POSSIBLE SOURCE MODELS FOR THE 1855 WAIRARAPA EARTHQUAKE, NEW-ZEALAND

The magnitude 8 Wairarapa, New Zealand, earthquake of 1855 was associated with surface rupture along the Wairarapa fault and regional uplift of the southwest of the North Island. Forward clastic dislocation modelling shows that movement on a steeply dipping Wairarapa fault alone cannot account for the recorded deformation data. Modelling of movement on the subduction interface that underlies the Wellington region as well as the Wairarapa fault also fails to produce a satisfactory fit to the data. Although a complex Wairarapa fault model may be able to explain the deformation pattern if its location. subsurface geometry, and slip distribution could be independently constrained, the best effort supported by available data, a flexed model incorporating a left side step of 8 km at the surface, incorrectly locates the deformation. The best fit to the data is obtained from a listric Wairarapa fault model involving rupture on 0 to 50 km width of the deeper part of the subduction interface. The shallower part of the subduction interface, east of the Wairarapa fault, apparently did not rupture in 1855, and the uplift mechanism for the overlying Aorangi Range remains unexplained. Partitioning of strike-slip and dip-slip components of the relative plate motions may involve separate earthquakes. Seismological verification of listric fault rupture mechanisms is required to determine the plausibility of the listric model presented here, because its implications are that the 1855 earthquake did not completely account for thc relative plate motion in the region.