The Taupo Seismic System

Abstract

The Taupo Volcanic Zone is an area of recent volcanism, with considerable shallow seismicityf including frequent earthquake swarms. The New Zealand Seismograph Network had only a few stations in this area, and only one station of low gain was near Lake Taupo, the source of a major eruption approximately 1800 years ago.

An improved seismic network was needed firstly to monitor the normal tectonic seismicity of the region, and secondly to enable accurate earthquake locations to be calculated quickly. In an area of known volcanic risk, the rapid determination of locations of felt earthquakes improves public confidence. Seismic signals from 13 vertical seismometers and one horizontal seismometer in the Taupo Volcanic Zone are transmitted to the DSIR Wairakei office. Three of these seismometers monitor seismicity in the Ohaaki area, where a geothermal power station is currently being built.

At the Wairakei recording centre, up to 16 seismic signals are sampled by a microprocessor-based digitizing system. Eight frequency-domain detectors monitor selected channels by continuously calculating spectra to see if the incoming signal has an earthquakelike spectrum. If any detector is triggered, the readings from all channels for a period starting 20 s before the trigger are recorded on magnetic tape, and the first arrival time for each channel is printed and sent to the analysis computer. These arrival times come from a time-domain phase picker (‘P-picker’) monitoring all channels.

A DEC MicroPDP-11 computer is used for earthquake analysis. Immediate locations can be calculated from the P-picker times, or an interactive graphics program can be used for more complete analysis later. Two visual recorders are used for setting up and for checking the system performance. The total capital cost of the system is less than half that of on-site visual recording, the analysis costs are much less, and locations can be calculated rapidly, rather than weeks after an earthquake.