Efthimios Sokos

• PhD
• Professor at University of Patras

Vibrations generated by anthropogenic activity propagate into the Earth’s subsurface as high-frequency seismic waves. The Covid-19 pandemic, which prompted widespread adoption of prevention policies in 2020, including social distancing measures, stay-at-home orders, travel restrictions and lockdowns, provided a unique opportunity to investigate on...

The South Aegean Volcanic Arc remains active, presenting not only primary volcanic hazards such as ashfall and lava flows but also secondary hazards from active submarine and coastal volcanoes with the potential to trigger tsunamis. These tsunamis pose a threat even to far-distant coastlines, as shown by the destructive history of large-scale erupt...

Plain Language Summary Intermediate‐depth earthquakes (60–300 km) occur in subduction zones and their origin is often attributed to dehydration processes as the subducted lithospheric plate drags hydrated rocks from the Earth's surface downwards. Several competing hypotheses related to mineral dehydration or thermal processes explain the earthquake...

Local soil conditions significantly influence the characteristics of observed seismic waves, often obscuring or distorting the original source signature and modifying recorded waveforms. Local backprojection (BP) imaging, a common technique for identifying the spatiotemporal high-frequency energy release during seismic ruptures, typically relies on...

We used full waveforms of the Hellenic Unified Seismic Network (HUSN) regional stations and a frequency range of 0.03-0.06 Hz. We calculated full moment tensors (MTs) and focused on their ISO and CLVD components. In the tested depth range of 1-20 km, the medians of ISO (=VOL) and CLVD are positive, but their 68% confidence intervals are broad due t...

We used full waveforms of the Hellenic Unified Seismic Network (HUSN) regional stations and a frequency range of 0.03-0.06 Hz. We calculated full moment tensors (MTs) and focused on their ISO and CLVD components. In the tested depth range of 1-20 km, the medians of ISO (=VOL) and CLVD are positive, but their 68% confidence intervals are broad due t...

ISOLA is a software package designed to calculate centroid moment tensors (MTs) by waveform inversion for single- and multiple-point-source models. Calculations are running with Fortran codes under Matlab Graphic User Interface. The codes have been continuously developed and widely used for over 20 yr. ISOLA2024 is the latest version upgraded in th...

Large-scale historical volcanic eruptions caused significant destruction in the Mediterranean region (e.g., Thera/Santorini explosion circa 1600 BCE). The South Aegean Volcanic Arc remains active, and in addition to primary volcanic hazards such as ashfall and lava flows, active submarine and coastal volcanoes have the potential to trigger tsunamis...

Modern seismometer electronics consist of an analog closed loop control scheme, which acts on a spring-mass electromechanical system operating under the force-balance principle. Although there are several options to implement this closed loop, the most successful is the one that incorporates a low-pass inverse filter at its open loop branch. Main c...

The Methana Volcano is located at the westernmost edge of the south Aegean volcanic arc, along the northeastern coast of Peloponnese in Greece. Historical records indicate that the most recent eruptive activity on Methana occurred around 230 BC. This, combined with present-day thermal activity and gas emissions, suggests the potential for future re...

Damages due to large earthquakes are influenced by broadband source effects that remain enigmatic. Here we develop a broadband (0–10 Hz) source model of the disastrous 2023 Mw7.8 Kahramanmaraş, Türkiye, earthquake by modeling recordings of 100 stations. The model combines coherent and incoherent rupture propagation at low and high frequencies, resp...

A new, improved approach in the design of broadband seismometers is presented. The design results in the implementation of a high performance, low cost, and simple-to-operate instrument. The proposed seismometer is based on a modified accelerometer followed by a continuous time integrator for providing velocity voltage output. It has a broadband re...

“PROION” is a research project, focused on the development of a platform for the continuous monitoring of high priority infrastructure (public infrastructure, dams, bridges, etc.) in the broader area of the Hellenic Supersite, named Enceladus. The project started on September 2020 and it was financially supported by the European Union and the Helle...

Patras Gulf pockmark field (Western Greece) is a tectonically controlled field that has been activated at least twice by strong earthquakes (M5.4, 14 July 1993 and M6.4, 8 June 2008), and episodic gas seepages have been recorded in the past using geophysical means. A distributed temperature sensor (DTS) system was deployed inside a shallow pockmark...

The Patras Gulf Pockmark field is located in shallow waters offshore Patras City (Greece) and is considered one of the most spectacular and best-documented fluid seepage activities in the Ionian Sea. The field has been under investigation since 1996, though surveying was partially sparse and fragmentary. This paper provides a complete mapping of th...

Key to studies of the rupture process is the identification of its major segmentation into asperities. Common multi-parametric methods invert for a continuous slip distribution; they usually rely on a predefined fault position, nucleation point, planarity, smoothing, etc. Here we propose more flexible low-parametric inversions - multi-point seismic...

The Aetolia-Akarnanian region, in Western Greece, is considered to be part of a micro-plate in formation, named the Ionian Island-Akarnanian Block (IAB), in the larger-scale Central Mediterranean tectonic context. The IAB accommodates the deformations between the surrounding tectonic structures that are the Corinth Gulf, the Hellenic subduction, th...

Rockfall analysis is a multiparametric procedure with many uncertainties and the outputs are largely dependent on some critical engineering geological parameters involved in the used simulation model. In this paper, three completely different limestone rock sequences, named Pantokratoras, Vigla, and Paxos limestones along the western coastal slopes...

Climate change constitutes a serious global challenge with consequences that are directly affecting infrastructure. Thus, there is a great need to develop reliable cost-effective systems, which integrate remote sensing data, in situ measurements and advanced methods for infrastructure monitoring. In this framework, the European Union and the Hellen...

The October 30, 2020, Mw7 Samos earthquake ruptured a north-dipping offshore normal fault, bounding the Samos basin; it accommodated ~N-S extension and can be viewed as a modern manifestation of the basin evolution. It caused 118 fatalities, generated a tsunami, and caused a co-seismic uplift of 20 to 35 cm of the NW part of Samos Island. Using bro...

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....

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 who...

A new class-A force-balance accelerometer (FBA) is designed, simulated, and evaluated. The focus of this work was to design a low-cost but high-performance instrument. The FBA has output voltage proportional to ground acceleration, flat response from direct current to 200 Hz, output range ± 10 V differential (40Vpp, peak-to-peak) and sensitivity 2....

We studied the kinematic behaviour of active landslides at several localities in the area of Panachaikon Mountain, Achaia (Peloponnese, Greece) using Sentinel (C-band) InSAR time series analysis. We processed LiCSAR interferograms using the SBAS tool, and we obtained average displacement maps for the period 2016–2021. We found that the maximum disp...

Automatic moment tensor (MT) determination is essential for real‐time seismological applications. In this article, Gisola, a highly evolved software for MT determination, oriented toward high‐performance computing, is presented. The program employs enhanced algorithms for waveform data selection via quality metrics, such as signal‐to‐noise ratio, w...

In the past 20 years, the Corinth Rift Laboratory (CRL) network has been closely monitoring the Western Gulf of Corinth (WGoC), the area being the main focus of the Corinth Rift Near Fault Observatory. The local seismological and geodetic network permitted the detection and location of microseismicity in high resolution, revealing complex sequences...

A dynamic finite-fault source inversion for stress and frictional parameters of the Mw 6.3 2017 Lesvos earthquake is carried out. The mainshock occurred on June 12, offshore the southeastern coast of the Greek island of Lesvos in the north Aegean Sea. It caused 1 fatality, 15 injuries, and extensive damage to the southern part of the island. Dynami...

Here we present a joint analysis of the geodetic, seismological and geological data of the March 2021 Northern Thessaly seismic sequence, that were gathered and processed as of April 30, 2021. First, we relocated seismicity data from regional and local networks and inferred the dip-direction (NE) and dip-angle (38°) of the March 3, 2021 rupture pla...

We investigate a seismic crisis that occurred in the western Gulf of Corinth (Greece) between December 2020 and February 2021. This area is the main focus of the Corinth Rift Laboratory (CRL) network, and has been closely monitored with local seismological and geodetic networks for 20 yr. The 2020–2021 seismic crisis evolved in three stages: It sta...

Άμεσος υπολογισμός Τανυστή Σεισμικής Ροπής σε σύγχρονες παράλληλες αρχιτεκτονικές Δείτε την παρουσίαση (στα Ελληνικά) https://lnkd.in/dsy8NRqY Gisola: A High Performance Computing application for real-time Moment Tensor inversion Watch Presentation (in Greek) https://lnkd.in/dsy8NRqY #fosscomm2021 #hpc #highperformancecomputing #gpu #openmp #opena...

Automatic Moment Tensor (MT) determination for regional areas is essential for real-time seismological applications such as stress inversion, shakemap generation, and tsunami warning. In recent years, the combination of two powerful tools, SeisComP and ISOLA (Sokos and Zahradník, 2008), paved the way for the release of Scisola (Triantafyllis et al....

The National Observatory of Athens data center for the European Integrated Data Archive (EIDA@NOA) is the national and regional node that supports International Federation of Digital Seismograph Networks and related webservices for seismic waveform data coming from the southeastern Mediterranean and the Balkans. At present, it serves data from eigh...

Following the installation of a temporary seismological network in western Greece north of the Gulf of Patras, we determined the quality of the sites of each of the 10 stations in the network. For this, we used the horizontal-to-vertical spectral ratio (HVSR) method and calculated an average curve over randomly selected days between 0 Hz and 10 Hz....

Aitolo-Akarnania prefecture, western Greece, is an area with strong earthquakes and large active fault systems. The most prominent are the Katouna sinistral strike slip fault and the Trichonis Lake normal fault system. Their proximity to large cities, and the lack of detailed information on their seismogenic potential, calls for multiparametric res...

Physical laws governing friction on shallow faults in the Earth and spatial heterogeneity of parameters are critical to our understanding of earthquake physics and the assessment of earthquake hazards. Here we use a laboratory-derived fault-friction law and high-quality strong-motion seismic recordings of the 2020 Elazığ earthquake, Turkey, to reve...

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic...

This study performs a reanalysis of the seismicity recorded during the 1996 Gjálp eruption that occurred at NW Vatnajökull, Iceland. The seismicity was recorded by the temporary HOTSPOT network consisting of 30 three-component broadband stations. In total 301 events were identified between 29 September and 12 October and their phases were manually...

With different styles of faulting, the eastern Ionian Sea is an ideal natural laboratory to investigate interactions between adjacent faults during strong earthquakes. The 2018 Mw 6.8 Zakynthos earthquake, well recorded by broadband and strong-motion networks, provides an opportunity to resolve such faulting complexity. Here, we focus on waveform i...

A collaborative strong motion array comprised mainly of low cost sensors has been deployed in the Patras city, Greece. Currently, it combines: 4 standard accelerometric stations operated by the National Observatory of Athens, Institute of Geodynamics (NOA) and 15 P-Alert MEMS acceleration devices deployed in public sector buildings, schools and pri...

The 2017 North Korea test is analyzed together with the previous 2009–2016 tests, and a generalized source model is derived using waveform data. Data are represented by lowfrequency records of 11 broadband near-regional stations (epicentral distances 140–310 km), bandpassed from 0.03 to 0.09 Hz. The events feature a significant degree of similarity...

ISOLA software package has been developed to invert local or regional full-wave seismograms for single- and multiple-point source models. The code was introduced in 2003; since then it has been continually upgraded, and, presently, it can be considered a well-established tool, used worldwide. Originally, the code name came from ‘isolated asperities...

Seismology illuminates physical processes occurring during underground explosions, not all yet fully understood. The thus-far strongest North Korean test of 3rd September 2017 was followed by a moderate seismic event (mL 4.1) after 8.5 minutes. Here we provide evidence that this aftershock was a non-tectonic event which radiated seismic waves as a...

Focal mechanisms of shallow crustal earthquakes with magnitude 4 or less remain unresolved in many regions of the world due to the sparseness of seismic networks. Relatively distant stations must be used, but modeling those waveforms is difficult due to imprecision of seismic-velocity models. The waveform inversions also suffer from instrumental pr...

The Hellenic Seas lie over the Aegean micro-plate, which is one of the world’s most seismically active areas and has experienced extreme tectonism throughout Quaternary time. This activity along with eustatic sea-level changes and water circulation patterns over the same time period, control the overall configuration of the seafloor and are the cau...

Automatic Moment Tensor (MT) calculation is exceptionally valuable in many realtime seismological applications, such as shake map generation or tsunami warning. Scisola, a new software for automatic MT retrieval, has been recently developed. This software binds the extensively used ISOLA MT inversion code with the widely known real-time seismologic...

During a 12-month period (August 1998 - June 1999), a dense microseismic network of 44 portable seismographs, was installed in Epirus - Northwestern Greece, by the Seismological Laboratory, of the University of Patras and recorded 1368 earthquakes. We selected a subset of 200 well recorded events, with duration magnitude ranging from 1.61 to 2.92 a...

Three years of continuous waveform data recorded at 22 stations from the Corinth Rift Laboratory and the Hellenic Unified Seismological Network are used to perform an ambient noise surface-wave tomography of the western Corinth Rift. All available vertical component time-series were cross-correlated to extract empirical Rayleigh-wave Green's functi...

Earthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. In contrast to teleseismic analysis of this doublet, we use regional seismic data providing robust two-point source models, further validated by regional back-projection and rupture-stop analysis. The...

On the 2 December 2002 an earthquake (Mw=5.5) occurred near the city of Vartholomio (western Greece) causing damage in more than 1000 buildings. The University of Patras Seismological Laboratory permanent network stations recorded the mainshock and the aftershocks. Furthermore, twenty-six sites were instrumented to study the aftershock sequence. We...

A series of strong earthquakes took place at the Trichonis lake area, during April 2007. Three events, with M~5.0 occurred within one day, causing damages to almost all nearby villages and especially at the small town of Thermo. The sequence lasted for more than one month with small to moderate size events. It was recorded by regional networks and...

In this paper we attempt to describe the experience we gained, working with a specially designed seismograph as an educational device. It is a modern instrument with short period response, GPS timing and digital recording on personal computer. The installation was done in the frame of the local seismology school network, called “Egelados”. The know...

A dense microseismic network was installed in Northwestern Greece for a period of eleven months. A total of 1368 events were recorded and located using a 1D model. These events were also used to derive a 3D velocity model for the area. This work presents results from further processing of the data using (a) simple location method of events in a 1D...

Patras is the third largest city in Greece and an ideal candidate for earthquake early warning (EEW) applications due to its high seismic hazard, its existing research infrastructure and the presence of critical structures such as the Rion-Antirion bridge. Patras is located a few hundred kilometres from the Hellenic Arc, where very strong and poten...

We present an integrated approach of the seismic structure and activity along the offshore SW Hellenic subduction from combined observations of marine and land seismic stations. Our imaging of the slab top topography from teleseismic receiver function analysis at Ocean Bottom Seismometers supports a trenchward continuation of the along-dip slab fau...

Moment Tensors (MTs) provide important information for seismotectonic, stress distribution and source studies. It is also important as a real time or near real time information in shakemaps, tsunami warning, and stress transfer. Therefore a reliable and rapid MT computation is a routine task for modern seismic networks with broadband sensors and re...

The Mw6.4 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...

On June 8, 2008, a Mw 6.4 earthquake occurred on a strike-slip fault in northwest Peloponnese, Greece, 35 km southwest of the city of Patras. We analysed part of the aftershock sequence of the 2008 earthquake, recorded by a portable network of six stations and a permanent station of the Hellenic Unified Seismological Network, in order to perform a...

On April 2007, an earthquake swarm occurred in the vicinity of Trichonis Lake, western Greece. The seismic activity started on April 10th , 2007 after the occurrence of three moderate size earthquakes Mw 5.0-5.2. We performed shear wave splitting measurements on seismic events recorded during the first week of the seismic activity by a portable net...

On June 8, 2008, a Mw 6.4 earthquake occurred on a strike-slip fault in northwest Peloponnese, Greece, 35 km southwest of the city of Patras. We analysed part of the aftershock sequence of the 2008 earthquake, recorded by a portable network of six stations and a permanent station of the Hellenic Unified Seismological Network, in order to perform a...

On April 2007, an earthquake swarm occurred in the vicinity of Trichonis Lake, western Greece. The seismic activity started on April 10th, 2007 after the occurrence of three moderate size earthquakes Mw 5.0-5.2. We performed shear wave splitting measurements on seismic events recorded during the first week of the seismic activity by a portable netw...

The Corinth Rift separates Peloponnesus to the south from mainland Greece to the north. It is one of the most active extensional intra-continental rifts in the world, with geodetically measured rates of extension varying from ∼5 mm/yr at the eastern part to ∼15 mm/yr at the western part. This work presents a first attempt to study the crustal veloc...

The feasibility of earthquake early warning (EEW) is now widely recognized. However, EEW systems that are in operation or under evaluation worldwide have significant variations and are usually operated independently of routine earthquake monitoring. We introduce a software that allows testing and evaluation of a well‐known EEW algorithm directly wi...

During a 12-month period (August 1998-June 1999), a dense microseismic network of 44 portable seismographs, was installed in Epirus – Northwestern Greece, by the Seismological Laboratory, of the University of Patras and recorded 1368 earthquakes. We selected a subset of 200 well recorded events, with duration magnitude ranging from 1.61 to 2.92 and...

The Corinth Rift in Central Greece has been studied extensively during the past decades, as it is one of the most seismically active regions in Europe. It is characterized by normal faulting and extension rates between 6 and 15 mm yr-1 in an approximately N10E° direction. On 2013 May 21, an earthquake swarm was initiated with a series of small even...

Extension of permanent seismic networks is usually governed by a number of technical, economic, logistic, and other factors. Planned upgrade of the network can be justified by theoretical assessment of the network capability in terms of reliable estimation of the key earthquake parameters (e.g., location and focal mechanisms). It could be useful no...

We study the 26 January and 3 February, 2014 (~Mw6) events in Cephalonia, combining weak and strong motion waveforms from regional and local stations. The hypocenter of the January 26 event is located at the southern-most tip of the Paliki Peninsula, at a depth of ~15 km. The centroid moment tensor (CMT) solution indicates rupture along a N20°E dex...

A moderate size earthquake occurred close to the town of Efpalio in the Corinth Gulf region on January 18, 2010. In the present paper, we have used this earthquake to study the dispersion of short-period Love waves along nine paths in a broader vicinity of the Corinth Gulf. The observed group velocities require very low S-wave velocities in the upp...

We present preliminary results of an ambient seismic noise analysis performed in the western Corinth Gulf, Greece. The Corinth Gulf is a continental rift which separates the central Greek mainland from Peloponnese. The rift is approximately 120 km long and 10-20 km wide, with a WNW-ESE orientation, extending from the Gulf of Patras in the west, to...

On 2010 January 18 and 22, two earthquakes of Mw 5.3 and 5.2, respectively, occurred near the town of Efpalio on the western Gulf of Corinth. We performed a shear wave splitting analysis using the cross-correlation method and calculated Vp/Vs ratios for events that occurred in the epicentral area of the Efpalio earthquakes, between 2009 January and...

The January 18, 2010, shallow earthquake in the Corinth Gulf, Greece (M w 5.3) generated unusually strong long-period waves (periods 4–8 s) between the P and S wave arrival. These periods, being significantly longer than the source duration, indicated a structural effect. The waves were observed in epicentral distances 40–250 km and were significan...

Thanks to local and near-regional data, the epicenter and centroid positions were considerably improved compared to the USGS and GCMT solutions. The spatial distribution of the seismic moment calculated from the accelerographic data in the frequency range 0.03-0.15 Hz agrees with the independent previous estimate of Yue et al. (2013). The non-negat...

11 , Efthimios SOKOS 12 , Kostas MAKROPOULOS 13 The Corinth Rift in Central Greece has been studied extensively during the past decades, as it is one of the most seismically active regions in Europe. It is characterized by normal faulting in an approximate WSW-ENE direction with an extension rate increasing from east (~6 mm/yr) to west (~14 mm/yr)...