PORTLAND, Ore. — Despite a swarm of earthquakes off the Oregon coast on Tuesday, officials with the National Weather Service said that there was no threat of tsunami for the region.
The NWS Portland office reported at 2:15 p.m. on Tuesday that there had been two earthquakes reported about 200 to 250 miles west of Newport at magnitude 5.3 and 5.5, neither of them provoking a tsunami warning.
According to data from the US Geological Survey, there had actually been more than a dozen quakes in the same general area on Tuesday, ranging in magnitude from 3.5 to 5.8.
Earthquake swarms are not an uncommon phenomenon, and they are distinct from aftershocks. The latter occur near the fault zone where a mainshock rupture occurs, causing what the USGS calls a "readjustment process" after the main slip of the fault. Though aftershocks tend to get less frequent with time, they can sometimes continue for long periods of time — even for years, in the instance of very large earthquakes.
Swarms consist of many smaller earthquakes with no identifiable mainshock. They tend to be short-lived, and can recur at the same locations. USGS says that they are usually associated with geothermal activity.