Report Cites Liquefaction As Key To Much Of Japanese Earthquake Damage

CORVALLIS, Ore. – The massive subduction zone earthquake in Japan caused a significant level of soil “liquefaction” that has surprised researchers with its widespread severity, a new analysis shows.

The findings also raise questions about whether existing building codes and engineering technologies are adequately accounting for this phenomenon in other vulnerable locations, which in the U.S. include Portland, Ore., parts of the Willamette Valley and other areas of Oregon, Washington and California.

A preliminary report about some of the damage in Japan has just been concluded by the Geotechnical Extreme Events Reconnaissance, or GEER advance team, in work supported by the National Science Foundation.

The broad geographic extent of the liquefaction over hundreds of miles was daunting to experienced engineers who are accustomed to seeing disaster sites, including the recent earthquakes in Chile and New Zealand.

“We’ve seen localized examples of soil liquefaction as extreme as this before, but the distance and extent of damage in Japan were unusually severe,” said Scott Ashford, a professor of geotechnical engineering at Oregon State University and a member of this research team.

“Entire structures were tilted and sinking into the sediments, even while they remained intact,” Ashford said. “The shifts in soil destroyed water, sewer and gas pipelines, crippling the utilities and infrastructure these communities need to function. We saw some places that sank as much as four feet.”

Some degree of soil liquefaction is common in almost any major earthquake. It’s a phenomenon in which saturated soils, particularly recent sediments, sand, gravel or fill, can lose much of their strength and flow during an earthquake. This can allow structures to shift or sink and significantly magnify the structural damage produced by the shaking itself.

But most earthquakes are much shorter than the recent event in Japan, Ashford said. The length of the Japanese earthquake, as much as five minutes, may force researchers to reconsider the extent of liquefaction damage possible in situations such as this.

“With such a long-lasting earthquake, we saw how structures that might have been okay after 30 seconds just continued to sink and tilt as the shaking continued for several more minutes,” he said. “And it was clear that younger sediments, and especially areas built on recently filled ground, are much more vulnerable.”

The data provided by analyzing the Japanese earthquake, researchers said, should make it possible to improve the understanding of this soil phenomenon and better prepare for it in the future. Ashford said it was critical for the team to collect the information quickly, before damage was removed in the recovery efforts.