Journal Articles by Dimitrios Giannopoulos
Geophysical Journal International, 2020
In this paper, we propose a two-step procedure for the automated detection of micro-earthquakes, ... more In this paper, we propose a two-step procedure for the automated detection of micro-earthquakes, using single-station, three-component passive seismic data. The first step consists of the computation of an appropriate characteristic function, along with an energy-based thresh-olding scheme, in order to attain an initial discrimination of the seismic noise from the 'useful' information. The three-component data matrix is factorized via the singular value decomposition by means of a properly selected moving window and for each step of the windowing procedure a diagonal matrix containing the estimated singular values is formed. The L 2-norm of the singular values resulting from the above-mentioned windowing process defines the time series which serves as a characteristic function. The extraction of the seismic signals from the initial record is achieved by following a histogram-based thresholding scheme. The histogram of the characteristic function, which constitutes its empirical probability density function, is estimated and the optimum threshold value is chosen corresponds to the bin that separates the above-mentioned histogram in two different areas delineating the background noise and the outliers. Since detection algorithms often suffer from false alarms, which increase in extremely noisy environments, as a second stage, we propose a new 'decision-making' scenario to be applied on the extracted intervals, for the purpose of decreasing the probability of false alarms. In this context, we propose a methodology, based on comparing among autoregressive models estimated both on isolated seismic noise, in addition to the detections resulting from the first stage. The performance and efficiency of the proposed technique is supported by its application to a series of experiments that were based on both synthetic and real data sets. In particular, we investigate the effectiveness of the characteristic function, along with the thresh-olding scheme by subjecting them to noise robustness tests using synthetic seismic noise, with different statistical characteristics and at noise levels varying from 5 down to-5 dB. Results are compared with those obtained by the implementation of a three-component version of the well-known STA/LTA algorithm to the same data set. Moreover, the proposed technique and its potential to distinguish seismic noise from the useful information through the proposed decision making scheme is evaluated, by its application to real data sets, acquired by three-component short-period recorders that were installed for monitoring the microseismic activity in areas characterized by different noise attributes.
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Science, Nov 9, 2020
Human activity causes vibrations that propagate into the ground as high-frequency seismic waves.M... more Human activity causes vibrations that propagate into the ground as high-frequency seismic waves.Measures to mitigate the coronavirus disease 2019 (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 noise reduction on record. Although the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of human activities.
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Annals Of Geophysics, 2013
On June 8, 2008, at 12:25 GMT, a Mw 6.4 earthquake, the Movri Mountain earthquake, occurred in th... more On June 8, 2008, at 12:25 GMT, a Mw 6.4 earthquake, the Movri Mountain earthquake, occurred in the area of northwest Peloponnese, western Greece. The epicenter was located in the municipality of Movri, 35 km southwest of Patras. For this study, a crustal anisotropy analysis was performed in the epicentral area of the Movri Mountain earthquake. Specifically, the shear-wave splitting phenomenon and its temporal evolution in relation to the Movri Mountain earthquake was studied, using the cross correlation method. The data analysis revealed the presence of shear-wave splitting in the study area. Both before and after the Movri Mountain earthquake, the polarization directions of the fast component of the shear waves followed a general NNW-SSE direction. The observed mean fast polarization direction was not consistent with the regional stress field, which showed a general E-W direction of the maximum horizontal compressive stress. The differences between the estimated fast polarization directions and the properties of the regional stress field suggest the presence of a local stress field in the area around the fault. An increase in time delays was observed soon after the Movri Mountain earthquake. The average value of the delay times before the earthquake was ca. 18 ±2.6 ms, while after the earthquake this was ca. 40 ±4.6 ms. This increase in the time delay indicates changes in the crustal properties, which were possibly caused by variations in the preexisting micro-crack system characteristics related to the Movri Mountain earthquake, and the possible involvement of over- pressured fluids.
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Geophysical Journal International, 2015
On 2010 January 18 and 22, two earthquakes of Mw 5.3 and 5.2, respectively, occurred near the tow... more 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 2010 December. The data analysis revealed the presence of shear wave splitting in the study area, as well as variations of the splitting parameters and Vp/Vs ratios. The average values of time-delay, fast polarization direction and Vp/Vs ratio for the time period before the Efpalio earthquakes, were calculated at 2.9 ± 0.4 ms/km, 92◦ ± 10◦ and 1.76 ± 0.04, respectively, while after the occurrence of the earthquakes, including the aftershock sequence, they were calculated at 5.5 ± 0.5 ms/km, 82◦ ± 9◦ and 1.88 ± 0.04. A few months after the occurrence of the Efpalio earthquakes, the
mentioned splitting parameters were calculated at 3.6±0.4 ms/km and 83◦ ±9◦. Vp/Vs ratio exhibited a mean value of 1.87 ± 0.04. The mean fast polarization directions were in general
consistent with the regional stress field, almost perpendicular to the direction of the extension of the Gulf of Corinth. The observed increase in the time-delays and Vp/Vs ratios after the
Efpalio earthquakes indicates changes in the crustal properties, which possibly resulted from variations in the pre-existing microcrack system characteristics. We suggest that a migration
of fluids in the form of overpressured liquids, which are likely originated from dehydration reactions within the crust, was triggered by the Efpalio earthquakes and caused the observed
variations. The findings of this work are consistent with those of previous studies that have indicated the presence of fluids of crustal origin in the study area.
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Three years of continuous waveform data recorded at 22 stations from the Corinth Rift Laboratory ... more 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 crosscorrelated
to extract empirical Rayleigh-wave Green's functions. Group velocity dispersion curves
were measured for each station-pair by applying frequency-time analysis and then inverted to build
2D group velocity maps between 1 and 6 s period. Finally, we locally inverted these velocity maps
using a neighborhood algorithm to assess the 3D shear-velocity model of the shallow crustal structure
of the western Corinth Rift. Across all studied periods the southern coast of the Corinth Gulf is
generally imaged as a region of lower velocities compared to the northern coast. At periods up to 3 s,
the spatial variation of the group velocities is correlated with the surface geology of the area. Lower
velocities are observed in areas where mostly Plio-Quaternary syn-rift sediments are present, such as
off-shore regions of the rift, the Mornos delta and the largest part of the southern coast. Higher
velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. At
periods above 3 s, where Rayleigh-waves begin to sense deeper structures below the sediments within
the underlying basement, our study highlights the presence of a distinct zone of lower velocities
across the southern part of the rift with an elongation in the WNW-ESE direction. The interpretation
of this low velocity includes two arguments, the present-day active tectonic regime and the possible
involvement of fluids circulation processes at depth within a highly fractured upper crust in the
vicinity of the major faults zones. In general, the results demonstrate good agreement with the major
geological and tectonic features of the area, as well as with previous local earthquake tomography
studies and support the assumption of fluid circulations at depth. This work intends to be the base for
further investigations towards the study of the Corinth Rift structure using long-time series of ambient
noise data.
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First Break, 2017
D. Giannopoulos, C. Orfanos, K. Leontarakis, A. Lois and N. Martakis show the results of an Ambie... more D. Giannopoulos, C. Orfanos, K. Leontarakis, A. Lois and N. Martakis show the results of an Ambient Noise Tomography performed in southwest Albania, using long time-series of ambient seismic noise
ecorded during a Passive Seismic Tomography project, to demonstrate
the efficiency and the usefulness of the Passive Seismic Interferometry technique.
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This study performs a reanalysis of the seismicity recorded during the 1996 Gjálp eruption that o... more 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 picked. A velocity model was estimated from P-phase travel times by using VELEST. Events were first located using the algorithm NON-LINLOC in order to obtain absolute locations. Precise relative locations were obtained with HYPODD by utilizing catalog and cross-correlation differential travel times. Results show that events clustered first along the SW rim of the Bárðarbunga caldera and later along the Gjálp fissure, with most hypocentral depths located between 3 and 8 km. Waveforms of the 10 largest events that followed the Bárðarbunga earthquake were inverted in order to obtain moment tensors. For all events we found that the deviatoric moment tensor fits the data better than pure double-couple or full moment tensor solutions. Events along the Bárðarbunga caldera exhibited reverse focal mechanisms, while those at the Gjálp fissure exhibited mostly strike-slip faulting. Seismic velocity variations calculated using ambient noise interferometry, point to the possibility that a small subglacial eruption occurred at Bárðarbunga before the main earthquake. This removed melt from the magma chamber causing its roof to collapse, and also resulted in the lateral migration of magma towards the Gjálp fissure. The 2014-2015 Bárðarbunga-Holuhraun eruption shares common characteristics with the 1996 Gjálp eruption, although the size of the latter was much smaller.
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Articles published in Conference Proceedings by Dimitrios Giannopoulos
Third European Conference on Earthquake Engineering and Seismology - 3ECEES, 2022
Algorithms that deal with the automatic P-and S-onset time identification problem are a topic of ... more Algorithms that deal with the automatic P-and S-onset time identification problem are a topic of ongoing research. Modern dense seismic networks used for earthquake location, focal mechanism determination, seismic tomography investigations, source studies, early warning, etc., demand accurate automatic arrival time estimations of compressional and transversal seismic waves. In this work, we present examples of the application of an automated, time-domain procedure for the estimation of P-and S-arrival times, which is based on the statistical and polarization characteristics of the seismic signals. In particular, for the P-onset time we utilize the kurtosis criterion, while for the Sonset time estimation a time domain technique based on eigevalue analysis and Higher Order Statistics is followed. The experiment took place using a number of seismic events occurred in the wider area of Corinth Gulf-West Attica, Greece where an earthquake early warning system is organized. Examples of successful but also incorrect estimations are presented and discussed in order to evaluate its efficiency in different cases of seismic signals and noise environments.
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Third European Conference on Earthquake Engineering and Seismology - 3ECEES, 2022
The pandemic of COVID-19 and the global policy implemented to attenuate the spread of the virus, ... more The pandemic of COVID-19 and the global policy implemented to attenuate the spread of the virus, provided a unique opportunity to study the footprint of the resulting social impact of COVID-19 pandemic on seismic data. In this study, a non-extensive statistical physics approach is examined for anthropogenic high-frequency seismic noise in view of COVID-19 pandemic lockdown measures and the consequent changes in human activity levels. A continuous monitoring of the entropic index q performed during a time period which includes a time interval of normal social activity and a period of anthropogenic quiescence due to the COVID-19 pandemic lockdown measures, enabled us to observe a dependance between the human activity levels and the degree of non-extensivity in the seismic signal. Average values of pre-lockdown q parameters estimated within the urban environment of the cities of Athens and Thessaloniki (Greece), were equal to 1.37 and 1.56, respectively, while the respective post-lockdown q values were found to be 1.25 and 1.49. In overall, the estimated q parameters could support the conclusion that the anthropogenic seismic noise is sub-extensive. Lower values of q during the lockdown period reflect a lower degree of interacting subsystems and degree of correlations in the high-frequency seismic noise wavefield. A short-term examination on a week scale of the temporal behavior of parameter q, revealed significant patterns of daily periodicity and diurnal variations resembling the diurnal cycle of human activities. According to the preliminary observations of this study, the non-extensive parameter q is shown to be a rather promising tool regarding the quantitative strategies for monitoring the effect of the anthropogenic activity levels in seismic data.
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Bulletin of the Geological Society of Greece, 2016
On April 2007, an earthquake swarm occurred in the vicinity of Trichonis Lake, western Greece. Th... more 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 network of 8 stations that was installed in the area by the University of Patras Seismological Laboratory. We were able to take measurements from 5 stations as the seismicity was concentrated mostly at the eastern part of the lake. Two splitting parameters were measured through the data processing, the time-delays between the fast and slow shear wave components and the polarization directions of the fast components. In general, the data analysis revealed the presence of shear wave anisotropy in the study area. The average value of normalized time-delays was calculated at 6.9 ±1.1 ms/km while the fast polarization directions had an average of 130 ±14 deg. The mean fast polarization directions were consistent with what is expected concerning the local stress field, as it was sub-parallel to the strike of the major faults at the eastern part of the lake and almost perpendicular to the direction of extension. Therefore, the findings can be interpreted by an anisotropic volume of stress-aligned micro-cracks within the upper crust according to the extensive dilatancy anisotropy model.
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Bulletin of the Geological Society of Greece, 2016
On June 8, 2008, a Mw 6.4 earthquake occurred on a strike-slip fault in northwest Peloponnese, Gr... more 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 shear wave splitting analysis. We determined fast polarization directions φ, and normalized time-delays dt between the fast and slow components. The average value of dt was calculated at 1.7 ±0.5 ms/km, while φ values varied between 155 ±8 deg and 11 ±9 deg , with an average of 170 ±9 deg. The observed φ values at all stations were inconsistent with the regional stress field, which is characterized-by a general E-W orientation of the maximum horizontal compressive stress. On the contrary, the observed fast directions appear to intersect the strike of the causative fault (210 deg) in smaller angles than the regional principal compressive stress axis. The findings of this work are consistent with those of previous studies in the area, reflecting possibly the presence of a locally rotated principal stress axis to more favorable angles to the strike of the fault of the 2008 strong earthquake.
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Abstracts/Extendend Abstracts - Presentations by Dimitrios Giannopoulos
37th General Assembly (GA) of the European Seismological Commission, 2021
The recently discovered coronavirus (COVID-19) first broke out in China in December 2019 and sinc... more The recently discovered coronavirus (COVID-19) first broke out in China in December 2019 and since then the virus has rapidly spread around the world causing thousands of deaths. The World Health Organization has declared the COVID-19 outbreak a pandemic. Greece, aiming to prevent the spread of the virus, imposed public health scenarios, including restrictive measures on the movement of people. The implementation of the measures concluded to a nationwide lockdown in March 23th 2020.
Human activity is considered as one of the principal sources of seismic noise energy for frequencies above 1Hz (human-induced seismic noise). The suspension of economic, cultural and numerous everyday-life activities, due to the “lockdown”, resulted in a significant and unprecedented reduction of the background noise levels, as this latter is continuously being monitored by the Hellenic Unified Seismological Network. In this study, we provide evidence of the effect of lockdown measures on seismic noise levels from different cities in Greece. The initiation of the lockdown measures caused a reduction of the noise levels, especially within urban environments. The magnitude of the observed variations largely depends on factors, such as the environment type (urban, suburban) or the proximity of the seismic stations to main roads, schools, public facilities. The observed noise levels are compared with those that are usually observed during public or summer holidays in Greece. The effect of the lockdown on the detection of small magnitude earthquakes by city-based seismometers is also discussed.
Since COVID-19 pandemic triggered restrictions on movement throughout the whole planet, “lockdown-induced” seismic noise reduction was upgraded to a global scale phenomenon. The observations of this study include evidence that permanent seismic stations, especially within urban environments, can be used as an efficient monitoring tool for non-seismic activities, highlighting the potential development of a new branch of seismology, that of Social Seismology.
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Geophysical Research Abstracts - EGU, 2015
We present the preliminary results of an ambient seismic noise analysis performed in the western ... more We present the 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 the Gulf of Alkionides in the east. It is considered as one of the most active extensional intra-continental rifts in the world, with the geodetically measured rates of extension varying from ∼5 mm/yr at the eastern part, to ∼15 mm/yr at the western part. We used data from three-component broad-band seismic stations operated under the framework of the Hellenic Unified Seismological Network (HUSN) and the Corinth Rift Laboratory (CRL). After the classical processing of continuous ambient seismic noise recordings, we used both auto-correlation and cross-correlation functions of single stations and station pairs, respectively, in order to retrieve empirical Green's functions (EGFs) of surface waves and estimate relative velocity changes. For estimating the relative velocity changes we used the moving-window cross spectrum analysis (MWCS) technique. This is the first attempt to characterize the ambient seismic noise properties in the area and study the possible relation between the detected relative velocity changes and the occurrence of moderate or strong earthquakes in the study area.
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Geophysical Research Abstracts - EGU, 2020
Methodologies dealing with the detection of micro-earthquakes and the accurate estimation of body... more Methodologies dealing with the detection of micro-earthquakes and the accurate estimation of body waves' arrival time constitute, during the last decades, a topic of ongoing research. The extraction and efficient analysis of the useful information from the continuous recordings is of great importance, since it is a prerequisite for reliable interpretations. Small magnitude seismic events, either naturally-occuring or induced, have been increasingly used in a wide range of industrial fields, with applications ranging from hydrocarbon and geothermal reservoir exploration, to passive seismic tomography surveys. A great number of algorithms have been proposed and applied up to now for seismic event detection, exploiting specific properties of the seismic signals both in time and in frequency domain, with the energy-based detectors (STA/LTA) to be the most commonly used, due to their simplicity and the low computational cost they require. A significant obstacle emerging at seismological identification problems lies on the fact that such processes usually suffer from a number of false alarms, which is significantly increased in extremely noisy environments. For that scope, we propose a "Decision-Making" mechanism, independent of the applied detection algorithm, which controls the results obtained during the detection process by minimizing false detections and providing the best possible outcome for further analysis. The specific scenario is based on the comparison among autoregressive models estimated on isolated seismic noise recordings, as well as on the detected intervals that resulted during the event identification procedure. A number of examples, associated with the implementation of the proposed scenario on real data, is presented with the scope of evaluating its performance. Several issues concerning the isolation of the seismic noise from the raw data, the estimation of the autoregressive models, the choice of the orders of the stochastic models etc., are discussed.
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SEG Technical Program Expanded Abstracts, 2020
In the context of a passive seismic survey that was launched in the South Lokichar Basin, southwe... more In the context of a passive seismic survey that was launched in the South Lokichar Basin, southwest of Lake Turkana, in Kenya, a seismic network consisting of 13 3-component broadband stations was deployed in the area. In this study, records of both ambient seismic noise and natural seismicity are jointly exploited, through ambient noise tomography and earthquake-based methodologies respectively, in order to obtain a reference shear-wave velocity (VS) crustal model. The applied passive seismic techniques allowed to constrain the VS distribution at depths ranging from the shallow parts of the crust down to Moho and the upper mantle. The lateral variations of the calculated VS model at depth are highly compatible with those obtained from previous experiments carried out across the Kenya rift. This agreement is a strong indication of the reliability of our results and leads us to infer that our model has the potential to serve as a reference VS crustal model for the broader region of the South Lokichar Basin.
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EGU General Assembly, 2019
The problem of seismic events extraction from the raw data constitutes one of the most important ... more The problem of seismic events extraction from the raw data constitutes one of the most important and vital tasks addressed in geosciences, since its solution plays a crucial role in events location, focal mechanism determination, early warning to name a few. Moreover, small-magnitude seismic events, either natural or induced microearthquakes, have increasingly been used in exploration seismology with applications ranging from hydrocarbon and geothermal reservoir exploration to high-resolution passive seismic tomography surveys. The large number of small-magnitude events recorded, as well as the increasing number of instruments used, makes the process of manual detection, selection and picking of seismic events a tedious, costly and time consuming task. Most of the techniques that are applied up to now for seismic event detection, are mainly based on energy criteria (STA/LTA detectors), due to their simplicity and the low computational cost they require. Nevertheless, usually there is a large number of parameters that have to be set in order to work properly and in some cases, their efficiency is dramatically reduced in noisy environments. In this work, we apply a thresholding type technique, tailored to fit real world situations where our knowledge on the statistical characteristics of the background noise process are unknown and a strict hypothesis testing framework cannot be followed. In such cases the replacement of the unknown probability density function under the null hypothesis by its empirical counterpart, constitutes a possibility. The detection algorithm is implemented in two stages: In the first stage by following sampling, autoregressive modeling and clustering procedures, segments from the initial seismic recording containing only noise samples, are isolated. Therefore, from these noise "blocks", an empirical probability distribution function (pdf) of the background noise process is estimated. In the second stage, using the resulted empirical function, a thresholding scheme is applied in order to solve the problem of the identification of seismic events in a non strict hypothesis testing framework. It has to be stressed that by following the above mentioned approach, since the empirical pdf has been estimated, any of the well known statistics that measures the discrepancy between two density functions can be used eg. Pearson's Chi-Squared goodness-of-fit statistic, Kullback-Leibler divergence, Kolmogorov-Smirnov test etc. The performance of the proposed technique is confirmed by its application in a series of experiments both in synthetic and real seismic data sets, and several issues concerning different thresholding scenarios as well as the efficiency of the above mentioned methodology are discussed.
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European Geosciences Union General Assembly 2019 , 2019
In the framework of the MAUPASACQ project, a dense seismic network of 440 three-component station... more In the framework of the MAUPASACQ project, a dense seismic network of 440 three-component stations was deployed throughout the Mauleon Basin, in the northern foothills of the Pyrenees, SW France. The network consisted of three different types of sensors, namely 190 SG-10 SERCEL geophone nodes, 197 5 Hz SEISMOTECH short-period sensors and 53 broadband sensors (Guralp CMG-40, Trillium Compact, Trillium 120), and operated continuously for 6 months, from April 1 until September 30, 2017. Numerous geological and geophysical observations provide evidence that the Mauleon Basin was created by the tectonic inversion of a precollisional rift system that was formed between the Iberian and European plates, involving processes of rift-related mantle exhumation. These observations, among others, characterize the area under investigation as an ideal locality to study and understand the structural and geodynamic evolution of the Pyrenean mountain range. The main scientific objective of the MAUPASACQ experiment is to provide new constraints on this topic, by applying different types of classical imaging techniques that incorporate a number of Passive Seismic methodologies (i.e. travel time tomography, ambient noise tomography, receiver functions, etc.). The scope of this work is to illustrate some preliminary observations deriving from the analysis of the ambient seismic noise field that was recorded in the Mauleon Basin. This was achieved by beamforming and by using the Rayleigh wave empirical Green's functions that were estimated by seismic interferometry. Beamforming analysis provided valuable information about the spatial distribution and the directionality of the ambient noise sources. The main source region for the primary microseisms (∼12-20 s) is located to the northwest (N315o), being present over a larger backazimuth range across the northwest quadrant, while the secondary microseisms (∼5-10 s) seem to present two maxima, one in the west (N270o) and a second one in the north-northwest (N350o), ranging between west-southwest and north directions. Furthermore, 1-day long recordings between all available vertical components were cross correlated, stacked over the whole recording period, resulting in more than 90000 reference noise cross-correlation stacks. A frequency-time analysis was applied to measure Rayleigh wave group velocity dispersion curves, which were used to invert for 2D velocity maps in the 1 to 10 s period range. The results reveal the presence of interesting and distinctive velocity structures, demonstrating a good agreement with the major geological features that were expected in the area. The preliminary observations of the current analysis highlight the sufficiency and the potential of the MAU-PASACQ array to use ambient seismic noise recordings as a standalone, as well as a complementary tool towards the investigation of the shear velocity structure beneath the Mauleon Basin. Acknowledgements The authors acknowledge all partners of the MAUPASACQ project, TOTAL EP/R&D, SEISMOTECH S.A., BRGM, CNRS and CSIC Barcelona, for the excellent collaboration, as well as the personnel responsible for the field operations related to the installation of the array and the data acquisition.
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Seventh EAGE Workshop on Passive Seismic 2018, 2018
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The Corinth Rift separates Peloponnesus to the south from mainland Greece to the north. It is one... more 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 velocity structure of the western Corinth Rift using ambient noise recordings. We used 3 yrs (01/2012-12/2014) of continuous waveform data recorded at 24 stations from the Corinth Rift Laboratory (CRL) and the Hellenic Unified Seismological Network (HUSN). All available vertical component time-series were cross-correlated to extract Rayleigh wave Green's functions. Group velocity dispersion curves between 0.5 and 7 s period were measured for each station pair by applying frequency-time analysis and then inverted to build group velocity maps of the study area. At the studied periods, the northern coast of the Corinth Rift is generally imaged as a region of elevated seismic velocities compared to the southern coast. More specifically, low velocities are observed in areas of Plio-Quaternary syn-rift sediments such as offshore regions of the rift, the Mornos delta and a large part of the southern coast. Higher velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. The preliminary results demonstrate good agreement with the major geological features of the area and agree relatively well with previous local earthquake tomography studies. This work will be the base for further investigations towards the study of the Corinth Rift structure using long time-series of ambient noise data.
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"The Corinth Gulf is often referred to as a natural laboratory for the study of continental rift ... more "The Corinth Gulf is often referred to as a natural laboratory for the study of continental rift tectonics. It is a fastspreading intra-continental rift, with the geodetically measured extension varying from 5 mm/yr at the eastern part, to 15 mm/yr at the western part. It is structured by a set of E-W striking, en-echelon active normal faults, presenting an interseismic N-S extension. The high strain rate in the western Corinth Gulf is accompanied with a high level of microseismicity. On 18th and 22nd of January, 2010, two strong earthquakes of Mw 5.3 and Mw 5.2, respectively, occurred near the village of Efpalio in western Corinth Gulf. These events were followed by a dense seismic sequence providing good quality data for shear wave splitting analysis. Thus three-component seismograms were analyzed in order to detect shear wave splitting parameters. The studied events occurred in the epicentral area of the two strong events during the period of 2009 and 2010. We used recordings from the local seismological stations of the Hellenic Unified Seismological Network (HUSN). The stations are equipped with three-component, broadband sensors. The cross correlation method was used in the analysis and results revealed the presence of shear wave splitting in the area. With this study, we attempt to present the relation between the anisotropic signature of the upper crust in western Corinth Gulf and the geotectonic framework of the area, and also, the relation between the temporal variation of the splitting parameters and the occurrence of the two strong events of Efpalio."
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Journal Articles by Dimitrios Giannopoulos
mentioned splitting parameters were calculated at 3.6±0.4 ms/km and 83◦ ±9◦. Vp/Vs ratio exhibited a mean value of 1.87 ± 0.04. The mean fast polarization directions were in general
consistent with the regional stress field, almost perpendicular to the direction of the extension of the Gulf of Corinth. The observed increase in the time-delays and Vp/Vs ratios after the
Efpalio earthquakes indicates changes in the crustal properties, which possibly resulted from variations in the pre-existing microcrack system characteristics. We suggest that a migration
of fluids in the form of overpressured liquids, which are likely originated from dehydration reactions within the crust, was triggered by the Efpalio earthquakes and caused the observed
variations. The findings of this work are consistent with those of previous studies that have indicated the presence of fluids of crustal origin in the study area.
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 crosscorrelated
to extract empirical Rayleigh-wave Green's functions. Group velocity dispersion curves
were measured for each station-pair by applying frequency-time analysis and then inverted to build
2D group velocity maps between 1 and 6 s period. Finally, we locally inverted these velocity maps
using a neighborhood algorithm to assess the 3D shear-velocity model of the shallow crustal structure
of the western Corinth Rift. Across all studied periods the southern coast of the Corinth Gulf is
generally imaged as a region of lower velocities compared to the northern coast. At periods up to 3 s,
the spatial variation of the group velocities is correlated with the surface geology of the area. Lower
velocities are observed in areas where mostly Plio-Quaternary syn-rift sediments are present, such as
off-shore regions of the rift, the Mornos delta and the largest part of the southern coast. Higher
velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. At
periods above 3 s, where Rayleigh-waves begin to sense deeper structures below the sediments within
the underlying basement, our study highlights the presence of a distinct zone of lower velocities
across the southern part of the rift with an elongation in the WNW-ESE direction. The interpretation
of this low velocity includes two arguments, the present-day active tectonic regime and the possible
involvement of fluids circulation processes at depth within a highly fractured upper crust in the
vicinity of the major faults zones. In general, the results demonstrate good agreement with the major
geological and tectonic features of the area, as well as with previous local earthquake tomography
studies and support the assumption of fluid circulations at depth. This work intends to be the base for
further investigations towards the study of the Corinth Rift structure using long-time series of ambient
noise data.
ecorded during a Passive Seismic Tomography project, to demonstrate
the efficiency and the usefulness of the Passive Seismic Interferometry technique.
Articles published in Conference Proceedings by Dimitrios Giannopoulos
Abstracts/Extendend Abstracts - Presentations by Dimitrios Giannopoulos
Human activity is considered as one of the principal sources of seismic noise energy for frequencies above 1Hz (human-induced seismic noise). The suspension of economic, cultural and numerous everyday-life activities, due to the “lockdown”, resulted in a significant and unprecedented reduction of the background noise levels, as this latter is continuously being monitored by the Hellenic Unified Seismological Network. In this study, we provide evidence of the effect of lockdown measures on seismic noise levels from different cities in Greece. The initiation of the lockdown measures caused a reduction of the noise levels, especially within urban environments. The magnitude of the observed variations largely depends on factors, such as the environment type (urban, suburban) or the proximity of the seismic stations to main roads, schools, public facilities. The observed noise levels are compared with those that are usually observed during public or summer holidays in Greece. The effect of the lockdown on the detection of small magnitude earthquakes by city-based seismometers is also discussed.
Since COVID-19 pandemic triggered restrictions on movement throughout the whole planet, “lockdown-induced” seismic noise reduction was upgraded to a global scale phenomenon. The observations of this study include evidence that permanent seismic stations, especially within urban environments, can be used as an efficient monitoring tool for non-seismic activities, highlighting the potential development of a new branch of seismology, that of Social Seismology.
mentioned splitting parameters were calculated at 3.6±0.4 ms/km and 83◦ ±9◦. Vp/Vs ratio exhibited a mean value of 1.87 ± 0.04. The mean fast polarization directions were in general
consistent with the regional stress field, almost perpendicular to the direction of the extension of the Gulf of Corinth. The observed increase in the time-delays and Vp/Vs ratios after the
Efpalio earthquakes indicates changes in the crustal properties, which possibly resulted from variations in the pre-existing microcrack system characteristics. We suggest that a migration
of fluids in the form of overpressured liquids, which are likely originated from dehydration reactions within the crust, was triggered by the Efpalio earthquakes and caused the observed
variations. The findings of this work are consistent with those of previous studies that have indicated the presence of fluids of crustal origin in the study area.
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 crosscorrelated
to extract empirical Rayleigh-wave Green's functions. Group velocity dispersion curves
were measured for each station-pair by applying frequency-time analysis and then inverted to build
2D group velocity maps between 1 and 6 s period. Finally, we locally inverted these velocity maps
using a neighborhood algorithm to assess the 3D shear-velocity model of the shallow crustal structure
of the western Corinth Rift. Across all studied periods the southern coast of the Corinth Gulf is
generally imaged as a region of lower velocities compared to the northern coast. At periods up to 3 s,
the spatial variation of the group velocities is correlated with the surface geology of the area. Lower
velocities are observed in areas where mostly Plio-Quaternary syn-rift sediments are present, such as
off-shore regions of the rift, the Mornos delta and the largest part of the southern coast. Higher
velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. At
periods above 3 s, where Rayleigh-waves begin to sense deeper structures below the sediments within
the underlying basement, our study highlights the presence of a distinct zone of lower velocities
across the southern part of the rift with an elongation in the WNW-ESE direction. The interpretation
of this low velocity includes two arguments, the present-day active tectonic regime and the possible
involvement of fluids circulation processes at depth within a highly fractured upper crust in the
vicinity of the major faults zones. In general, the results demonstrate good agreement with the major
geological and tectonic features of the area, as well as with previous local earthquake tomography
studies and support the assumption of fluid circulations at depth. This work intends to be the base for
further investigations towards the study of the Corinth Rift structure using long-time series of ambient
noise data.
ecorded during a Passive Seismic Tomography project, to demonstrate
the efficiency and the usefulness of the Passive Seismic Interferometry technique.
Human activity is considered as one of the principal sources of seismic noise energy for frequencies above 1Hz (human-induced seismic noise). The suspension of economic, cultural and numerous everyday-life activities, due to the “lockdown”, resulted in a significant and unprecedented reduction of the background noise levels, as this latter is continuously being monitored by the Hellenic Unified Seismological Network. In this study, we provide evidence of the effect of lockdown measures on seismic noise levels from different cities in Greece. The initiation of the lockdown measures caused a reduction of the noise levels, especially within urban environments. The magnitude of the observed variations largely depends on factors, such as the environment type (urban, suburban) or the proximity of the seismic stations to main roads, schools, public facilities. The observed noise levels are compared with those that are usually observed during public or summer holidays in Greece. The effect of the lockdown on the detection of small magnitude earthquakes by city-based seismometers is also discussed.
Since COVID-19 pandemic triggered restrictions on movement throughout the whole planet, “lockdown-induced” seismic noise reduction was upgraded to a global scale phenomenon. The observations of this study include evidence that permanent seismic stations, especially within urban environments, can be used as an efficient monitoring tool for non-seismic activities, highlighting the potential development of a new branch of seismology, that of Social Seismology.