Article,
Seismic tomography of the Tongariro Volcanic Centre, New Zealand
Geophysical Journal International, 163 (3): 1180--1194 (December 2005)
DOI: 10.1111/j.1365-246X.2005.02716.x
Abstract
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.
