Continuous swarm-like seismicity: the Wanganui, New Zealand, earthquakes

During the last 40 years of seismic observation earthquakes have occurred persistently near Wanganui in the western part of the North Island, displaying continuous swarm-like seismicity. Altogether 445 shallow earthquakes of magnitude above 3.5 have been found for the period 1940–1980 and only three shocks reached magnitude slightly above 5. A large majority of small shocks are located within the compact central zone and their composite focal mechanisms seem to indicate extensional normal faulting. Fewer but larger events are scattered outside the central zone and the mechanism of the three largest shocks implies compressional thrust faulting. A continuous decrease in source size and increase in stress drop, as well as a longterm decreasing tendency in b-values and increasing tendency in the rate of occurrence, are observed. Small earthquakes in the central zone might be considered as a response to local stresses, probably of thermal origin, and the larger shocks as a response to stress redistribution in the central zone and to stresses resulting from subduction processes at the boundary between the Indian and Pacific plates. The time variations of the source parameters and b-values might imply that stress accumulation on a large scale is in progress. In this case the Wanganui activity would be situated at the northern limit of the seismic gap.