The week’s deepest strong quake struck 142 kilometers under Indonesia, west of Tual

Seen from the surface, a magnitude 5.7 earthquake in eastern Indonesia on October 28 barely registered: the U.S. Geological Survey’s ShakeMap put the peak intensity at 3.21, and no one had filed a felt report by the morning after. Seen from below, it was the outlier of the week, a rupture 142 kilometers under the Banda Sea, 271 kilometers west-southwest of Tual. No other event in a four-day global list of 26 earthquakes of magnitude 5.0 and above started anywhere near that deep.

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The gap between the energy released and the shaking felt is the whole story of a quake at this depth, and a disagreement between two agencies over its size makes the picture less tidy than the stated figures suggest.

A hypocentre 142 kilometers down

The USGS puts the origin at 22:06:46 UTC, which was 5:06 a.m. on October 29 in western Indonesia, and lists the depth at 142 kilometers. Within the USGS query window that opens on October 26, the next deepest event was the magnitude 5.4 quake near Guam at 101 kilometers. Most of the other 24 sit at 10 kilometers, the default depth catalogs assign when the data cannot pin one down, or at a few tens of kilometers.

Seismic waves from a source 142 kilometers down must travel at least that far before they reach anyone standing directly above it, and the offshore epicentre adds more. Amplitude falls with distance, and the high-frequency shaking that rattles buildings fades fastest. A shallow quake of magnitude 5.7 under land can crack plaster and topple shelves. The same energy released at this depth spreads across a far wider area at a much lower intensity, which is why the ShakeMap figure of 3.21 corresponds to light shaking and why the USGS tsunami flag stayed at zero.

Magnitude 5.7 or 6.4

Indonesia’s own agency read the event differently. When this article was prepared, the BMKG list of recent earthquakes carried an entry at 05:06:47 WIB on October 29, the same second as the USGS origin, with a magnitude of 6.4 and a depth of 160 kilometers, located 60 kilometers northwest of Larat in the Tanimbar Islands. BMKG marked it as felt.

A 0.7-unit gap in magnitude is large, since each step of one unit is roughly a thirty-fold change in released energy. Differences of that size between a regional and a global agency are not unusual in the first hours, when each uses a different magnitude scale and a different set of stations. The depth disagreement runs 18 kilometers in the same direction, with BMKG deeper. Both sets of figures were still preliminary at the time of writing. The claim survives either reading: at 142 or 160 kilometers, it is the deepest event in the window. The felt label is a second point of difference, because the USGS had received no responses from its own online survey while the BMKG list flags the event as felt, a label the USGS record does not mirror.

Why rock that deep can still break

Rock at 142 kilometers is hot and under pressures that should make it flow rather than fracture, yet intermediate-depth earthquakes occur in subducting plates between roughly 30 and 300 kilometers. Laboratory work by Thomas Ferrand and colleagues, published in Nature Communications in 2017, tested one explanation. The team squeezed and deformed synthetic antigorite-olivine rock at upper-mantle conditions as it dehydrated, and recorded acoustic emissions that serve as small-scale analogues of earthquakes. Their conclusion was that “dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.”

The authors are candid that the mechanism is unsettled: they describe the mechanics of these events as “enigmatic.” The ingredients are nonetheless familiar. A plate that has spent tens of millions of years on the seafloor soaks up water into minerals such as antigorite, and as the slab sinks and heats, that water is driven out of the rock at the depths where these earthquakes cluster. The laboratory result suggests the shift itself, not just the escaping fluid, can set off the brittle failure that the surrounding hot mantle would otherwise forbid.

The slab that carries it

The setting is the curved Banda arc, where the Indo-Australian plate descends beneath the Eurasian side of the collision zone. A broadband array study by Miller and colleagues in Seismological Research Letters notes that seismic zones in the sinking slab can be traced to depths below 600 kilometers beneath much of the arc, that the Australian continental margin converges with the arc at roughly 7.5 centimeters a year, and that the slab appears to deform unevenly, possibly tearing near the continental edge. The Miller team also mapped a gap in activity between about 70 and 350 kilometers under Wetar, a reminder that slab seismicity is patchy.

A magnitude 5.7 at 142 kilometers does not resolve any of that. It adds one more point to the catalog of ruptures inside the slab, and the disagreement between the USGS figure of 5.7 and the BMKG figure of 6.4 will be settled only when the agencies publish final moment-tensor solutions that can be compared line by line.

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

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