At 1.35 p.m. on 2 March 1987, a magnitude 5.2 earthquake struck the Bay of Plenty region, cutting power and sending many people outdoors. Minutes later a much stronger quake rocked the region. This main shock, at 1.42 p.m., had a magnitude of 6.3 and was centred north of Edgecumbe. Four aftershocks with magnitudes greater than 5 occurred in the next six hours, and smaller aftershocks were felt for weeks.
The Edgecumbe earthquake was the first since the 1968 Īnangahua quake to cause major damage. Although not of an exceptional magnitude, it was damaging because it was very shallow. No one was killed, but several dozen people suffered serious injuries. One woman was hurt by a falling piano in her home, and another was hit by a bull thrown out of its pen at a stock yard.
Industrial sites were badly affected. At the Tasman Pulp and Paper Mill in Kawerau a loaded logging truck toppled onto its side. Mill workers escaped from falling debris through a maze of twisted stairs and walkways. One man was thrown over the rail of a catwalk to the concrete floor 3 metres below. Another was trapped under 11 huge circular saws, each of them 2 metres in diameter. Following the main shock, an engineer checking for structural damage fell 5 metres from a platform.
At Bay Milk Products in Edgecumbe, huge stainless steel milk silos collapsed, spilling thousands of litres of milk. Two milk tankers were thrown on their sides. At the N.Z. Distillery Company, tanks of spirits collapsed, saturating the ground with vodka and gin.
In Kawerau 40 houses were evacuated because of an unstable hill above them. A major worry was the large earthfill hydro dam at Matahina on the Rangitāiki River. Staff found minor cracks in the roadway and concrete abutments. They opened the floodgates to lower the lake level, but controlled the flow to ensure the river would not overtop its stopbanks downstream. Below the dam, the residents of Te Mahoe were evacuated as a precaution. The dam was repaired in 1988, and subsequently strengthened in the late 1990s to withstand earthquake motion.
Some roads cracked or acquired ‘judder bars’ as the ground buckled. Railway tracks were twisted and bent, and a diesel-electric locomotive toppled over.
In the hours after the Edgecumbe earthquake, accounts of the disaster became wildly exaggerated. The tremors produced a massive dust cloud on Moutohorā (Whale Island), an extinct volcano off the coast. Soon there were ‘eyewitness reports’ that the island had erupted. A Singapore newspaper quoted ‘the Police’ as saying that 95% of homes in the Kawerau, Whakatāne and Edgecumbe areas were uninhabitable. In the United States, Washington radio stations reported that 400,000 people had been made homeless. The New Zealand Embassy in Washington was inundated with calls from worried people seeking news.
The Edgecumbe Fault
The most spectacular effect of the Edgecumbe earthquake was the 7-kilometre-long rift that appeared across the Rangitāiki Plains – the Edgecumbe Fault. A fissure up to 3 metres wide and 3–4 metres deep opened up along much of the fault, although some sections were marked just by zones of cracks. A woman who had been picking fruit was thrown from a ladder by the quake. Soon after, she was driving hurriedly down a road to check her home when her car became airborne and flew 6 metres across the rift, landing at the bottom of the scarp.
The earthquake had been caused by movement along the fault; the land to the north-west had dropped by up to 2 metres. The region which sank downward is now more prone to flooding.