Five quakes up to magnitude 5.0 rattled the Mid-Indian Ridge in about eight hours

Between 18:30 UTC on November 1 and 02:43 UTC on November 2, the seismic networks that feed the U.S. Geological Survey logged five earthquakes of magnitude 4.6 to 5.0 on the Mid-Indian Ridge. The strongest, a magnitude 5.0, struck just four minutes after the first. Every one was listed with a depth of 10 km and a tsunami flag of zero.

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The sequence runs across roughly eight hours and a little over 1,500 km of seafloor, which makes it two clusters rather than one tight swarm. Nobody lives on the spreading axis where these ruptures occurred, and the USGS lists no felt reports for any of the five.

Five events, minute by minute

The USGS weekly feed of magnitude 4.5 and larger earthquakes contains the full set, all labeled Mid-Indian Ridge. A magnitude 4.6 (us7000tuqv) opened the run at 18:30 UTC on November 1. The 5.0 (us7000tuqx) followed at 18:34, and a 4.8 (us7000tur0) arrived at 18:53. After a lull of about three and a half hours came a 4.6 (us7000turv) at 22:22, and the last, a 4.7 (us7000tuts), landed at 02:43 UTC on November 2.

The agency’s entry for the magnitude 5.0 event confirms it as the largest of the group, with a significance score of 385, ahead of the 4.8 at 354 and the 4.7 at 340.

Magnitudes in this range are small by global standards. The planet records many earthquakes of magnitude 4.5 and above every week, which is the very cutoff the USGS feed uses, and most of them occur in remote ocean basins where no instrument other than a seismometer registers them. What sets the November 1 run apart is concentration in time. Five entries from one named region inside a single 8-hour 13-minute window is dense for a stretch of ocean floor with no population, no platform and no shipping lane to speak of, and it is the density, rather than any single magnitude, that put the sequence into the feed summary.

Two patches of ridge, 1,500 km apart

The coordinates in the feed split the five into two groups. The first two earthquakes sit near 23.9 degrees south, 69.5 degrees east, about 130 km from the point where three ocean spreading ridges meet. The entry on the Rodrigues triple junction, at 25 degrees south and 70 degrees east, says the Central Indian Ridge, the Southwest Indian Ridge and the Southeast Indian Ridge converge there. That places the opening pair on the northern, Central Indian Ridge side of the junction, an inference drawn from the coordinates, since the USGS uses a single regional label for the whole system.

The other three earthquakes cluster near 35.4 to 36.0 degrees south and 78.1 to 78.4 degrees east. That is much farther along the ridge system, in territory consistent with the Southeast Indian Ridge, which the reference entry on the Southeast Indian Ridge traces for about 6,000 km from the Rodrigues junction toward the Macquarie triple junction.

Two linked groups of quakes this far apart are unlikely to share a single fault. Dynamic stress from a large rupture can nudge other faults, but stress transfer at these distances and magnitudes is a research question here, not a documented effect, and the USGS record makes no such claim. Significance scores in the feed track the same ordering as magnitude. The 5.0 scored 385, the 4.8 scored 354, the 4.7 scored 340, and both 4.6 events scored 326. All five carry a tsunami value of zero and none lists a felt count, so the feed registers instrument detections only. The reading that the opening pair and the closing three belong to separate faults is an interpretation of the coordinates, and a reviewed catalog could refine it by adding smaller aftershocks below the 4.5 cutoff that the weekly summary omits.

What spreading ridges do to rock

Mid-ocean ridges are where plates pull apart and new seafloor forms, and they produce frequent moderate earthquakes as the brittle crust fractures. The Central Indian Ridge is classed as intermediately fast-spreading, opening at about 49 mm per year near the triple junction. The Southeast Indian Ridge opens faster, at about 65 mm per year across its full width, with rates of 69 to 75 mm per year reported farther east.

Plates separating at those rates keep stress building on the faults along the axis and on the transform fractures that offset it, which is why moderate events show up in the weekly feed with some regularity. A run of five within eight hours is the notable part, and the feed itself offers no explanation for it. The USGS catalog shows every depth at exactly 10 km. A uniform value like that suggests the agency fixed the depth rather than resolving it from the waveforms, so the figure should be read as shallow rather than precise.

No agency statement tied the eight-hour run to a volcanic or magmatic episode, and the open question for seismologists is whether later reviewed solutions move the epicenters or depths. Until the USGS revises the listings, the five-event count and the 5.0 maximum stand as the agency’s catalog values as of November 2.

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

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