Geodetic measurement of deformation in the Taupo Volcanic Zone, New Zealand: the North Taupo Network revisited

A previous geodetic estimate of 18 mm/yr horizontal extension for the Taupo Volcanic Zone (TVZ) immediately north of Lake Taupo for the period 1949-86 is re-examined for several reasons: this rate has not been confirmed by GP S surveys in the 1990s; newly compiled precise levelling data now allow us to estimate the extent of non-tectonic deformation attributable to the Wair akei geothermal field; and the precise levelling and lake-levelling data re veal a spatial variation in tectonic subsidence that casts doubt on the ear lier assumption of homogeneous horizontal strain. We use the vertical and h orizontal data to derive a Mogi point source model for the geothermal field , and this model allows us to correct the observed horizontal velocities of survey points. Statistical analysis of the corrected horizontal velocities shows that the strain across the TVZ is not homogeneous. When these factor s are accounted for, an extension rate of 8 +/- 2 mm/yr(1 SE) can be applic able for both 1949-86 and 1986-97. This is about half the previous estimate , which we now consider to be incorrect. The distribution of deformation di ffers between these periods, and the seismicity of the region shows tempora l variations on a similar time-scale (decades). The extension rate is much greater than can be accounted for by seismic strain release, and the occurr ence of historical earthquakes up to M = 6 indicates that a significant par t of the measured extension represents seismic strain accumulation. The spa tial heterogeneity of the strain partitions the region identically to that derived from geological studies of fault activity. In particular, there is a spatial concentration of extension and tilt about the Whangamata fault sy stem.