Seismicity of Ruapehu volcano, New Zealand, 1971–1996: a review

From 1971 until 1995, the style of seismicity at Ruapehu changed little, reflecting a period of relatively low eruptive activity and consequent long-term stability within the vent system. Volcanic earthquakes and volcanic tremor were both dominated by a frequency of about 2 Hz. Volcanic earthquakes accompanied all phreatic and phreatomagmatic eruptions, but not small hydrothermal eruptions that originated within Crater Lake. Furthermore, more than half of the ML>3 volcanic earthquakes and changes in the reduced displacement of 2 Hz volcanic tremor by as much as a factor of 20 occurred without any accompanying eruptive activity. Three and 7 Hz volcanic tremor were also recorded, although never at lower-elevation seismometers. At times, this tremor was stronger at the summit seismometer than the 2 Hz tremor. Their source regions were independent of the 2 Hz source, and located at shallower depths. Volcano-tectonic earthquakes were generally unrelated to eruptive activity. The seismicity accompanying the 1995–1996 eruptive activity was significantly different from that of the period 1971 to 1995, and included volcanic tremor with a frequency of less than 1 Hz, simultaneous changes in the amplitude of the previously independent 2 Hz and 7 Hz volcanic tremor, and finally a change in the frequency content of volcanic earthquakes and volcanic tremor from 2 Hz to wideband. Path transmission effects play an important role in determining the characteristics of seismograms at Ruapehu. The presence of Crater Lake affects both the style of eruptions and the accompanying seismicity.