Sunday's shock came in at magnitude 5.4, then downgraded to 5.0. Why does that happen?

Image from AEC.

ANCHORAGE (KTUU) - Yet another aftershock rocked Anchorage and the Mat-Su Valley just before 8:00 a.m. Sunday, and was originally released as a magnitude 5.4, then downgraded to 5.0.

Many of the thousands of aftershocks we've seen since the Nov. 30 earthquake have had their magnitude change. Many people have commented on the Channel 2 Facebook page, frustrated and confused over the change in magnitude from one moment to the next.

Seismologist and Associate Professor of Engineering with the University of Alaska Anchorage's Civil Engineering program Utpal Dutta has the answer sought after by many for whom shaking has become the new normal.

He says seismic instruments installed around Anchorage rapidly read the data as the ground shakes, and automatically feed it to seismic data monitors.

"All the stations send the information, so there is automatic determination of the magnitude level," Dutta said. "Which might be a little bit sometimes higher, sometimes lower. But then, a seismologist interferes and checks all of the data."

So machines relay the first magnitude reading, and seismologists re-interpret the seismic data to determine a more precise magnitude.

Apart from the differences in magnitude reports, ground-shaking numbers are adding up for 2019. Sunday morning's shake was one of 1,455 seismic events in Alaska so far this year (just 13 days in), according to the Alaska Earthquake Center.

The last magnitude 5.0 aftershock hit Dec. 31st northwest of Anchorage.

As hard as the ground shook Sunday morning, aftershocks since the Nov. 30 quake have only accounted for around ten percent of the total energy released from the entire seismic sequence, according to AEC.

Dutta says all aftershocks come from the same rupture patch of the original earthquake. He says even though the frequency of aftershocks is slowing down, we will continue to experience them into the foreseeable future.