Geological investigations of the 1987 Edgecumbe earthquake, New Zealand

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Seismic reflection and outcrop data from the onshore Hikurangi forearc reveal the styles and history of deformation for a c. 3000 km 2 region between Dannevirke and Hawke' s Bay. The data cover the forearc basin, including its western and eastern boundaries, and delineate folds and faults in a late Miocene-Recent sedimentary sequence. Five seismic horizons, including the basement/Neogene cover unconformity (variable age), base Waipipian (3.7 Ma), base Mangapanian (3.2 Ma), base Nukumaruan (2.6 Ma), and base Castlecliffian (1.6 Ma) were identified using outcrop and well ties. These horizons were traced across the study region to provide information on the geometry, spatial distribution, and timing of structures. To the west, the rangefront fault is predominantly reverse and separates uplifted Torlesse basement of the axial ranges from Neogene forearc basin sediments. Structures within the forearc basin are dominated by north-northeast-striking reverse faults and associated asymmetric folds which parallel the subduction margin and often have sinuous traces. Faults are planar to depths of at least 1-2 km and typically dip at 30-80°NW (most often at 40-70°). Many faults in the basin terminate northwards and fault-normal spacings decrease from 2-8 km in the Dannevirke area to c. 20 km near Hastings. Angular unconformities and syntectonic strata constrain the timing of deformation on fault/fold pairs. Faults within the forearc basin were active over two main periods, at c. ?3.1-2.5 Maandc. 1.6 Ma-Recent. Active fault traces are confined to the edges of the forearc basin. In the west, the Mohaka and Ruahine Faults have mainly recent right-lateral offsets, but interpretation of a seismic reflection line which crosses the Mohaka Fault indicates minimal (<300 m) right-lateral displacement over the last 3 m.y., and lateral slip may not significantly predate the late Quaternary. In the east, a zone of reverse faults (including the Longlands, Poukawa, Tukituki, and Oruawharo Faults) began forming ate. 1 Ma and remains active. Margin-normal shortening across the forearc basin and basin-bounding structures ranges up to 5 mm/yr (120- *

Section: Axial Rangesmentioning

confidence: 99%

Structure and deformational history of the inner forearc region, Hikurangi subduction margin, New Zealand

Beanland

Melhuish

Nicol

et al. 1998

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“…This study may provide an opportunity to test this assumption. The recurrence interval of the EdgecumbeFault is thought to be c. 700 years (Beanland et al 1989). Although this site may not be regularly observedfor the next700 years, continued observation for as long as possible will provide data on the erosion rate and patterns of modification.…”

Section: Discussionmentioning

confidence: 99%

“…These scarps are in poorly sorted colluvium and alluvium which reached an angle ofrepose of 25-30° (Wallace 1980). The scarps at Edgecombe are formed in a layered sequence of soil and volcanic ash overlying loose to compact pumice gravels (Taupo Pumice Alluvium) (Beanland et al 1989). The angle at which such free faces remain stable is largely unknown.…”

Section: Discussionmentioning

confidence: 99%

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Monitoring degradation of the Edgecumbe Fault scarp, Rangitaiki Plains, New Zealand

Fellows

Scott

1989

A portion of the Edgecumbe Fault scarp has been fenced and protected to provide a record of the major surface rupture. The site will be monitored to observe the natural degradation of the fault scarp, and it is also the sole remaining record, unmodified by humans or stock, of the surface rupture formed during the 1987 Edgecumbe earthquake.Measurements of the degradation are being made across seven profile lines perpendicular to the rupture at regular intervals. At present the monitoring results have not indicated any change in the free faces.The diffusion model has been shown by a number of workers to be applicable to scarps and is assumed to be the appropriate model for fault-scarp degradation in the Rangitaiki Plains. An assumption of this model is that the time taken for the free face of the scarp to degrade to an angle of repose is short compared with the time span over which diffusion occurs. Data obtained from monitoring of the site over a significant time period, combined with the known recurrence interval of the Edgecumbe Fault, will provide information to test this.In the long term, monitoring may also reveal details of the processes that carry the scarp to an angle of repose.

“…A historical New Zealand earthquake which resulted in triggered ground surface rupture is the 1987 M w 6.3 Edgecumbe earthquake, which ruptured five faults over an area c. 15)7 km (Beanland et al 1989). The largest displacements (52.5 m vertical) occurred on the Edgecumbe Fault, which is considered to be the primary fault rupture.…”

Section: Triggered Rupture?mentioning

confidence: 99%

Post c. 300 year rupture of the Ohariu Fault in Ohariu Valley, New Zealand

Litchfield

Dissen

Hemphill‐Haley

et al. 2010

An important component of quantifying seismic hazard and risk in regions such as Wellington is to characterise both the longterm rate of occurrence of the region's major earthquake-generating active faults, as well as potential interactions between faults (e.g., the potential for earthquake triggering). This paper describes paleoseismic data from two trenches (Ohariu Valley) and a natural streambank exposure (Horokiri valley) c. 24 km apart, which constrain the timing and size of recent surface rupture events on the northeast striking Ohariu Fault c. 6 km northwest of Wellington City. One event is recorded at all three sites and radiocarbon age constraints indicate it correlates with the 1050Á1000 cal. years BP event previously identified elsewhere along the fault. A younger, smaller (decimetre-scale) event is recorded in one of the Ohariu Valley trenches, and the timing is constrained by two radiocarbon ages to post-310 cal. years BP. This event may either be a small, primary Ohariu Fault rupture, or a triggered event associated with a large earthquake on a nearby fault. If it is a triggered rupture, then a possible mechanism is dynamic triggering associated with one of the recent large-great earthquakes on the Wellington (post-300 yr), Wairarapa (AD1855), or Awatere (AD1848) Faults. Small rupture events do not necessarily contribute to the recurrence interval classification of the Ministry for the Environment Active Fault Guidelines, but they could be important for sensitive structures crossing the fault and for lifelines crossing multiple active faults.