Eruption-induced modifications to volcanic seismicity at Ruapehu, New Zealand, and its implications for eruption forecasting

Broadband seismic data collected on Ruapehu volcano, New Zealand, in 1994 and 1998 show that the 1995-1996 eruptions of Ruapehu resulted in a significant change in the frequency content of tremor and volcanic earthquakes at the volcano. The pre-eruption volcanic seismicity was characterized by several independent dominant frequencies, with a 2 Hz spectral peak dominating the strongest tremor and volcanic earthquakes and higher frequencies forming the background signal. The post-eruption volcanic seismicity was dominated by a 0.8-1.4 Hz spectral peak not seen before the eruptions. The 2 Hz and higher frequency signals remained, but were subordinate to the 0.8-1.4 Hz energy. That the dominant frequencies of volcanic tremor and volcanic earthquakes were identical during the individual time periods prior to and following the 1995-1996 eruptions suggests that during each of these time periods the volcanic tremor and earthquakes were generated by the same source process. The overall change in the frequency content, which occurred during the 1995-1996 eruptions and remains as of the time of the writing of this paper, most likely resulted from changes in the volcanic plumbing system and has significant implications for forecasting and real-time assessment of future eruptive activity at Ruapehu.