Capturing the rare long-period (LP) volcanic earthquakes occurring in and near Long Valley caldera, California, is important to ongoing volcanic-hazards monitoring. It is difficult, however, because LP events are weak, emergent, and almost devoid of energy above a few hertz. Automatic systems designed for tectonic earthquakes routinely fail to capture LP events.
We applied a PC-based teleseism-specific event-detection computer program to capturing these events. Retuning the software for LP events involved only changing parameters originally designed for change in this algorithm. Our retuned algorithm has captured every known 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 events, saving those events that triggered enough of these stations (typically 10 of 16) within a specified time window. The principal difficulty has been the algorithm's sensitivity to regional earthquakes, which have waveforms similar to LP events. During our test, the 1992 Landers, 1994 Northridge, and 1994 Double Springs Flat (Nevada) earthquakes each have swamped the detector, requiring careful, active management of PC disk resources.
The efficacy of this retuned algorithm and the poor performance of tectonic-earthquake detectors during some volcanic emergencies make this algorithm an attractive candidate for volcano monitoring.