Sumba, the island near the eastern end of Indonesia’s Sunda arc, took a magnitude 5.9 earthquake offshore on Oct. 3, 2026. The U.S. Geological Survey located it 39 kilometers west-southwest of Tambolaka, the main town of Southwest Sumba, at 22:55:55 UTC on Oct. 3, which was 6:55 a.m. on Oct. 4 local time in Tambolaka. USGS recorded the origin at about 46 kilometers depth.
The agency’s review is complete, with the magnitude computed as a moment magnitude from W-phase waves and the solution built on 82 seismic stations. It assigned a green alert level and recorded no tsunami.
The USGS record for event us6000tzer
The USGS catalog entry for event us6000tzer places the epicenter at 9.567 degrees south and 118.908 degrees east, with a reviewed magnitude of 5.9 and a depth of 45.951 kilometers. The same record carries an estimated maximum intensity of about 5.1 on the Modified Mercalli scale and a community-reported intensity of 4.5 from USGS’s felt-report system. Felt reports were few, and the count shifted between reads of the record, so no total is cited here.
The record also shows a tsunami flag of zero, the green alert level and a review status of reviewed, the label USGS applies once analysts have checked the automatic solution. The solution’s azimuthal gap, the largest angle around the epicenter with no recording station, was 63 degrees.
Moment magnitude and why the number can be trusted
USGS describes the mww magnitude as its authoritative measurement: a centroid moment tensor inversion of the W-phase, the long-period waves lasting roughly 50 to 2,000 seconds. It is computed for all earthquakes of magnitude 5.0 or larger and, when available, supersedes other magnitude types. That is the label on the Tambolaka solution.
Magnitude is a single value, while shaking varies place to place. The agency’s explainer on magnitude, energy and intensity states that “an earthquake has a single magnitude,” whereas shaking depends on distance, surface material and other factors, and that each whole step in magnitude means about 32 times more energy released. Soft ground amplifies shaking more than bedrock, so a 5.9 offshore of a coastal town can feel different from one street to the next, and the intensity figures in the catalog entry are estimates for the region, not measurements at any one address in Tambolaka.
The Sunda arc and the Lesser Sunda convergence
The quake belongs to one of the most active plate boundaries on Earth. In its seismicity summary of Java and vicinity, USGS describes the Sunda margin as 5,600 kilometers long, with the India and Australia plates subducting beneath the Sunda plate at roughly 50 to 70 millimeters a year. The Sunda volcanic arc runs from 105 to 122 degrees east, and the Banda arc picks up from 122 to 128 degrees east, so Sumba sits near the junction where one gives way to the other.
A separate USGS tectonic summary of the Lesser Sunda region adds detail: the Australian plate moves northeastward relative to the Sunda plate at about 75 millimeters a year, and two opposing thrust systems operate in the area, the Java Trench to the south and the Flores Thrust to the north. The summary points to the 1977 magnitude 8.3 event and the December 1992 magnitude 7.8 earthquake about 350 kilometers east of Sumbawa, which generated a tsunami that killed more than 2,500 people.
Depth matters in reading where the energy came from. The same Java-region summary sorts earthquakes into three depth bands: shallow events from 0 to 20 kilometers, which occur within the overlying plate; intermediate events from 70 to 300 kilometers, which occur inside the descending slab; and deep events from 300 to 650 kilometers. A hypocenter near 46 kilometers sits between the first two bands, and the catalog record by itself does not assign it to either mechanism. Assigning a fault type would take the moment tensor solution, which the entry examined here does not detail.
A busy week on the 4.5 feed
The Tambolaka shock was one of 115 earthquakes of magnitude 4.5 or larger listed worldwide in the USGS past-week feed when it was read for this article. Near Sumba, the feed lists a magnitude 4.5 event 39 kilometers southwest of Tambolaka and three magnitude 4.8 to 5.0 events north of Ruteng, on Flores, a couple of hundred kilometers to the north-northeast along the same island chain.
A catalog snapshot of that kind is a count of what instruments recorded, and it says little about damage on the ground. The USGS record for this quake does not report casualties or structural damage, and none of the agency’s products consulted for this article describes any.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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