| Abstract |
A recent subsidence bowl that has formed in the Crown-Invergarry Road area of Taupo (Wairakei- Tauhara geothermal field) is interpreted to originate from compaction of a thin elliptical lens of ìPost Oruanui Sedimentsî at about 35 (f 10) m depth, with a minor diameter of about 160m. The local increase in rate of subsidence, which commenced about 1997, but has been more stable since March 2001, is most likely to be caused by a water level decline in the upper-most groundwater aquifer, resulting in drainage of a lens of highly compressible mudstones or embedded cavities. The most likely future scenario predicts that the duration of the anomalous subsidence rates will be another eight years, adding approximately 150% to the existing accumulated subsidence. Since 1997 this has reached a maximum of 0.25 m near the centre (RM59), causing 1.8 mad tilt at about 80 m radius, and curvatures of +0.05 mradm near RM59, and -0.035 mradm at the bowlís edge. Predicted accumulated curvatures, using reasonable future scenarios, are generally well below building code tolerances. Properties that are about lOOm to 140m from the centre of the bowl will probably experience the greatest extensional curvature effects which may cause minor distortion to rigid structures. |