The Lake Taupo, New Zealand, earthquake swarms of 1983

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Abstract
The results of microearthquake surveys made during two earthquake swarms that occurred near Lake Taupo, New Zealand, are presented. The swarm of 1983 February was concentrated in an area of 30 km2 while that in 1983 June–July covered an area of 300 km2. Composite focal mechanisms show a predominance of strike-slip faulting with a consistent, but small thrust component. This is in sharp contrast with normal surface faulting observed at the time of the second swarm. For the February swarm the strike-slip motion is right-lateral on a northeast nodal plane and for the June-July swarm it is left-lateral on a northeast plane. The observed surface deformation is an order of magnitude larger than that expected due to seismic slip alone which, together with the gradual development of the surface faulting, suggests that the surface faulting was largely aseismic and may have been in response to subsurface deformation and earthquake shaking. No evidence is found for the swarms being caused by other than ongoing regional deformation. A coda magnitude relation suitable for the Taupo Volcanic Zone is Mc = 0.03 ‡ 0.03 + (1.00 ‡ 0.03) log2 t + (0.041 + 0.007) R.