Earthquake swarms to the west of Mt Ruapehu preceding its 1995 eruption

Some months prior to the 1995 eruption of Mt Ruapehu (New Zealand), a serie s of shallow earthquake swarms occurred about 15-20 km west of the summit o f Ruapehu. Several earthquakes in these swarms were felt, and the largest e vent was M-L 4.8. Cmstal earthquakes of M-L greater than or equal to 3.0 wi thin 20 km of the summit of Ruapehu have been rather uncommon in recent yea rs. Furthermore, the two periods of strongest activity were both just befor e times when the temperature of Crater Lake showed rapid increases. The sec ond of these rapid heating phases was immediately followed by increases in the Mg2+ ion concentration in Crater Lake, indicating that chemical interac tions were occurring between fresh magmatic material and the lake water. Th e coincidence between seismicity and lake changes suggested a link with the following eruption. A 1-D simultaneous inversion to locate the earthquakes more accurately showed that most of the earthquakes fell into three spatia l clusters, each cluster having a small horizontal cross-section. The predo minant depth was about 10-16 km. The b-value of this swarm was 0.74, quite compatible with ordinary tectonic earthquakes. Each cluster of earthquakes lies close to the normal Raurimu Fault which runs predominantly north-south to the west of Ruapehu, with an east-trending branch splaying off near its northern end (see Fig. 1b). Composite focal mechanisms of events in the tw o more southern clusters are oblique-normal, while the other cluster to the north has an oblique-reverse mechanism. The two oblique-normal mechanisms suggest that extension has occurred on part of the fault. This stress patte rn was also observed in the focal mechanism solutions of events that occurr ed after the eruption, when a denser network of portable seismographs cover ed the region. Although we cannot definitely connect the occurrence of thes e swarms to the eruptions later in 1995, there is a strong suggestion that the seismicity was connected to the process of magma movement, which temper ature and chemical changes in Crater Lake suggest was occurring during the first half of 1995. (C) 1999 Elsevier Science B.V. All rights reserved.