Earthquakes, active fault displacement and associated vertical deformation near Lake Taupo, Taupo Volcanic Zone

15 refs; 17 figs; 1 table; 7 plates

Two swarms of earthquakes affected the northern Lake Taupo area in 1983. The first in the Kinloch-Whangamata area lasted for six weeks in February/March and was monitored for four days by seismographs installed by Seismological Observatory. Over 30 tremors per day were recorded and minor damage resulted on some farms. Tilt-levelling patterns and lake level stations, part of a regional earth deformation network monitored by Geological Survey, showed uplift of the whole north Taupo region during and following the earthquake swarm. In mid June a second swarm started between Whakaipo and Kaiapo Bays further to the east and on 22-23 June the ground cracked over a distance of 1200 m along the Kaiapo Fault, 4 km west of Taupo. The displacement observed by ground surveys and geodetic relevelling was 50 mm down to the west with about 30 mm of extension. Earthquakes ceased after a further two weeks, during which time seismographs were again deployed. The fault movement is much less than the last recorded movement of 1 m observed in the more severe earthquakes of 1922, when the north side of Lake Taupo dropped by as much as 3 m. Nor were the earthquakes as numerous or as intense as those recorded in the last swarm of 1964-65, when no fault displacement was observed. However, valuable information was obtained about the nature of the faulting at depths of 1-6 km on the fault planes. The Geological Survey earth deformation monitoring network also passed its first major test after almost a decade of observations. These swarms appear to be regular occurrences in the Taupo Volcanic Zone and probably reflect the widening of the zone (averaging 5-10 mm/year) and emplacement of magma at deeper levels. Regular level and tilt observations combined with a network of seismographic stations appear to offer the best chance of predicting volcanic eruptions in the Taupo Volcanic Zone. (auths)