On August 8, 2024, a magnitude 7.1 earthquake struck approximately five to ten kilometers off the shores of Miyazaki, a city on Japan’s southern island, Kyushu. The earthquake, which occurred at a depth of 25 kilometers, is believed to have originated from the Nankai Trough subduction zone interface. Although the region is no stranger to seismic activity, the recent event has raised concerns about the possibility of a larger megathrust earthquake.
Context and Historical Seismic Activity
The Nankai subduction zone is notorious for producing massive earthquakes, with magnitudes ranging from 8 to 9, occurring approximately every 100 to 200 years. However, the recent magnitude 7.1 shock occurred in a zone characterized by repeating magnitude ~7 earthquakes every 25 to 30 years, typically without leading to megaquakes. This pattern of seismicity suggests that the moderate shocks may relieve enough stress to prevent larger events in this specific area.
Historically, the region has seen significant earthquakes, such as the 2016 Kumamoto earthquake, which caused extensive damage in central Kyushu. Fortunately, the shaking from the recent Miyazaki event was less severe on land, and although some damage was reported, there were no fatalities. A modest 0.5-meter tsunami was recorded, which did not result in significant impact.
Megaquake Advisory and Its Implications
In response to the 2011 Tōhoku earthquake, which was unexpectedly larger than anticipated, Japan’s government increased the potential maximum size of a Nankai Trough event from magnitude 8.6 to 9.1. This change led to the institution of a “megaquake advisory” (kyodai jishin chūi / 巨大地震注意) policy, which triggers a cautionary alert when an earthquake of magnitude 7.0 or greater occurs within the hypothesized rupture zone.
Following the August 2024 event, the Japan Meteorological Agency (JMA) issued an advisory to warn authorities and residents of the increased probability of a large quake. This advisory is expected to last for a week and aims to promote awareness and preparedness among the population.
Assessing the Risk of a Megathrust Event
Despite the issuance of the advisory, the likelihood of the recent magnitude 7.1 event leading to a megathrust earthquake appears to be low. Historical and paleoseismic records indicate that no great earthquakes have occurred within 100 kilometers of the recent event’s epicenter.
The recurring pattern of magnitude ~7 earthquakes in this region suggests that these moderate shocks may be sufficient to accommodate the subduction rate, potentially preventing the buildup of stress required for a larger event.
However, it is important to note that the recent earthquake has increased stress on nearby faults, including those that ruptured during the great 1946 Nankai earthquake. This stress increase, though small, could trigger additional seismic activity in the coming days or weeks. Additionally, the earthquake’s proximity to volcanic regions, such as Mt. Kirishima, raises concerns about potential volcanic unrest.
Conclusion: Weighing the Benefits and Costs of the Advisory
While the probability of a megathrust event in the near future remains low, the advisory issued by the JMA is a prudent measure that promotes preparedness and mitigates potential risks. The advisory serves as a reminder of the importance of disaster management planning and the need for continued vigilance in seismically active regions.
The decision to issue the advisory, despite the low probability of a larger event, reflects the government’s commitment to public safety and the lessons learned from past seismic disasters. The advisory’s impact on daily life, commerce, and transportation must be balanced against the potential benefits of heightened awareness and preparedness in the face of unpredictable natural hazards.
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