Three earthquakes of magnitude 6.3 or stronger have struck off Vanuatu in the past week, and the strongest, a magnitude 6.9, hit on October 24, according to the U.S. Geological Survey. The other two both measured 6.3: one on October 20 and one on October 24, a few minutes after the 6.9. All three carry a green alert level in the agency’s records, and none of the entries describes damage or casualties.
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Four days separate the first quake from the second, and the second and third arrived about four and a half minutes apart. Each of the three titles in the USGS catalog measures its distance from Port-Olry, so the figures compare directly.
The three quakes, event by event
The first came on October 20 at 04:20 UTC. The USGS lists it as a magnitude 6.3 earthquake 197 kilometers northwest of Port-Olry, at a depth of 16 kilometers, measured as a moment magnitude. Its record carries a tsunami value of 1 and links to a tsunami information statement from the National Tsunami Warning Center, and the feed counts one felt report and an estimated shaking intensity of about 4.1 on the Modified Mercalli scale.
The largest followed on October 24 at 02:31 UTC. The agency titles it “M 6.9 – 197 km WNW of Port-Olry, Vanuatu” and places it at a depth of 10 kilometers, a figure a USGS analyst fixed in the reviewed solution instead of computing from the waveforms. Four people filed felt reports, and the estimated shaking intensity is about 5.1. The record again carries a tsunami value of 1 and a green alert level, which is the lowest tier on the agency’s loss-estimate scale.
Only 4 minutes and 29 seconds later, at 02:35 UTC on the same day, came a second magnitude 6.3 shock, 170 kilometers northwest of Port-Olry. Its depth is also listed as 10 kilometers, but unlike the other two it was sized with a body-wave magnitude instead of a moment magnitude, and its tsunami value is 0. No felt reports are attached to it, and its estimated intensity is about 4.5.
The three reviewed solutions rest on different amounts of data. The October 20 quake was located using 138 seismic stations, the 6.9 used 57, and the second 6.3 used 69, according to the station counts in the feed. The record also flags the 6.9’s 10-kilometer depth as assigned by an analyst, so that figure is a working value and not a measured one.
What the tsunami flag and the green alert mean
The tsunami field in the feed is an integer, and the USGS documentation lists it without defining it. In the records themselves the pattern is plain enough: two of the three events, the October 20 shock and the 6.9, carry a value of 1, and the 6.3 that followed the 6.9 carries 0. The feed gives no wave heights, no run-up measurements and no coastal observations, so the flag by itself says only that the tsunami product is attached to those two records.
The green alert sits in a separate field that comes from the agency’s PAGER system, which estimates likely fatalities and economic losses. Green is the lowest category, and all three Vanuatu events hold it. That label is an automated first estimate and can change as the agency reviews an event, but all three records in the feed are marked as reviewed. For the 6.9, the record holds only the location, the depth, the four felt reports and the intensity estimate.
Three of the week’s five significant quakes
The seven-day list of significant earthquakes, generated by the USGS on the evening of October 26, holds only five events from anywhere in the world. Three of them are the Vanuatu quakes. The other two are a magnitude 7.0 near Nagqu in China, on October 23, which carries an orange alert level, and a magnitude 6.5 in the Bonin Islands region of Japan on October 26, which struck at a depth of 492 kilometers.
The Vanuatu entries carry significance scores of 611, 734 and 611 in the feed. Three of five slots in a global weekly list going to one stretch of the Pacific says how active that margin was across these seven days, and it says nothing about what comes next.
The two other entries show how differently the same list can read. The Nagqu quake struck at a depth of 18 kilometers on land, carries a tsunami value of 0 and an orange alert, a tier above green. The Bonin Islands event was deeper than any of the Vanuatu shocks by a wide margin, yet it still carries a tsunami value of 1 and a green alert. Neither pattern follows magnitude alone, which is why the individual records matter more than the peak magnitude.
The feed is a rolling window, so these entries will drop off the list within days as the week advances. The individual event pages stay available, and the USGS can revise magnitudes, depths and alert levels after further review. For now, the record shows three quakes, one of magnitude 6.9 and two of 6.3, between October 20 and October 24, with a tsunami flag on two of them and no reported damage in any of the entries.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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