Soil damage more sweeping than earlier thought

3/11 study stresses liquefaction risk

Kyodo

Soil liquefaction in the March 2011 earthquake was more widespread than previously thought and not restricted to areas of reclaimed land, disaster risk research released Thursday shows.

Nearly 9,700 zones in 189 municipalities across 13 eastern and northeastern prefectures experienced soil liquefaction due to the Great East Japan Earthquake, researchers found.

While reclaimed land along coastlines was especially susceptible, liquefaction also occurred inland along rivers as well as on land developed for housing, according to the study by Kazue Wakamatsu, a professor at Kanto Gakuin University and an expert in disaster prevention, and Shigeki Senna, a researcher at the National Research Institute for Earth Science and Disaster Prevention.

At least 91 zones had liquefied before, previous to 3/11.

“Liquefaction was found to be more widespread than expected,” Wakamatsu said. “It is necessary to measure the degree of (liquefaction) risk nationwide to prepare for future disasters.”

Soil liquefaction is a process by which water-saturated sediment loses strength and behaves like a liquid. It damages buildings as well as groundwater and gas pipes.

“We must realize that hills and higher ground in inland areas are also at risk of the seismic phenomenon, even though these places have long been believed immune from liquefaction and excluded from hazard maps drawn up by local municipalities,” Wakamatsu said.

In putting together their report, the two researchers interviewed local officials and analyzed aerial photos. They divided the 13 prefectures in their study up into 250-sq.-meter zones.

Most of the 9,678 zones that experienced soil liquefaction were rocked by strong tremors measuring upper 5 on the Japanese seismic scale to 7. The zone farthest from the epicenter was in Hiratsuka, Kanagawa Prefecture, some 440 km away.

Around 90 percent, or 9,117 zones, are in the Kanto region, which has many low-lying areas and tracts of reclaimed land.

A joint survey by the Ministry of Land, Infrastructure, Transport and Tourism and the Japan Geotechnical Society said in 2011 that liquefaction occurred in 3,332 zones in the Kanto region following the quake, but the latest study spotted 2.7 times as many.