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Monitoring nuclear test sites with GERESS

Institute of Geophysics Ruhr-University Bochum, D-44780 Bochum, Germany

Abstract

Since the installation of the 25-element GERman Experimental Seismic System (GERESS) array in southeastern Germany in 1990, 36 presumed nuclear tests were recorded by this system: 16 from Nevada, 12 from the Tuamotu Archipelago, 7 from Lop Nor, and 1 from Novaya Zemlya. Also, a chemical calibration experiment at Nevada Test Site (NPE) was observed at the array in 1993. In addition, two presumed nuclear events from Nevada, four from the Tuamotu Archipelago, and six from eastern Kazakhstan were recorded on temporary stations close to the GERESS array during the site survey (1988 to 1989). These latter 12 events were carefully included in this study to broaden the data set. This has been done since the number of underground nuclear tests observable at the GERESS site is not expected to increase significantly in the future because most nuclear powers have adhered to a moratorium on nuclear testing. In addition, we also do not expect that peaceful nuclear explosions (PNE) will be detonated in the future due to reasons of environmental hazard. Concentrating on Nevada Test Site and the Tuamotu Archipelago, a cross-correlation method and a cluster analysis indicate subgroups of the data. These empirical methods clearly discriminate presumed nuclear tests on the islands of Mururoa and Fangataufa. In addition, detection thresholds at GERESS were estimated based on bodywave magnitudes published by ISC: m_b = 4.3 for the Nevada Test Site, m_b = 3.9 for the Tuamotu Archipelago, m_b = 4.6 for Lop Nor, m_b = 4.3 for Novaya Zemlya, and m_b = 4.4 for eastern Kazakhstan. Clearly, these path-specific detection thresholds deviate substantially from those derived from global amplitude-distance relations (i.e., Gutenberg-Richter). This indicates that a network of seismic stations used for global monitoring purposes may need careful calibration.