A magnitude 4.2 under Puget Sound drew 9,401 ‘did you feel it’ reports

A magnitude 4.2 earthquake that struck beneath the Puget Sound region on Tuesday afternoon, September 29, produced 9,401 felt reports to the U.S. Geological Survey. The shaking began at 21:05:56 UTC, which was 2:05 p.m. in Washington, and the rupture started about 28 kilometers, or 17 miles, underground. No injuries or significant damage were reported.

For a quake of that size, the volume of reports is the more telling number. A magnitude 4.2 is a moderate event, yet thousands of residents across the region took the time to tell the USGS what they felt, and the sheer count shows how widely a quake at that depth was noticed from Seattle to Olympia.

Epicenter, depth and magnitude on the USGS record

The USGS event record places the epicenter 6 kilometers west-northwest of Wauna, Washington, on the Kitsap Peninsula, at a depth of 27.92 kilometers. The agency lists the magnitude as 4.2 on the local scale, though the computed value is 4.15, and reviewed status means a seismologist has checked the automatic solution. The first automatic estimate was 4.3, Fox 13 Seattle reported, before it was revised downward. Fox 13’s separate Kitsap report placed the depth at about 17.3 miles and the epicenter roughly 10 miles northwest of Gig Harbor, while Gig Harbor Now gave a slightly different 5 kilometers northwest of Wauna, so the quoted location varies slightly between outlets.

The USGS record carries a green alert level and no tsunami flag. Its community intensity is 3.8 and its instrumental intensity, a figure computed from seismometer data, is 3.452, both in the range of weak shaking on the Modified Mercalli scale.

Two numbers on that record measure different things. Magnitude describes the energy released at the source, while intensity describes how hard the ground shook at a given place. The community value of 3.8 is built from what people reported, and the instrumental value of 3.452 is computed from seismometer readings, so a gap between them is expected rather than a discrepancy. Both sit in the weak-shaking range, which fits the pattern of reports: widely noticed, rarely damaging, and over in a few seconds.

The 9,401 felt reports

The felt-report count comes from a USGS system in which people submit what they experienced, and the agency converts the answers into intensity values by location. The count for this event stood at 9,401 reports when the record was last read, after an update on September 30. Fox 13 had cited more than 9,200 earlier, so the total climbed as late responses arrived.

The reports came from across the region. Gig Harbor Now said social media users described the shaking from as far away as Seattle and Olympia, and Fox 13 listed Tacoma and Bremerton among the places with weak shaking, while many other people felt nothing.

Harold Tobin, director of the Pacific Northwest Seismic Network at the University of Washington, told KIRO 7 that the quake was one of the bigger ones in the area since March 2025. Fox 13’s separate Kitsap report calls it the strongest quake in western Washington since 2023 on the Olympic Peninsula, a slightly different benchmark from Tobin’s. The shaking lasted about two to three seconds, and no wireless emergency alert went out because the event fell below the alert threshold. Krista Carlson of the City of SeaTac said it felt like a shudder, and the quake was felt at the Angle Lake light rail station.

Aftershock odds and the regional context

Tobin said the most likely scenario is that this 4.2 is a 4.2, and he advised the first response be to drop, cover and hold on. USGS aftershock forecasts were revised in the days after the quake. One report cited a 38 percent chance of a magnitude 3 or larger aftershock within a week, per Fox 13’s first story, while a later Fox 13 article gave 25 percent for magnitude 3 and 4 percent for magnitude 4 or larger within a week, and a 1-in-200 chance of a magnitude 5. Forecast percentages of this kind are re-run as the sequence unfolds, which is why the figures changed between the two stories, and neither number is a prediction that such an aftershock will occur.

The same coverage notes the area’s history: the 6.8 Nisqually earthquake of 2001, the 6.5 Puget Sound earthquake of 1965 and the 6.8 Olympia earthquake of 1949. Against that record, a 4.2 is a small event, and the regional picture that week was busy: Fox 13 counted a 2.6 on the western flank of Mount Adams on September 27 and three quakes of magnitude 4.9, 5.4 and 5.2 near Vancouver Island on September 27 and 28, none of them tied by the reports to the Kitsap event.

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

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