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DEPARTMENT OF GEOLOGY OSLO UNIVERSITY, PO BOX 1047, BLINDERN, N-0316 OSLO3, Norway
NTNF/NORSAR, PO BOX 51, N-2007 KJELLER, Norway
Abstract
A new three-component (3C) seismic signal detector is introduced that has enabled us to produce a preliminary single-station bulletin in an automatic mode. We consider automatic station bulletins as very important because in the high-frequency range local events (mostly chemical explosions like quarry blasts) are truly numerous, say 20 to 50 per day, and cannot be handled efficiently by an analyst in the conventional way. A short-term-average / long-term-average (STA/LTA) test statistic is used either alone or in combination with 3C signal polarization (for P phases), which allows a lowered threshold. An added advantage with the polarity estimate is that a corresponding slowness vector estimate is obtained. The detector is operated in three consecutive steps: (1) the detector is run on a suite of bandpass filtered records, (2) the initial detection log is compressed while retaining parameters from the "best" filter, and (3) the detected and identified phases are grouped into event families and epicenter parameters are estimated. The 3C detector has been tested on 6 days of recordings from the station C4 in the NORESS array located in SE Norway and results were compared to the array bulletin. For teleseismic signals the array has an advantage in SNR of about 15 dB. For local and regional events (distance < 1500 km) the 3C SNR in the frequency range 5 to 15 Hz is about 6 to 8 dB below that of the array. The 3C detector picks practically all S phases detected by the array but occasionally misses the preceding Pn. However, some new events missed by the array were also found. At local and regional distances, the 3C azimuth and distance estimates are generally within 20° and 20 km respectively of the NORESS solution and the origin time estimates are within a few seconds. The 3C epicenter solutions are less stable than that of the array mainly due to phase identification problems and Pn azimuth errors at low SNR. Naturally, the benefit of analyst screening and re-processing would improve the 3C bulletin quality, although the performance is considered satisfactory at the present stage of development.
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