MONITORING NUCLEAR TEST SITES WITH GERESS

Since the installation of the 25-element GERman Experimental Seismic S ystem (GERESS) array in southeastern Germany in 1990, 36 presumed nucl ear tests were recorded by this system: 16 from Nevada, 12 from the Tu amotu Archipelago, 7 from Lop Nor, and 1 from Novaya Zemlya. Also, a c hemical calibration experiment at Nevada Test Site (NPE) was observed at the array in 1993. In addition, two presumed nuclear events from Ne vada, four from the Tuamotu Archipelago, and six from eastern Kazakhst an were recorded on temporary stations close to the GERESS array durin g the site survey (1988 to 1989). These latter 12 events were carefull y included in this study to broaden the data set. This has been done s ince the number of underground nuclear tests observable at the GERESS site is not expected to increase significantly in the future because m ost 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 haza rd. Concentrating on Nevada Test Site and the Tuamotu Archipelago, a c ross-correlation method and a cluster analysis indicate subgroups of t he data. These empirical methods clearly discriminate presumed nuclear tests on the islands of Mururoa and Fangataufa. In addition, detectio n thresholds at GERESS were estimated based on body-wave magnitudes pu blished by ISC: m(b) = 4.3 for the Nevada Test Site, m(b) = 3.9 for th e Tuamotu Archipelago, m(b) = 4.6 for Lop Nor, m(b) = 4.3 for Novaya Z emlya, and m(b) = 4.4 for eastern Kazakhstan. Clearly, these path-spec ific detection thresholds deviate substantially from those derived fro m global amplitude-distance relations (i.e., Gutenberg-Richter). This indicates that a network of seismic stations used for global monitorin g purposes may need careful calibration.