The 6.0 magnitude quake was located on a fault line underneath the Pacific Ocean, 173 miles (279 kilometers) from the city of Bandon in the state of Oregon, the US Geological Survey said.
There were no immediate reports of any damage, and computer modeling by the USGS showed "there are likely to be no affected structures in this region."
Seismologists put the depth of the quake initially at 10 kilometers, a default setting for shallow earthquakes that is frequently revised as more data becomes available.
There are a number of fault lines running along, through and near the US West Coast, which form part of a larger network around the world's biggest ocean known as the Pacific Ring of Fire.
The quake, which was recorded at 2015 GMT, came coincidentally as Oregon was conducting a two-day earthquake preparedness drill.
The drill is called Iron-OR 24, and is intended to give cities an opportunity to test how they would respond in the event of a major quake, which could knock out utilities and transport networks, including by downing bridges.
Starting Tuesday, agencies were working out what impact a 9.0 quake might have, and how they might keep services going, the local ABC affiliate reported.
"We are trying to get better at response generally so every time we gather ourselves in this way, we get to know each other better, we learn more and we get more prepared for any response, any emergency the city may face," the outlet quoted Amanda Westervelt of Portland Bureau of Emergency Management as saying.
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