A magnitude 6.0 earthquake rattled central Japan at 22:15 UTC on October 31, which was 7:15 on the morning of November 1 in Japan. Its source lay 352 kilometers below the seabed south-southeast of Shima, on the Pacific side of Mie Prefecture. That is among the deepest depths at which earthquakes of this size occur.
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Twenty-nine people reported feeling it to the U.S. Geological Survey, and no tsunami was flagged. The reviewed record carries a green alert, the lowest tier on the agency’s damage-estimate scale, and the event has a status of reviewed, meaning an analyst has checked the automatic solution.
The numbers behind the October 31 quake
The reviewed USGS record, summarized on the agency’s event page, places the epicenter at about 33.82 degrees north and 136.99 degrees east, 58 kilometers south-southeast of Shima. The magnitude is a moment-magnitude value. Programmatic data in the agency’s event record gives a depth of 352.4 kilometers, a significance score of 563, a green alert level indicating little expected damage or loss, and a tsunami flag of zero.
Shima is a city on the Shima Peninsula, part of the Ise-Shima coast in Mie Prefecture, which faces the open Pacific south of the Kii Peninsula’s eastern shore. The epicenter lay offshore, so the nearest land was the point on the map from which the agency measured the 58 kilometers. In Japan the shaking arrived at 7:15 on a Sunday morning, an hour when many households were at home.
Japan’s own agency, the Japan Meteorological Agency, publishes separate magnitude and intensity figures at its earthquake information site, and different agencies often differ by a tenth or two in magnitude. The page returned only a navigation shell when read for this article, so no Japanese figure is quoted here and the numbers above are the USGS values alone.
Why 29 reports is a small number for a 6.0
A shallow quake of this size close to a coast would be felt by thousands. Depth changes the arithmetic. The USGS explanation of magnitude and intensity says a quake has a single magnitude but many shaking values, depending chiefly on distance from the rupture and also on local geology and depth. With 352 kilometers of rock between the source and the surface, energy spreads out before it arrives. The agency’s community-reported intensity came to 3.1 and its instrumental estimate to 3.0, both in the range of light, noticeable shaking. On the Modified Mercalli scale used in the agency’s intensity work, a level around 3 is the kind of shaking people notice indoors, particularly on upper floors, without damage to buildings.
The count of 29 appears in the agency’s weekly list of magnitude 4.5 and larger events, which carries it in the felt column of the same row as the depth. For comparison, the 24 Vanuatu entries in the same list never exceed five felt reports, but those quakes struck far from towns. In energy terms, a 6.0 is roughly 32 times a 5.0 and about a thousand times a 4.0, so size alone does not decide how many people notice. The significance scores tell a similar story: the Vanuatu 6.0 of November 1 scored 554 and this one scored 563, although one struck at 10 kilometers depth at sea and the other 352 kilometers down.
Deep shocks and the descending plate
Seismologists commonly call quakes deeper than about 300 kilometers deep-focus events, and this one clears that line by roughly 50 kilometers. Earthquakes at such depths are generally linked to slabs of oceanic plate sinking into the mantle, since ordinary crustal faults do not reach that far. Japan sits above such a boundary, where the Pacific plate descends beneath the islands. The USGS record read for this article does not tie the October 31 event to a named structure, so that link is the general pattern for deep shocks in the region and not a finding about this particular rupture.
Deep earthquakes seldom disturb the sea surface, because the rock that moves is far below the crust where the seafloor sits. The USGS flagged no tsunami for this event, and the warning centers post any formal bulletins through tsunami.gov, where no threat for the region was shown when the page was read.
The weekly list read for this article shows no other event labelled near Shima, which fits the usual pattern that deep earthquakes produce few aftershocks, though a few small ones below the 4.5 cutoff would not appear there. The sources do not carry a Japanese agency assessment, a reported injury count or any damage statement, so none is claimed here. What they establish is narrow: a magnitude 6.0, very deep, lightly felt, reviewed by the USGS.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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