Magnitude 4.5 Earthquake Near Honchō, Japan: SJC2 Cable Shows Temporary Anomaly

What Happened

On October 21, 2026, at 04:04 local time, an earthquake of magnitude 4.5 occurred 2 km north of Honchō, Japan. The epicenter was located near the coast, causing concern about the condition of several submarine cables within a radius of 500 km from the seismic activity zone. Although the magnitude of the earthquake was relatively mild, its proximity to key nodes in the submarine cable infrastructure requires careful analysis.

Cables in the Zone

Several large submarine cables are affected by the earthquake. Among them is the Southeast Asia-Japan Cable 2 (SJC2), which entered service in 2025 and passes through a node in Chikura, Japan, just 83.2 km from the epicenter. Also nearby are FLAG Europe-Asia (FEA) via Miura (72.4 km), Tata TGN-Pacific via Emi (74.0 km), as well as FLAG North Asia Loop and Asia United Gateway East via Wada (76.6 km). Other cables, such as Asia Pacific Gateway (APG), Trans-Pacific Express (TPE), and New Cross Pacific (NCP), pass through the Maruyama node (80.0 km).

What Our Measurements Show

Among all the mentioned cables, anomalies were only recorded on SJC2. Under normal conditions, the average RTT (round-trip time) value for this cable is 130.3 ms. However, immediately after the earthquake, the indicator rose to 283.0 ms, which represents an increase of +152.8 ms and exceeds the baseline by 2.7 standard deviations. This may indicate temporary data transmission difficulties along this route. No significant deviations were recorded on other cables such as FEA, TGN-Pacific, and APG, confirming their stability against seismic impacts.

Context and Risks

Japan is a crucial node for submarine internet infrastructure in the Asia-Pacific region. Any damage to cables in this area could lead to significant delays in data transmission between Asia, Australia, North America, and Europe. The anomaly on SJC2, although temporary, highlights the vulnerability of new cable systems to seismic events. At the same time, the stable operation of older systems such as FEA and TGN-Pacific demonstrates the importance of route diversification and the use of proven solutions.

Further Monitoring

We will continue to closely monitor the condition of all submarine cables in the Honchō area, particularly SJC2, where an anomaly was previously detected. Special attention will be given to restoring RTT stability and checking the cable's integrity. Additionally, we will keep an eye on any potential subsequent seismic events that may affect the infrastructure. We will inform our users of any new data and changes in cable operation as soon as possible.