Shear Wave Splitting at Rotokawa and Ngatamariki Geothermal Fields for 2015

Abstract

In order to investigate the fracture orientation in the Ngatamariki and Rotokawa geothermal fields and compare them to the orientation of maximum horizontal stress, we determine the shear wave splitting for 1919 earthquakes across 22 stations during 2015.
The seismic catalogue provides a large number of microearthquakes from both fields. Due to the volume of unique events (4670), shear wave phases are picked automatically. We carry out automatic shear wave splitting measurements using the Multiple Filter Automatic Splitting Technique (MFAST). The shear wave splitting measurements are interpreted in the context of the stress in the region of Rotokawa and Ngatamariki by applying principles from circular statistics.
Preliminary results indicate that the mean fast azimuth measured from most stations in Ngatamariki agree with the expected NE-SW direction of maximum horizontal stress for the region. Stations located in Rotokawa show some tendency towards mean fast azimuths in a N-S direction, however they are generally more variable, perhaps indicating a more complex relationship between maximum horizontal stress and shear wave splitting measurements within the field.