None has been large enough to prompt tsunami fears. The sixth and seventh quakes, occurring late Monday Pacific time, were the weakest, measuring 3.9 and 4.2.
The sequence of quakes, while fairly unusual, is not worrisome, said Robert Dziak, a research oceanographer with the National Oceanic and Atmospheric Administration based in Newport, Ore.
The earthquakes occurred on the Blanco Fracture Zone, a 200-mile-long fault beginning about a hundred miles west of Newport.
The fault forms the southern boundary between the Juan de Fuca Plate and the Pacific Plate and frequently generates earthquakes, Dziak said.
In addition to being too small to generate a tsunami, the series of quakes was caused by the plates moving horizontally past each other, he said.
That’s in contrast to Japan’s great earthquake of 2011, which caused a massive tsunami. There, the crust dropped down below another.
In Oregon, the series of quakes took place on the west side of the Juan de Fuca Plate. The Cascadia subduction zone, which threatens a megaquake here, is on the east side of the plate.
“It’s not clear that they’re linked seismologically,” Dziak said. “The link as a precursor event is not clear.”
The first quake, a magnitude 5.8, hit at 11:52 p.m. Pacific Sunday.
It was followed Monday by a magnitude 4.3 at 12:01 a.m.; a magnitude 5.5 at 3:46 a.m. and a magnitude 4.4 at 7:46 a.m.
A magnitude 5.9 quake shook the same area at 1:11 p.m. Monday.
The earthquakes were relatively shallow at about 6 miles deep. It usually takes an earthquake of more than 7 to trigger a tsunami.
The quakes come as the Oregon Legislature is considering whether to fund a $337 million seismic renovation of the State Capitol Building and whether to fund seismic upgrades at schools.