Have we just dodged a big earthquake?

BY MICHAEL FOX

Wellington may just have dodged a devastating earthquake.

Scientists estimate that an average of one magnitude-7 quake will strike New Zealand every 10 years and a magnitude 8 once a century. A 7.8 magnitude quake hit Fiordland in July.

If the same quake had struck in a more populated area such as Wellington, its impact could well have resulted in thousands of deaths.

GNS seismologist John Ristau says New Zealand has had a "dream run" for the past five decades, with "a remarkably low number of large damaging quakes for a country that has similar earthquake activity to California".

However, there is no guarantee the dream run will continue. In the past three weeks, Samoa and American Samoa, as well as parts of Tonga, have been ravaged by an earthquake-generated tsunami and two earthquakes devastated the Indonesian island of Sumatra.

Vanuatu was rocked by repeated quakes, including three measuring 7.6, 7.8 and 7.3 last week, and in late August a 5.2 quake rattled Wellington.

More than 150 people, including eight New Zealanders, died in Samoa, Tonga and American Samoa after the tsunami. A total of 310 people were injured in Samoa as entire villages were wiped out.

In Sumatra, the magnitude 7.6 Padang earthquake, which struck on September 30, claimed more than 1100 lives and injured 1200. About 300 people are missing and more than 135,000 homes and building were destroyed.

The disasters "remind us that Asia-Pacific is the world's disaster hot spot. A person living in Asia-Pacific is four times more likely to be affected by natural disasters than someone living in Africa and 25 times more likely than someone living in Europe or North America", said Noeleen Heyzer, executive director of the United Nations Economic and Social Commission for Asia and the Pacific.

Three-quarters of the world's earthquakes happen around the Pacific Rim, a 40,000-kilometre string of geological activity stretching from Indonesia to Chile where several plates, known as the Pacific Ring of Fire, collide.

However, Victoria University Earth Sciences programme director Tim Stern says he is not so sure the recent spate of activity is unique, saying there have been similar active periods in the past.

Between 1960 and 1964 the eastern side of the Pacific experienced two of the biggest tremors in known history: the magnitude-9.5 Valdivia earthquake, or Great Chilean Earthquake, in 1960 – the most powerful yet recorded – and the magnitude 9.2 Alaska earthquake in 1964.

GNS figures also show that, between 1929 and 1934, New Zealand was hit by five major earthquakes of magnitude 7 or more – 10 times greater than the long-term average for similar-sized quakes.

The spate included the magnitude 7.8 Murchison quake in 1929 that killed 17 people and hundreds of farm animals and caused slips and floods, the magnitude 7.8 Napier earthquake that killed 256 people in 1931, and the magnitude 7.6 Pahiatua quake in 1934 that caused widespread property damage.

"I wouldn't be so sure to say that this is unprecedented," Professor Stern says.

New research suggests the recent activity may also be linked. A study by the Carnegie Institution in Washington says a big earthquake could potentially cause other earthquakes thousands of kilometres away.

Seismologists looked into a 1992 California earthquake and the 2004 Sumatra quake that triggered the Boxing Day tsunami, and investigated their impact on the San Andreas fault in California.

The study, published in the journal Nature, said scientists detected an increase in small earthquakes along the fault, suggesting it was possible that the strength of faults and earthquakes was affected by seismic events thousands of kilometres away. The team said the research could help seismologists eventually predict when earthquakes could happen.

However, scientific opinion over this is split. GNS Science seismologist Warwick Smith says he thinks the recent quakes were geographically too far apart to be related.

"We know quite a lot about how, when one large earthquake occurs, relieving stress in the Earth, it can either relieve or increase stress on adjoining faults, depending on the geometry.

"I don't believe these stresses can be transmitted over the thousands of kilometres between such places as Samoa, Vanuatu and Indonesia. So I regard these as random coincidences, all evidence of the very active plate margins they are on."

Experts say a major disaster at some point is inevitable in New Zealand, as one of the world's largest faultlines, the 600km-long Alpine Fault, runs through the South Island, and Lake Taupo is one of the world's biggest volcanos.

According to GNS, the Alpine Fault – part of the boundary of the Pacific and Australian plates – has ruptured four times in the past 900 years, the last time being about 1717. Each time it has produced an earthquake of about magnitude 8.

Some suggest the lack of any major disasters in New Zealand in recent memory has led to apathy about disaster warnings, starkly illustrated by reports of Kiwis heading to the beach to watch the tsunami arrive after Civil Defence's alert last month.

Otago University senior geology lecturer Michael Palin says scientists can prepare warning systems all they want, but there is a psychological aspect involved in getting people to respond to warnings.

"It's a very dicey thing because, if you get too many warnings and nothing comes of it, people end up ignoring them and, if the warnings are made in not the right style, then things like going down to the beach to watch the tsunami come in kind of become the norm. It's a very difficult business."

Researchers from GNS and Niwa have been to American Samoa to study the impact of the tsunami on buildings and infrastructure there. The team spent five days exploring how different types of construction handled the huge waves.

In earthquake-prone Japan, all buildings must meet earthquake-safe building codes. This includes the instalment of devices that automatically shut off gas at the first tremor.

Demand for quake-resistant technology has been high since the deadly 1995 quake in Kobe that killed 6434 people, caused a highway to collapse and buildings to topple.

In China, after a magnitude 7.8 quake in Tangshan province took 250,000 lives in 1976, focus turned to improving the earthquake resistance of the country's buildings.

During the 7.9 magnitude Sichuan earthquake in May last year, apartments and buildings collapsed, leading to 70,000 being killed and five million left homeless.

In New Zealand, design standards for earthquake loadings on buildings were first introduced in 1935 after the 1931 Napier quake.

Big Earthquakes in our region this year (above magnitude 6.5)

January 3: Near north coast of Papua New Guinea, 5 deaths, 250 injured. 7.6 (magnitude)

January 15: SE of Loyalty Islands (near New Caledonia). 6.7

February 11: Kepulauan Talaud, Indonesia, at least 64 injured. 7.2

February 18: Kermadec Islands. 6.9

March 19: Tonga region, small tsunami. 7.6

June 23: New Ireland region, PNG. 6.7

July 15: West coast NZ, small tsunami. 7.8

August 10: Santa Cruz islands, Solomon Islands. 6.6

August 16: Kepulauan Mentawai region, Indonesia, 9 injured 6.7

August 28: Banda Sea. 6.9

August 30: Samoa islands. 6.6

September 2: Java, Indonesia, at least 72 killed, many injured. 7.0

September 30: Samoa islands, large local tsunami, at least 150 killed (8 New Zealanders) 8.1

September 30: Southern Sumatra, Indonesia. Thousands feared dead 7.6

October 1: Southern Sumatra 6.6

- Source: US Geological Survey

 

- © Fairfax NZ News