Shallow M5.9 earthquake near Sumisujima volcano triggers 50 cm (1.6 feet) tsunami, Japan

The epicenter was located 363.3 km (225.8 miles) SSE of Shimoda (population 21 402), 412.8 km (256.5 miles) SSE of Shizuoka (population 690 881), and 600 km (370 miles) south of Tokyo (population 14.2 million), Japan.

8 000 people are estimated to have felt weak shaking.

JMA advised that waves up to 1 m (3.3 feet) above tide levels could occur along the coasts of the Izu (population 21 500) and Ogasawara (population 2 500) island chains.

Tsunami warnings or advisories are still in effect, as of 07:32 UTC on September 24.

According to the agency, a tsunami of about 50 cm (1.6 feet) was detected in the Yaene district on Hachijo Island about 30 minutes after the quake. Smaller waves were detected on three other islands — Kozushima, Miyakejima, and Izu Oshima.

The USGS issued a Green alert for shaking-related fatalities and economic losses. There is a low likelihood of casualties and damage.

Overall, the population in this region resides in structures that are resistant to earthquake shaking, though vulnerable structures exist. The predominant vulnerable building types are heavy wood frame and reinforced/confined masonry construction.

Recent earthquakes in this area have caused secondary hazards such as landslides that might have contributed to losses.

Fumihiko Imamura, a Tohoku University seismologist, said the tsunami is believed to be related to undersea volcanic activity and urged coastal residents to keep their guard up while the tsunami advisory was in place.

The nearest volcano with recently issued Volcanic Warning is Sumisujima (Smith Rocks). JMA upgraded the alert level for the volcano from the Volcanic Forecast (potential for increased activity) on September 19, 2024, urging caution for the sea areas below Tokyo.

On July 18, 2024, the Japan Coast Guard reported that an area of discolored water, brown to yellow-green in color, was visible about 300 m (9.8 feet) E of the eastern coast of Sumisujima. The report noted that areas of discolored water had occasionally been seen in the vicinity of the island since 2017, though usually characterized as pale blue to white and smaller in size. Sailors were warned to exercise caution in the area.

The last eruption of this volcano took place on June 21, 1916 CE (VEI 0).

Sumisujima is a steep-sided basaltic pinnacle that forms part of the outer southern flank of an 8 – 9 km (5 – 5.6 miles) wide submarine caldera that truncates a 20 km (12.4 miles) wide seamount.

The caldera near Sumisujima, formed between 60 000 and 30 000 years ago, is a significant geological feature showcasing a diverse range of volcanic activity.

The Shirane dacitic central cone, which is 3 km (1.9 miles) wide and 800 m (2 624 feet) high, is a prominent structure within the caldera, rising to just 8 m (26 feet) below the ocean surface. The caldera’s steep walls, ranging from 600 to 700 m (1 969 – 2 297 feet) in height, and its outer flanks expose a variety of volcanic rocks, including basaltic, andesitic, and rhyolitic formations.

Two large submarine cones, Sumisu Knolls No. 1 and 2, are located to the west of the caldera. Historically, submarine eruptions have been documented near Sumisujima, with the last recorded in 1916. Since the 1970s, frequent water discoloration has been observed, indicating ongoing volcanic activity.

Notably, in October 1992, a 6 km (3.7 miles) long zone of discolored water was observed extending from the shallow Shirane rock mass near the eastern rim of the caldera, which rises to within 7 m (23 feet) of the ocean surface.

Estimated population exposure to earthquake shaking

Regional seismicity

References:

1 M5.6 – Izu Islands, Japan region – USGS – September 23, 2024

2 M5.6 – Izu Islands, Japan region – EMSC – September 23, 2024

³ Small tsunami wash ashore on remote Japanese islands after quake – Japan Today – September 24, 2024

⁴ Geological summary for Sumisujima – GVP – Accessed on September 24, 2024