Aftershocks Persist in Wake of Bigger Quakes

Aftershocks from strong earthquakes last week continued to ripple throughout California over the weekend.

A magnitude 5.0 earthquake struck 129 miles off the coast of Eureka at 2:27 a.m. Sunday, just south of the epicenter of a 7.2 magnitude quake Tuesday night that prompted a tsunami warning for coastal areas from British Columbia to the Mexican border.

A magnitude 3.9 quake rumbled nearby about five hours later. The Humboldt County Sheriff’s Department received no immediate reports of damage.

Following a 5.2 quake near Anza in Riverside County and a 4.9 temblor near Yucaipa in San Bernardino County, several so-called micro-earthquakes between 2.6 and 1.0 shook the area Saturday night, experts said.

“It’s typical that we’ll have aftershocks and they will continue for some time,” said Caltech seismologist Anthony Guarino.

Major earthquakes can often cause smaller aftershocks for years, he said. “We just stopped having aftershocks last year sometime” related to the 1994 Northridge earthquake, which had a magnitude of 6.7.

Since the 7.2 quake rocked the Juan de Fuca plate off the coast of Eureka, there have been 15 temblors above 3.0 in the area, including a 6.7 quake Thursday night, Guarino said.

It is an active area, where the Juan de Fuca, North American and Pacific Plates all come together, the seismologist said.

The quake Sunday morning was too small to generate a tsunami warning, Guarino said.

Officials at the West Coast and Alaska Tsunami Warning Center in Palmer, Alaska, had issued a warning Tuesday night within minutes of the earthquake because it was above 7.0. They canceled the warning about an hour later, when scientists determined that the horizontal movement of the fault did not change the level of the ocean floor.

“There have been instances in the past where a magnitude 6.2 or 6.5 have created a tsunami,” Guarino said, but only because they created landslides under water. “With a magnitude 5, we wouldn’t expect that to happen.”