Weight of water set off quakes, suggests expert

By AINSLEY THOMSON

The weight of floodwater in the eastern Bay of Plenty is believed to have triggered the swarm of earthquakes that have rocked the region.

Since Sunday the area has experienced hundreds of earthquakes. They began after more than 250mm of rain fell in 48 hours.

The storm killed two women and caused thousands of people to be evacuated from their homes.

Institute of Geological and Nuclear Sciences duty seismologist Martin Reyners said the weight of the floodwater - estimated at up to a billion tonnes - could have triggered the quakes.

Dr Reyners said the crust in the region was already stretched and the weight of the water would have placed an extra load on it, bringing on the earthquakes.

"If you put on a load, like the load of the floodwater, you can actually destabilise the seismic regime, and if an earthquake is about to happen, it can bring it forward in time."

This type of activity is called induced seismicity.

Dr Reyners said the region was tectonically stretching by about 10mm a year.

There are faultlines to the east and west of the area that was flooded, but only ones on the western side were triggered.

The worst of the quakes measured 5.4 on the Richter scale and was centred a few kilometres north of Lakes Rotoehu and Rotoma, on the edge of the large Okataina volcanic crater which contains the Rotorua lakes and last blew in the devastating Tarawera eruption of 1886.

Yesterday, a quake of 4.4 was felt around the same area.

Dr Reyners said there had been hundreds of quakes, but most were too small to be felt by humans.

"It is a significant earthquake sequence and we will definitely look at the relationship between the floods and earthquakes in more detail."

Civil Defence said the state of emergency in Whakatane and Opotiki districts would remain in force until at least tomorrow.

Whakatane District Council spokeswoman Mary Hermanson said several rural communities in the eastern Bay of Plenty were still completely isolated and had food flown in yesterday. Some will not have road access restored for at least a month.

Three people with chronic medical conditions were airlifted from Ruatahuna yesterday as a precaution.

In Whakatane, flooded areas were drained and residents allowed to secure their homes.

There were still 100 houses in the suburb of Awatapu that were affected by flooding - meaning about 250 people could not go home last night.

Mrs Hermanson said several hundred residents of rural Edgecumbe and Thornton on the Rangitaiki Plain probably would not be able to return until the weekend.

The level of the Rangitaiki River was dropping and Civil Defence staff were having success channelling floodwaters back into the river.

But a temporary road across the flood plain to reach a major breach in the stopbank would take several days to complete.

- Additional reporting NZPA