EDITORIAL | Earthquake Preparedness Not Just for Nankai Trough Region

On August 15 the Japan Meteorological Agency lifted the emergency warning it issued on August 8. The warning declared that the "possibility of a large-scale earthquake is considered relatively higher than under normal conditions."

This timely warning also provides a good opportunity to review our mental and practical disaster preparedness for earthquakes and tsunamis. Likewise, the warning recalls the "Showa earthquakes" that struck Japan during the turbulent period before and after the end of World War II. 

The Showa Earthquakes

There were successive large earthquakes of magnitudes 7 to 8 in the five years bracketing the war's end. Moreover, the death toll in each case exceeded 1,000 people.

Chronologically, these five earthquakes were: 

• the Tottori Earthquake of 1943 (Magnitude 7.2, 1,083 deaths) 
• the Showa Tonankai Earthquake of 1944 (M7.9, 1,233 dead or missing) 
• the Mikawa Earthquake of 1945 (M6.8, 2,306 deaths) 
• the Showa Nanaki Earthquake of 1946 (M8.0, 1,330 deaths) 
• the Fukui Earthquake of 1948 (M7.1, 3,769 deaths)

Of these, the Showa Tonankai and Showa Nankai earthquakes were part of a series of tremors of magnitude 8 or higher that occurred two years apart. In those cases, the Nankai Trough was their epicenter. 

This "staggered" occurrence pattern was last seen in 1854 at the end of the Edo period. At that time, however, there was only a 32-hour gap between the Ansei Tokai and Nankai earthquakes. 

Responding to Earthquake Advisories

Should a massive earthquake of magnitude 8 or greater occur in the expected focal area of ​​the Nankai Trough, an emergency "Massive Earthquake Alert" would be issued. That is stronger than the "Warning" issued this time. Based on the history of activity in the Nankai Trough, it would also indicate that the possibility of a linked tremor occurring is far higher than with a "Megaquake Advisory" issued when an earthquake of M6.8 or greater occurs. 

Every effort should be made to assist the areas affected by the recent M8 earthquake. Meanwhile, we must be on high alert for a major earthquake in the adjacent epicenter area. "Extra Information" alerts regarding this risk can be expected to continue for about a week during major earthquake advisories. Nevertheless, given the past variable intervals between quakes, vigilance may need to be extended for a prolonged period. 

Responding to "staggered" quakes is extremely difficult. Governments, local governments, communities, and households should consider their options in advance, rather than just wait until an emergency occurs. 

Vigilance Beyond the Immediate Area

There is also another lesson from the Showa era earthquakes. Inland earthquakes of M7 or higher are likely before and after ocean trough earthquakes. 

The latest warning was aimed at 707 municipalities in 29 prefectures in the Nankai Trough earthquake disaster prevention measures promotion area. Nonetheless, the whole of Japan remains earthquake-prone. Therefore, even in areas far from the Nankai Trough epicenter, it is important to reconfirm earthquake preparedness measures.

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(Read the editorial in Japanese.)

Author: Editorial Board, The Sankei Shimbun