Two separate magnitude 5.5 earthquakes struck Japan on Nov. 7, about five hours apart, and both were catalogued by the U.S. Geological Survey with a green PAGER alert and no tsunami flag. They arrived barely a day after a larger magnitude 6.1 shock out in the Pacific off the same stretch of coast, which made Saturday an unusually busy one for seismographs in central and eastern Honshu.
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The three events sit in different places and at different depths, and the USGS records treat them as independent. Sorting out which is which matters, because the two 5.5s were several hundred kilometers from each other.
The 6.1 offshore on Nov. 6
The earliest of the three was the largest. The USGS logged a magnitude 6.1 quake at 10:39 UTC on Nov. 6, located 144 kilometers southeast of Hasaki, a coastal town in Ibaraki Prefecture, at a depth of about 29 kilometers. Its moment-magnitude solution came out at 6.1, and the agency attached a green PAGER level, the lowest of its impact tiers, with a maximum modeled shaking intensity of 0 on land. Only 21 people had filed felt reports for it, which fits an epicenter that far offshore.
Its distance from shore is the reason the shock produced so little modeled damage despite its size. The event page carries a tsunami flag of zero, meaning the agency did not tag it as tsunami-generating.
First magnitude 5.5, 74 kilometers off Hasaki
The first of the Nov. 7 pair hit at 11:07 UTC, which was 8:07 p.m. in Japan. The USGS places the magnitude 5.5 event 74 kilometers northeast of Hasaki at a depth near 32 kilometers. PAGER came back green, with a maximum modeled intensity of 4, the level where hanging objects swing and dishes rattle. Twenty people reported feeling it through the agency’s felt-report system, topping out at an intensity of 3.4 on the community scale.
This quake sits closer to the Kanto coastline than the 6.1 did, and the small count of reports suggests people nearer the water noticed it while most of the region slept through or ignored it. The USGS does not tie the two shocks together in its event pages.
Second magnitude 5.5 near Hamakita
The other 5.5 came at 16:04 UTC, just after 1 a.m. on Nov. 8 in Japan, and the USGS lists it 10 kilometers northwest of Hamakita at a depth of roughly 38 kilometers. Hamakita is a district of Hamamatsu in Shizuoka Prefecture, well to the southwest of the Hasaki area, which is why the agency catalogs it separately. The event page shows a green PAGER alert, no tsunami flag, and 57 felt reports with a maximum community intensity of 5.6, a clear step above what the earlier quake produced.
For families with relatives in the area, the location gap is the more practical detail. A quake of the same magnitude under populated land generated nearly three times as many reports and a higher perceived intensity than the one that struck offshore, even though the second hit in the middle of the night, when fewer people are awake to notice a shake. The PAGER tool is built to estimate fatalities and economic loss, and its green rating for both events points to low expected impact.
The timing is easier to follow in local terms. The 6.1 came at 7:39 p.m. in Japan on Nov. 6, the first 5.5 at 8:07 p.m. on Nov. 7, and the second 5.5 at 1:04 a.m. on Nov. 8. From the largest to the last, the sequence covered roughly 29 hours. Each of the three carried a depth between 29 and 38 kilometers, which is shallow enough for noticeable shaking where it strikes populated land and common for this part of the Pacific margin.
For travelers and relatives, the practical readings are modest. A green PAGER level is the agency’s lowest alert, tsunami flags were zero on all three records, and the felt-report totals stayed in the dozens. Anyone planning travel through Shizuoka or the Ibaraki coast can check the Japan Meteorological Agency for local intensity values and any advisories, since that office issues the official warnings in Japan.
Japan’s own seismological reporting runs through the Japan Meteorological Agency, whose intensity readings use a different scale and magnitudes that can differ by a few tenths from the USGS figures. The numbers here are the American agency’s and should be read that way. Local scale values, any damage reports and the national agency’s count of the quakes in its own catalog were not available in the records read for this article.
Three magnitude 5.5-or-larger events in about 30 hours is a notable cluster, but nothing in the three event records links the shocks along a single fault. The open question is whether the offshore pair near Hasaki are connected to each other, which seismologists would settle with relocated hypocenters and focal mechanisms that the USGS has not published for these events.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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