Qualitative precursory pattern before several strong earthquakes in Greece.

Μελετήθηκε η χρονική μεταβολή τριών σεισμικών παραμέτρων, δηλαδή, του αριθμού των σεισμών Ν, της παραμέτρου b-value και της εκλυόμενης σεισμικής ενέργειας, πριν από την εκδήλωση ισχυρών σεισμών στην Ελλάδα, για το χρονικό διάστημα 2000- 2008. Η ανάλυση έγινε με τη χρήση δεδομένων του καταλόγου σεισμών του Γεωδυναμικού Ινστιτούτου του Εθνικού Αστεροσκοπείου Αθηνών. Συγκεκριμένες φάσεις της χρονικής διακύμανσης των σεισμικών παραμέτρων, γύρω από σχετικές μέσες τιμές τους, σε δεδομένο χρονικό διάστημα, μπορούν να συσχετιστούν με τις φάσεις προετοιμασίας και εκδήλωσης ενός ισχυρού σεισμού. Η επαναλαμβανόμενηκανονικότητα της αυτής της συσχέτισης μπορεί να θεωρηθεί ως ποιοτικό μοτίβο (pattern) πρόδρομης συμπεριφοράς. H συνεχής παρακολούθηση της μεταβολής της σεισμικότητας και η αναγνώριση των φάσεων του παραπάνω μοτίβου, μπορεί να συμβάλει στην αξιολόγηση του σεισμικού κινδύνου και της πρόγνωσης μιας επικείμενης ισχυρής σεισμικής δραστηριότητας, σε μια δεδομένη περιοχή.The temporal variation of the seismicity, based on the analysis of three seismic parameters i.e., number of earthquakes, b-value and energy released, were investigated before several strong earthquakes occurrence in Greece the time period 2000-2008. The seismic parameters estimates were obtained by the means of new tool, suited to analyze earthquake catalogue, and visualize their spatio-temporalvariation behaviour. The seismic data used were taken from the earthquake catalogue of the Geodynamic Institute of National Observatory of Athens, Greece. The obtained temporal variation series shows significant changes around their relative mean values, which specific phases can be related to the strong earthquakes preparation stages. This relation shows remarkable temporal regularity so that itcan be establish considered as a precursor seismicity pattern. These results suggest that identification of this behaviour, by the continuous monitoring of the temporal variation of the seismic parameters, can contribute to the assessment of the current seismic hazard and to the impending strong earthquake parameters evaluation, in a given area

MAPPING OF CODA ATTENUATION AT THE EXTEND OF THE NATIONAL SEISMOLOGICAL NETWORK OF GREECE

Coda decay rates of 538 vertical components corresponding to local earthquakes which occurred in Greece during the period 1998 to 1999 were used to deduce the coda quality factor (Qc) characteristics in the Hellenic area. The seismograms have been selected from a broader sample of 776 records obtained at 8 stations of the National Seismographic Network operated by the Institute of Geodynamics of the National Observatory of Athens. Earthquake magnitudes range from 2.5 to 4.0; epicentral distances and depths are smaller than 100 km and 40 km, respectively. Using the Single Back Scattering model, the dependence of Qc on frequencies between 1 and 10 Hz has been investigated at each station and the usual Qc =Qo f relationships have been deduced. The spatial distribution of Qo has been drawn using waves that sample approximately equivalent ellipsoidal volumes with semiminor axis up to 100 km. The corresponding map shows a decreasing trend in SN direction

The spatiotemporal analysis of the minimum magnitude of completeness Mc and the Gutenberg–Richter law b-value parameter using the earthquake catalog of Greece

Spatiotemporal mapping the minimum magnitude of completeness Mc and b-value of the Gutenberg–Richter law is conducted for the earthquake catalog data of Greece. The data were recorded by the seismic network of the Institute of Geodynamics of the National Observatory of Athens (GINOA) in 1970–2010 and by the Hellenic Unified Seismic Network (HUSN) in 2011–2014. It is shown that with the beginning of the measurements at HUSN, the number of the recorded events more than quintupled. The magnitude of completeness Mc of the earthquake catalog for 1970–2010 varies within 2.7 to 3.5, whereas starting from April 2011 it decreases to 1.5–1.8 in the central part of the region and fluctuates around the average of 2.0 in the study region overall. The magnitude of completeness Mc and b-value for the catalogs of the earthquakes recorded by the old (GINOA) and new (HUSN) seismic networks are compared. It is hypothesized that the magnitude of completeness Mc may affect the b-value estimates. The spatial distribution of the b-value determined from the HUSN catalog data generally agrees with the main geotectonic features of the studied territory. It is shown that the b-value is below 1 in the zones of compression and is larger than or equal to 1 in the zones dominated by extension. The established depth dependence of the b-value is pretty much consistent with the hypothesis of a brittle–ductile transition zone existing in the Earth’s crust. It is assumed that the source depth of a strong earthquake can probably be estimated from the depth distribution of the b-value, which can be used for seismic hazard assessment. © 2016, Pleiades Publishing, Ltd

Study of the seismicity temporal variation for the current seismic hazard evaluation in Val d'Agri, Italy

This study examines the temporal variation of the seismicity in the Val d'Agri (southern Italy) and adjacent areas, for the current seismic hazard evaluation. The temporal variation of the seismicity is expressed as time series of the number of earthquakes, <i>b</i> value of Gutenberg–Richter relationship or <i>b</i> value of the frequency–magnitude distribution and the seismic energy released in the form of log<i>E</i>^2/3. The analysis was performed by means of a new research tool that includes visualizing techniques, which helps the interactive exploration and the interpretation of temporal variation changes. The obtained time series show a precursory seismicity pattern, characterized by low and high probability periods, which preceded earthquakes of magnitude <i>M</i> ≥ 4.0. The 75% of the examined cases were successfully correlated with a change in seismicity pattern. The average duration of the low and the high probability periods is 10.6 and 13.8 months respectively. These results indicate that the seismicity temporal variation monitoring in a given area and the recognition of the low and high probability periods can contribute to the evaluation, in regular monthly intervals, of current seismic hazard status

Hellenic Unified Seismological Network: an evaluation of its performance through SNES method

In this paper we analyze the location performance of the Hellenic (Greek) Unified Seismological Network (HUSN) by SNES method (Seismic Network Evaluation through Simulation). This method gives, as a function of magnitude, hypocentral depth and confidence level, the spatial distribution of the: number of active stations in the location procedure and their relative azimuthal gaps and confidence intervals in hypocentral parameters regarding both the geometry of the seismic network and the use of an inadequate velocity model. Greece is located on a tectonically active plate boundary at the convergence of the Eurasian and African lithospheric plates and exhibits a high level of seismicity. The HUSN monitors the seismicity in Greek territory from 2007. At present it is composed by 88 seismic stations appropriately distribute in the area of Greece. The application of the SNES method permitted us to evaluate the background noise levels recorded by the network stations and estimate an empirical law that links the variance of P and S travel time residuals to hypocentral distance. This latter permitted us to assess the appropriateness of the velocity model used by the HUSN in the location routine process. We constructed SNES maps for magnitudes (M_L) of 2, 2.5 and 3, fixing the hypocentral depth to 10 km and the confidence level to 95%. We also investigated, by two different vertical sections, the behavior of the errors in hypocentral parameters estimates as function of depth. Finally we also evaluated, fixing the hypocentral depth to 10 km and the confidence level to 95%, the Magnitude of Completeness. Through the application of the SNES method, it is demonstrated that the HUSN provides the best monitoring coverage in western Greece with errors, that for M_L=2.5, are less than 2 and 5 km for epicenter and hypocentral depth respectively. At magnitude 2.5, this seismic network is capable of constraining earthquake hypocenters to depths of about 160 km and more, and provides a threshold of completeness down to magnitude 2 for most of Greek territory. Some seismogenic areas of southern Greece, that probably are not adequately covered by HUSN were delineated. The upgrading of the network in these areas could be optimized using the SNES technique.Published1417-14302.5. Laboratorio per lo sviluppo di sistemi di rilevamento sottomariniJCR Journalreserve

Hellenic Unified Seismological Network: an evaluation of its performance through SNES method

In this paper we analyze the location performance of the Hellenic (Greek) Unified Seismological Network (HUSN) by SNES method (Seismic Network Evaluation through Simulation). This method gives, as a function of magnitude, hypocentral depth and confidence level, the spatial distribution of the: number of active stations in the location procedure and their relative azimuthal gaps and confidence intervals in hypocentral parameters regarding both the geometry of the seismic network and the use of an inadequate velocity model. Greece is located on a tectonically active plate boundary at the convergence of the Eurasian and African lithospheric plates and exhibits a high level of seismicity. The HUSN monitors the seismicity in Greek territory from 2007. At present it is composed by 88 seismic stations appropriately distribute in the area of Greece. The application of the SNES method permitted us to evaluate the background noise levels recorded by the network stations and estimate an empirical law that links the variance of P and S travel time residuals to hypocentral distance. This latter permitted us to assess the appropriateness of the velocity model used by the HUSN in the location routine process. We constructed SNES maps for magnitudes (M_L) of 2, 2.5 and 3, fixing the hypocentral depth to 10 km and the confidence level to 95%. We also investigated, by two different vertical sections, the behavior of the errors in hypocentral parameters estimates as function of depth. Finally we also evaluated, fixing the hypocentral depth to 10 km and the confidence level to 95%, the Magnitude of Completeness. Through the application of the SNES method, it is demonstrated that the HUSN provides the best monitoring coverage in western Greece with errors, that for M_L=2.5, are less than 2 and 5 km for epicenter and hypocentral depth respectively. At magnitude 2.5, this seismic network is capable of constraining earthquake hypocenters to depths of about 160 km and more, and provides a threshold of completeness down to magnitude 2 for most of Greek territory. Some seismogenic areas of southern Greece, that probably are not adequately covered by HUSN were delineated. The upgrading of the network in these areas could be optimized using the SNES technique