Today’s M4.2 aftershock: Is this normal?

Jackie DewolfeRecent earthquakes

We will continue to update this post with questions and more information.

This shakemap for tonight’s M4.2 aftershock shows that shaking would have been moderate to light depending on one’s location.

What do we know so far?

Utah was shaken again following a magnitude 4.2 aftershock with an epicenter in Magna at 8:56 p.m.

Many Utahns are asking if this is normal to have aftershocks almost a month after the main M5.7 earthquake that happened March 18. We received reports of people feeling shaking in Tooele, Davis, and of course, Salt Lake counties.

Since March, we’ve been telling Utah that we can expect aftershocks for weeks afterward. So is it normal? Yes.

How many earthquakes have there been since March?

Aftershocks can occur for weeks or even months following a mainshock. As of April 13, there have been 1,180 aftershocks, with 38 of those being greater than a M3.0.

This is at least the fourth M4 aftershock since March 18. As expected with events like this, the frequency and consistency of the aftershocks has gradually decreased.  

What about earthquakes in Utah history?

Utah has experienced 17 earthquakes greater than M5.5 since pioneers settled Utah in 1847, and geologic studies of Utah’s faults indicate a long history of repeated large earthquakes of magnitude 6.5 and greater prior to settlement.

How do we know today’s quake was an aftershock and not a new earthquake?

Remember that all aftershocks are earthquakes. But because this particular earthquake has a similar epicenter as the March Magna quake, it’s part of the same sequence, and so, an aftershock.

What kind of damage can this cause?

We don’t expect this aftershock to cause any damage, but Salt Lake County will conduct a damage investigation in the morning.

How many aftershocks have we had?

As of early this week, there have been more than 1,000 aftershocks since the March 18 earthquake.

I thought we were all done with earthquakes for a while. Now my nerves are shot.

We know that aftershocks can trigger feelings of anxiety and nervousness. Please know that not only are the aftershocks normal and expected, the feelings they trigger are also normal. We wrote about earthquake anxiety and some of the ways to help take control of those feelings here.

What is the state doing at this point?

The State Emergency Operations Center is activated to a Level 1 – Full activation, because of our response to the COVID-19 pandemic. We remain activated at a Level 1 and will await any reports from local government. We have not received any damage reports so far. We expect Wednesday will be a day we use to help Utah get ready for the Great Utah ShakeOut on Thursday.

What is the Great Utah ShakeOut?

It’s the best. It’s a day for Utahns everywhere to practice what to do in an earthquake. We hold it on the third Thursday of April each year. So for this year, it’s April 16. We had more than 1 million people participate in 2019.

In March, we heard many stories of people who got up out of their beds and ran. Please don’t do this.

When the earth shakes, here’s what you do:

  • If you’re in bed, stay there and cover your head with your pillow. This will keep you safe, assuming nothing heavy is above or near your bed.
  • If you’re anywhere else, DROP to the ground, take COVER and HOLD ON.

Running or walking during an earthquake can cause you to fall and falls can lead to serious injuries. You also put yourself in danger to be hit by falling or flying objects, which are the most likely ways to be killed in an earthquake. Keep yourself safe and practice with us on April 16, 2020.

How can we do the ShakeOut during the COVID-19 pandemic?

It’s time to get creative.
Teachers can hold a virtual earthquake drill during a Zoom meeting.
Businesses can discuss continuity plans.
Families can still ShakeOut together.
Organizations can also reschedule any of their drills originally planned for April 16. Hold your drill on another date and you can still be counted for 2020. See how you can participate here.

Joe Dougherty is the public information officer for the Utah Division of Emergency Management.