George Kaviris - Profile on Academia.edu (original) (raw)

Papers by George Kaviris

Applied Sciences, 2022

Coastal areas constitute a very dynamic environment, balancing between numerous natural and anthr... more Coastal areas constitute a very dynamic environment, balancing between numerous natural and anthropogenic processes liable to sometimes hazardous geomorphic phenomena. Especially in tectonically active coastal regions and areas of high economic value, slope failures can have significant impacts and therefore need careful and detailed examination. This work uses Unmanned Aerial System (UAS)-aided photogrammetry and Terrestrial Laser Scanning (TLS) in tectonically active segments of the coastal zone of the Ionian Islands in Greece, to explore how their capabilities can help to improve our understanding of the structural integrity of the slopes. Results show that the two approaches are able to extract large numbers of discontinuity facets, in a more practical, rapid and safe way than conventional methods of rock slope stability analysis extending to unreachable yet important parts of the slope. Through this holistic record of the structural condition of the slope the two applications a...

Annals of Geophysics, Jun 27, 2023

This special issue of Annals of Geophysics "Seismic anisotropy and shear-wave splitting: Achievem... more This special issue of Annals of Geophysics "Seismic anisotropy and shear-wave splitting: Achievements and perspectives" originates from a session (S10) of the 37 th General assembly of the European Seismological commission ESC 2021 Conference which was planned to take place on 21 September 2021, in Corfu Greece, but due to the Covid19 pandemic was Virtual. The main theme of the session and of this special issue was the crucial role of seismic anisotropy in investigating the Earth's interior from the upper crust to the inner core. Shear-wave splitting, one of the most effective ways to study seismic anisotropy, can identify the properties and the geodynamics of the upper mantle, and identify the presence of fluid-saturated microcracks, oriented according to the stress regime, in the upper crust. Azimuthal anisotropy and radial anisotropy can be assessed from earthquake or ambient noise recordings to detect the seismic layered features and to rebuild the 3D seismic structure.

Applied sciences, Jun 26, 2023

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

On the origin of upper crustal shear-wave anisotropy at Samos Island, Greece

Acta Geophysica, May 11, 2021

Shear-wave splitting is associated to different sources in the upper crust. Preferentially orient... more Shear-wave splitting is associated to different sources in the upper crust. Preferentially oriented minerals, stress-aligned microcracks and tectonic structures have all been identified as causes of seismic anisotropy in the upper crust. However, distinguishing between them and discovering the actual origin of the splitting effect has important implications; changes in the anisotropic properties of the medium related to the behavior of fluid-filled microcracks could have potential connections to the occurrence of an impending significant earthquake. The recent 2020 Samos Mw = 6.9 event and its associated sequence was a great opportunity to study shear-wave splitting in the area. The spatial constrains in such studies, i.e., the requirement of events located very close to the receivers, did not permit exploring local anisotropy in the past, due to a severe lack of suitable data. To establish a background of splitting, we searched for any appropriate earthquake in a five-year period preceding the mainshock. We performed an automatic analysis on over 200 event-station pairs and obtained 164 high-quality splitting observations between January 2015 and November 2020. Results indicated a strong connection to local structures; Sfast polarization axes seem to align with faults in the area. However, we also observed a period of increasing and decreasing time-delays, associated with an Mw = 6.3 earthquake that occurred on June 2017 near Lesvos Island. The latter behavior implies the possibility of stress-induced anisotropy in the area. Thus, the Samos Island could be represented by two different sources of splitting; structures to the NW and microcracks to the SE.

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

Pure and Applied Geophysics, Jun 10, 2021

Earthquake diffusion and the migration behaviour of seismic clusters are commonly studied to prov... more 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.

Annals of Geophysics, May 5, 2023

The area of Damasi-Tyrnavos (Thessaly, Central Greece), in the vicinity of Larissa, was character... more The area of Damasi-Tyrnavos (Thessaly, 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 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. Anisotropy directions at stations installed at the northern and eastern parts of the study area strike 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.

Physics of the Earth and Planetary Interiors, Jun 1, 2020

The Florina basin (NW Greece) embodies sources of natural CO 2 emissions, which are aligned paral... more The Florina basin (NW Greece) embodies sources of natural CO 2 emissions, which are aligned parallel to the NE-SW striking regional major normal faults. The study area is characterized by low to moderate seismicity, as historical information and instrumental data reveal. During February 2013 to January 2014 an outburst of seismic activity emerged, consisting of more than 2000 located earthquakes, with a mainshock of M w = 4.1 on 17 February 2013 (16:48 UTC). A local temporary network was installed in July 2013, whose recordings provided the necessary data for performing a Shear-wave Splitting (SwS) study of the upper crust. After the application of strict selection criteria, 532 SwS measurements were obtained for two stations, using a mixture of manual and fully automatic methods. Two polarization directions of the fast shear-wave were identified at one of the stations, with the main one striking NE-SW and the secondary in a NW-SE direction. The first direction is in agreement with the local stress field, as it is identified by focal mechanisms, and with the strike of faults along which soil gas CO 2 concentrations are distributed and can be explained by the Anisotropic Poro-Elasticity (APE) model. The second direction is consistent with the strike of mapped local faults, inherited since the Late Miocene extension, which are responsible for the formation of the Florina basin. The SwS results at the second station revealed an almost N-S dominant polarization direction, which is not consistent with the local stress field. This direction, obtained from events belonging to the southern group of the sequence, that was probably triggered by fluid intrusion, might be interpreted by spatiotemporal patterns of seismicity and CO 2 upwelling.

Pure and Applied Geophysics, Jul 22, 2021

Seismic hazard estimation relies classically on databased ground motion prediction equations (GMP... more Seismic hazard estimation relies classically on databased ground motion prediction equations (GMPEs) giving the expected motion level as a function of several parameters characterizing the source and the sites of interest. However, records of moderate to large earthquakes at short distances from the faults are still rare. For this reason, it is difficult to obtain a reliable ground motion prediction for such a class of events and distances where also the largest amount of damage is usually observed. A possible strategy to fill this lack of information is to generate synthetic accelerograms based on an accurate modeling of both extended fault rupture and wave propagation process. The development of such modeling strategies is essential for estimating seismic hazard close to faults in moderate seismic activity zones, where data are even scarcer. For that reason, we selected a target site in Upper Rhine Graben (URG), at the French-German border. URG is a region where faults producing micro-seismic activity are very close to the sites of interest (e.g., critical infrastructures like supply lines, nuclear power plants, etc.) needing a careful investigation of seismic hazard. In this work, we demonstrate the feasibility of performing near-fault broadband ground motion numerical simulations in a moderate seismic activity region such as URG and discuss some of the challenges related to such an application. The modeling strategy is to couple the multi-empirical Green's function technique (multi-EGFt) with a k-2 kinematic source model. One of the advantages of the multi-EGFt is that it does not require a detailed knowledge of the propagation medium since the records of small events are used as the medium transfer function, if, at the target site, records of small earthquakes located on the target fault are available. The selection of suitable events to be used as multi-EGF is detailed and discussed in our specific situation where less number of events are available. We then showed the impact that each source parameter characterizing the k-2 model has on ground motion amplitude. Finally we performed ground motion simulations showing results for different probable earthquake scenarios in the URG. Dependency of ground motions and of their variability are analyzed at different frequencies in respect of rupture velocity, roughness degree of slip distribution (stress drop), and hypocenter location. In near-source conditions, ground motion variability is shown to be mostly governed by the uncertainty on source parameters. In our specific configuration (magnitude, distance), the directivity effect is only observed in a limited frequency range. Rather, broadband ground motions are shown to be sensitive to both average rupture velocity and its possible variability, and to slip roughness. Ending up with a comparison of simulation results and GMPEs, we conclude that source parameters and their variability should be set up carefully to obtain reliable broadband ground motion estimations. In particular, our study shows that slip roughness should be set up in respect of the target stress drop. This entails the need for a better understanding of the physics of earthquake source and its incorporation in the ground motion modeling.

Computers & Geosciences, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The 2020-2021 Seismic Swarm in the Western Gulf of Corinth: Insights on the Triggering Mechanisms Through High Resolution Seismological and Geodetic Data Analysis

SSRN Electronic Journal

The CRL School in the European Education community and the modern classroom

In the Gulf of Corinth area in Central Greece, for about 30 years, a concerted e... more In the Gulf of Corinth area in Central Greece, for about 30 years, a concerted effort is being performed to better understand the Geophysical processes (Earthquakes, Landslides, Tsunamis) that take place. It is one of the narrowest and fastest extending continental regions worldwide and has one of the highest seismicity rates in the Euro-Mediterranean region. The area is studied by research teams from all over Europe and a network, the Corinth Rift Laboratory (C.R.L.), has been established. Aiming to perform the most complete and thorough studies, many instruments have been installed (~70 Seismometers, Accelerometers, GNSS satellite receivers, etc.). The Corinth Rift Laboratory (CRL) is a Near Fault Observatory (NFO) within the European Plate Observing Systems (EPOS). Since 2016, every year, the "School of the Corinth Rift Laboratory" (CRL School), the educational component of this natural Observatory, is being held in the CRL area with the support of EGU &#8211; GIFT, to educate Primary and Secondary Education Teachers and University Students in the &#8216;real&#8217; research issues. The aims of the school are to: <ul> <li>Bring the Research/University community in contact with Primary and Secondary Education Teachers to enrich their knowledge, with an impact on the most efficient and targeted education and its dissemination to students.</li> <li>Intensify osmosis and collaboration between Teachers from different schools in European countries.</li> <li>Be next to the on-site research and related activities carried out in the scientific fields of Seismology, Geophysics, Geodesy, Tectonics and Geology for the physical laboratory of CRL.</li> <li>To inspire Teachers for new actions and for new activities and tasks that they can assign to students.</li> <li>To encourage them to participate in international scientific conferences in specific thematic areas in education (such as E.G.U.)</li> </ul> During the C.R.L. School, School Teachers participate in activities as: <ul> <li>Introduction to Seismology, Geophysics, Geodesy, Tectonics and Geology, focused on the Gulf of Corinth through lectures, hands-on and field trips by Researchers, University Professors, Academic students and Primary &#8211;Secondary Education Teachers.</li> <li>Seismological, Geodetic and Geological measurements and visits to permanent operational stations.</li> <li>Hands-on for locating earthquakes in the Gulf of Corinth area using real waveform data.</li> <li>Operate and use special educational equipment for measuring and understanding seismic vibrations.</li> <li>Acquaintance with modern educational tools and methodology for the school classroom.</li> <li>Further analysis of measurements in student classes, in collaboration with C.R.L. partner schools and educators participating in C.R.L. and E.G.U..</li> <li>Earth Observation with emphasis on ground deformation measurements from satellite microwave radars and its application to C.R.L., with presentations and in the laboratory.</li> <li>Geohazards with emphasis on seismic hazard and implications in technical Geology, as the Rio - Antirrio Bridge case.</li> </ul> Teachers' participation is completely free with a grant from E.G.U..

Geosciences

Lefkada and Corfu old towns are located in the western part of Greece, in the Ionian Sea. Their p... more Lefkada and Corfu old towns are located in the western part of Greece, in the Ionian Sea. Their proximity to the Hellenic subduction zone (HSZ) is the reason for their intense seismicity. The main goal of this study was the estimation of the geotechnical characteristics of the subsurface, with the contribution of applied geophysical techniques. Therefore, seismic refraction tomography (SRT) and multichannel analysis of surface waves (MASW) were applied. A total of thirty-three (33) seismic and geoelectrical profiles were performed in both towns in order to evaluate the geotechnical characteristics of the subsurface formations. Additionally, subsurface resistivity distributions were investigated with the application of electrical resistivity tomography (ERT). Some important elastic moduli were calculated through the combination of estimated seismic wave velocities and laboratory density measurements. The horizontal distribution of seismic velocities and mechanical properties (σ, E, K...

Journal of Geophysical Research: Solid Earth

Moderate‐to‐large earthquakes in rifts may occur on leading boundary faults or inner antithetic f... more Moderate‐to‐large earthquakes in rifts may occur on leading boundary faults or inner antithetic faults. Here we show a rare case of the 2020–2021 seismic sequence in the Corinth rift, that culminated in the shallow rupture of the antithetic fault, neither preceded nor followed by the leading fault rupture. The hypocenter of the largest shock (Mw 5.3 of 17 February 2021) was located at ∼8 km depth. However, seismic waveform data, supported by satellite‐geodetic and tide gauge measurements, pointed to rupture at shallow depth (∼3 km), where no earthquakes were previously observed. We show that the earthquake most probably ruptured two orthogonal, conjugate fault segments: a weak nucleation phase occurred in the microseismically highly active sub‐horizontal detachment layer, followed – a few seconds later – by a larger, shallow moment release on a high‐angle, south‐dipping normal fault. The latter is the Mornos offshore fault, antithetic to the leading, north‐dipping Psathopyrgos fault...

The 2020 Perachora peninsula earthquake sequence (Εast Corinth Rift, Greece): spatiotemporal evolution and implications for the triggering mechanism

Acta Geophysica

The Near Fault Observatory community in Europe: a new resource for faulting and hazard studies

Annals of Geophysics

The Near Fault Observatories (NFOs) community is one of the European Plate Observing System (EPOS... more The Near Fault Observatories (NFOs) community is one of the European Plate Observing System (EPOS, http://www.epos-eu.org) Thematic Communities, today consisting of six research infrastructures that operate in regions characterised by high seismic hazard originating from different tectonic regimes. Earthquakes respond to complex natural systems whose mechanical properties evolve over time. Thus, in order to understand the multi-scale, physical/chemical processes responsible for the faulting that earthquakes occur on, it is required to consider phenomena that intersect different research fields, i.e., to put in place multidisciplinary monitoring. Hence, NFOs are grounded on modern and multidisciplinary infrastructures, collecting near fault high resolution raw data that allows generation of innovative scientific products. The NFOs usually complement regional backbone networks with a higher density distribution of seismic, geodetic, geochemical and other geophysical sensors, at surfac...

Applied Sciences

The Arkalochori village in central Crete was hit by a large earthquake (Mw = 6.0) on 27 September... more 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 combining seismological and remote sensing data processing, followed by extensive field validation. Detailed geological mapping, fault surface measuring accompanied with tectonic analysis, fault photorealistic model creation by unmanned aerial system data processing, post-seismic surface deformation analysis by DInSAR image interpretation coupled with accurately relocated epicenters recorded by locally established seismographs have been carried out. The combination of the results obtained from these techniques led to the determination of the contemporary tectonic stress regime that caused the earthquake in central Crete, which was found compatib...

An integrated UAS and TLS approach for monitoring coastal scarps and mass movement phenomena. The case of Ionian Islands

Mediterranean tectonically-active coastal areas are a highly-dynamic environment... more Mediterranean tectonically-active coastal areas are a highly-dynamic environment balancing internal tectonic dynamics with external geomorphic processes, as well as manmade influences. Especially in touristic areas characterized by high built-up pressure and land value, where these dynamics are even more concentrated, the evolution of coastal environments needs careful and high-resolution study to identify localized risk and the processes they derive from. Recently, new advanced remote sensing techniques such as Unmanned Aerial Systems (UAS)- and Terrestrial Laser Scanners (TLS)-aided monitoring have improved our capabilities in understanding the natural processes and the geomorphic risks (i.e. mass movement phenomena). An integrated study comprising Unmanned Aerial Vehicles (UAV) and Light Detection And Ranging (LIDAR) sensors was conducted in coastal areas of the southern Ionian Islands (Western Greece) aiming to the mitigation of earthquake-triggered landslide risk and to responsible coastal development. Located at the northwesternmost part of the Hellenic Arc, this area is characterized by high seismicity and has been affected by destructive earthquakes mainly due to the Cephalonia Transform Fault Zone (CTFZ), which constitutes one of the most seismic active structures in the Eastern Mediterranean region. One of the most common environmental effect triggered by these earthquakes are landslides distributed along fault scarps in developed and highly visited coastal areas. Furthermore, this area is highly susceptible to hydrometeorological hazards inducing intense geomorphic processes, including Medicanes among others. These technologies allow a highly-detailed view of landslide processes, providing insights on the structures and factors controlling and triggering failures along coastal scarps as well as highlighting susceptible zones and high-risk areas with accuracy and mitigating adverse effects with precision and clarity. Overall, by providing a better understanding of the risks the approach used allows a more sustainable development of these coastal segments enhanced by risk mitigation. The study was conducted in the framework of the project &#8220;Telemachus - Innovative Operational Seismic Risk Management System of the Ionian Islands&#8221;, co-financed by Greece and the European Union (European Regional Development Fund) in Priority Axis &#8220;Environmental Protection and Sustainable Development&#8221; of the Operational Programme &#8220;Ionian Islands 2014&#8211;2020&#8221;.

Journal of Geodynamics, 2020

Bulletin of the Geological Society of Greece, Sep 13, 2021

The main goal of an Earthquake Early Warning System (EEWS) is to estimate the expected peak groun... more The main goal of an Earthquake Early Warning System (EEWS) is to estimate the expected peak ground motion of the destructive S-waves using the first few seconds of P-waves, thus becoming an operational tool for real-time seismic risk management in a short timescale. EEWSs are based on the use of scaling relations between parameters measured on the initial portion of the seismic signal, after the arrival of the first wave. Herein, using the abundant seismicity that followed the 3 March 2021 Mw=6.3 earthquake in Thessaly we propose scaling relations for PGA, from data recorded by local permanent stations, as a function of the integral of the squared velocity (IV2p). The IV2p parameter was estimated directly from the first few seconds-long signal window (tw) after the P-wave arrival. Scaling laws are extrapolated for both individual and across sites (i.e., between a near-source reference instrument and a station located close to a target). The latter approach is newly investigated, as local site effects could have a significant impact on recorded data. Considering that further study on the behavior of IV2p is necessary, there are indications that this parameter could be used in future on-site single-station earthquake early warning operations for areas affected by earthquakes located in Thessaly, as it presents significant stability.

Elastic-Anelastic properties beneath the Aegean inferred from long period Rayleigh Waves

AGUFM, Dec 1, 2007

ABSTRACT This work is towards contributing to the better knowledge of the deep structure of the A... more ABSTRACT This work is towards contributing to the better knowledge of the deep structure of the Aegean by introducing experimental elastic and anelastic parameters via the study of long period Rayleigh waves. For this scope path- average phase velocities and attenuation coefficients of fundamental Rayleigh waves crossing the Aegean were extracted over the period range 10-100 s. It is mean worth that it is the first time that anelastic parameters of the long period wavefield are determined for the region. The wavetrains were recorded at the broadband stations installed some years ago in the Aegean region for the SEISFAULTGREECE project. The stochastic inversion algorithm has been used to derive 36 path-average models of shear velocity and 19 path-average models of inverse shear Q down to 200 km. Average over the study region shear Q values at depths from 0 to 200 km range between 29±13. The observed low shear Q likely indicate that fluids reside in lower crustal, as well as upper mantle depths. Furthermore, the elastic and anelastic 1-D path-average models were combined in a continuous regionalization tomographic scheme to obtain a 3-D model of shear velocity variation down to 200 km and a 3-D model of inverse shear Q variation down to 120 km. The most prominent features in the tomograms are: a) A low shear velocity zone in the back-arc region, especially in the central and north Aegean. This region is located south of the North Aegean Trough (the western edge of the North Anatolian Fault) and correlates well with the derived anelastic tomograms which present high attenuation in this area. b) A high velocity/low attenuation zone in South Aegean indicating the subducted African lithosphere beneath the Aegean. The zone in central and north Aegean characterized by low velocities/high attenuation is compatible with a region of high extensional strain rates, recent volcanism and high heat flow. These observations suggest a hot or perhaps partially molten upper mantle and/or distributed deformation beneath the study region, probably related with the slab roll-back that has accompanied back-arc extension. ACKNOWLEDGMENTS The present study was co-funded by the European Social Fund and National Resources - (EPEAEK II) PYTHAGORAS, contract No. 70/3/7306.