Seismic tomography of the Tongariro Volcanic Centre, New Zealand

Abstract

Summary Tomographic inversion of local earthquake P- and S-wave traveltime data is used to investigate 3-D P-wave velocity (Vp) and P-wave/S-wave velocity ratio (Vp/Vs) variations at the Tongariro Volcanic Centre (TgVC), New Zealand. P-wave model resolution is generally high throughout the TgVC at depths of around 3–9 km below sea level. Near-surface resolution is lower since rays at shallow depths are predominantly subparallel. Resolution decreases below 12 km depth due to clustering of seismicity at greater depths. The final 3-D Vp model shows low-velocity volumes from 3 to 7 and 8 km depth, respectively beneath the active volcanoes Ruapehu and Ngauruhoe. Smaller low-velocity volumes are observed beneath the active Tongariro volcano and the extinct Tama Lakes vents. Synthetic tests indicate that the resolution of the 3-D Vp/Vs model is insufficient to permit any reliable interpretation. Accurate earthquake depths are determined for the first time within the TgVC. Seismicity is largely restricted to two clusters west of Ruapehu, possibly with origins on eastward striking branches of the normal Raurimu Fault, two lineations near Waiouru in the south-east of the study area and a fault belt between Tongariro and Lake Taupo in the north of the study area. Earthquake depths indicate possible shallowing of the brittle-ductile transition beneath Ruapehu and Ngauruhoe.