Seismicity associated with the 1995–1996 eruptions of Ruapehu volcano, New Zealand: narrative and insights into physical processes

Seismicity associated with the 1995–1996 eruptions of Ruapehu reflects processes associated with the intrusion of magma to shallow depth in the volcanic edifice, its subsequent eruption, and changes in the volcanic plumbing system resulting from the eruptions. The sequence consisted of two distinct periods of eruptive activity, the first lasting from September 17, 1995 until late-October or early-November 1995 and the second from June 16, 1996 until late-July 1996. Immediately prior to and during the early stages of the 1995 eruptions, seismicity was similar to that recorded at Ruapehu during the previous 25 years, with the exception of the occurrence between September 17 and 25, 1995 of sub-1 Hz tremor, which is thought to represent magmatic intrusion. In early-October, a new pattern of seismicity was established with the disappearance of the 2 Hz tremor resonator source and a change to wideband (2–10 Hz) tremor and wideband volcanic earthquakes. This change coincided with the ejection of the last vestiges of Crater Lake and the change in eruptive style from phreatomagmatic to magmatic. Seismicity associated with the 1996 eruptions was very similar to that which accompanied the later part of the 1995 activity, implying the same eruption processes and that no significant changes had occurred in the volcanic plumbing system during the intervening period. Geochemical, geodetic, and seismic precursors to the 1995 eruptions were both minor and inconsistent, highlighting the difficulty in forecasting these eruptions. Furthermore, deep volcano-tectonic earthquakes were extremely rare throughout the 1995 and 1996 eruptive sequences, suggesting that stresses associated with magmatic intrusion were minor. This most likely resulted from the existence of either an open or a ductile pathway from the deep magma source to the surface prior to the eruptions and because the volume of magma intruded and subsequently erupted was relatively small (<0.05 km3).