SAKIGAKE管理人 Introduction
Early in the morning of August 6, 1977, felt earthquakes began occurring frequently around Mount Usu in Hokkaido. This eruption — the first in 32 years since the 1944–45 eruption that formed Showa-Shinzan — began the following day, August 7, at 9:12 AM, with a pumice eruption (Plinian eruption) from the summit. The eruption plume reached a height of 12,000 meters within an hour.
The eruption occurred a mere 30 hours after the onset of precursory earthquakes, but residents had already evacuated in advance, and there was no loss of life immediately following the eruption. However, the true threat of this eruption lay not in the eruption itself, but in what came after.
This article looks back at the events of the 1977 Mount Usu eruption and considers the lesson that “successful initial disaster response” and “preparedness for prolonged risk” are two separate matters.
Just One Week of the “First Phase Eruption”
Following the start of the eruption on August 7, large and small eruptions continued intermittently, and the “first phase eruption” temporarily subsided over the course of about one week, by the early hours of August 14. During this period, pumice and volcanic ash accumulated thickly around Mount Usu, causing damage to homes at the mountain’s base amounting to approximately 8 completely destroyed, 4 partially destroyed, and 370 partially damaged. Damage to crops and forests also extended over a wide area, with ashfall confirmed across most regions of Hokkaido, including Soya, Kushiro, and Nemuro.
For many people, the word “eruption” may bring to mind this one week of intense activity. However, in reality, the disaster did not end here.
The Eruption That “Should Have Ended” Continued for Another Four and a Half Years
Even after the first phase eruption subsided on August 14, activity at Mount Usu did not settle down. From November 16 of that year through October 27 of the following year, 1978, phreatic and phreatomagmatic explosions occurred intermittently. Furthermore, seismic activity and crustal deformation themselves continued for approximately four and a half years, until March 1982.
Over the course of this prolonged crustal deformation, a new mountain body — “Usu-Shinzan” — formed at the center of the crater floor. Rather than a single-point event, the 1977 eruption was defined by being a disaster that unfolded as a “line,” continuing over a period of years — a defining characteristic of this event.
More Serious Loss of Life Occurred After the Eruption
And the heaviest lesson of this eruption is the fact that loss of life occurred not “immediately after the eruption,” but from a “post-eruption mudflow.”
The large amounts of volcanic ash and pumice that accumulated on the mountainside as a result of the eruption subsequently turned into mudflows triggered by rainfall. In 1979, this mudflow disaster claimed 2 lives, with 1 person missing. Lives that had been protected by prompt evacuation at the time of the eruption were lost to a “secondary disaster” that occurred after time had passed since the eruption.
This fact teaches us an important perspective in disaster response: “it is precisely after volcanic activity has settled down that a different kind of risk begins to emerge.”
A Total Damage Cost of Over 50 Billion yen — the “Prolonged Cost” of Disaster
The damage from the 1977 eruption extended across a wide range of sectors — agriculture, civil engineering, forestry, and tourism — reaching a total scale of over ¥50 billion. This was not the result of the immediate damage from the eruption alone, but the accumulation of approximately four and a half years of crustal deformation and the subsequent mudflow damage.
Disaster response required not only emergency response immediately following the eruption, but also the management of long-term recovery costs — including ash removal, infrastructure restoration, and response to agricultural damage. In fact, ash removal at the time was commissioned to the Hokkaido Agricultural Development Public Corporation, and a system was introduced through which affected farmers who had lost income could earn wages by participating in the removal work.
Implications for Corporate Disaster Prevention and BCP
The lessons of the 1977 Mount Usu eruption offer an essential perspective for corporate BCP development: “a successful initial response” and “continued safety thereafter” are separate matters.
First, view disasters as a “line,” not a single “point.” Disasters such as eruptions, earthquakes, and heavy rains can continue to generate secondary risks not only at the moment they occur, but for months or even years afterward. A BCP must account not only for the initial response phase, but also for a mid-to-long-term recovery and vigilance phase.
Second, do not underestimate the risk that emerges “after volcanic activity has settled down.” The 1979 mudflow disaster was not the eruption itself, but a secondary disaster caused by subsequent rainfall. Companies, too, must recognize that the risk of secondary damage from complacency often rises precisely once a crisis appears to have passed.
Third, anticipate the cost structure of a prolonged disaster response in advance. Much of the damage exceeding ¥50 billion resulted not just from the immediate aftermath of the eruption, but from the cumulative buildup of complex damage over an extended period afterward. In corporate crisis management as well, financial preparedness must account not only for initial response costs, but also for mid-to-long-term recovery and compensation costs.
Introducing a Related Solution
What the 1977 Mount Usu eruption demonstrated is that preparing for “the eruption itself” alone is not enough — continuous monitoring that anticipates secondary disasters such as subsequent crustal deformation and mudflows is essential. We would like to introduce the following solution handled by SAKIGAKE JAPAN Corp.
DIPPS Cloud | Portable Landslide Monitoring Stake System
“DIPPS Cloud” is a portable monitoring stake system that detects subtle ground movement in real time simply by inserting stakes into slopes or cliffs, and notifies the cloud wirelessly.
As the lessons of the 1977 eruption demonstrate, even after a volcanic warning has been lifted, slopes where ash and pumice have accumulated continue to carry the risk of mudflows and debris flows triggered by rainfall for an extended period. DIPPS Cloud enables continuous slope monitoring during this often-overlooked “post-eruption” period, when it matters most. When an anomaly is detected, the display on the management screen changes from green to red, and an instant email notification is sent to relevant parties.
- Easy installation: Just insert the stake into the ground — no large-scale construction required
- Centralized cloud management: Monitor multiple locations on a single screen from a PC or smartphone
- Works even outside mobile coverage: Mountain areas covered via satellite communication and Starlink connectivity
- Approximately 2-year battery life: Continuous monitoring with minimal maintenance burden
“The eruption had subsided. But the risk continued.” Building on this lesson, please consider DIPPS Cloud as a means of visualizing the disaster risks that persist even after the initial response has concluded.
For details and inquiries, see below:
DIPPS Cloud Product Page
Conclusion
The 1977 Mount Usu eruption — beginning with just 30 hours of precursory warning, settling into a temporary lull after one week of intense activity, only for crustal deformation to continue for roughly four and a half years, and for a mudflow disaster to claim 3 lives in 1979 — demonstrated to us the complexity of a disaster that unfolds over a prolonged period.
Rather than finding reassurance in information alone — that “the eruption has subsided” or that “the warning has been lifted” — maintaining a posture that anticipates the risks lying beyond is the greatest lesson left by this eruption. As professionals in corporate disaster prevention consulting and disaster risk management, SAKIGAKE JAPAN Corp. is fully committed to supporting your disaster preparedness and BCP development.