A magnitude 7.1 earthquake struck Kumamoto Prefecture on Tuesday, producing a seismic intensity of seven in Uki City and Hikawa Town [1, 3].

The event has raised urgent concerns regarding the stability of the Hinagu fault zone, specifically sections that remain unruptured. Because these areas lie beneath the sea, any future significant seismic activity could displace the seabed and trigger a tsunami.

The earthquake occurred at 4:27 p.m. JST on July 28, with a depth of approximately 10 kilometers [3]. In the wake of the tremor, Junichi Nakajima, a professor at Tokyo University of Science, identified a specific area of concern for future monitoring.

"The area that requires the most attention is further south of the Hinagu fault zone," Nakajima said [2].

Nakajima said that the southern portion of the fault zone is located beneath the Ariake Sea. He said that if a large earthquake occurs in that specific location, caution regarding tsunamis becomes necessary [2].

Experts are focusing on the "unruptured zone," where tectonic stress has accumulated but not yet been released. If a magnitude seven-class earthquake were to occur in this region, the resulting movement of the seafloor could push water toward the coast, creating a hazardous surge for coastal communities [2, 4].

Local authorities and residents are being urged to remain vigilant as geologists assess the impact of Tuesday's quake on the surrounding fault systems. The proximity of the fault to the Ariake Sea increases the complexity of the risk, as seabed displacement is a primary driver of localized tsunami events [2, 4].

“The area that requires the most attention is further south of the Hinagu fault zone.”

The warning highlights a critical geological vulnerability in Kumamoto. While the recent quake released significant energy, the existence of an unruptured zone beneath the Ariake Sea means the region remains at risk. The transition from a land-based seismic threat to a maritime one introduces the danger of tsunamis, necessitating updated evacuation plans for coastal areas that may have previously focused only on shaking.