Velocity Modelling and Earthquake Relocation in the Rotorua and Kawerau Geothermal Areas, Taupo Volcanic Zone, New Zealand

Abstract

<p>We analyse catalogued phase arrival times and seismograms from two sets of earthquakes that occurred in and around the Rotorua and Kawerau geothermal systems in the Taupo Volcanic Zone, New Zealand. These data sets contain 504 and 1875 shallow (less than or equal to 20 km deep) earthquakes, respectively, and span the 21 year period between the beginning of 1984 and the end of 2004. We use arrival time data from these earthquakes to calculate reliable 1-D P- and S-wave seismic velocity models and accompanying station correction terms for Rotorua and Kawerau. These models are well constrained at depths of 4 to 15 km and are consistent with models obtained in seismic refraction studies for the same region. Using a combination of precise cross-correlation-derived and cataloguebased arrival times, we calculate accurate hypocentres for 155 and 400 earthquakes in Rotorua and Kawerau, respectively. These hypocentres are far less scattered than those in the earthquake catalogue. In Rotorua, the earthquakes cluster near the geothermally active parts of Rotorua City and also beneath the Mount Ngongotaha rhyolite dome. In Kawerau, the earthquakes align along northeast-trending lineations, consistent with the predominant alignment of faults within the region. Earthquakes in Okataina caldera to the southwest of Kawerau are tightly clustered, particularly beneath Puhipuhi Hill. We use cross-correlation to identify clusters of earthquakes in Rotorua and Kawerau based on the similarity of their recorded seismograms. Apart from two clusters in the northern part of Rotorua City, these are generally indicative of relatively short-lived bursts of activity within these regions. We compute composite focal mechanisms for four of these clusters, all of which indicate rupture on normal faults, consistent with the extensional tectonic regime of the Taupo Volcanic Zone.</p>