Geological structure of the Rangitaiki Plains near Edgecumbe, New Zealand, from seismic data

Abstract

Seismic reflection profiles in the vicinity of the 1987 March 2 Edgecumbe earthquake have been used to determine the geological structure in the top 2 km under the southern Rangitaiki Plains. The greywacke basement is offset by about 1200 m on a fault plane that is estimated to have a 38° dip near the major fault associated with the earthquake at McCracken Road. The surface expression of the faulting, however, seems to be on one of a number of splay faults originating from the fault surface on the grey wacke basement. The Matahina Ignimbrite mapped on the seismic section in the western half of the plains is generally flat lying but shows several faults with offsets of up to a few metres. There are, therefore, three major faults in the Rangitaiki Plains—one on each of the margins of the plains and the basement fault in the vicinity of McCracken Road. The other faults within the basin that are seen on the seismic sections seem to have less tectonic significance and may be simply due to the relaxation of the sediments to movement on the major faults.