The legacy of the 1987 Edgecumbe earthquake, New Zealand

A series of six earthquakes of Mw 5.0 to 6.5 struck the Bay of Plenty, New Zealand, on 2nd March 1987 creating a surface rupture several kilometres long across the Rangitaiki plains, crossing the Rangitaiki River and causing up to two metres of ground subsidence. Fifty percent of the houses in Edgecumbe, a major earth dam and flood protection structures were severely damaged with the standard of flood protection reduced from 100-year to a 20-year return period level. Two concrete flood protection walls subsided 0.6m to 0.8m. These were subsequently replaced/heightened without rectifying foundation damage. One of these walls failed during a flood in April 2017, flooding Edgecumbe and causing severe damage. The failure has been attributed to the foundation damage from the earthquake 30 years earlier. Earthquake damage to Matahina Dam was mainly settlement of rock fill and downstream displacement of the structure, resulting in increased seepage, but internal damage was revealed during investigations and was subsequently repaired.