A magnitude 4.2 quake rattled Puget Sound near Gig Harbor, with a 37% chance of an aftershock this week

A magnitude 4.2 earthquake shook the Puget Sound lowlands at 2:05 p.m. Pacific time on Tuesday, September 29, its epicenter about 5 kilometres west-northwest of the community of Wauna and just northwest of Gig Harbor, Washington. The U.S. Geological Survey put the rupture roughly 28 kilometres underground, and its forecast gave a 37% chance of a magnitude 3 or larger aftershock within the coming week.

No injuries or damage were reported in the first hours. The council in Seattle paused its afternoon meeting and then hurried through the agenda. Pierce County Emergency Management reported no damage or impacts as of 3 p.m., and the National Tsunami Warning Center ruled out any tsunami risk for the Seattle area.

A quake 28 kilometres down, under a basin of millions

The USGS event record lists a magnitude of 4.17, rounded to 4.2, a depth of 27.88 kilometres and an origin time of 21:05:56 UTC, with no tsunami. The agency’s significant-events feed later showed more than 8,000 felt reports filed through its public felt-report system, a count that had stood under 2,000 in an earlier reading of the same record and was still climbing. Community-reported intensity averaged 3.8 on a scale that tops out at 10, in the range of weak shaking.

Residents described something stronger than the number suggests. Stephanie Holloway told KUOW the quake “shook the whole house and the chandelier in the living room,” and Courtney Clark Childers said her house “just shook and shook” her back and forth, having assumed at first that it was routine military training in the area. The KUOW report also carries the aftershock odds that give this event its wider interest.

What the 37% number counts, and what it leaves out

The 37% figure is the probability of at least one aftershock of magnitude 3 or larger between now and the end of the week, according to the USGS forecast as relayed by KUOW. The same forecast, as reported, gave an 8% chance of a magnitude 4 aftershock and less than 1% for anything bigger than the mainshock. A magnitude 3 quake is typically felt only by people close to the epicenter and rarely breaks anything.

The numbers come from the Reasenberg-Jones model that the USGS runs for every significant sequence. Its method page explains that the model starts from generic parameters drawn from comparable tectonic regions, and that aftershock rates fall roughly in proportion to elapsed time, so there are about ten times as many aftershocks on the first day as on the tenth. Odds for a first week are therefore front-loaded, and by that logic the largest share of the 37% falls in the first day or two after the mainshock. A quiet Friday would not mean the forecast had been wrong, because a 37% chance leaves a 63% chance of no such aftershock at all.

One comparison is telling. The USGS monthly significant-events feed lists a much smaller magnitude 3.8 earthquake near Castle Rock, Washington, at 18 kilometres depth, which still drew 1,025 felt reports. Western Washington reports its quakes eagerly, and the Puget Sound count on Tuesday was a multiple of that.

The ShakeAlert decision and the question of which fault

KOMO reported that the ShakeAlert early warning system “automatically detected the earthquake and (correctly) deemed it too small” to send alerts to cell phones. Sound Transit had 1 Line trains share a single track at Angle Lake Station while the system checked for damage.

Harold Tobin, director of the Pacific Northwest Seismic Network, told the Spokesman-Review that the region is earthquake country and that a damaging quake remains possible, though Tuesday’s was not one, and that smaller aftershocks are likely while a larger one is improbable. The same story placed the source on the Tacoma fault. At 28 kilometres, though, the rupture sits at the shallow edge of the 30-to-70-kilometre band the seismic network gives for deep intraplate events, and the reports available on Tuesday night did not settle which structure slipped.

Why deep quakes here have done the most damage

The seismic network’s own description of deep earthquakes attributes them to stress inside the Juan de Fuca plate as it descends beneath North America. Because the energy spreads out from that depth, the resulting shaking is widespread rather than concentrated, which is why a magnitude 4.2 could be felt from Olympia to Everett.

The record for larger versions is costly. A 1949 event caused more than $100 million in damage, including harm to the Capitol building in Olympia. A 1965 quake caused more than $50 million, and the 2001 Nisqually earthquake, magnitude 6.8, caused about $2 billion. The network estimates that deep quakes strike damagingly about every 30 years and puts the chance of a magnitude 6.5 or larger in Washington within 50 years at 85%, a figure that Tuesday’s small event neither raises nor lowers.

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

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