Seismometers worldwide recorded a magnitude 5.9 earthquake at 16:48 UTC on Oct. 6, centered 264 kilometers south-southwest of Severo-Kuril’sk in Russia’s Kuril Islands and about 26 kilometers deep. It matched the strongest quake of the previous seven days without exceeding it: a second magnitude 5.9 shock had struck near Tambolaka, Indonesia, on Oct. 3.
The U.S. Geological Survey’s reviewed solution places the epicenter at 48.455 degrees north, 154.809 degrees east, in open water off the island chain. No one reported feeling it.
A shallow thrust event on the Kuril arc
The USGS record for event us6000u0hy lists a magnitude of 5.9 on the moment W-phase scale, a depth of 26.047 kilometers, reviewed status and a tsunami flag of zero. Its detailed product file shows that 216 stations contributed to the magnitude, and that the moment tensor returns a scalar moment of 7.62 times 10 to the 17th newton-meters, with nodal planes whose rake values sit near 90 degrees, the signature of thrust faulting.
Thrust faulting is what the region is known for. The Kuril-Kamchatka arc is where the Pacific plate is subducted roughly westward beneath a microplate, a geometry Eos described in its coverage of the magnitude 8.8 Kamchatka earthquake of July 29, 2025, which it reported as tied for the sixth largest recorded by modern instruments. The Oct. 6 shock is far smaller than that rupture, and USGS does not link the two.
The two nodal planes in the solution make the geometry concrete. Plane one has a strike of 41.32 degrees, a dip of 59.33 degrees and a rake of 89.56 degrees; plane two has a strike of 222.18 degrees, a dip of 30.68 degrees and a rake of 90.74 degrees. The double-couple share is 84 percent. Either plane describes the same motion, one block of crust pushed up and over another, and seismologists need other data, such as aftershock alignment, to say which plane actually slipped.
Two magnitude 5.9 shocks share the top spot
The USGS magnitude 4.5-plus weekly feed listed 115 earthquakes when read on Oct. 6 and ranked the Kuril event first at 5.9, the Tambolaka shock second at 5.9 and a magnitude 5.8 event south-southeast of Vilyuchinsk, Russia, third. Fourth and fifth were both magnitude 5.6: one 74 kilometers south of Yonakuni, Japan, and one 94 kilometers southwest of Tamarindo, Costa Rica. A separate pull of the catalog for Sept. 29 through Oct. 7 above magnitude 5.7 returned only the top three.
The weekly feed’s window matters for the ranking. Its earliest entry is a magnitude 5.4 event in the Balleny Islands region at about 05:08 UTC on Sept. 30, so the list covers Sept. 30 through Oct. 6, and the Oct. 6 feed was generated at 17:25 UTC, less than an hour after the Kuril quake.
The Tambolaka record gives magnitude 5.9 on the same moment W-phase scale, a depth of 45.951 kilometers and an origin time of 22:55 UTC on Oct. 3, with 4 felt reports and a community intensity of 4.5. Because both events carry the same one-decimal magnitude, neither is the week’s single largest. Third place, the magnitude 5.8 Vilyuchinsk event, struck at 16:34 UTC on Oct. 2 at a depth of 29.5 kilometers, with a strike-slip mechanism.
PAGER alert level, tsunami flag and the same-day Alaska quake
USGS calls the moment W-phase magnitude its authoritative magnitude when computed, produced for every earthquake of magnitude 5.0 or larger worldwide. That makes the two 5.9 values comparable, since both come from the same method and both are reviewed.
For the Kuril event, the agency’s PAGER system issued a green alert, its lowest tier, and predicted a maximum Modified Mercalli Intensity of about 3.8, which corresponds to weak shaking. The tsunami flag on the record is zero. The Tambolaka event also carries a green alert, with a modeled maximum intensity of about 5.1.
A different quake dominated the felt reports that day. The catalog pull for Oct. 5 to 7 lists a magnitude 5.5 earthquake 92 kilometers north-northwest of Aleneva, Alaska, at 87.6 kilometers depth, with 326 felt reports, against none for the larger Kuril shock. The Alaska event’s magnitude type is local magnitude, a different scale from the Kuril shock’s moment W-phase value, which is one reason the two are not ranked against each other here.
The magnitude tie is a statement about a one-decimal rounding of two reviewed solutions, and USGS can still revise either value by a tenth, which would break it.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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