Three earthquakes of magnitude 5 struck the Reykjanes Ridge in the North Atlantic on Oct. 21, at 19:06, 21:38 and 21:58 UTC, according to the U.S. Geological Survey. The middle shock was the largest at magnitude 5.3, and the first and last each measured 5.0. All three were placed near 56.8 to 57.0 degrees north and 34.2 degrees west, a stretch of the ridge far from any coastline.
Help yourself and the planet: The Cool Down makes it easy to help yourself while helping the planet, with smart, simple changes to save more and waste less in your everyday life.
We may earn a commission if you sign up.
That location is the detail most easily lost in the name. Reykjanes is also the name of the peninsula at Iceland’s southwestern tip, near 63.8 degrees north and 22.7 degrees west, and a straight-line calculation from the USGS coordinates to that tip comes to roughly 1,000 km. The cluster sits on the same submarine ridge system that runs out to sea from the peninsula, but it did not shake Iceland’s towns.
Three USGS events in under three hours
The USGS list of magnitude 5 and larger earthquakes for Oct. 19 through Oct. 22 holds 33 events, and three of them carry the place name Reykjanes Ridge. The earliest, event us6000u3x2, was a magnitude 5.0 at 19:06:05 UTC with a reviewed moment-magnitude solution built from 120 phases. Two and a half hours later came event us6000u3yj, a magnitude 5.3 at 21:38:55 UTC, followed 20 minutes after that by event us6000u3yp, a magnitude 5.0 at 21:58:48 UTC.
The third differs in method: its magnitude is a body-wave value, labeled mb, whereas the first two are moment magnitudes, labeled Mww. Each is listed at a depth of 10 km, a figure the USGS marks as assigned by an operator rather than constrained by the data, so the true depths could differ.
The spacing is uneven. About two hours and 33 minutes separate the first and second shocks, while the second and third are only about 20 minutes apart, so two of the three arrived within a single half hour in the evening of Oct. 21 UTC, all three within about two hours and 52 minutes of one another. The USGS lists the 5.3 as located with 107 stations and the final 5.0 with 96, against the 120 behind the first event.
What the first shock’s moment tensor shows
The reviewed detail for the first event includes a moment-tensor solution with a derived depth of 11.5 km and a scalar moment of 4.49 times 10 to the 16th newton-meters. The double-couple share is only about 21 percent, which means the solution does not describe a clean slip on one planar fault, and the USGS offers no tectonic summary for the event beyond the place name. Nothing in the record explains why three shocks of this size arrived close together.
The same page lists zero felt reports and a maximum intensity of 1, the lowest rung, along with a tsunami flag of zero and no PAGER alert. That is what a magnitude 5.0 in the middle of the ocean, with no inhabited land nearby, would be expected to produce, and no damage or injury report accompanies any of the three events. The ridge is a spreading boundary under the North Atlantic, where seafloor routinely produces moderate shocks that few people ever feel, so the events matter mainly to seismologists tracking how often such clusters occur.
Iceland’s own earthquake desk and a separate shock on Oct. 19
The Icelandic Met Office publishes automatic earthquake maps for the ridge, and its Reykjanes ridge page labels the results of the last 48 hours as preliminary, with the epicenter map and the magnitude-time graph supplied only as images. No Met Office statement about the three shocks was available in text form, so this account rests on the USGS records, whose times and magnitudes come from the global network rather than from the Icelandic system.
A separate magnitude 5.3 earthquake, logged two days earlier, is a useful contrast. The USGS event for Oct. 19 is described as 116 km west-northwest of Höfn, Iceland, at 14:44 UTC, and its origin is near 64.6 degrees north and 17.5 degrees west, several degrees of latitude north of the ridge cluster. It is a different earthquake in a different place, with the same magnitude as the Oct. 21 midpoint shock, and it should not be counted among the three.
Whether the Oct. 21 shocks form a swarm, a foreshock-mainshock sequence or three independent events is not something the records read can settle. The USGS entries are reviewed, but reviewed status means the analysts confirmed the solutions, not that the cause has been identified. Further shocks, if any, would appear in the same USGS list, and a revision to the 5.3 would change only the numbers, not the fact that three events of magnitude 5 or greater occurred on the single date of Oct. 21.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
Help yourself and the planet: Get The Cool Down →
We may earn a commission if you sign up.
More from Morning Overview
- Lake Powell sank to a record 3,517 feet, nearing the level that stops Glen Canyon Dam’s turbines
- Seven of Earth’s nine planetary boundaries are breached, and all seven are worsening
- The NSA says three phone features should be off whenever you aren’t using them
- Hurricane Hunter radar shows four warning signs that a tropical cyclone is about to strengthen, a University of Miami study found
