Principal component analysis and modeling of the subsidence of the shoreline of Lake Taupo, New Zealand, 1983–1999: Evidence for dewatering of a magmatic intrusion?

Principal component analysis was applied to a set of relative water level measurements made at 22 sites around Lake Taupo, New Zealand, in 37 surveys during 1986–1996. Only a single mode was significantly above noise levels. This mode showed a subsidence of the lake shore that decayed exponentially with time, with a time constant of about 12 years. The mode was well modeled by a Mogi point dilatation at a depth of 8 ± 1 km located beneath the point with the greatest rate of subsidence, which was about 8.5 ± 1 mm yr−1, in the center of the northern lake shore. The model implies that the source contracted by 0.02 ± 0.002 km3 during 1986–1996. Three model parameters, the time constant, the depth, and the volume contraction, place constraints on the physical process or processes that caused the contraction and resulting lake shore subsidence. We infer that magma was intruded into the crust beneath the north shore of the lake at some time prior to the start of the lake shore observations in 1979 and that the contraction was due to water leaving the melt because of decompression. For this to work, the water must diffuse quickly away from the source, and we accordingly infer that the permeability of the overlying crust must be 10−15 m2 or greater. To match the observed source contraction, and assuming a 1% by weight fluid loss, the magma would have to have contained about 2.5 km3 or more of melt.