Faulting and subsidence during the Edgecumbe earthquake, March 2, 1987, New Zealand.

Abstract

A M6.3 earthquake occurred on March 2nd 1987 beneath the Rangitaiki Plain at the NE end of the Central Volcanic Zone, North Island, New Zealand. Surface faulting and regional subsidence of up to 2 m are consistent with shallow depth (about 12 km) extensional normal faulting. The most prominent rupture developed along a previously unrecognized active fault trace over a distance of 7km with maximum vertical displacement of 1.5 m and extension of about 1.2 m. The vertical movement almost doubled the height of the pre-earthquake scarp. Compressional features developed on either side of the main rupture and many sand fountains were erupted over the area of regional subsidence where the water table was near the surface. Continuity of stratigraphy across the main fault which contains tephra layers and pumice alluvium indicates that possibly two faulting events took place prior to 1.8 Ky B.P. with the same sense of movement as the March 2nd earthquake. Buried tephras and 6.5 Ky shells indicate an average subsidence rate of about 2 mm/yr over this period for the same area that underwent subsidence during the latest event. The effects of the March 2nd earthquake and previous earthquakes are consistent with the extension that is taking place across the Central Volcanic Zone.