How many earthquakes have we had in the Bluffdale area? The University of Utah Seismograph Stations (UUSS) has located 102 earthquakes that occurred in the Bluffdale, Utah, area from February 13 until noon on February 21. 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 101 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. 88 of the earthquakes are aftershocks. The largest aftershock was an M 2.5 event that occurred on Wednesday, February 20, at 8:08 pm MST. There have been five aftershocks of M 2.0 and larger, including the M 2.5.
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. However, it is also possible that they are occurring on a minor, unnamed fault. It is difficult to say which fault an earthquake is on unless the 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. The probability of a larger earthquake decreases with increasing magnitude difference. For example, the probability of an M 4.0 earthquake being a foreshock to a potentially damaging earthquake of M 5.5 or greater is much smaller than one-in-twenty.
Should I be worried? Small earthquakes occur every day in Utah, although most of them are too small or too far from population centers to be felt. 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.
On September 02, 2017, in eastern Idaho near the town of Soda Springs a magnitude 5.3 earthquake occurred that was widely felt throughout southeastern Idaho and northern Utah. This earthquake has been followed by a very active aftershock sequence. These earthquakes are slightly outside of the University of Utah Seismograph Stations area of responsibility, but the area is of interest to UUSS and the USGS Earthquake Hazards Program.
UUSS in partnership with USGS have deployed two UUSS 3-channel strong-motion systems and six USGS 6-channel seismic systems (broadband and strong-motion) within 50 km of the seismicity.
August 03, 2017 UPDATE: The University of Utah Seismograph Stations (UUSS) is monitoring an earthquake swarm which is currently active on the western edge of Yellowstone National Park. The swarm began on June 12th, 2017 and, as of 13:00 MDT on August 2nd, 2017, is composed of 1,562 events with the largest magnitude of ML 4.4 (MW 4.4) (Figure 1). The swarm consists of one earthquake in the magnitude 4 range, 8 earthquakes in the magnitude 3 range, 134 earthquakes in the magnitude 2 range, 505 earthquakes in the magnitude 1 range, 879 earthquakes in the magnitude 0 range, and 35 earthquakes with magnitudes of less than zero. These events have depths from ~0.0 km to ~14.0 km, relative to sea level. At the time of this report, there were 125 felt reports for the M4.4 event that occurred on June 16, 2017 at 00:48:46.94 UTC (June 15, 2017 at 18:48:46.94 MDT). The M4.4 event has an oblique strike-slip moment tensor solution (Figures 1 & 2). In addition, four other earthquakes in the swarm have been reported felt.
Figure 1. Location of the earthquakes that are part of the swarm as of August 3, 2017 at 01:00 PM MDT (red symbols).
Figure 2. Moment Tensor solution for the M4.4 event showing the fit between data (black) and synthetics (red dashed).
Figure 3. Animation of the June 2017 Yellowstone earthquake swarm. Earthquakes appear as red circles as they happen, then transition to blue. After they have occurred, they appear as black circles. The size of the circles are proportional to the earthquakes magnitude.
Earthquake swarms are common in Yellowstone and, on average, comprise about 50% of the total seismicity in the Yellowstone region.
UUSS will continue to monitor this swarm and will provide updates as necessary.
On April 22, 2017, a magnitude 3.8 earthquake occurred approximately 4 km northwest of Rangely, Colorado at 11:48 AM local time (05:48 PM UTC). There were 15 felt reports from the town of Rangely, CO. Two aftershocks, approximately 1 km NNE of the mainshock, were located by UUSS. The first aftershock (ML 2.6) occurred on April 27 at 03:11 AM local time (09:11 AM UTC), and the second aftershock (ML 3.3) occurred on May 3 at 01:42 AM local time (07:42 AM UTC). Based on the moment tensor solution for the mainshock this was a predominantly strike-slip earthquake on steeply dipping planes with the strike either northwest or northeast. From the distribution of the aftershock locations, we tentatively favor the northeast striking plane. Eighteen earthquakes within 20 km of the mainshock , with magnitude greater than 2.0, have been catalogued since 1962. The largest historical earthquake (ML 4.6, March 20, 1995) was located 2.3 km NE of the 2017 mainshock.
The Rangely area was one of the first focus sites for the study of fluid-induced earthquakes. Some of the first documented induced earthquakes occurred near Rangely in the 1960s and 1970s. During this time water-flood expansion was being used for secondary oil recovery. It was a good place to test the correlation between fluid injection and seismic events with a controlled experiment (Rayleigh et al., 1976), and the experiment showed a direct link. The seismicity during the experiment occurred on a ENE-WSW trending plane. This is rotated from the current seismicity, but the locations of the seismic events have also migrated through time. Water based fluid injection ended in 1983; since 1986 injection of CO2 has been used for secondary oil recovery.
Given the proximity of the recent seismicity to the Rangely Oil Field, it is fair to ask if the recent sequence is also induced. Analysis of this sequence is ongoing, but initial work includes the following results. An STA/LTA detector (detection threshold 3.5) was run across continuous waveforms from the two nearest stations (O20A and RDMU) for the time period April 22, 2017–May 04, 2017. Requiring simultaneous detections on both stations, in order to reduce the number of false detections, resulted in one new detected event that occurred on May 3. Using cross-correlation, we found similar waveforms (CC > 0.5) from the four events (mainshock, two aftershocks, and the new detected event) recorded at station O20A, suggesting possible common source properties.
The lack of close seismic stations makes it difficult to clearly associate these seismic events with oil production efforts.
On February 26, 2016, an earthquake of magnitude 4.0 struck about 31 km (19 miles) east of Jackson, WY at about 5:00 PM local time. The quake was reported felt by over 150 people in the Teton region. There were no reports of any damage. The earthquake occurred in the Gros Ventre range east of Grand Teton National Park near the location of previous seismic activity. In 2010 there was a swarm of earthquakes, including a M4.8 earthquake, ~11 km (7 miles) north of this event.
The Teton region is part of the Intermountain Seismic Belt, a region of relatively high seismicity in the Intermountain West that extends from northern Arizona to western Montana. Most of the seismicity in the Teton region occurs east of Grand Teton National Park in the Gros Ventre range while there is very little earthquake activity on the Teton Fault.
News accounts for this event can be found at the following links: