Photo/IllutrationThe Asahi Shimbun

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The Kansai area escaped devastating structural damage from the June 18 earthquake because it had intense but short-period ground motion that mainly affected smaller objects, disaster experts said.

They said if the quake that hit northern Osaka Prefecture had longer period waves of the same intensity, medium-sized buildings and wooden houses that survived could have been flattened.

The magnitude-6.1 earthquake, which registered a lower 6 on the Japanese intensity scale of 7, caused furniture to tip over and outdoor walls to collapse, but most of the buildings and homes remained standing after the shaking stopped.

The Osaka quake was lower in magnitude than the Great Hanshin Earthquake in 1995 and the Kumamoto prefecture earthquake in 2016, both of which recorded magnitude 7.3.

Those two quakes also had long-period ground motion and caused widespread destruction in Kobe and parts of Kumamoto Prefecture.

Ground shaking in an earthquake is a complex mixture of rattling short-wave period jolts and slower long-wave period sways. How the fault slips and the condition of the land can strengthen either the short-wave and long-wave ground motion.

The shorter waves tend to affect smaller objects, while the long-wave motion can cause buildings to collapse.

Yuki Sakai, professor of disaster prevention engineering at the University of Tsukuba, analyzed wave periods recorded in the June 18 earthquake in Takatsuki, Osaka Prefecture.

Sakai believes the intense short-wave period ground motion of 0.5-second or less caused concrete block walls to collapse, toppled furniture and damaged roofing tiles.

He also found that the intensity of long-wave period ground motion of one to two seconds, which was prominent in the 1995 Great Hanshin Earthquake, was weak in the Osaka earthquake.

“People should not assume their buildings with low seismic resistance can always withstand an earthquake with a lower-6 intensity, even though they did not collapse this time,” Sakai warned.

Masumi Yamada, assistant professor of earthquake engineering at the Disaster Prevention Research Institute of Kyoto University, observed collapsed gates, stone lanterns and gravestones at temples in Ibaraki, Osaka Prefecture.

She pointed out that most structures that fell over were either “old” or had “a center of gravity too close to one side.”

Yamada also found that concrete block walls with sturdy support structures survived the quake.

A concrete block wall in Takatsuki that collapsed and fatally crushed a schoolgirl was not adequately supported.

According to Yamada, stronger earthquakes generally have seismic waves with longer periods. An earthquake of magnitude 7 or so tends to produce ground motion with one- to two-second wave periods.

Quakes of a magnitude similar to that in the June 18 trembler tend to produce seismic waves with shorter periods.

When short-wave period ground motion is strong, the degree of acceleration of ground motion, expressed in gal, also tends to be higher.

In the Great Hanshin Earthquake, 891 gal was observed.

Although the magnitude of the June 18 earthquake was smaller, it still recorded highly strong ground motion of 806 gal in Takatsuki.

(This article was written by Tomoyuki Suzuki and Shigeko Segawa.)