Seismicity and seismic structure of the tongariro volcanic centre area

Abstract

A network of ten smoked-drum seismographs was operated in the Tongariro Volcanic Centre area for fourteen days in order to study the shallow seismicity and P-wave velocities beneath the area. Sixty-seven shallow tectonic earthquakes and an explosion in the crater lake of Mt Ruapehu recorded by the network comprised the principal data used in the study. Most of the microearthquakes recorded by the network occurred beneath the southern part of the area; more than half were concentrated 15-29 km beneath Waiouru. The distribution of shallow microearthquake epicentres located by the N.Z. seismograph Network showed similar concentrations of activity and an additional concentration in the northern part of the area. The microearthquake and macroearthquake distributions each showed low rates of activity beneath Ruapehu and Tongariro massifs. The microearthquake hypocentres were located between 9-40 km deep and exhibited an increase in depth with increasing distance from Mt Ruapehu. Composite focal mechanism solutions for two groups of earthquakes, beneath the western and southern sides of Ruapehu massif, indicated normal faulting with tensional axes oriented orthogonal to Mt Ruapehu. The distribution of hypocentres and the focal mechanism orientations were both consistent with uplift and expansion beneath the southern part of the area. An apparent refractor velocity of about 5.4 km/s was determined from travel-times of explosions in the crater lake of Mt Ruapehu. The velocity agreed with velocities determined from seismic refraction profiles in the area by other workers. Average P-wave velocities were determined for a one-dimensional model from simultaneous inversion of explosion and earthquake travel-times. The velocities determined for the area between about 2-25 km beneath Mt Ruapehu were 5.2-6.5 km/s. An average apparent refractor velocity beneath the network of 6-4 km/s was determined from travel-times of four shallow earthquakes about 70 km north of Mt Ruapehu. These velocities could all be explained by progressively metamorphosed greywacke basement.