Months later, earthquake ‘swarm’ continues in Port Heiden

A seismograph at Mission San Juan Bautista in San Juan Bautista, California, on Feb. 23, 2012. The mission is located along the San Andreas fault.
A seismograph at Mission San Juan Bautista in San Juan Bautista, California, on Feb. 23, 2012. The mission is located along the San Andreas fault. (Creative Commons photo by Ray Bouknight)

Earthquakes have rattled through Port Heiden more often than usual this year.

Michael West is the state seismologist with the Alaska Earthquake Center.

“If we look back over the past 15 years or so at earthquake activity in the area of Port Heiden that’s shallow in the earth, what we can say is that the vast majority of those, meaning more than half of them, have occurred thus far in 2016,” West said.

West explains how some earthquakes propagate.

“Sometimes a fault will rupture in a single significant earthquake. But other times, faults will rupture in a lot of smaller earthquakes,” West said.

He calls these quakes near Port Heiden “a swarm.”

“You can think of it as like a little cluster of earthquakes that, added together, might be sort of equivalent to a single, larger earthquake.”

This swarm, West said, began abruptly on April 2 with a magnitude 6.2 earthquake.

“Following that, there was a rather normal, expected sequence of aftershocks. That is hundreds of smaller earthquakes, maybe magnitude 3s, a few magnitude 4s, in response, in response to that earthquake. But the earthquakes in that area have continued over the recent months and kind of peaked again in the summer and are continuing on into today, though at a somewhat lesser rate than in the summer.”

Sometimes swarms stop abruptly, West notes, but more often they fade away. And while the earthquakes in the Port Heiden area seem to have been decreasing since August, it’s also possible that they could pick up again. It’s tricky to know where the swarms are headed.

KDLG - Dillingham

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