A 6.0-magnitude earthquake struck off the coast of Hiroo, located in Hokkaido’s southeastern region, in the early hours of Monday, just a day after a similar tremor jolted the nearby city of Kushiro, according to the United States Geological Survey (USGS).

The USGS pinpointed the epicentre approximately 64 kilometres south-southeast of Hiroo, in the Pacific Ocean, at a depth of about 27 kilometres. The agency classified the event as minor in terms of impact potential and issued a “green alert” — indicating a low probability of casualties or substantial economic damage.

No tsunami warnings were issued, and Japanese authorities confirmed that the quake did not trigger any significant sea-level changes. Despite concerns over possible landslides or infrastructure damage, initial assessments reported no such incidents.

Monday’s quake followed closely on the heels of a 6.1-magnitude tremor that hit Kushiro on Sunday night. That event, which was also shallow in depth, did not cause injuries or damage but left many residents unsettled. Experts believe Saturday’s quake may have been a foreshock to the more recent seismic activity off Hiroo.

Tremors from Monday’s quake were described as weak to light and were reportedly felt by nearly two million residents in the surrounding areas. The Japan Meteorological Agency (JMA) has advised locals to remain alert in case of further aftershocks.

A Hiroo resident took to X (formerly Twitter) to express concern, stating, “The shaking felt stronger than expected and lasted longer. We’re worried about more aftershocks. All we can do is hope the pressure releases gradually.”

Japan, situated on the seismically active Ring of Fire, is no stranger to such events. Most buildings in Hokkaido are constructed with earthquake resistance in mind. However, the USGS noted that some vulnerable structures, particularly those made of adobe or unreinforced brick and mud, still exist in parts of the region.

Authorities have urged the public to review emergency preparedness measures, though life in Hiroo and Kushiro has largely returned to normal.