Latter, J.H.; Hurst, A.W. 1998 Seismicity caused by magma intrusion, episodic degassing, and resonance in gas-filled chambers at Ruapehu Volcano, New Zealand : a record of 20 years of activity, 1971 to 1990. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 98/29 82 p.
Abstract: Seismicity in the immediate area of Crater Lake, Ruapehu, has been documented for the period 1971 to 1990 inclusive. Apart from periods when the seismographs were inoperative, a complete record has been kept of all earthquakes of magnitude ML >/= 2.0. The rate of occurrence, and magnitude-frequency relationship of such earthquakes has been studied throughout the period. Both parameters show considerable variations. Stress in the focal area of the earthquakes is interpreted as high during the 1971 and 1975 eruptive periods, masking any tendency for the b-value to increase as a result of increased temperature. By contrast, temperature effects dominate over those of stress during a period of marked seismicity between 1980 and 1983, when high temperature in the focal area is inferred from an increased b-value at a time when stress was also clearly high. Variations in energy release for volcanic and near-vent volcano-tectonic earthquakes, and for volcanic tremor, have been estimated annually throughout the period. The mode of occurrence of seismicity accompanying eruptions suggests that a chain reaction process operates, releasing stress exponentially until a climax is reached. A progressive change, interpreted as shallowing of earthquakes, is identified, beginning in 1980, and accelerating through 1981 to 1983. This process continued, at a reduced rate, up to and beyond 1990. (auth)