A magnitude 5.7 quake hit off Italy’s Aeolian Islands, then a 5.2

A magnitude 5.7 earthquake struck the southern Tyrrhenian Sea off Italy’s Aeolian Islands on Oct. 23 at 10:19 UTC, and a magnitude 5.2 followed about three and a half hours later, at 13:55 UTC. The U.S. Geological Survey placed the larger shock 61 kilometers west-northwest of Leni and the second 50 kilometers west-northwest of Malfa, two towns on the island of Salina. Both are reviewed solutions, and neither carries a tsunami flag.

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The pair was not the whole sequence. USGS also lists a magnitude 4.9 shock at 09:51 UTC, 53 kilometers west-northwest of Leni and 28 minutes before the 5.7, and a magnitude 4.7 at 15:32 UTC, 52 kilometers west-northwest of Malfa, after the 5.2. Four of the 22 earthquakes of magnitude 4.5 or larger that USGS listed worldwide from the start of Oct. 23 through early Oct. 24 belonged to this one patch of sea, so anyone reading a single number for the day is reading one point in a tight cluster rather than an isolated event.

What USGS recorded for each shock

For the 5.7, the USGS event record gives a moment magnitude (Mww) of 5.7 at 38.72 degrees north, 14.15 degrees east. The depth is listed as 10 kilometers, a value assigned by the analyst, while the moment tensor solution points to about 11.5 kilometers. Nine people filed felt reports through the agency’s community felt-report system, with a maximum reported intensity of 4.6 on its scale, and the ShakeMap, the agency’s modeled map of ground shaking, peaked at an intensity of about 3.8. The moment tensor is about 99 percent double-couple, with a scalar moment of 3.88 x 10^17 newton-meters.

The 5.2 is thinner on data. The USGS record for the second quake shows a moment magnitude of 5.2, a depth fixed at 10 kilometers, a single felt report with a maximum intensity of 2.7, no alert level and a tsunami flag of zero.

INGV’s Mw 5.5 and why agencies differ

Italy’s own seismic network reads the main shock a little lower. The earthquake list of the National Institute of Geophysics and Volcanology, known as INGV, carries an Mw 5.5 event at 12:19:15 Italian time on Oct. 23, which is 10:19:15 UTC because Italy is still on daylight time until Oct. 25. INGV places it in the Tirreno Meridionale, the southern Tyrrhenian Sea, at 38.79 degrees north, 14.17 degrees east and 9 kilometers deep.

Those coordinates sit within a few kilometers of the USGS epicenter, close enough that the two agencies are plainly describing the same rupture, and the origin times agree to within a second. A gap of 0.2 between a 5.7 and a 5.5 is ordinary when two institutes run their own analyses on different station sets, and it is the reason any magnitude printed for this event should name the agency behind it. Readers comparing Italian and American coverage of the day will see 5.5 in one and 5.7 in the other without either being wrong.

The same comparison works for the earlier shock. INGV’s list shows an ML 4.3 at 11:51:38 Italian time, which appears to correspond to the USGS magnitude 4.9 at 09:51 UTC, a difference of 0.6 that is larger than the one for the main shock. Smaller events are scored with different magnitude scales by the two agencies, and the gap shows how much that choice matters below magnitude 5.

Shaking, alert level and what the records leave open

Only one event in the USGS feed of Oct. 23 carried an alert level, and it was the 5.7, which received a green PAGER rating with an estimated maximum intensity of 4. That is the agency’s automated estimate of potential impact, built from the shaking model, and it is a long way from the intensities that damage buildings. The magnitude 4.9 that came first drew no alert and zero felt reports at the time of the USGS query.

The records read for this article do not describe damage, injuries or disruption on Salina, Lipari or the other islands, and none are asserted here. The small number of felt responses, nine for the largest shock and one for the second, reflects how few people report to the USGS system from this part of Italy and how little of the area is densely populated at sea, so it is a weak guide to how strongly residents of the islands or the Sicilian and Calabrian coasts felt either quake.

Magnitudes of reviewed events can still shift by a tenth or two as agencies update their solutions, and aftershock counts keep growing in the days after a shock of this size. The magnitude 4.7 at 15:32 UTC is an example of the activity that followed the 5.2 within two hours, and together with the 4.9 it makes four shocks of magnitude 4.7 or larger in the cluster on USGS’s list, counting the two headline events.

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

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