San'in Region Struck by Consecutive Strong Earthquakes, Shindo 5+ Registered in Shimane and Tottori

On the morning of January 6th, the San'in region experienced multiple earthquakes, with epicenters located within the region. The quakes registered a maximum seismic intensity of Shindo 5+ in Shimane and Tottori prefectures, immediately followed by another quake of Shindo 5-. This series of tremors caused strong shaking for local residents and led to some disruptions in railway services, though no significant damage has been reported to date.

The initial earthquake occurred between approximately 10:17 and 10:18 AM. Its epicenter was in eastern Shimane Prefecture, with an estimated magnitude (M) of 6.2 and a shallow depth of about 10 kilometers. This tremor registered a seismic intensity of Shindo 5+ in Sakaiminato City, Hino Town, and Kofu Town in Tottori Prefecture, as well as Matsue City and Yasugi City in Shimane Prefecture. Merely 10 minutes later, around 10:28 AM, another earthquake struck, again with its epicenter in eastern Shimane Prefecture. This second quake had a magnitude of 5.1 and a depth of approximately 10 kilometers, observing Shindo 5- in eastern Shimane Prefecture and Yonago City in Tottori Prefecture, among other areas. Subsequent aftershocks of Shindo 4 and Shindo 3 continued, and the Japan Meteorological Agency (JMA) has issued a warning for continued vigilance against further strong aftershocks. There is no tsunami threat.

Residents in Yonago City, which experienced Shindo 5-, reported 'It was a strong horizontal shake, a Shindo 5- after a long time' and confirmed that 'Lifelines are normal.' In Matsue City, comments such as 'Shindo 5+ and Shindo 5- came one after another' reflected widespread surprise at the rapid succession of strong tremors. Shindo 5- was also observed in western Tottori Prefecture, causing a temporary suspension of operations on parts of the JR Sakai Line; however, no severe individual-level damage has been reported.

The extremely shallow epicenter of these earthquakes (approximately 10 kilometers) prompted remarks such as 'It was incredibly shallow' and 'Could it be a fault?', drawing attention to the specific characteristics of seismic activity in the region. Some residents recalled the Noto Peninsula earthquake from January of this year and the Tottori Earthquake of 1943, with certain opinions suggesting this event serves as an opportunity to re-evaluate preparations for a potential Nankai Trough earthquake. Authorities emphasize that similar intensity earthquakes could occur over the next few days, necessitating continued caution.

The Context

Japan is located on the Pacific Ring of Fire, a region known for frequent seismic activity and volcanic eruptions. The country experiences thousands of earthquakes annually, making earthquake preparedness a fundamental aspect of daily life and infrastructure planning. The San'in region, where these recent earthquakes occurred, is on the southwestern coast of Honshu, facing the Sea of Japan. While less frequently associated with major megathrust earthquakes than Japan's Pacific coast, it is still susceptible to inland crustal earthquakes, often characterized by shallower epicenters.

In Japan, earthquake intensity is measured using the JMA seismic intensity scale, known as 'Shindo' (震度), which ranges from 0 to 7. Unlike magnitude (M), which quantifies the total energy released at the earthquake's source, Shindo measures the degree of shaking experienced at a specific location on the Earth's surface. Shindo 5+ (強) indicates 'strong' shaking, where most people find it hard to move, and some heavy furniture may topple. Shindo 5- (弱) indicates 'moderately strong' shaking, where many people are frightened, and unsecured objects may fall. Both levels are considered significant enough to cause concern and potential damage. The mention of a shallow epicenter (10 km) is significant as shallower quakes typically result in more intense shaking on the surface directly above the epicenter, even if their magnitude isn't exceptionally high.

The article makes reference to the Noto Peninsula earthquake, which occurred on January 1, 2024, causing extensive damage and a tsunami on Japan's west coast. This recent major event has heightened public awareness and vigilance regarding seismic activity across the country. The 1943 Tottori Earthquake (M7.2) is a historical benchmark for the region, reminding residents of past destructive events. The concern about a potential Nankai Trough earthquake refers to a hypothetical megathrust earthquake along a subduction zone off Japan's Pacific coast, which is predicted to be catastrophic. While the recent San'in earthquakes are distinct from a Nankai Trough event, their occurrence often prompts a broader re-evaluation of disaster preparedness among the public.