The Edgecumbe Earthquake of 1987

The magnitude 6 earthquake which struck the Bay of Plenty area on the 2nd of March 1987 is remembered as the “Edgecumbe Earthquake” even though it was centred close to the coastal town of Matata. The damage it caused was most severe in Edgecumbe and nearly as severe in Kawerau, Matata, Thornton and Te Teko, with surface ruptures spreading across the Rangitaiki Plains.

During the preceding month, swarms of earthquakes had unnerved the residents of Kawerau and Edgecumbe and distinct foreshocks were felt at Matata and Thornton prior to the main earthquake.

Monday March 2nd 1987 was a typical early autumn business day throughout the Bay of Plenty region. Schools, offices, factories and other establishments were conducting business as usual, despite the swarms of small earthquakes which had been felt at Edgecumbe and Kawerau during February.

Just after lunch for many workers, a sharp jolt of magnitude 4.9 struck the area at about 1:35 p.m. This shallow foreshock caused a power cut and alarmed several people, causing the evacuation of some premises, seemingly to allow weak or poorly constructed buildings to be checked before they were re-occupied.

Seven minutes later a strong 6th magnitude earthquake struck near the coastal town of Matata at a depth of 8 km. This was the main shock in the sequence and has been variously reported as magnitude 6.1 [Geonet] and 6.3 [“Awesome Forces”, and “Rocked and Ruptured”]. The magnitude of the quake was moderate by New Zealand standards, but its very shallow depth meant that it was strongly felt and caused considerable damage nearby.

“Rocked and Ruptured” describes the sequence of events thus: “The rupture began nearly 10 km down, travelled to the surface, and produced at least eleven ruptures over an area 16 km by 10 km. They are all normal faults and most were associated with existing faults, ground fissures, and surface warping. The largest rupture was a metre wide and snaked northeast for about 7 km.”

As the ground shook in response to the ruptures, pockets of land liquefied, sand fountained into the air under pressure, the Rangitaiki River flowed backwards [“Awesome Forces”] and man-made structures shook violently.

The main fault scarp ruptured the surface for 7 km crossing McCracken Road where a gap of 1.5 metres opened up as the land on one side dropped with respect to the other. At its highest, the vertical wall of the scarp was 2.4 metres high.

In Kawerau one of the massive paper-making machines at the Tasman Pulp and Paper Mill was jolted from its mounting, the bridge over the Whakatane River partly subsided and water and sewerage systems in Whakatane were badly damaged as pipes were broken. The milk in the huge tanks at the Bay Milk Factory sloshed back and forth, causing the tanks to rupture and collapse in a spectacular fashion.

In Edgecumbe, an 80 tonne DC-class locomotive was toppled from the railway tracks as they were twisted and torn apart by the shuddering ground. Twenty tonne power transformers were torn from their mountings as the ground moved underneath them creating more difficulty in restoring power to the area.

The ground shaking cracked roads and footpaths, damaged river embankments, caused numerous landslides and threw the contents of cupboards to the floor as houses shook, chimneys collapsed and some buildings moved from their foundations. Benchtop appliances were catapaulted to the floor, large appliances such as fridges wobbled drunkenly across floors as if there was someone inside, and water pipes burst showering people and house contents with hot and cold water.

The population had a few minutes to take stock before the first of over 200 aftershocks shook the region. Just ten minutes after the main quake another very shallow earthquake of magnitude 5.5 compounded the damage and caused further landslides which blocked many roads. Clouds of dust which were seen to rise from Whale Island off the coast near Whakatane were thankfully not caused by a volcanic eruption, but by the collapse of part of the coastal cliffs.

Despite having experienced three very shallow earthquakes with magnitudes between 4.9 and 6.3 in the space of 17 minutes, no-one was killed although several were injured. Damage was severe near the earthquake epicentre but mainly confined to a 20 km radius of the epicentre [“Rocked and Ruptured”]. Damage was estimated at $300 million [“Rocked and Ruptured”] and the Earthquake Commission paid out nearly $136 million for houses and small businesses [“Awesome Forces.”]

A lingering legacy of the earthquake is the changed drainage resulting from the western side of the fault scarp having dropped part of the Ragitaiki Plains by 2 metres.

The frightening experience lives on through the simulation of one of the aftershocks in the Earthquake House at Te Papa in Wellington.

[Compiled from correspondence and eyewitness accounts, and data from the following publications:
Geonet media release 27 February 2007,
“Awesome Forces” Geoff Hicks and Hamish Campbell, Te Papa Press, 1999; and
“Rocked and Ruptured” Jefley J. Aitken, Institute of Geological and Nuclear Sciences Ltd., 1999.]

One Response to “The Edgecumbe Earthquake of 1987”

  1. Ratu says:

    This Is Very Helpful Thank You

Leave a Reply