What can NZ learn from Japan about earthquakes?

Quake damage in Christchurch.
SIMON BAKER

Quake damage in Christchurch.

The Japanese Government is so certain of an impending earthquake that it has already given it a name - the Tokai Earthquake. Paul Gorman compares how tectonic forces have shaped New Zealand and Japan, and looks at how the Japanese use science to minimise their impact.

The repeating two- notes up, one-note down xylophone jingle calls out in the dark but it doesn't register. The iPad sounds again. This time I'm vaguely aware of the sound and am starting to become conscious when it rings for the third time.

I fumble for my glasses and paddle with my hand to find the device. It goes a fourth time.

"Earthquake is coming!" all four messages on the screen read. "Intensity: 2," is the warning. "In 0 seconds," the last one says.

I scroll up to find the first alarm which I slept through. At that stage it was 28 seconds away.

I am using Yurekuru, one of the earthquake early- warning apps which millions of Japanese have downloaded to give them seconds, sometimes tens of seconds, or a minute's notice of an approaching quake.

As I'm already back in Christchurch, this warning of a magnitude 5.5 tremor off the east coast of Japan early one morning is of little relevance to me. But it would have been for people living in northeastern parts of the island of Honshu.

NZ ONLY IN THE B-LEAGUE

In New Zealand we are acutely aware of our location on the Pacific Ring of Fire and what that means in terms of earthquakes and volcanoes. But we are really only in the B-league of tectonic activity compared with the A-leaguers - Japan and Indonesia.

Here we have one major plate boundary to worry about - the margin where the Pacific and Australian plates meet offshore of the North Island's east coast then runs through the South Island as, largely, the Alpine Fault. We have several active volcanoes and a slumbering super- volcano underneath Lake Taupo too.

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Japan, however, sits at a much more complex junction of plates, with several intersecting boundaries underneath or near the coast. It also has more than 100 active volcanoes to worry about.

There are similarities though in the tectonic setting. The plates subducting below the main land mass of both countries provide a deep marine trench not that far offshore which is capable of producing mega-thrust quakes, which in turn generate highly damaging tsunami.

MEGA-THRUST QUAKES

Japan had its mega-thrust quake in March 2011, when a magnitude 9.0 subduction zone event - the most powerful known to have hit the country and the world's fourth largest since modern records began in 1900 - produced a one-in-1000 year tsunami that killed 15,900 people.

The country is now waiting for a large quake and tsunami from what is believed to be an imminent rupture in the Nankai Trough, off the south coast near Nagoya and Mt Fuji. In fact, the Japanese Government has already named this future event the Tokai Earthquake.

In New Zealand, the section of the Hikurangi Trough running north off the east coast of the Wairarapa to offshore of East Cape is also known to have generated a great subduction earthquake of magnitude 9.0 or higher, followed by tsunami, about 7100 years ago.

Researchers have been able to put an approximate year on this event by finding and dating distinctive tsunami deposits in lagoons at several sites along the North Island's east coast and the northeast coast of the South Island. GNS Science principal scientist and geophysicist Stuart Henrys says there is also historic evidence for other subduction quakes along the margin, in which small parts of the plate interface ruptured, generating quakes of around magnitude 8.0.

"The northern Hikurangi margin, from Hawke Bay north, has been compared to the Japan trench, the site of the 2011 earthquake. Here there are more seamounts being subducted, for example.

"However, it is not a one- to-one comparison. The trench is deeper in Japan than offshore Gisborne, and in Japan the Pacific Plate being subducted is thinner than the Hikurangi Plateau being subducted along the North Island.

"The southern Hikurangi margin is more similar to Nankai, where the Philippines Sea Plate is being subducted. The trench-filled sediments and fault structures offshore Nankai look very similar to offshore Wairarapa."

OUR SUBDUCTION ZONE

A major New Zealand, Japan and United States research programme is now under way east of the North Island to learn more about our subduction zone.

Henrys says he is also collaborating with the Earthquake Research Institute at the University of Tokyo and also Kyoto University's Disaster Prevention Research Institute and Tohoku University's International Research Institute of Disaster Science (IRIDeS), set up after the March 2011 Great East Japan Earthquake.

In May last year, 32 underwater seismometers were deployed in Poverty Bay, between 25km and 100km east of Gisborne and at depths from 100m down to 4000m. The data they record can only be downloaded and analysed once the instruments are retrieved at the end of June.

Back in Japan, there are still a few scientists working in the fraught field of quake prediction. Others have accidentally stumbled into it.

University of Hokkaido geophysicist Professor Kosuke Heki hit the scientific headlines in the wake of the March 2011 quake. While studying GPS data to see how the total electron count (TEC) in the highest parts of the Earth's atmosphere had been affected by waves from the quake, he found a strange pattern.

40 MINUTES WARNING

About 40 minutes before the quake began, the TEC in the ionosphere rose by about 8 per cent above expected levels. Somewhat perplexed, he looked back at the trend for other recent giant quakes, including the February 2010 magnitude 8.8 event in Chile and the December 2004 magnitude 9.1 quake in Sumatra. He found the same increase about the same time before the quakes occurred.

Heki says there has been considerable academic debate both supporting and opposing his research.

To have 40 minutes warning of a massive quake would be very useful indeed and could help save many lives. "So, why 40 minutes?" he says. "I just don't know."

He says if the link were to be proved more firmly in the future it could be a useful warning tool. However, there are drawbacks in that the correlation only appears to exist for the largest earthquakes, whereas big quakes of less than magnitude 8.0 are far more frequent and still cause death and devastation. Geomagnetic storms can also render the system impotent, with fluctuations in the total electron count masking any pre-quake signal.

Government funding for research into quake prediction has decreased in recent years, Heki says.

"There is a very strong official angle not to organise these kinds of projects. Before Kobe (January 1995 earthquake) scientists were working on earthquake prediction, getting money from the government. But after 1995 they backed off, because they hadn't predicted it."

One earthquake even the Government isn't shy about predicting is the Tokai quake. This is expected to occur along the Suruga and Nankai troughs. These lie just offshore of Nagoya and Mt Fuji, where the Philippines Sea Plate is subducting below the Eurasian and Okhotsk plates on which the mainland is situated.

The quake is forecast to be around magnitude 8.0 and is given a 70 per cent chance of happening in the next decade. It would generate a tsunami, which some estimate could be up to 4.5m-high in this region, and cause extremely strong ground shaking.

The Government has already designated the Tokai region as "areas under intensified measures against earthquake disaster".

The Japanese Meteorological Agency (JMA), which monitors quake and volcanic activity as well as the weather, explains on its website how it is forecasting this quake. While prediction is still in the research stage, the agency says, "it is thought fully possible to predict the Tokai Earthquake".

The JMA has installed an intensive seismic and crustal strain network to detect any unusual deformation that may signal the pre-slip of the locked faults, although it points out the pre-slip "may be too slight to be detected, so it is not possible to say that the Tokai Earthquake will be predicted without fail".

An advisory committee of seismologists and members of governmental agencies would be convened if either monthly assessments or anomalies indicate a quake is imminent and an official warning would then be issued by the prime minister.

More routinely, the meteorological agency is charged with issuing early quake warnings, which are then disseminated on radio and television and through cellphones and other personal devices. Senior coordinator for seismic motion modelling, Masaki Nakamura, explains how the warnings provide precious seconds for people to take cover or evacuate, bullet trains to be slowed and halted, and factory lines and lifts stopped.

New computer models are being developed to improve the accuracy of the warning system, which is currently most useful when it involves strong tremors from large offshore, plate-boundary quakes.

The system, which began in October 2007, uses the difference in speed between a quake's P (primary)-waves and its S (secondary)-waves - which cause the damage - to determine the arrival time of the S-waves. P-waves travel about 7km per second and S-waves about 4km per second.

However, he says if several quakes occur almost at the same time or in the same place, the warnings may be inaccurate because the system cannot distinguish between them. Similarly, intensity warnings may lose accuracy for quakes that are more than 100km deep and warnings may not arrive ahead of the strongest ground motion for places close to a quake's epicentre.

He says there were problems with the March 2011 quake. Warnings were under strength because of the length of time the rupture continued. Eight seconds after detection the system calculated it as a magnitude 7.2, and after 108 seconds it was registering as magnitude 8.1.

"We underestimated the intensity because the main rupture was more than 60 seconds after the initial, and the source region was very large," Nakamura says.

Tohoku University's IRIDeS Director Professor Fumihiko Imamura agrees the second quake, which made the event a magnitude 9.0, threw the early warning model into a spin.

"In Sendai two quakes were felt, for a total of three minutes. In Tokyo, there was motion for five minutes on soft ground. The early warnings very much underestimated the situation."

With fine tuning taking place on the Japanese warning models, it would seem a good time for New Zealand to consider a similar system. Our two countries are of much the same size, even though we only have about 4 per cent of Japan's population. But the JMA would not specify a likely cost, saying it was too difficult to separate the system from general seismological operational costs.

Several days later on the bullet train to Kobe, after a night of haze-clearing rain, we were blessed with incredible views of the gigantic Fuji-san, at 3776m Japan's tallest mountain and 52m higher than Aoraki-Mt Cook.

As breathtaking as it was, as tranquil as it looked, one has to remember why it is where it is. It's right on the junction of some of the most complex and dangerous geology in Japan.

Not for nothing is it so big. It appears to sit in judgment, overseeing the location of what could well be Japan's next big devastating quake.

Paul Gorman is science editor at The Press. He visited Japan on an Asia New Zealand Foundation and Foreign Press Center of Japan fellowship.

 - The Press

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