| Title | SEISMOLOGICAL STUDIES OF GEOTHERMAL SYSTEMS IN THE TAUPŌ VOLCANIC ZONE |
|---|---|
| Authors | M. Savage, J. Townend, S. Sewell, C. Hopp, S. Mroczek, F. Civilini, B. Keats |
| Year | 2019 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Microseismicity, Anisotropy, Brittle-ductile transition, Ambient noise, Temporal variations |
| Abstract | In the last decade, in partnership with Mercury NZ Limited (formerly Mighty River Power), Victoria University of Wellington postgraduate students have used seismic data from New Zealand’s geothermal fields to elucidate the structure and geomechanical properties of several geothermal systems in the Taupō Volcanic Zone. Recent projects have focussed on the Rotokawa and Ngatamariki fields, but earlier studies looked at Kawerau. Double difference earthquake locations reveal that under Kawerau the majority of seismicity is shallower (4 km maximum depth) than in the surrounding regions (6-7 km). This suggests that the major heat source is situated beneath the centre of the field and that at levels deeper than 4 km the ground is too hot to support brittle failure. Between 2012 and 2015, matched filter methods doubled the number of earthquakes detected in Rotokawa and Ngatamariki to about 9000. The focal mechanisms of almost 1000 of those events were inverted to determine the prevailing stress field. At Ngatamariki, the new locations exhibit a strong spatio-temporal association with borehole drilling and stimulation. Areas of high b-values (a measure of the ratio of the numbers of small to large earthquakes) correspond to areas of elevated pore fluid pressure and a broad distribution of fractures. Focal mechanism inversion yields a normal faulting stress state with a NW/SE axis of minimum compressive stress (S3), but distinct regions vary from the average. Shear wave splitting also yields NE/SW fast directions, consistent with the presence of stress-aligned cracks and NW/SE extension. Changes in delay time and Vp/Vs ratios correlate with changing volumes of production. Changes in isotropic shear velocity were measured from stacking noise cross-correlations. There were faster velocities in regions of injection, and gradual increases in shear-wave velocity from 0.06% to 0.08% over a year, but it remains difficult to distinguish the effects of rainfall changes and production. We observed abrupt decreases in velocity by as much as 0.07% in Ngatamariki immediately after regional and local earthquakes, which returned to normal over a period of several weeks. |