Evaluation of seismic hazard in the Rangitaiki Plains, New Zealand

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Abstract
Histories of movement on several active faults have been determined and used to evaluate possible future seismic hazard in the Rangitaiki Plains. The eruption of Kaharoa Ash c. 800 years B. P. provides a datum to establish the history of faulting. The Matata, Edgecumbe, Onepu, and Rotoitipakau Faults, all located in the southern and western Rangitaiki Plains, have moved at least once during the last 800 years. In contrast, faults in the eastern part of the Rangitaiki Plains do not show evidence of post-Kaharoa movement. The past level of seismic hazard has, therefore, been lower in the eastern part of the plains compared with southern and western areas. We assume that rupture of the northeast-trending normal faults in the Rangitaiki Plains will produce isoseismal patterns similar to that observed during the 1987 March 2 Edgecumbe earthquake. By overlaying the MM IX isoseismal of the Edgecumbe earthquake over each fault in the Rangitaiki Plains, and taking into account geological data on the recurrence interval of movement on each of the faults, we find that the average return period for MM IX intensity shaking varies from c. 175 years at Edgecumbe and Te Teko, c. 200 years at Kawerau, 300 years at Matata, to > 800 years at Whakatane. If the past record of fault movement is indicative of future activity, the future seismic hazard is higher in the west than in the east of the plains. The occurrence of moderate-magnitude earthquakes of M 6–6. 5 on the northeast-trending normal faults presents a major seismic hazard in the area of the Rangitaiki Plains and raises the level of seismic hazard previously estimated from more regional studies.