To guard against the onslaught of natural disasters, we need to learn the lessons of the past. Authorities should look into how and why the experience gained from similar disasters failed to prevent the deaths of dozens of people in mudslides that engulfed hilly residential areas of Hiroshima — after torrential rains hit in the early hours of Wednesday — so that the extent of damage can be limited in future disasters.

With its largely mountainous terrain, Japan is prone to landslides that cause heavy damage. According to the Cabinet Office, roughly 1,000 reported landslides take place each year on average, and more than 500,000 locations nationwide have been identified as susceptible to landslides.

At least 91 people were killed or listed as missing after the collapse of rain-sodden slopes released torrents of mud, rock and debris that devoured houses built at the foot of mountains in the northern outskirts of the city.

The victims included a firefighter in a rescue effort. He was found buried in the mud holding a 3-year-old boy.

The severe landslides and flooding was caused by torrential rain that struck a narrow area in a short period. In Hiroshima’s Asakita Ward, one of the hardest-hit areas, a record 217.5 mm of rain fell in three hours from 1:30 a.m. Wednesday, which exceeded the total rainfall for August in an average year.

With current weather forecast technology, it is difficult to predict exactly when and where such localized torrential rain will hit, even though such storms have been striking with increasing frequency in recent years. According to the Meteorological Agency, rainfall of at least 50 mm per hour took place 241 times a year on average in the decade to 2013, compared with 174 times a year from 1976 to 1985.

Whether rains will cause landslides depends on the local terrain and soil conditions. The soil in many parts of Hiroshima prefecture has a tendency to crumble easily when it becomes saturated, and the capital city is particularly vulnerable to landslides. Population growth has led to many new residential areas being built on unstable ground in the city’s mountainous outskirts. Torrential rain in 1999 caused mudslides that left 32 people dead or missing.

The 1999 disaster prompted the national government to create a system in which local authorities designate zones of “special caution” against landslides. Within these zones, development of residential land requires local government permits. Many of the areas hit hardest in this week’s landslides were not designated as such. According to a city official, local residents tend to resist the designation because it reduces the value of residential property.

Work to designate landslide-prone areas around the country is reportedly slow because authorities first try to obtain the consent of residents.

In Wednesday’s landslides, Hiroshima officials admitted that the evacuation advisory for residents was issued too late. The local meteorological observatory issued a heavy rain warning for the city of Hiroshima at 9:26 p.m. on Tuesday, but at that time it was raining just mildly in the area later hit by landslides. The rain turned heavier after midnight.

The Meteorological Agency and the Hiroshima Prefectural Government issued an alert for possible landslides at 1:15 a.m. Wednesday, and the municipal government set up an emergency alert headquarters shortly thereafter.

Around 3:20 a.m. the local authorities got the first report that two boys had been buried in a landslide. It was nearly an hour later — at 4:15 a.m. — that the city started issuing evacuation advisories for residents in the affected areas.

Officials of the city say they hesitated to issue the evacuation advisories because the torrential rain came too suddenly and in narrow areas of the city. They admit that damage could have been reduced if the advisories had been issued earlier.

In some municipalities hit by typhoons and downpours in recent years, local authorities were found to have delayed issuing evacuation advisories for local residents even after they received landslide alerts from prefectural governments and the Meteorological Agency. Some authorities said they waited because of the danger in having people evacuate their homes at night in stormy weather.

When massive landslides caused by a typhoon left 39 people dead or missing on Izu-Oshima Island in October 2013, the town mayor — who was away from the island and was not informed of the alert until hours later — did not issue an advisory for the same reason.

In the Hiroshima landslides, people evacuating their houses on mountain slopes in the rain after midnight would have been exposed to danger. Still, experts say any alerts coming from local authorities would help residents to take precautions. Municipalities must use early warnings to urge residents to evacuate while it’s still safe to do so.

The Hiroshima municipal government says it will look into what went wrong with its disaster response and improve the city’s anti-disaster procedures. What’s also important is that the lessons the city learns from the catastrophe be shared with other municipalities vulnerable to similar disasters.

It’s also crucial that residents in disaster-prone areas be made fully aware of what could happen when their neighborhood is hit by violent weather — and be able to respond appropriately in emergencies.

When the Meteorological Agency issued a special alert earlier this month for powerful Typhoon Halong, which left 11 people dead nationwide, few people are said to have followed evacuation orders issued for more than 500,000 residents in the cities of Yokkaichi and Suzuka in Mie Prefecture.

Are people who live in landslide-caution areas being properly informed of potential dangers? Local authorities must monitor how residents respond to their alerts and take the necessary steps to improve it.

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