of the Chalkidiki in northern Greece.

How reliable results can be obtained using three-dimensional seismic refraction traveltime tomography in shallow depth applications? Nowadays, there is an increasing demand for 3d seismic tomographic surveys due to their ability on describing complex structures (some of them of economical importance). To evaluate the current 3d tomographic algorith...

Three-dimensional velocity structure of the upper crust was determined by inversionof P-wave travel times in the region of Attiki Prefecture (Greece), located at the eastern part of the Greek mainland, which is traditionally considered as an area of low seismicity. An earthquake of Ms = 5.9 occurred there, on September 7, 1999, causing extensive da...

Within the frames of a geophysical project aimed at assessing the Marathon Dam and its surrounding area, it was decided to implement the one-sided acoustic refraction tomography method in the main interior tunnel of this concrete dam, in order to get a picture about the dynamic properties of the concrete. This method was chosen since the implementa...

Geophysical investigations were conducted at the Marathon Dam site near Athens, Greece, in order to detect possible degraded areas that are potentially liable to water infiltration or leakage, and to evaluate the dynamic properties of the subsurface materials. The investigations were also extended in the dam interior in order to evaluate the qualit...

Three-dimensional P-velocity structure of the upper crust was determined by inversion of P-wave travel times in the Athens broader area (Greece), where a destructive earthquake of 5.9 Ms magnitude occurred on September 7, 1999. The investigated area is located at the eastern part of Greek mainland and was previously considered as a region of low se...