RELIABLE AUTOMATIC DETECTION OF LONG-PERIOD VOLCANIC EARTHQUAKES AT LONG-VALLEY CALDERA, CALIFORNIA

Capturing the rare long-period (LP) volcanic earthquakes occurring in and near Long Valley caldera, California, is important to ongoing volc anic-hazards monitoring. It is difficult, however, because LP events a re weak, emergent, and almost devoid of energy above a few hertz. Auto matic systems designed for tectonic earthquakes routinely fail to capt ure LP events. We applied a PC-based teleseism-specific event-detectio n computer program to capturing these events. Retuning the software fo r LP events involved only changing parameters originally designed for change in this algorithm. Our retuned algorithm has captured every kno wn LP event at Long Valley from October 1992 through the end of 1994. We monitored up to 16 stations known to produce good records of LP eve nts, saving those events that triggered enough of these stations (typi cally 10 of 16) within a specified time window. The principal difficul ty has been the algorithm's sensitivity to regional earthquakes, which have waveforms similar to LP events. During our test, the 1992 Lander s, 1994 Northridge, and 1994 Double Springs Flat (Nevada) earthquakes each have swamped the detector, requiring careful, active management o f PC disk resources. The efficacy of this retuned algorithm and the po or performance of tectonic-earthquake detectors during some volcanic e mergencies make this algorithm an attractive candidate for volcano mon itoring.