Weak spot in Christchurch is its brick buildings「地震による死者、９２人に」

Weak spot in Christchurch is its brick buildings

2011/02/24

Search efforts were temporarily halted Wednesday but later resumed in the building where a group from Toyama College of Foreign Languages was studying. (Shiro Nishihata)Cars crushed by rubble in the streets of Christchurch (Shiro Nishihata)

For the southern New Zealand city of Christchurch, its stone and brick buildings proved to be its greatest weakness when an earthquake struck Tuesday.

Unable to absorb the energy generated by the shallow temblor, many structures collapsed.

New Zealand officials said at least 75 people died as a result of the magnitude 6.3 earthquake and that 300 others were still unaccounted for, including 24 Japanese.

According to Tomotaka Iwata, a professor specializing in seismology at Kyoto University's Disaster Prevention Research Institute, ground acceleration of 1,800 gals was recorded in some areas, meaning that a force twice that of gravity whipped those locations in an instant.

"If an earthquake that is not large in magnitude occurs at a shallow depth, it can cause strong shaking in a narrow area along a fault line," Iwata said.

Yuki Sakai, a professor of earthquake engineering at the University of Tsukuba, said that in the absence of detailed data, the temblor was likely one called a short-period earthquake "judging from the strong ground acceleration."

Normally, earthquakes that strike directly under an urban area have short periods of less than half a second. Sakai said the Christchurch earthquake may have had much shorter periods of between 0.2 and 0.3 of a second.

Such short-period shaking will produce devastating damage to stone or brick buildings that have little give.

In addition, the area around Christchurch is not known as an earthquake zone, with the only known past example being a magnitude 7 earthquake in September 2010.

While active faults are prevalent along the Alpine Fault that extends for about 600 kilometers from the southeastern portion of North Island to the northern part of South Island, no active faults had been confirmed in and around Christchurch.

Toshikazu Yoshioka, who heads the Active Fault Evaluation Team at the National Institute of Advanced Industrial Science and Technology, said: "Not only does New Zealand have few records of past earthquakes, but it is difficult to locate active faults because there was a lot of sediment in and around the latest temblor site. While there was no basis to think that no major earthquake would hit, I was told that anti-quake measures were not as advanced as in Wellington, where there are many active faults."

Many of Christchurch's buildings were built of stone and brick by settlers from western Europe who moved there in the 19th century.

Kojiro Irikura, professor emeritus at Kyoto University, said, "Compared with wooden structures, buildings made of brick and stone are more susceptible to collapse because they cannot absorb the energy."

After viewing photos of Christchurch, Taiki Saito, a chief research engineer at the International Institute of Seismology and Earthquake Engineering at the Building Research Institute, said, "In many cases, brick buildings facing street corners or intersections collapsed."

He added, "Just by looking at the structures that collapsed, anti-quake measures do not appear to have been taken, and there were many cases in which the floors were wooden (making the interiors susceptible to collapse)."

The earthquake is believed to have occurred along a fault directly under Christchurch.

According to the U.S. Geological Survey, the quake struck only about 5 kilometers below the surface.

According to an analysis by Yuji Yagi, an associate professor of geodynamics at the University of Tsukuba, a fault about 15 kilometers long and 10 kilometers wide in an east-west direction shifted a maximum of 70 centimeters.

Yagi said, "While the scale is only one-tenth that of the Great Hanshin Earthquake, the large shaking occurred directly under the city."

Like Japan, New Zealand has frequent earthquakes. Strong pressure is applied to the Earth's surface by the collision of the Pacific Plate to the east of the nation with the Indo-Australian Plate to the west.

The tectonic plates have a very complicated structure with the Pacific Plate subducted under the Indo-Australian Plate in the northern part of New Zealand, while the Australian Plate is subducted under the Pacific Plate to the south.

For that reason, not only do major earthquakes occur along the boundary of the tectonic plates, but they also occur along the many active faults that lie further inland. Tuesday's earthquake is believed to be of the latter type.

According to the U.S. Geological Survey, there have been four earthquakes with a magnitude of 7 or larger near New Zealand since 2000.

In the earthquake last September, an active fault about 70 kilometers long lying to the west of the active fault where Tuesday's earthquake occurred caused the earthquake that had about 10 times the energy of the latest temblor.

Kunihiko Shimazaki, professor emeritus at the University of Tokyo who chairs the Coordinating Committee for Earthquake Prediction, said, "The latest earthquake may have been related in some way to last year's quake and can be considered an aftershock in a wide sense of the term."

Masanori Hamada, an earthquake engineering professor at Waseda University who conducted research on-site following last year's earthquake, said, "Although soil liquefaction occurred over a wide area last year, it was mainly in the suburbs. The damage this time was probably much greater because it occurred closer to the city center and at a shallow depth of 5 kilometers."

ＮＺ首相「地震による死者、９２人に」

2011年2月24日13時52分

　【クライストチャーチ＝塚本和人】ニュージーランドのキー首相は２４日午後、同国南部クライストチャーチを直撃した地震による死者が９２人に上っていることを記者団に明らかにした。