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3.9 Earthquake Prompts Discovery Of New Fault In Box Elder County

Rob McDermott
McDermott measuring layers of sediment in a trench dug across San Andreas Fault. Data like this can be used to construct a history of when quakes occurred in the past - critical for knowing how often big quakes recur in an area.

Last week, the 3.9 earthquake centered near Tremonton came as a big surprise to Box Elder County geologists - caused by one of many unknown faults in northern Utah.

“A couple of weeks ago we had a magnitude 3.9 earthquake that happened right outside the town of Howell, Utah. The interesting thing about that earthquake is there are no mapped faults specifically where that earthquake happened,” said Rob McDermott, a Ph.D. candidate in geosciences at Utah State University.

McDermott said that a magnitude 3.9 earthquake is on the low end of the Richter scale. It equates to a 2-inch movement along the strike-slip interface of the newly discovered fault.

“The fault is there, we just didn’t know it was there. A fault can be a tricky thing to find, and they’re not always well exposed at the surface. There are faults in the area that a geologist can go out and say, ‘Yes, there’s a fault there.' There’s not an active scarp along where the Howell earthquake occurred,” McDermott said.

There are other unknowns in northern Utah’s geology. Because the Wasatch Fault is more dangerous (capable of producing an earthquake with a magnitude of up to 7.5), it holds a lot of the focus. As a consequence, fault maps are out of date. There isn’t scientific consensus on how often big earthquakes happen on Cache and Box Elder County faults. 

“The current active fault map was made in the late 80’s. There’s also faults that we know that they’re there, but we don’t know anything about them. Since then our technology has advanced. We can use certain tools like LIDAR to map faults that are easy to miss in the field. It comes down to how many scientists there are, and funding, really,” McDermott said.