Seismic Event Identification of Earthquakes and Explosions in Germany Using Spectral Lg Ratios

At the German NDC initial work on seismic event identification has focused on the application of spectral amplitude ratios for Lg in order to discriminate naturally occurring seismic events from other events associated with mining and quarry activities. Only about 10% of all seismic events occurring in Germany and adjacent areas are due to natural seismicity and are mostly constrained to the Alpine regions and areas along the Rhinegraben, Rhenish massif, Swabian Jura, and the Bohemian massif (Vogtland region). Using data from the broadband GRSN network, spectral amplitude ratios are calculated from maximum trace amplitudes in the 1–2 Hz and 6–8 Hz frequency bands, which are within the passbands of the deployed STS-2 instruments and the recorded 20 Hz data streams. These amplitude ratios then must be corrected with an appropriate attenuation model in order to remove propagation paths effects. For event identification, a scoring scheme is applied across the GRSN network, based on stationdependent scoring thresholds. In a case study aimed at testing the identification scheme, events are investigated from a quarry in southern Germany that provided ground-truth information for six events in 1997 to demonstrate the suitability of this identification approach. Except for one event with a rather strong earthquake signature, i.e., a low spectral Lg ratio, these events could be screened out from the earthquake population by their large Lg ratios. In a second step, aimed at applying the identification scheme, all events in Germany and neighboring areas that occurred in 1995 were processed, with approximately 800 out of more than 1200 events showing explosion-type Lg ratios, while only 10% remain in the earthquake population. However, specific mining areas appear to consistently produce earthquaketype spectral ratios indicative of particular blasting practices.