Location of zones of anomalously high s-wave attenuation in the upper crust near ruapehu and ngauruhoe volcanoes, New Zealand

Abstract

Many earthquakes within the crust near Ruapehu and Ngauruhoe volcanoes, recorded at epicentral distances less than 20 km on vertical seismometers, show S-waves of lower dominant frequency than the P-waves. A large number also have amplitudes in the S-group less than those of the P-waves. Whereas the reduced amplitude of S-waves relative to that of P-waves can be a source mechanism effect, the corresponding reduction in dominant frequency should be independent of the source radiation pattern. The most plausible reason for such a reduction in dominant S-wave frequency is that the waves have passed through a zone of partially molten rock. The data are therefore interpreted in terms of the presence of magma in restricted zones near the volcanoes. Using ray paths from 232 hypocentres to three permanent seismograph stations, together with paths from three additional earthquakes to one permanent and two temporary stations, an interpretation in three dimensions has been made of the source of the anomalous attenuation at depths between 2 and 10 km below datum (Ruapehu Crater Lake). Wave paths which lie largely at depths shallower than 2 km cannot be used, as almost all such paths show evidence of enhanced S-wave attenuation, and this is attributed to the presence of superficial pyroclastic and unconsolidated laharic material within 2 km of the surface. At Ruapehu, the data suggest the presence of three principal intrusions, one underlying much of the southwest slopes and reaching as far east as Crater Lake, one beneath the eastern side of the Summit Plateau, and one beneath part of the northeast slopes of the volcano. All three are essentially vertical or steeply dipping structures, detectable to a depth of between 7 and 9 km. The first appears to extend to within about 5 km of the surface, whereas the other two have intruded to within 2 or 3 km. Other, less well-defined, and comparatively small bodies exist beneath both the western and eastern slopes of Ruapehu. In the Ngauruhoe area, few earthquakes have occurred and all have been at depths less than 6 km. Therefore, only shallow attenuating areas can be defined. A small area of anomalous S-wave attenuation occurs beneath the northwest slopes of Ngauruhoe, and another, elongated, body appears to coincide with a fault zone west of the volcano. Both of these lie at depths of about 3 km below datum (less than 2 km below surface in one locality). Finally, areas of high attenuation, at depths of 4–5 km below datum, appear to define a narrow east-west zone about 6 km long in the immediate area of Whakapapa village. Other zones exist east of the volcanic axis, defining a line which cuts the axis on the north east slopes of Ruapehu, at a point where a parasite crater formed a few thousand years ago.