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A collapsed slope is seen in Otsuchi, Iwate Prefecture, on May 17, behind temporary housing that is still under construction. Residents are still taking shelter because of the mudslide. (Mainichi)

Ongoing inspections of ground stability in five northeastern prefectures shaken badly by the Great East Japan Earthquake have uncovered deep cracks and unstable ground in 1,061 spots, it has been learned.

Forty-one of those spots require emergency safety maintenance but work is not expected to finish before the rainy season -- and the onset of typhoons in some areas. At least one area has already experienced a mudslide.

The Ministry of Land, Infrastructure, Transport and Tourism ordered the five prefectures to conduct ground stability inspections on landslide-prone locations -- where the quake registered an upper 5 or higher on the Japanese intensity scale -- before the start of the rainy season in June. Iwate, Miyagi, Fukushima, Tochigi, and Ibaraki prefectures all had 20 or more municipalities with the quake measuring this strong. Of the 28,074 areas to be inspected across the five prefectures, 24,733 have already been surveyed.

Inspectors found that the terrain in 41 spots had changed significantly and required emergency maintenance. Seventeen of these spots were in Tochigi Prefecture and 16 were in Fukushima Prefecture. There were also 1,020 locations, including 400 in Miyagi Prefecture and 200 in Fukushima Prefecture, where ground condition changes were minor, and officials decided maintenance would be performed "if necessary."

However, even in locations where emergency maintenance was deemed necessary, work has yet to begin. Prefectures have so far taken only temporary measures such as covering the areas with plastic sheets and setting up sirens to warn residents if slopes begin to slide.

Meanwhile, some residents in the town of Otsuchi, Iwate Prefecture, have already been visited by danger. On the night of May 15, a slope along a town road collapsed, and an evacuation order was issued for 18 households. This area was subject to an inspection, and although cracks were found, the damage was judged "minor" and the only response taken was putting it under continued supervision. From May 12 to 13, a total of 28 millimeters of rain fell in the town, but it was not enough for authorities to issue warnings about heavy rainfall or mudslides.

Professor Susumu Yasuda, a geotechnical engineering specialist at Tokyo Denki University said, "After the 2008 Sichuan earthquake and the 1999 earthquake in Taiwan there were many mudslides and collapsing slopes. This is because major earthquakes create cracks that make it easy for rainwater to soak deep into the ground. Depending on the circumstances, a deep collapse all the way from the bedrock up can occur."