A COMPARISON OF SELECT TRIGGER ALGORITHMS FOR AUTOMATED GLOBAL SEISMIC PHASE AND EVENT DETECTION

Digital algorithms for robust detection of phase arrivals in the prese nce of stationary and nonstationary noise have a long history in seism ology and have been exploited primarily to reduce the amount of data r ecorded by data logging systems to manageable levels. In the present e ra of inexpensive digital storage, however, such algorithms are increa singly being used to flag signal segments in continuously recorded dig ital data streams for subsequent processing by automatic and/or expert interpretation systems. In the course of our development of an automa ted, near-real-time, waveform correlation event-detection and location system (WCEDS), we have surveyed the abilities of such algorithms to enhance seismic phase arrivals in teleseismic data streams. Specifical ly, we have considered envelopes generated by energy transient (STA/LT A), Z-statistic, frequency transient, and polarization algorithms. The WCEDS system requires a set of input data streams that have a smooth, low-amplitude response to background noise and seismic coda and that contain peaks at times corresponding to phase arrivals. The algorithm used to generate these input streams from raw seismograms must perform well under a wide range of source, path, receiver, and noise scenario s. Present computational capabilities allow the application of conside rably more robust algorithms than have been historically used in real time. However, highly complex calculations can still be computationall y prohibitive for current workstations when the number of data streams become large. While no algorithm was clearly optimal under all source , receiver, path, and noise conditions tested, an STA/LTA algorithm in corporating adaptive window lengths controlled by nonstationary seismo gram spectral characteristics was found to provide an output that best met the requirements of a global correlation-based event-detection an d location system.