NORMAL FAULTING IN A BACK ARC BASIN - GEOLOGICAL AND GEODETIC CHARACTERISTICS OF THE 1987 EDGECUMBE EARTHQUAKE, NEW-ZEALAND

The ML6.3 Edgecumbe earthquake was associated with a 7-km surface rupture of the NE striking Edgecumbe fault and 10 secondary fault ruptures. Average net slip of the main fault at the ground surface was 1.7 m with an azimuth of N30°W. Regional subsidence of up to 2 m affected a large part of the Rangitaiki Plains and the Whakatane graben widened by about 0.7 m in a NW-SE direction. Dislocation modelling suggests the fault has a dip of about 40° and extends to more than 6 km depth with 2.7 m normal slip. The slip was large for an earthquake of this magnitude in comparison with other normal faulting events. There is preliminary evidence for both regional postseismic deformation and continued fault slip. The Edgecumbe earthquake illustrates active extension of the Taupo volcanic zone, a back arc basin associated with the Hikurangi subduction zone beneath the North Island of New Zealand. The faulting was in many ways typical of worldwide historical normal faulting earthquakes and suggests that back arc spreading, where onshore, is accommodated by tectonic processes that are similar to those in other extensional environments. -from Authors