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Enhancements of Seismic Detection Capabilities Resulting from the Bilateral Cooperation of the NDCs of Austria and the Czech Republic 1 2 2 1 1 Ulrike Mitterbauer , Josef Havír , Jan Švancara , Peter Melichar , Wolfgang Lenhardt 1 Austrian National Data Centre at the Central Institute for Meteorology and Geodynamics, Vienna 2 Czech National Data Centre at the Institute of Physics of the Earth, Masaryk University, Brno The cooperation between the Austrian and the Czech National Data Centres (NDCs) has been in existence since the establishment of the CTBTO Preparatory Commission in 1997. Among the goals of the bilateral cooperation are the improvements of seismic events location accuracy in the Central European Region utilizing all data collected by both NDCs and readiness to support the CTBTO with supplementary data and information which are formally not part of the IMS data as needed for the IDC and IMS evaluations. Comparative studies of the IDC and the NDC event locations for the area of Austria and the Czech Republic have been carried out using the IDC event locations reported in the Reviewed Event Bulletins (REBs) for the period 2004 - 2008. ISS - International Scientific Studies 2009, Vienna - Austria Comparison of Epicenteres of Tectonic Events Located by the Czech NDC and Located by the IDC (2004-2008; ML>=3.0) L3 L3 M1 M1 In the period 2004-2008 29 tectonic events with local magnitude ML=>3.0 were located by the Czech and Austrian NDCs within the area of interest (Fig. L2). Due to higher seismicity in Austria there are more events from Austria than from the territory of the Czech Republic; the same pattern of more events from Austria is correctly reflected in the REB. The comparison revealed that only 17 of the 29 events were included in the REB, the remaining 12 events were not located by the IDC. Most events which were not reported in REBs had local magnitude smaller then 3.3, the highest local magnitude of an event not included in the REB was 3.7. The magnitudes of these “missing” events are generally below or close to the expected average global detection capability of the IMS seismic network. Histogram of Located Events Present and Missing in the REB To assess the real IDC location accuracy 17 events have been analysed. These events were reported both in the REB and in the earthquake catalogues of the NDCs. Most events showed that a difference between the REB and NDC locations is smaller than 30 km, with an average difference of 18 km (median is 14 km). The largest difference occurred for an event at the Austrian/Hungarian border and amounted to 59 km. Conclusions - 108 Events were located within the NDC in the period 2004 - 2008 with a local magnitude ML - 29 of those events had a local magnitude ML - Only 17 of them were reported in the REB. - Most of the events reported by the NDC as well as by the IDC have a difference less than 30 km within their locations. - The largest difference was observed for an event at the Austrian/Hungarian boarder and it amounted 59 km. Based on the Protocol to the CTBT the on-site inspection should be carried out in the area where the suspected nuclear test occurred, whereby the size of the on-site inspection area should not exceed 1000 km². The area of 1000 km² can be seen as an equivalent circle area with a radius of 17 – 18 km. Therefore the estimated location difference between the IDC and the NDC locations for the area of interest is at the limit of the uncertainty expected by the Treaty provisions for on-site inspection. >= 2. >= 3. Correlation The length of misfit was compared with some characteristics of the events. Only low correlations were found for comparison between the length of misfit and seismic gaps, local magnitude and the number of stations used for location. The colour signature in the following Figures (R1, R2, R3) refers to the region where the event had occurred, the numbers create a connection to the events displayed in Figure M1. 3. Length of misfit vs. Number of stations (REB-bulletin ) 2.Length of misfit vs. Magnitude 1. Location misfit L2 L2 Tectonic Events Located by the Czech NDC (2004-2008; ML>=2.0) Figure L1 shows all epicentres of events located by the Czech NDC in the period between 2004 - 2008 with a local magnitude >= 2.0. In total 108 events were detected. Events with the highest magnitudes were recorded in the western part of Slovenia (ML=5.3), in the western part of Czech Rep. (ML=4.1) and in the southern part of Poland (ML=4.6) Triangles indicate stations which were registering the events. Events are displayed proportional to their local magnitude (Ml). Tectonic Events Located by the Czech NDC (2004-2008; ML>=3.0) Selected Events L1 L1 Czech NDC Austrian NDC 4.1 4.6 5.1 Events located by NDCs as well as by IDC Events located only by NDCs IDC NDC IDC NDC AT Figures M2 and M3 show an assortment of events which were localized as well within the NDC and within the IDC. Triangles indicate stations which were used for localization either by the IDC or by the NDC. Event 1 25.5.2004 - 07.30 Delta 59 km NDC/IDC ML 3.2 NDC ML 3.5 IDC = = = Event 5 18.7.2008 - 22.54 Delta 27 km NDC/IDC ML 3.4 NDC ML 3.6 IDC = = = Stations used by Localization by Czech NDC M3 M3 M2 M2 Events located by IDC Events located by NDCs Austria Slovenia Hungary Slovakia Czech Republic The average distance between REB and NDC locations is 18 km, no significant correlation between azimuth and misfit was found. A low correlation between length of misfit and local magnitude was observed, usually a local magnitude ML>4 is needed to obtain a mislocation less than 10 km. The correlation between length of misfit and number of stations is indistinct for a small number of stations. Generally the length of misfit decreases if the number of stations used for location increases significantly. R1 R1 R2 R2 R3 R3 Event was located in
