Many of the speakers at the latest 2017 NZSOLD Workshop referred to the Matahina Dam and the effects of the Edgecombe earthquake on the structure as an important case study in their line of research.
CASE STUDY: THE EFFECTS OF THE 1987 EDGECOMBE EARTHQUAKE ON THE MATAHINA DAM
On the afternoon of 2 March 1987 at 1.35 p.m, a magnitude 5.2 earthquake rocked the Bay of Plenty region. Minutes later, at 1:42pm, a much stronger 6.3 magnitude quake centred between Matata and Edgecumbe struck.
The Edgecumbe earthquake was the first in New Zealand since the 1968 Īnangahua quake to cause major damage. At a shallow depth of just 8 kilometres, 50% of the towns housing was effected and several dozen people were seriously injured.
The most visually striking effect of the earthquake was a 7 kilometre long rift, that opened up in the Rangitaiki Plains, now known as the Edgecumbe Fault. And of major concern was the large earthfill hydro dam at Matahina on the Rangitāiki River.
The Matahina earth dam and hydroelectric power scheme on the Rangitaiki River, was commissioned in 1967. The largest of its type in the North Island it stands a mighty 86 metres high set in this known tectonically active part of the country.
The powerful force of the Edgecombe earthquakes had caused damage to the Matahina dam, including minor cracks that caused leakage at its concrete abutments.
As a preventative measure at the time, floodgates were opened to lower dam lake levels, with a controlled flow to ensure the river would not spill over its stopbanks downstream.
Below the dam, residents of the small township of Te Mahoe were evacuated as a precaution.
The Matahina dam was repaired in 1988, and subsequently reassessed by GNS to find out more about the seismic hazards posed to the dam and its reservoir.
Assessment included geological and fault mapping, gravity profiling, and seismic reflection profiling, revealing that there have been four earthquakes of magnitude 7.2 or greater, at or near the dam site, in the past 11,500 years. Also, that the Waiohaou fault zone was active and closer to the dam than previously thought.
Based on these assessments, a US$40m strengthening project to upgrade the Matahina dam and hydro facility to meet international safety and performance standards was carried out.