5.8-magnitude quake and swarm rattle waters off Oregon coast, USGS warns of aftershocks

A 5.8-magnitude earthquake struck off the coast of Oregon late Monday, followed by a series of smaller tremors that continued into Tuesday, the U.S. Geological Survey said.

The USGS reported the first tremor at 10:30 p.m. local time (1:30 a.m. ET) about 102 miles west-southwest of Port Orford and identified it as "the potential mainshock of an earthquake sequence." At least five additional quakes were recorded in the same area, the strongest measuring magnitude 4.9. The agency estimated a 6% chance that one or more aftershocks stronger than magnitude 5 could occur in the next week and said as many as 50 aftershocks of magnitude 3 or higher were possible. One such tremor, magnitude 3.1, struck at 1:05 a.m. local time (4:05 a.m. ET) on Tuesday.

The earthquakes occurred along the Cascadia Subduction Zone, where the oceanic Juan de Fuca Plate is being forced beneath the North American Plate. That fault system is known for producing frequent small offshore tremors and poses a long-recognized risk of very large, destructive earthquakes. Scientists say major earthquakes of magnitude 8.0 or larger have occurred in the region every 400 to 600 years, with the most recent coming in 1700.

Research published in April by Virginia Tech underscores the potential consequences of a very large Cascadia event when combined with rising sea levels. The study found that an earthquake of magnitude 8.0 or higher could cause coastal land to subside by as much as 6.5 feet within 30 minutes, expanding the coastal floodplain in parts of southern Washington, northern Oregon and northern California. The authors estimated that such subsidence would enlarge the one-percent-annual-chance floodplain from 35 square miles to 116 square miles, putting an additional 14,350 residents, 22,500 structures and 777 miles of roadway at risk.

The seismic activity on Monday came hours after Oregon Gov. Tina Kotek announced new seismic safety rules for the state. The order requires all new state-owned buildings larger than 10,000 square feet to meet the strictest earthquake standards, and mandates that older state-owned structures be upgraded to current life-safety codes by 2060. "Preparing for an earthquake is a long game," Kotek said. "If we look at 50 years, we can spread that cost out—replace or retrofit two per cent of our buildings each year, and in 50 years we'll have upgraded every single one. Yes, it's a multibillion-dollar effort, but planning it over decades makes the funding challenge more manageable."

State Rep. Dacia Grayber, a first responder and advocate for earthquake readiness, said the region has not yet experienced its worst-case earthquake and urged sustained preparation. "One of the things that keeps me up at night is … we are the only post-industrial region pretty much in the world that hasn't lived through our worst natural disaster," she said. "We've seen this train barreling down on us, yet we only react when emergencies start to personally affect us."

The USGS continues to monitor the sequence and will update assessments of aftershock probabilities as new data arrive. Seismic activity along the Cascadia margin remains an area of active study because of its potential for widespread impacts on coastal communities and infrastructure.