Geological investigations of the 1987 Edgecumbe earthquake, New Zealand

The Edgecumbe earthquake, 1987 March 2 ( M L 6. 3), was associated with renewed tectonic rupture on the Edgecumbe Fault, renewed movement on the Onepu and Rotoitipakau Faults, and several new surface breaks—the Awaiti, Otakiri, Te Teko, and Omeheu Faults. These northeast trending tectonic ruptures are widely distributed across the Rangitaiki Plains, range in length from 0. 5 to 7 km, and are mostly downthrown to the northwest. They are associated with warping and prominent fissuring. Maximum displacement, 2. 5 m vertical and 1. 8 m extensional, occurred near the middle of the Edgecumbe Fault trace. Trenching investigations revealed that perhaps two faulting events have occurred in the past 1850 years in addition to the 1987 event; the earlier one is unsubstantiated, but may have occurred at about 1850 years B. P., and another occurred at about 800 years B. P. (the time of deposition of the Kaharoa Ash). The 800 year B. P. event was associated with warping, but little fissuring. An average slip vector, derived mainly from laterally offset cultural features, trends 330°, plunges 55°, and represents up to 3. 1 m of normal fault slip on a plane of average strike about 055°. Our proposed fault model has a 55° dipping plane curving upwards to become almost vertical within 10 m of the ground surface. This 55° dip is probably representative to at least 100 m depth. The tectonic effects of the earthquake were influenced by the soft, wet sediments forming the Rangitaiki Plains and the relatively shallow hypocentre, but were otherwise typical of normal faulting events expected in the Taupo Volcanic Zone.