Mislocation vectors for the Tripoli seismic array, Greece, and structural effect implications from backazimuth and slowness residual analysis

Η αποτελεσματικότητα της Σεισμικής Διάταξης Τριπόλεως (TRISAR) στον προσδιορισμό των εστιακών παραμέτρων των σεισμών περιορίζεται από το μικρό ανάπτυγμα και αριθμό αισθητήρων αυτής. Η λεπτομερής ανάλυση των σφαλμάτων στον αυτόματο προσδιορισμό επκέντρων δημιουργεί υπόνοιες για επιδράσεις της δομής του φλοιού και της τοπικής γεωλογίας. Για τη διερεύνηση της δυνατότητας αυτόματης διόρθωσης των αποτελεσμάτων του ανύσματος βραδύτητας, υπολογίστηκαν τα ανύσματα διόρθωσης για ένα εκτεταμένο δείγμα δεδομένων. Οι θεωρητικές τιμές προσδιορίστηκαν βάσει των σεισμικών καταλόγων του Εθνικού Αστεροσκοπείου Αθηνών και του Διεθνούς Σεισμολογικού Κέντρου (ISC). Τα ανύσματα διόρθωσης, που προέκυψαν, έχουν σημαντικό μήκος, σύμφωνο με τα μεγάλα σφάλματα που παρατηρούνται για το αζιμούθιο και τη βραδύτητα. Οι μικρότερες τιμές διαπιστώνονται ΒΑ της διάταξης, για επικεντρικές αποστάσεις μικρότερες των 200 km. Όπως είναι αναμενόμενο, οι διορθώσεις αφορούν κυρίως στο αζιμούθιο και δεν είναι αρκετές ώστε να επηρεάσουν την τελική λύση, δεδομένου ότι τα μεγαλύτερα σφάλματα του αυτόματου αλγόριθμου αφορούν στον προσδιορισμό της επικεντρικής απόστασης. Ωστόσο, καθίσταται σαφής η δυνατότητα αυτόματης διόρθωσης του ανύσματος βραδύτητας, αναμένεται δε μία βελτιωμένη γεωμετρία της σεισμικής διάταξης να δώσει πολύ καλύτερα αποτελέσματα, κυρίως λόγω της μειωμένης διασποράς, που θα επιτυγχάνειTripoli Seismic Array, Greece, performance in terms of event location is restricted by its very small aperture and limited number of sensors. Detailed investigation of errors in automatic location results suggests structural and local geology effects. In order to investigate the possibility to correct for systematic errors automatically, mis location vectors were calculated for an extended data-set. Theoretical values were calculated based on earthquake catalogues compiled by the National Observatory of Athens and the ISC. Resulting mis location vectors are characterized by significant vector length, consistent with the large observed backazimuth and slowness residuals, the smaller values being met in the area NE of the array and for epicentral distance values less than 200 km. As expected, resulting corrections mostly concern backazimuth values and are not able to sufficiently affect the final epicentre solution, as the largest automatic algorithm errors are observed in epicentral distance determination. However, the possibility to automatically correct for systematic deviations is verified, and future research with an extended array configuration is expected to provide clearer results, due to significantly lower scatter

The March 2004 Kalamata seismic sequence: A case of efficient seismicity monitoring in the area of Peloponnese, southern Greece, by the Tripoli Seismic Array

On March 1, 2004, a moderate earthquake occurred in the vicinity of the town of Kalamata, southern Peloponnese, Greece. The Tripoli Seismic Array (TRISAR), located in the centre of Peloponnese, recorded the mainshock as well as the large number of aftershocks that followed. Only a small number of these events were located by regional seismographic networks. Analysis of the Kalamata seismic sequence and evaluation of the results is presented in this article, as an example of the efficiency of TRISAR in seismicity monitoring and location in the area of Peloponnese. © Springer Science+Business Media, Inc. 2006

Preliminary analysis of the 21 February 2008 Svalbard (Norway) seismic sequence

The Svalbard Archipelago is situated in the northwestern part of the Barents shelf, in close proximity to the passive continental margin. This intraplate region is characterized by some of the highest seismicity in the entire Barents Sea and adjoining continental shelf, surpassed only by the Knipovich ridge (e.g., Engen et al. 2003; International Seismological Centre 2001), which, as a spreading plate boundary, is the structure that dominates the regional stress field. Most of the seismic activity (Figure 1) is characterized by smaller events, which often occur in small concentrations sparsely distributed in time. However, earthquakes of moderate to stronger magnitudes do occur in the Svalbard area, such as the 4 July 2003 mb 5.7 event close to Hopen Island (e.g., Stange and Schweitzer 2004)

3-D crustal velocity structure in northwestern Greece

The three-dimensional crustal velocity structure in the area of the northwestern Greek mainland was determined by P wave travel time inversion, applying a two-step tomography procedure. The data set consists of the travel-time residuals of 584 well located earthquakes. In order to improve the initial (reference) velocity model, before the inversion of travel times, the "minimum 1-D" model was determined. Several tests were conducted to estimate model stability and hypocenter uncertainties. The velocity distribution in the shallow layers (4 and 7 km) is strongly affected by the crustal thickness variation and the complex tectonics. A first, well-defined velocity discontinuity appears at a depth of 3-6 km, along the Hellenides Mountain chain. A second low velocity anomaly is detected at a depth of 9-12 km and may be connected with the Alpidic orogenesis. Another interesting feature appears beneath the Amvrakikos Gulf (horstgraben structure), where relatively low velocities (<6.0 km-1) appear to a depth of 20 km. Finally, a well-pronounced velocity boundary is found at a depth of 16 km. In general, low velocities are predominant along the Dinarides-Hellenides Mountain chain, rather typical for the upper crust. © Birkhäuser Verlag, Basel, 2005

Tectonic structure of Central-Western Attica (Greece)based on geophysical invesigations - preliminary results

In an effort to investigate the deep geological structure in the broader area of central-western Attica, that suffered severe damage during the destructive Athens earthquake of September 7th, 1999, the Department of Geophysics-Geothermics of the Faculty of Geology and Geoenvironment of Athens University, in collaboration with the Geodynamic Institute of National Observatory of Athens and the Department of Geological Sciences of Durham University, carried out a combined geophysical survey. For the first time in Attica, seismic and gravity geophysical methods were applied along profiles, in such an extensive scale. Within the framework of this investigation the following tasks were accomplished: a) Three (3) seismic lines of about 30 kilometres of total length, two (2) in the area of Thriassion plain and one (1) along the Parnitha-Krioneri-Drosia-Ekali-Dionysos axis (Attica plain) and b) 338 gravity measurements distributed along eight (8) gravity profiles, four (4) of which in Thriassion plain, three (3) in Petroupoli-Aharnes-Thrakomakedones region (Attica plain) and one (1) along Parnitha-Krioneri-Drosia-Ekali-Dionysos axis (Attica plain). Preliminary results of the geophysical investigation combined with existing geological and tectonic data are presented in this paper. Significant variation in the elevation of the alpine basement has been detected, expressed by manifestation of low and high subsurface areas which are well correlated with existing fault zones. In the area of Thriassion plain the thickness of post-alpine sediments is estimated of a few hundred meters (<500 meters) and huge thickness of several hundred meters (~800 meters) of post-alpine sediments were detected in Thrakemakedones and Krioneri areas. The relief of alpine basement is more intense in fault zones areas, such as Thriassion plain (WNW-ESE), Pathitha and Kifissos (NE-SW) as well as along the fault zone outline the Fili and Aharnes graben and is covered by postalpine formations