Anastasia Kiratzi

The active collision of the Apulian continental lithosphere with the Eurasian plate characterizes the tectonics of the Epirus region in northwestern Greece, invoking crustal shortening. Epirus has not experienced any strong earthquakes during the instrumental era and thus there is no detailed knowledge of the way the active deformation is being expressed. In March 2020, a moderate size (Mw 5.8) earthquake sequence occurred close to the Kanallaki village in Epirus. The mainshock and major aftershock focal mechanisms are compatible with reverse faulting, on NNW-ESE trending nodal planes. We measure the coseismic surface deformation using radar interferometry and investigate the possible fault geometries based on seismic waveforms and InSAR data. Slip distribution models provide good fits to both nodal planes and cannot resolve the fault plane ambiguity. The results indicate two slip episodes for a 337° N plane dipping 37° to the east and a single slip patch for a 137° N plane dipping ...

Instrumental magnitudes in Greece have been reported as: a) Mmagnitudes based on the records of the Wiechert or Mainka seismographs,b) MLGR magnitudes based on the records of the Wood-Anderson(WA) seismographs (To = 0.8 sec, Veffective ~ 1000) or othershort period seismographs calibrated against WA records and,c) MLSM magnitudes based on strong motion records(accelerograms). Comparison of such magnitudes with momentmagnitudes, Mw,

Evia Island (Greece) lies in a transition zone from strike-slip faulting in the east, due to the strands of the North Anatolian Fault (NAF) that enter to the Aegean Sea, to normal faulting in the west along central Greece. In June 2003 a series of moderate events occurred in central Evia whose source parameters are investigated. These earthquakes caused serious damage to almost 20 residencies mainly in the town of Psachna. The sequence could be identified as an earthquake swarm with earthquake magnitudes in the range of 3 < M < 4.9. We used the Ρ and S arrivals at the stations of the National Seismic Network to relocate the events using the double-difference algorithm. All Ρ and S phase pickings were made by us using the broad band records from the network operated by the Geodynamic Institute of Athens. The relocated epicenters define a pronounced ENE- WSW zone, parallel to the high topography of the area. All depths are shallow from 1 to 8 Km. Regional waveform modeling was a...

The Kos-Bodrum 20 July 2017 Mw6.6 earthquake ruptured an E-W normal fault dipping ~50° to the south. It caused destruction mainly in monumental structures in the city of Kos. We examine the source and rupture properties that shaped the near-fault ground motions. A key factor controlling the level of ground motions is the dip angle of the fault (between 30° to 60° in published mechanisms), alongside the dip polarity (to the north or to the south). The preferred slip model has two major asperities, with peak slip ~1.6m, located west and east of the hypocenter. The rupture propagated predominantly bilateral at a speed of 2.8 km/s. The ShakeMap shows an E-W spatial extension, in the mid-distance between the cities of Bodrum and Kos. The near-fault unique NS velocity time-series at Bodrum station, which is the fault normal direction, shows a late pulse, that is to say not at the beginning of the record, of period ~0.7s. In the absence of strong motion records in the city of Kos, we selected two near-fault horizontal time-series from the 2016 Norcia earthquake in Italy, as representative for the postulated bedrock motions. Adopting the preferred soil-profile and properties of Psycharis and Taflampas (2017) we perform 1D site response analysis, using the Bodrum and Norcia horizontal records as bedrock input motions, to estimate the surface motions. From all spectral comparisons, we conclude that the mainshock was characterized by enriched spectral ordinates for spectral periods >0.6s up to 2s, and that the effect of the soil in Kos city, is significant in this range.

We carry out a probabilistic seismic hazard assessment (PSHA) for Lesvos Island, in the northeastern Aegean Sea. Being the most populated island in the northern Aegean Sea and hosting the capital of the prefecture, its seismic potential has significant social-economic meaning. For the seismic hazard estimation, the newest version of the R-CRISIS module, which has high efficiency and flexibility in model selection, is used. We incorporate into the calculations eight (8) ground motion prediction equations (GMPEs). The measures used are peak ground acceleration, (PGA), peak ground velocity, (PGV), and spectral acceleration, (SA), at T=0.2 sec representative of the building stock. We calculate hazard curves for selected sites on the island, sampling the southern and northern parts: Mytilene, the capital, the village of Vrisa, Mithymna and Sigri. Hazard maps are also presented in terms of all three intensity measures, for a mean return period of 475 years (or 10% probability of exceedanc...

Abstract We investigate the rupture process of the 14 February 2008, M6. 7 earthquake sequence that occurred offshore the westernmost tip of the Peloponnese peninsula in southern Greece. The sequence occurred close to the western boundary of the subducting African lithosphere with the overlying Aegean crust. Three were the strongest events of the sequence: 14 Feb 2008 M6. 7 depth 30 km, low-angle (10o) thrust, its strongest aftershock on the same day and two hours later of M6. 1 depth 33km, again low angle thrust and on ...

Abstract Geological and geophysical data are often combined with seismological observations to define faults that are related to past, as well as recent catastrophic earthquakes. Such a combination of information from different disciplines is rather required in areas such as the Aegean, where the sparseness of permanent seismological networks on one hand, and the high seismotectonic complexity on the other hand, pose constraints on the efficient definition of seismogenic sources by using seismological methods alone. The ...

Near real time moment tensors (MT) of earthquakes in Greece and its surroundings, which have been distributed by the Aristotle University of Thessaloniki (AUTH) since March 2006 to August 2007, are compared to fast moment tensors computed by other agencies. We compare the best double- couple parts of the solutions for 30 events of Mw=3.6 to 5.8, for which AUTH

The Kos-Bodrum 20 July 2017 Mw6.6 earthquake ruptured an E-W normal fault dipping ~50° to the south. It caused destruction mainly in monumental structures in the city of Kos. We examine the source and rupture properties that shaped the near-fault ground motions. A key factor controlling the level of ground motions is the dip angle of the fault (between 30° to 60° in published mechanisms), alongside the dip polarity (to the north or to the south). The preferred slip model has two major asperities, with peak slip ~1.6m, located west and east of the hypocenter. The rupture propagated predominantly bilateral at a speed of 2.8 km/s. The ShakeMap shows an E-W spatial extension, in the mid-distance between the cities of Bodrum and Kos. The near-fault unique NS velocity time-series at Bodrum station, which is the fault normal direction, shows a late pulse, that is to say not at the beginning of the record, of period ~0.7s. In the absence of strong motion records in the city of Kos, we selected two near-fault horizontal time-series from the 2016 Norcia earthquake in Italy, as representative for the postulated bedrock motions. Adopting the preferred soil-profile and properties of Psycharis and Taflampas (2017) we perform 1D site response analysis, using the Bodrum and Norcia horizontal records as bedrock input motions, to estimate the surface motions. From all spectral comparisons, we conclude that the mainshock was characterized by enriched spectral ordinates for spectral periods >0.6s up to 2s, and that the effect of the soil in Kos city, is significant in this range.

The sequence of 14 October 2008 Mw 5.1 offshore Evia Island occurred at the westernmost termination of the North Anatolian Fault Zone into the Aegean. The focal mechanisms of six of the stronger events clearly show normal faulting with considerable dextral strike slip component. We obtained the slip model for the strongest GMT 02:06 event, we identified the NE-SW trending

We study the surface deformation of a moderate size M5+ earthquake swarm-type activity which burst at the tip of the Biga peninsula (western Turkey) in early 2017. No previous M5+earthquakes have been recorded in the instrumental period on land, however, offshore normal faults, have ruptured to produce strong (M > 6) earthquakes. We use the Interferometric Synthetic Aperture Radar (InSAR) technique and exploit a data set of Sentinel-1 and ALOS-2 images, to successfully detect the surface deformation caused by three M5+ events of the sequence and constrain their source models, furtherly strengthened by seismic waveform analysis. The sequence occurred at shallow depths (<12 km) and is related with the activation of a normal fault. Our geodetic inversions constrained the rupture distribution of the main events and led us to conclude that they belong to a single fault plane, striking N110 • E and dipping ∼40 • to the SW, compatible with the regional tectonics. Furthermore, the InSAR analysis revealed that no aseismic transients occurred during the Biga swarm. InSAR applications in seismic swarms are limited mainly due to the low displacement signal they produce, which is not always easily detectable by radar satellites. Another obstacle is the time frequency of radar satellites acquisitions that often does not allow the temporal isolation of distinct seismic events. However, here we present a study that exploits InSAR data to isolate seismic sources of a swarm and proposes its use for the understanding of shallow seismic swarms in a systematic manner.

We use 576 earthquakes of magnitude, Mw, 3.3 to 6.8 that occurred within the region 33◦ N–42.5◦ N, 19◦ E–30◦ E in the time period 1969 to 2007 to investigate the stability of the relation between moment magnitude, Mw, and local magnitude, ML, for earthquakes in Greece and the surrounding regions. We compare Mw to ML as reported in the monthly bulletins of the National Observatory of Athens (NOA) and to ML as reported in the bulletins of the Seismological Station of the Aristotle University of Thessaloniki. All earthquakes ...

On 3 March 2021, the Mw6.3 Tyrnavos earthquake shook much of the Thessalia region, leading to extensive damage in many small towns and villages in the activated area. The first main shock was followed in the next day, on 4th of March 2021, by an “equivalent” main shock with Mw6.0 in the adjacent fault segment. These are the largest earthquakes to strike the northeastern part of Thessalia since the M6.3, 1941 Larissa earthquake. The main shocks triggered extensive liquefaction mainly along the banks of the Titarisios tributary where alluvial flood deposits most probably amplified the ground motions. Our seismic monitoring efforts, with the use of recordings of the regional seismological network along with a dense local network that was installed three days after the seismic excitation initiation, led to the improved understanding the geometry and kinematics of the activated faults. The aftershocks form a north–northwest–trending, east–northeast–dipping, ~40 km long distribution, enco...

The M w 6.4 earthquake sequence of 2015 in western Greece is analyzed using seismic data. Multiple point source modeling, nonlinear slip patch, and linear slip inversions reveal a coherent rupture image with directivity toward the southwest and several moment release episodes, reflected in the complex aftershock distribution. The key feature is that the 2015 earthquake ruptured a strong asperity, which was left unbroken in between two large subevents of the M w 6.2 Lefkada doublet in 2003. This finding and the well-analyzed Cephalonia earthquake sequence of 2014 provide strong evidence of segmentation of the major dextral Cephalonia-Lefkada Transform Fault (CTF), being related to extensional duplex transform zones. We propose that the duplexes extend farther to the north and that the CTF runs parallel to the western coast of Lefkada and Cephalonia Islands, considerably closer to the inhabited islands than previously thought. Generally, this study demonstrates faulting complexity across short time scales (earthquake doublets) and long time scales (seismic gaps).

For routine practice, we need simple tools to reliably identify earthquakes with large isotropic (ISO) components. This study aims to highlight a possible indicator. Non-double-couple (non-DC) components of moment tensors (MTs) play a key role in our understanding of faulting earthquake processes and/or in identifying explosions. As opposed to DC components of the calculated seismic source model, the non-DC components (compensated linear vector dipole and ISO) are more vulnerable to errors in location, inaccurate velocity modeling, and noise. Methods for analyzing resolvability of ISO are relatively complicated. We propose a simple procedure to identify an earthquake with a strong ISO component. Recent MT determinations include space and time grid search of the centroid position, mainly the depth and time. The centroid is identified with a trial source position that maximizes correlation between real and synthetic waveforms. In synthetic tests with varying ISO percentage, we compare the correlation-depth dependence for two types of MT inversion: full and deviatoric. We show that in the inversion of data with a significant ISO component under the deviatoric assumption (i.e., when ISO is neglected), we might obtain an inaccurate centroid depth. However, when we make the grid search twice, under the deviatoric-MT and full-MT assumptions, and compare the results, we can obtain an indication of the significant ISO and avoid depth bias. This straightforward method is applied to two shallow earthquakes in Greece (the 27 January 2012 M w 5.3 Cretan Sea earthquake and the 26 June 2009 M w 4.9 Santorini earthquake).

(UTC 12:28:38.26) a magnitude Mw 6.3 earthquake occurred offshore Lesvos Island in SE Aegean Sea, which was widely felt, caused 1 fatality, and partially ruined the village of Vrisa on the southeastern coast of the island. I invert broad band and strong motion waveforms from regional stations to obtain the source model and the distribution of slip onto the fault plane. The hypocentre is located at a depth of 7 km in the upper crust. The mainshock ruptured a WNW-ESE striking, SW dipping, normal fault, projecting offshore and bounding the Lesvos Basin. The strongest and most aftershocks clustered away from the hypocentre, at the eastern edge of the activated area. This cluster indicates the activation of a different fault segment, exhibiting sinistral strike-slip motions, along a plane striking WNW-ESE. The slip of the mainshock is confined in a single large asperity, WNW from the hypocentre, with dimensions 20 km × 10 km along fault strike and dip, respectively. The average slip of the asperity is ~50 cm and the peak slip is ~1 m. The rupture propagated unilaterally towards WNW to the coastline of Lesvos island at a relatively high speed (~ 3.1 km/s). The imaged slip model and forward modelling was used to calculate peak ground velocities (PGVs) in the near-field. The damage pattern produced by this earthquake, especially in the village of Vrisa is compatible with the combined effect of rupture directivity, proximity to the slip patch and the fault edge, spectral content of motions, and local site conditions.

ABSTRACT: We applied the strong ground motion stochastic simulation method for finite faults to produce synthetics at grid points of a net (0.5×0.5 km), which covers the broader area of the city of Thessaloniki. We assume medium magnitude scenario earthquakes, which however occur on nearby to the city active faults. More specifically, we simulate strong ground motion from scenario earthquakes on the faults of a) Asvestochori and b) Pylaia-Panorama to the north and northeast of the city center, respectively and c) N. Mesimvria to the west. In all our simulations we used a common model for the propagation of the seismic waves, which consisted of empirical relations validated in previous studies. Among the examined earthquake scenarios the one on Asvestochori fault appears to be the worst for the building environment of Thessaloniki and especially for the low-storey structures. The other two scenarios appear to be overall of equivalent severity for the built environment of the city alt...

ABSTRACT : We present the first results from the pilot application of a methodology which produces, in near-real time, synthetic maps of strong ground motion parameters from moderate to large magnitude earthquakes in Greece. The methodology involves the calculation of the earthquake moment tensor, the discrimination of the fault plane among the two nodal planes of the focal mechanism, the calculation of the slip distribution onto the fault plane and its use to forward model velocity time histories in the broader epicentral area. The pilot applications of the methodology resulted in synthetic maps for eight moderate magnitude events, which occurred in the broader Aegean area in 2006-2007. Results are compared to corresponding synthetic maps that were produced based on empirical attenuation relations valid within the study area.

Temporal monitoring of terrain movements and satellite observations are used here to monitor geophysical Natural Hazards in northern Greece for the period 1992-2010, applying Persistent Scatterer Interferometry and Small Baseline Subset techniques, in an attempt to address their causes: anthropogenic or natural due to geological and geomorphological evolution. Our study highlights new areas that were previously unknown to be at risk and validates the remote sensing estimates using drill data, geomechanics and hydrogeology results. Site-specific results are presented for the broader urban area of Thessaloniki, Thessaloniki plain (Nea Malgara-Kimina, Chalastra) and the broader area of Katerini.

We present the first results from the pilot application of a methodology which produces, in near-real time, synthetic maps of strong ground motion parameters from moderate to large magnitude earthquakes in Greece. The methodology involves the calculation of the earthquake moment tensor, the discrimination of the fault plane among the two nodal planes of the focal mechanism, the calculation of the slip distribution onto the fault plane and its use to forward model velocity time histories in the broader epicentral area. The pilot applications of the methodology resulted in synthetic maps for eight moderate magnitude events, which occurred in the broader Aegean area in 2006-2007. Results are compared to corresponding synthetic maps that were produced based on empirical attenuation relations valid within the study area.

Near real time moment tensors (MT) of earthquakes in Greece and its surroundings, which have been distributed by the Aristotle University of Thessaloniki (AUTH) since March 2006 to August 2007, are compared to fast moment tensors computed by other agencies. We compare the best double-couple parts of the solutions for 30 events of Mw=3.6 to 5.8, for which AUTH and at least two other agencies have released fast MTs, and find that 97% of the AUTH fast solutions are comparable to the majority of other similar computations. The preliminary Mw distributed in AUTH MT alerts is found, on average, 100% equal to independent Mw computations.