How many earthquakes have we had in the Bluffdale area?
The University of Utah Seismograph Stations (UUSS) has located 160 earthquakes that occurred in the Bluffdale, Utah, area from February 13 until 10:00 am MST on March 5 (Figure 1). The largest of these earthquakes was the magnitude (M) 3.7 mainshock that occurred at 5:09 am MST on Friday, February 15. Of the remaining 159 earthquakes, 13 occurred before the M 3.7 and, in retrospect, are considered to be foreshocks. The largest foreshock, and the only one larger than M 2.0, was an M 3.2 event that occurred seven minutes before the mainshock. 146 of the earthquakes are aftershocks. The largest aftershock was an M 3.1 event that occurred on Saturday, February 23, at 2:31 am MST. There have been seven aftershocks of M 2.0 and larger, including the M 3.1.
Was the M 4.0 earthquake that occurred on Wednesday, February 20, near the town of Kanosh in Central Utah related to the recent Bluffdale earthquakes?
No, the February 20 M 4.0 earthquake in central Utah is not related to the Bluffdale earthquakes. The distance between these areas of recent earthquake activity is more than 120 miles. The M 3.7 Bluffdale mainshock was too small to trigger other earthquakes at such a large distance.
Are these earthquakes occurring on the Wasatch Fault?
Within the uncertainties in the data, it is possible that the Bluffdale earthquakes are occurring on the nearby Wasatch fault (Figure 3). However, it is also possible that they are occurring on a minor, unnamed fault. It is generally difficult to know for sure which fault an earthquake is on, due to uncertainties in the locations of both faults and earthquakes below the ground surface. The main exceptions are when an earthquake is large enough for the fault displacement that caused the earthquake to break the ground surface and create a fault scarp. In Utah, an earthquake usually needs to be larger than M 6.0-6.5 for a surface break to occur.
Do these small earthquakes make a big one less likely?
No, small earthquakes do not relieve enough stress buildup in the earth to reduce the likelihood of a large earthquake. In fact, every earthquake that occurs has a small, roughly one-in twenty, chance of being a foreshock to a larger earthquake within five days. A “Larger” earthquake means any earthquake bigger than the one that just occurred, even if it is only 0.1 magnitude units bigger. The probability of an earthquake being a foreshock to an earthquake that is one or two magnitude units larger is much smaller than one-in-twenty.
Are the recent Blufdale earthquakes unusual?
No, not in the context of statewide earthquake activity. Small earthquakes occur every day in Utah, although most of them are too small or too far from population centers to be felt. On the average, the Utah region has one M ≥ 4.0 earthquake per year and one M ≥ 3.0 earthquake per month, not counting foreshocks and aftershocks. The 2019 Bluffdale earthquakes are within an east-west trending band of seismicity across the southern end of the Salt Lake Valley that has had earthquakes off and on since at least 1971, including events of M 4.1 in 1992 and M 3.2 in 2016 (Figure 3). The recent earthquakes near Bluffdale serve as a reminder that Utah is earthquake country and a large, damaging earthquake could occur at any time. Therefore, everyone living in Utah should strive to be prepared for large earthquakes.
What can I do to be prepared?
An excellent source of information on earthquake preparedness is the publication “Putting Down Roots in Earthquake Country”.