USGS logged a magnitude 6.1 earthquake 144 km southeast of Hasaki, Japan

A magnitude 6.1 earthquake struck at 10:39 UTC on Nov. 6, centered 144 km southeast of Hasaki, Japan, based on the U.S. Geological Survey’s reviewed solution. The rupture began 29 km below the seafloor off the Pacific side of the Kanto region. The USGS record carries a tsunami value of zero and a green PAGER alert, the lowest tier in its impact scale.

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In Tokyo the clock read 7:39 p.m. on a Friday evening when the fault slipped. Only 21 felt reports had reached the agency when its record was last refreshed, a modest tally that fits a source well away from the coastline.

The 6.1 solution: moment magnitude, 29 km depth and a reviewed status

The event appears in the USGS feed of earthquakes at magnitude 4.5 and above that began collecting at 06:00 on Nov. 5, ranked by size. Its magnitude type is mww, a moment magnitude calculated from long-period W-phase waves, which is the method seismologists favor for quakes above roughly magnitude 5.5 because it does not saturate the way older scales do. The agency’s status field reads reviewed, meaning an analyst checked the automatic solution rather than leaving it as a first guess, and the entry was last updated at 12:23 UTC, a little under two hours after the origin time.

Magnitude is logarithmic. Each whole step up the scale means roughly ten times the recorded ground-motion amplitude and about 32 times the energy released, so a 6.1 releases around 32 times what a 5.1 does. The USGS significance score for the event is 581, a composite of magnitude, felt reports and estimated impact that the agency uses to sort its feeds.

The machine-readable record for event us7000twg9 lists the origin time as 10:39:27 UTC and the depth as 29 km. A source at that depth is shallow by subduction-zone standards, since slabs plunging beneath Japan host earthquakes at hundreds of kilometers. Shallow ruptures transmit shaking efficiently to the surface, but distance from the coast does most of the protective work here. The USGS event page carries the same solution with maps and the full list of contributing networks.

The place name in a USGS event label is a convenience, not a claim of proximity. The agency attaches the nearest settlement from a gazetteer and then adds a distance and compass direction, so “144 km southeast of Hasaki” means the epicenter lies far out to sea from that coastal town, in open water east of the Kanto coastline. Anyone comparing the label with Japanese reports should expect the Japan Meteorological Agency to name an offshore region instead, such as a sea area off Ibaraki or Chiba, because its bulletins use regional zones rather than nearest-town distances.

PAGER green and a zero on the tsunami field

PAGER is the USGS system that produces fatality and economic-loss estimates after significant earthquakes worldwide. It assigns one of four colors, and green means the model expects limited or no damage and casualties. For the Nov. 6 event the model returned green, a result consistent with a mid-sized quake whose epicenter lies well off the coast and whose nearest populated shoreline is more than a hundred kilometers away.

The tsunami field in the USGS record is zero. That value signals that the agency did not flag the event as tsunami-generating from its own data; formal warnings and advisories for Japan are the responsibility of the Japan Meteorological Agency, whose own bulletin for this event could not be retrieved while this article was prepared.

Felt reports are crowd-submitted, and the number moves as people file late. The tally stood at 21 when the record was read for this article, and the reports are a better guide to where shaking was noticed than to how strong it was.

Offshore Kanto and the 2026 record in southern Ibaraki

This stretch of coast has produced other notable shaking this year. A report from J-RISQ, the rapid-response system run by Japan’s National Research Institute for Earth Science and Disaster Resilience, logged a magnitude 5.9 quake at 02:00 Japan time on Aug. 23 beneath southern Ibaraki Prefecture at a depth of 70 km. The Japan Meteorological Agency supplied that magnitude, and the maximum observed seismic intensity was 5-, a level on Japan’s own shaking scale at which people struggle to move and unsecured furniture topples. The J-RISQ page does not mention damage or injuries.

Comparing the two events needs care. The August quake was graded by the Japan Meteorological Agency and sat deeper, under land, while the Nov. 6 shock was graded by the USGS and sits offshore at 29 km. Agencies compute magnitude with different methods and station networks, and their numbers for one event can differ by a tenth or two. The Japanese agency’s own magnitude and intensity map for the Nov. 6 event are the open items worth watching, since they would show whether any coastal station recorded strong shaking.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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