Regional horizontal deformation associated with the March 2, 1987, Edgecumbe Earthquake, New Zealand

Abstract

Following the March 2 1987 Edgecumbe earthquake (magnitude ML 6.3) angle and distance measurements were made from 4 previously observed survey stations to a further 10 stations to determine the regional horizontal deformation associated with the event. The surface faulting associated with the earthquake was largely confined to the Rangitaiki Plain, a low lying flood plain on which four of the survey stations are located. These stations show large relative displacements which imply local compression of at least 100ppm. The stations off the Plain show a more consistent pattern of behaviour which may be characterised as the stations to the NW of the Plain having moved about 0.7m in direction N20°W with respect to the stations to the SE. The displacements can be modelled by a 28km long by 2.3km wide rectangular fault striking at N40°E, with a 39°dip to the NW and with the top at depth 3km, on which 1.4m right‐lateral strike‐slip and 5.4m normal dip‐slip movement occurred. Though the model fault plane is unrealistically narrow, its extrapolation is consistent with the hypocentre location, aftershock distribution, and focal mechanism observed for the event.