Nikolaos S . Melis
National Observatory of Athens, Geodynamic Institute, Faculty Member
Warning systems are key elements in disaster risk reduction. Today, early warning systems, based on rapid earthquake detection, facilitate towards taking urgent action to mitigate damages, prior to the arrival of strong ground motion... more
Warning systems are key elements in disaster risk reduction. Today, early warning systems, based on rapid earthquake detection, facilitate towards taking urgent action to mitigate damages, prior to the arrival of strong ground motion and/or tsunami. However, even when this is not possible, considering the location of the earthquake source and the monitoring facilities, the rapid assessment still is valuable for decision makers to direct emergency response within minimal time to the affected area. Monitoring networks, such as broad band and strong motion seismic networks, as well as, cGPS and tide gauge networks, can facilitate to detect and evaluate the earthquake event and then assess the effect (i.e. the extent of damage area or tsunami origination, etc.). Scientific information that can also be extracted rapidly, could include, apart from the location (hypocenter) and the earthquake magnitude, the following: moment tensor estimation (point to the corresponding fault zone with its...
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SUMMARY On 2020 May 2, an Mw = 6.6 earthquake struck about 63 km south of Ierapetra in Crete, Greece. The earthquake generated a small tsunami which agitated local harbours. We studied this event in the context of earthquakes with seismic... more
SUMMARY On 2020 May 2, an Mw = 6.6 earthquake struck about 63 km south of Ierapetra in Crete, Greece. The earthquake generated a small tsunami which agitated local harbours. We studied this event in the context of earthquakes with seismic records in 1908, 1910, 1923, 1952, 2009 and 2013, all of similar magnitudes located south of Crete. Based on an energy-to-moment ratio, our analysis suggests that this event was neither slow nor fast, hence appropriate for using scaling laws to infer seafloor deformations. We also performed a field survey, three days after the event and present field observations from seven locations, including the island of Chrisi, where our highest measurement of 0.95 m was located. Runup along the coast of southern Crete ranged from 0.24 to 0.87 m. One tide gauge record is available for this event, and we did image analysis to obtain accurately timed water surface elevations from eyewitness videos and images. We undertook high-resolution hydrodynamic simulations...
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<p>Once a niche technology, Distributed Acoustic Sensing (DAS) has gained increasing popularity over the last decade, due to its versatility and ability to capture extremely dense seismic datasets in a wide range of... more
<p>Once a niche technology, Distributed Acoustic Sensing (DAS) has gained increasing popularity over the last decade, due to its versatility and ability to capture extremely dense seismic datasets in a wide range of challenging environments. While DAS has been utilised in some particularly remote locations, such as on glaciers and volcanoes, it also holds a great deal of potential closer to home; beneath our cities. As DAS is able to be used with existing telecommunication fibres, urban areas contain huge potential networks of strain or strain-rate sensors, right beneath our feet. This data enables us to monitor the local environment, recording events such as earthquakes, as well as characterising and monitoring the shallow subsurface. DAS experiments using dark fibres are unintrusive and highly repeatable, meaning that this method is ideal for long-term site monitoring.</p><p>In collaboration with the OTE Group (the largest telecommunications company in Greece), we have collected urban DAS data beneath North-East Athens, utilising existing, in-situ telecommunication fibres. This large dataset contains a wide range of anthropogenic signals, as well as many seismic events, ranging from small, local events, to an internationally reported Magnitude 6.4 earthquake in Crete.</p><p>We conduct a preliminary analysis of the dataset, identifying and assessing the earthquake signals recorded. This will be compared with the event catalogue of the local, regional network in Athens, to determine our sensitivity to events of different magnitudes, and in a range of locations. We hope to gain an understanding of how DAS could be combined with the existing network for seismic monitoring and earthquake detection.</p><p>Moving forward, we aim to also apply ambient noise methods to this dataset in order to extract dispersion measurements, and ultimately invert for a shallow velocity model of the suburbs of Athens.</p>
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We experimentally demonstrate a microwave frequency dissemination system operating as a sensitive interferometric sensor of seismic waves on commercially deployed fiber networks in Attika, Greece. Efficient detection of seismic waves from... more
We experimentally demonstrate a microwave frequency dissemination system operating as a sensitive interferometric sensor of seismic waves on commercially deployed fiber networks in Attika, Greece. Efficient detection of seismic waves from distant epicenters (>400km) is presented
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ABSTRACT Santorini caldera has experienced several explosive eruptions in the past, the most well-known of these being the Late Bronze Age (ca. 1628 BC) eruption that may have been responsible for the demise of the Minoan civilization.... more
ABSTRACT Santorini caldera has experienced several explosive eruptions in the past, the most well-known of these being the Late Bronze Age (ca. 1628 BC) eruption that may have been responsible for the demise of the Minoan civilization. Since the early 1950's the volcano has been dormant without exhibiting any significant activity except from discharge of low-temperature hydrothermal fluids. In January 2011 both deformation and seismic activity increased considerably signaling a period of unrest which however, did not result in an eruption. One permanent and seven temporary seismic stations equipped with three-component sensors were deployed by the National Observatory of Athens. These were combined with seismic stations from the University of Thessaloniki, seven with only a vertical component and four with three-component sensors and all operated under the Hellenic Unified Seismic Network, thus densely monitoring the Santorini Volcano. These seismic stations have recorded the seismic activity from its start up to now. About 290 micro-earthquakes recorded by at least 5 stations were analyzed for the purpose of obtaining accurate epicentral and hypocentral locations using both catalog and differential travel times from waveform cross-correlation. All of these events exhibit clear P- and S-phases indicating that they resulted from shear failure of rock rather than fluid-flow within volcanic conduits. Results show two well-defined clusters in Palea and Nea Kameni islands within the caldera with hypocentral depths ranging between 5-10 km. Interestingly, one more cluster of events with depths between 15-19 km appears near the area of Cape Coloumbo and developed almost simultaneously with the clusters within the caldera. The Mogi source depth inferred from geodetic observations previously is shallower (~4 km) and does not coincide spatially with the clusters within the caldera. This points to the possibility that seismicity and deformation may be excited by deeper pressure changes. Shear wave splitting measurements have also been performed using all available waveform data in order to understand the nature and spatial variation of the stress field during the unrest. Fast polarization directions exhibit some orientations consistent with the regional NW-SE extension in the area, but also orientations along NE-SW that signify the presence of a local stress field as well.
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SUMMARY We present a theory and conceptual examples for fibre-optic deformation sensing based on phase changes of transmitted light. As a first result, we establish an exact relation between observable phase changes and the deformation... more
SUMMARY We present a theory and conceptual examples for fibre-optic deformation sensing based on phase changes of transmitted light. As a first result, we establish an exact relation between observable phase changes and the deformation tensor along the fibre. This relation is nonlinear and includes effects related to both local changes in fibre length and deformation-induced changes of the local refractive index. In cases where the norm of the deformation tensor is much smaller than 1, a useful first-order relation can be derived. It connects phase changes to an integral over in-line strain along the fibre times the local refractive index. When spatial variations of the refractive index are negligible, this permits the calculation of phase change measurements from distributed strain measurements, for instance, from distributed acoustic sensing (DAS). An alternative form of the first-order relation reveals that a directional sensitivity determines the ability of a point along the fib...
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Summary Fiber-optic sensing based on transmission offer an alternative to scattering-based Distributed Acoustic Sensing (DAS). The ability to interrogate fibers that are thousands of kilometers long opens opportunities for studies of... more
Summary Fiber-optic sensing based on transmission offer an alternative to scattering-based Distributed Acoustic Sensing (DAS). The ability to interrogate fibers that are thousands of kilometers long opens opportunities for studies of remote regions, including ocean basins. However, by averaging deformation along the fiber, transmission systems produce integrated instead of distributed measurements. They defy traditional interpretations in terms of simple seismic phases, thereby inherently requiring a full-waveform approach. For this, we develop a formalism to calculate sensitivity kernels of transmitted optical phase changes with respect to (Earth) structure using optical phase delay measurements. We demonstrate that transmission-based sensing can effectively provide distributed measurements when optical phase delays are analysed in different time windows. The extent to which a potentially useful sensitivity coverage can be achieved depends on the fiber geometry, and specifically on...
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In this study the application of active damping on a short-period Spreng-nether S-7000 passive sensor is presented. This sensor uses a unique magnetic sus-pension mechanism that eliminates spring resonance effects and ensures improved... more
In this study the application of active damping on a short-period Spreng-nether S-7000 passive sensor is presented. This sensor uses a unique magnetic sus-pension mechanism that eliminates spring resonance effects and ensures improved thermal behavior. The resilient properties of the magnetic spring are devoid of non-linearities seen in the metallic springs. Although the magnetic suspension is highly susceptible in external magnetic fields, the magnetic scheme applied on the S-7000 seismometer ensures reliable magnetic shielding and it can be considered as a stable suspension. The multiple frequency-dependent feedback, which is the dominant tech-nique in the design of broadband sensors, modified the original velocity response envelop of the old instrument. The theoretical response obtained from the new trans-fer function, in accordance with the experimental response obtained from calibration data, characterizes the modified S-7000 seismometer as a flat velocity (0.011 to 20 Hz) sens...
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Since November 2003 a collaborative group between Greek, Polish, and Slovak colleagues installed a dense network of non-permanent GPS stations and extensometers to monitor active faults in the eastern end of the Gulf of Corinth, central... more
Since November 2003 a collaborative group between Greek, Polish, and Slovak colleagues installed a dense network of non-permanent GPS stations and extensometers to monitor active faults in the eastern end of the Gulf of Corinth, central Greece. The network includes eleven GPS stations across the Kaparelli fault and the Asopos rift valley to the east and two TM-71 extensometers that were installed on the Kaparelli fault plane. The motions recorded by the TM-71 instruments show agreement with long-term fault kinematics. The GPS network has been measured in three campaigns (2004, 2005 & 2006) with very good accuracies (1-4 mm in the horizontal plane). Given that the total offset on the Kaparelli fault is small, and the geological data suggesting a segmented character of this fault, we expect in the near future to differentiate fault slip and strain accumulation among segments.
In this letter we present preliminary results concerning precursors to the recent significant (Mw 6.7) earthquake (EQ) that took place in the region south of the Greek Island of Zakynthos (October 25, 2018). We show, in terms of... more
In this letter we present preliminary results concerning precursors to the recent significant (Mw 6.7) earthquake (EQ) that took place in the region south of the Greek Island of Zakynthos (October 25, 2018). We show, in terms of fracture-induced electromagnetic emissions (EME), that the Earth system around the focal area was at critical state up to a few hours before the occurrence of the EQ. Importantly, departure from the critical state, in terms of the symmetry breaking phenomenon, was also identified in the EME recorded after the appearance of the last signatures of criticality. The analysis was performed by means of the method of critical fluctuations (MCF).
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For a proportion of individuals judged clinically to have a recessive Mendelian disease, only one heterozygous pathogenic variant can be found from clinical whole exome sequencing (WES), posing a challenge to genetic diagnosis and genetic... more
For a proportion of individuals judged clinically to have a recessive Mendelian disease, only one heterozygous pathogenic variant can be found from clinical whole exome sequencing (WES), posing a challenge to genetic diagnosis and genetic counseling. One possible reason is the limited ability to detect disease causal structural variants (SVs) from short reads sequencing technologies. Long reads sequencing can produce longer reads (typically 1000 bp or longer), therefore offering greatly improved ability to detect SVs that may be missed by short-read sequencing. Here we describe a case study, where WES identified only one heterozygous pathogenic variant for an individual suspected to have glycogen storage disease type Ia (GSD-Ia), which is an autosomal recessive disease caused by bi-allelic mutations in the gene. Through Nanopore long-read whole-genome sequencing, we identified a 7.1 kb deletion covering two exons on the other allele, suggesting that complex structural variants (SVs)...
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In this brief report we present evidence for the dynamics of the earthquake (EQ) preparation processes that led to two strong events (Mw>6) that took place in Durres (Albania) and Chania (West-Crete, Greece) on 26 and 27-11-2019,... more
In this brief report we present evidence for the dynamics of the earthquake (EQ) preparation processes that led to two strong events (Mw>6) that took place in Durres (Albania) and Chania (West-Crete, Greece) on 26 and 27-11-2019, respectively. Specifically, MHz fracto-electromagnetic emissions (EME) recorded by our telemetric stations presented critical fluctuations prior to each one of these EQs. Subsequently, the EME possibly related to each one of these EQs evolved differently towards the occurrence of the EQ event: the ones possibly related to Chania EQ departed from critical state according to the symmetry breaking phenomenon, while the ones possibly related to Durres EQ departed from critical state according to the tricritical crossover phenomenon. The analysis was performed by means of the method of critical fluctuations (MCF).
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We present in this contribution preliminary results obtained through seismic data analyses and numerical simulations of the response of the Acropolis of Athens, Greece. We use earthquake and ambient vibration data recorded by the... more
We present in this contribution preliminary results obtained through seismic data analyses and numerical simulations of the response of the Acropolis of Athens, Greece. We use earthquake and ambient vibration data recorded by the high-quality accelerometer array operated by the Institute of Geodynamics, National Observatory of Athens (NOA-IG), in collaboration with the Acropolis Restoration Service (YSMA). The array consists of 10 strong-motion sensors (Guralp CMG-5TD) recording in continuous mode on 24-bit digitisers, sampling the main geological units constituting the Acropolis and including two sites devoted to monitoring monumental elements of the Parthenon. We compare the experimental amplification functions (SSRs, HVSRs, 5%-damped SRs) with those obtained from numerical simulations of the seismic response of the hill. The 3D computational mesh is designed in CUBIT while the simulations are carried out using the discontinuous Galerkin spectral element (SEM) computer program SPE...
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We present a dataset of 77 strong ground motion records within 200 km epicentral distance from the 30 October 2020, M7.0 Samos Island (Aegean Sea) earthquake, which affected Greece and Turkey. Accelerograms from National Networks of both... more
We present a dataset of 77 strong ground motion records within 200 km epicentral distance from the 30 October 2020, M7.0 Samos Island (Aegean Sea) earthquake, which affected Greece and Turkey. Accelerograms from National Networks of both countries have been merged into a single dataset, including metadata that have been uniformly derived using a common preliminary source model. Initial findings from the analysis and comparative examination of acceleration time histories, Fourier amplitude spectra and 5%-damped response spectra are discussed along with significant source, propagation path and site effects. The long-period amplifications observed in most records in Izmir bay triggered failures and severe damages in weak structures. Yet, the spectral accelerations are observed to lie below the current and previous design spectra corresponding to the damaged regions. Peak ground motions are used to construct a purely instrumental-based macroseismic intensity map, which is capable of reflecting the actual earthquake damage caused by this considerably large event. Finally, peak ground motions are compared to various ground motion models (GMMs) and deviations are highlighted. Our overall preliminary analysis reveals a strong energy signature of the Samos earthquake in the period range 0.5–1.5 s at many sites, both on rock and soil, whereas records in the heavily hit Izmir city, at an epicentral distance circa 70 km, provide strong indication for additional amplification due to basin effects. At relatively large distance from the earthquake source (> 120 km), several recorded amplitudes are significantly lower than those predicted by many GMMs, implying that further studies are necessary toward the improvement of regional attenuation models.
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We present a Hellenic database of intensity measures from uniformly processed strong ground motion recordings, together with metadata on earthquake source attributes and recording site conditions. The database consists of information from... more
We present a Hellenic database of intensity measures from uniformly processed strong ground motion recordings, together with metadata on earthquake source attributes and recording site conditions. The database consists of information from 471 earthquakes between 1973 and 2015 that produced 2993 usable recordings from 333 sites. A key element of this work is a unified presentation of data from two major data providers that operate in Greece (Institute of Engineering Seismology and Earthquake Engineering and the Institute of Geodynamics, National Observatory of Athens) along with a university-operated local urban array (University of Patras). Consistent procedures were applied to develop source parameters that include hypocenter locations, moment magnitudes (directly estimated or derived using a conversion procedure), fault-plane solutions, and finite-fault parameters (generally, for events with M>6.0). The time-averaged shear-wave velocity in the upper 30 m parameter is provided f...
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The preparation process of two recent earthquakes (EQs) occurred in Cephalonia (Kefalonia) island, Greece, (38.22° N, 20.53° E), 26 January 2014, <i>M</i><sub>w</sub> =6.0, depth =21 km, and (38.25° N, 20.39° E), 3... more
The preparation process of two recent earthquakes (EQs) occurred in Cephalonia (Kefalonia) island, Greece, (38.22° N, 20.53° E), 26 January 2014, <i>M</i><sub>w</sub> =6.0, depth =21 km, and (38.25° N, 20.39° E), 3 February 2014, <i>M</i><sub>w</sub> =5.9, depth =10 km, respectively, is studied in terms of the critical dynamics revealed in observables of the involved non-linear processes. Specifically, we show, by means of the method of critical fluctuations (MCF), that signatures of critical, as well as tricritical, dynamics were embedded in the fracture-induced electromagnetic emissions (EME) recorded by two stations in locations near the epicenters of these two EQs. It is worth noting that both, the MHz EME recorded by the telemetric stations on the island of Cephalonia and the neighboring island of Zante (Zakynthos), reached simultaneously critical condition a few days before the occurrence of each earthquake. The critical characte...
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Using a recently completed database of uniformly processed strong-motion data recorded in Greece, we derive a ground-motion prediction model (GMPM) for horizontal-component peak ground velocity, peak ground acceleration, and 5% damped... more
Using a recently completed database of uniformly processed strong-motion data recorded in Greece, we derive a ground-motion prediction model (GMPM) for horizontal-component peak ground velocity, peak ground acceleration, and 5% damped pseudoacceleration response spectra, at 105 periods ranging from 0.01 to 10 s. The equations were developed by modifying a global GMPM, to account for more rapid attenuation and weaker magnitude scaling in the Greek ground motions than in the global GMPM. Our GMPM is calibrated using the Greek data for distances up to 300 km, magnitudes from 4.0 to 7.0, and time-averaged 30 m shear-wave velocities from 150 to 1200 m/s. The GMPM has important attributes for hazard applications including magnitude scaling that extends the range of applicability to M 8.0 and nonlinear site response. These features are possible because they are well constrained by data in the global GMPM from which our model is derived. An interesting feature of the Greek data, also obser...
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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... more
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 eight permanent broadband and strong-motion networks from Greece and Cyprus, individual stations from the Balkans, temporary networks and aftershock deployments, and earthquake engineering experimental facilities. EIDA@NOA provides open and unlimited access from redundant node end points, intended mainly for research purposes (see Data and Resources). Analysis and quality control of the complete seismic data archive is performed initially by calculating waveform metrics and data availability. Seismic ambient noise metrics are estimated based on power spectral densities, and an assessment of each station’s statistical mode is achieved within ea...
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Research Interests: Physics and Active Fault
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Research Interests: Geology and Seismology
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We present in this paper a structural health monitoring study of the Egyptian lighthouse of Rethymnon in Crete, Greece. Using structural vibration data collected on a limited number of sensors during a 3-month period, we illustrate the... more
We present in this paper a structural health monitoring study of the Egyptian lighthouse of Rethymnon in Crete, Greece. Using structural vibration data collected on a limited number of sensors during a 3-month period, we illustrate the potential of the stretching method for monitoring variations in the natural frequencies of the structure. The stretching method compares two signals, the current that refers to the actual state of the structure, with the reference one that characterizes the structure at a reference healthy condition. For the structure under study, an 8-day time interval is used for the reference quantity while the current quantity is computed using a time window of 24 h. Our results indicate that frequency shifts of 1% can be detected with high accuracy allowing for early damage assessment. We also provide a simple numerical model that is calibrated to match the natural frequencies estimated using the stretching method. The model is used to produce possible damage sce...
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This paper reports an attempt to use ultra-low-frequency (ULF) magnetic field data from a space weather monitoring magnetometer array in the study of earthquake (EQ) precursors in Greece. The data from four magnetometer stations of the... more
This paper reports an attempt to use ultra-low-frequency (ULF) magnetic field data from a space weather monitoring magnetometer array in the study of earthquake (EQ) precursors in Greece. The data from four magnetometer stations of the HellENIc GeoMagnetic Array (ENIGMA) have been analyzed in the search for possible precursors to a strong EQ that occurred south of Lesvos Island on 12 June 2017, with magnitude Mw = 6.3 and focal depth = 12 km. The analysis includes conventional statistical methods, as well as criticality analysis, using two independent methods, the natural time (NT) method and the method of critical fluctuations (MCF). In terms of conventional statistical methods, it is found that the most convincing ULF precursor was observed in the data of ULF (20–30 mHz) depression (depression of the horizontal component of the magnetic field), which is indicative of lower ionospheric perturbation just 1 day before the EQ. Additionally, there are indications of a precursor in the ...
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The preparation process of two recent earthquakes (EQs) occurred in Cephalonia (Kefalonia) island, Greece, (38.22° N, 20.53° E), 26 January 2014, <i>M</i><sub>w</sub> =6.0, depth =21 km, and (38.25° N, 20.39° E), 3... more
The preparation process of two recent earthquakes (EQs) occurred in Cephalonia (Kefalonia) island, Greece, (38.22° N, 20.53° E), 26 January 2014, <i>M</i><sub>w</sub> =6.0, depth =21 km, and (38.25° N, 20.39° E), 3 February 2014, <i>M</i><sub>w</sub> =5.9, depth =10 km, respectively, is studied in terms of the critical dynamics revealed in observables of the involved non-linear processes. Specifically, we show, by means of the method of critical fluctuations (MCF), that signatures of critical, as well as tricritical, dynamics were embedded in the fracture-induced electromagnetic emissions (EME) recorded by two stations in locations near the epicenters of these two EQs. It is worth noting that both, the MHz EME recorded by the telemetric stations on the island of Cephalonia and the neighboring island of Zante (Zakynthos), reached simultaneously critical condition a few days before the occurrence of each earthquake. The critical characte...