S U M M A R YTomographic inversion of local earthquake P-and S-wave traveltime data is used to investigate 3-D P-wave velocity (V p ) and P-wave/S-wave velocity ratio (V p /V s ) 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 V p 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 V p /V s 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.

S U M M A R YThe subglacial Grímsvötn central volcano, lying within a volcanic zone directly above the core of the Iceland mantle plume, is one of the most active in Iceland. Local, regional and teleseismic earthquake data recorded on a temporary seismometer array across western Vatnajökull icecap during the summer of 1998 have provided a three-dimensional image of the shallow crustal structure of the volcano. Microearthquake activity at depths of 1-4 km along the Grímsvötn caldera rim coincided with inflation of a shallow magma chamber beneath the caldera, which culminated in a 0.1 km 3 eruption in December 1998. Tomographic inversion of these earthquakes define the extent of a low-velocity body beneath Grímsvötn with a volume of ∼20 km 3 extending to ∼3 km below the surface. This low-velocity body is flanked by high velocities under the caldera rim. Delays in the P-wave arrival times through the Grímsvötn caldera from regional and teleseismic earthquakes and from two detonations ∼150 km east of Grímsvötn are 0.10-0.15 s greater than the delays through the uppermost 3-4 km of crust shown by local earthquake arrivals. This suggests the presence of a further low-velocity body at depths greater than 3-4 km beneath Grímsvötn, presumed to be due to the presence of melt. Using the distribution of local seismicity and shear wave attenuation we estimate the maximum lateral extent of the region containing partial melt to be 7-8 km E-W and 4-5 km N-S. P-wave delays require a thickness of less than 1 km of pure/high percentage partial melt, assuming a sill-like magma chamber.