Tornillos beneath Tongariro Volcano, New Zealand

Abstract Unusual low-frequency seismic events have recently begun to occur beneath Tongariro Volcano, New Zealand. Because the seismic waveforms appear similar to waveforms called tornillos, which have been recorded at Galeras Volcano in Colombia and have been found to precede small to medium-sized ash eruptions there by several weeks to months, we are monitoring them closely. At Tongariro, these events are characterised by small-amplitude, monochromatic or multichromatic waveforms that decay slowly with time, features that suggest a source involving magmatic or hydrothermal fluids within a zone surrounded by hard rock. The frequency content of monochromatic events is typically between 1 and 3 Hz and is often found to be peaked at f =1.2 Hz. Event durations range from about 10 to 80 s; the appearance of the shorter-duration events is identical to so-called LP or long-period events; the LPs and tornillos appear to be co-located so that the only difference between LPs and tornillos is the extended duration of the latter. These events have been identified in the seismograms of the permanent monitoring station KAVZ since at least January, 2001, occurring at a rate of about one per month. During June–August, 2001, the rate of occurrence gradually increased, peaking in September, 2001, at several events per day. We deployed two broadband and several short-period seismometers at close distances to the expected source location. The resulting seismograms show a complex mixture of seismic waves, originating from shallow depth beneath the area of the Te Maari Craters, located approximately 0.5 km from the nearest station. The close range, broadband records reveal some interesting details about the tornillo source. The tornillo waveform is found to consist of two parts: a low-frequency ( f ∼1.2 Hz) part and a higher-frequency ( f >3 Hz) part. The high-frequency energy travels at body wave speeds ( c ∼3.0 km/s) while the low-frequency energy travels at the speed of shear or surface waves in the shallow crust ( c ∼1 km/s). Interestingly, for some events, the low-frequency waves arrive before the high-frequency body waves at the closest station. This, and comparison with several LP events, suggests a spatial separation of the high- and low-frequency sources, with the low-frequency energy originating at shallow depth and acting as a trigger for the release of tectonic stress at greater depth, in the form of high-frequency body waves.