Japanese authorities on Thursday issued the first ever “mega-quake advisory” to citizens living along the Pacific coastline in the southern half of the island nation, warning of a “relatively distant” but still serious enough probability of a massive earthquake from the Nankai Trough, a tectonic boundary capable of breaking at the highest intensities possible, NHK reports.

Geological officials believe a 7.1 magnitude quake that struck Thursday off Kyushu, the southernmost of the four main Japanese islands, may have been a precursor to a larger event on the scale of the apocalyptic, 9.1 magnitude 2011 Tōhoku earthquake and tsunami that devastated the Pacific coastline of the northern half of the largest island, Honshu, a quake that was the fourth most powerful ever recorded by modern seismology. While the specifics of why the Thursday Kyushu quake was a possible precursor were not made clear, there’s certainly a logic to anticipating that another event would occur along a different section of the same boundary that produced the 2011 one. The Pacific plate subducts under the Japanese archipelago but not all of its edge moves at the same pace. It’s possible that the southern half of Japan is overdue for such a reckoning.

US State Department officials are taking the warning seriously enough to pass it along to Americans in Japan, tweeting “The Japan Meteorological Agency has issued an alert warning about the increased risk of a possible large earthquake around the Nankai Trough. This follows a magnitude 7.1 quake off the coast of Kyushu… but does not indicate that an earthquake is imminent. Please stay alert and take disaster prevention measures. The US Embassy in Tokyo will be releasing informational content over the coming days to re-emphasize good emergency preparedness.”