Seismicity in Taupo volcanic zone geothermal systems

Abstract

The geothermal systems at Rotorua and Kawerau are located within the volcanically and geothermally active Taupo Volcanic Zone (TVZ) in the central part of the North Island of New Zealand. Previous attempts to investigate the subsurface structure of these areas have met with varying levels of success. Most faults are buried and their locations are generally inferred from the locations of surficial hydrothermal features or offsets in shallow drilled strata.

We analyse catalogued data from two sets of earthquakes that occurred in and around the Rotorua and Kawerau geothermal systems. These data sets contain 504 and 1875 shallow (_ 20 km deep) earthquakes, respectively, and span the 21 year period between the beginning of 1984 and the end of 2004.

Arrival time data from these earthquakes are used to calculate one-dimensional P- and S-wave seismic velocity models for Rotorua and Kawerau. The resulting solutions are well constrained, particularly at depths between 4 and 15 km, and are consistent with models obtained in seismic refraction studies for the same region.

Seismogram and arrival time data are used to calculate earthquake 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. Here, alignments of earthquakes reveal possible fault structures in the upper crust whose existence has previously been suggested based on surface observations. In Kawerau, the earthquakes within the geothermal field align along northeast-trending lineations, consistent with the predominant alignment of faults within the region.

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Technical Abstract

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 (≤ 20 km deep) earthquakes, respectively, and span the 21 year period between the beginning of 1984 and the end of 2004.

Arrival time data from these earthquakes are used to calculate 1-D P- and S-wave seismic velocity models and accompanying station correction terms for Roturua and Kawera. Numerous starting models are trialled in multiple inversions to fully probe the set of possible solutions in each area. The resulting solutions are well constrained, particularly at depths between 4 and 15 km, and are consistent with models obtained in seismic refraction studies for the same region. At these depths we calculate Vp/Vs ratios which range between 1.68 and 1.73 in Rotorua, and 1.64 and 1.71 in Kawerau.

Using a combination of cross-correlation-derived and catalogue-based arrival times, hypocentres are calculated for 155 and 400 earthquakes in Rotorua and Kawera, 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. Here, event alignments reveal possible seismogenic structures in the upper crust which are previously unknown although inferred, and are consistent with focal mechanisms determined for the area. In Kawera, the earthquakes within the geothermal field align along northeast-trending lineations, consistent with the predominant alignment of faults within the region.