Yellowstone records strongest swarm of earthquakes since 2017

July saw the region’s most vigorous swarm of earthquakes since the Maple Creek earthquake swarm of June-September 2017, the Yellowstone Volcano Observatory (YVO) reported in its monthly report, released Aug. 2, 2021.

While this level of seismicity is above average, it is not unprecedented and does not reflect magmatic activity, YVO noted.

Earthquakes in Yellowstone are mostly caused by movements along preexisting faults and can be stimulated by increased pore pressure due to groundwater recharge from snowmelt. If magmatic activity were the cause of the earthquakes, we would expect to see other indicators, such as changes in deformation style or thermal/gas emissions, but no such changes have been detected.

In July 2021 there was only one eruption of Stimboat Geyser, so the total number of eruptions for the year is 13. The time between eruptions has increased in the last few months, which may indicate that the current period of frequent eruptions is gradually coming to an end.

In July 2021, the University of Utah seismographic stations responsible for operating and analyzing Yellowstone’s seismic network recorded 1,008 earthquakes in the Yellowstone National Park area. This number is preliminary and is likely to increase as dozens more small earthquakes since July 16 require further analysis.

This is the highest number of earthquakes in a month since June 2017, when more than 1,100 earthquakes were recorded.

The largest event of the past month was a small magnitude 3.6 earthquake that occurred 17.7 km (11 miles) below Yellowstone Lake, 11.9 km (7.4 miles) southeast of Fisherman’s Bridge at 6:45 p.m. MDT on July 16. This event was part of a vigorous sequence of earthquakes in the same area that began on July 16.

The July seismicity in Yellowstone was marked by seven swarms of earthquakes:

1) A swarm of 764 earthquakes occurred below Yellowstone Lake (with several small earthquakes from July 16 still needing analysis). It began on July 16 and includes the largest event of the month (magnitude 3.6, detailed above). This swarm consists of four earthquakes of magnitude 3 and 85 earthquakes of magnitude 2. These numbers will be updated once analysis is complete. This sequence has decreased to a few earthquakes per day, but may continue into August.

2) A swarm of 40 earthquakes ~19 km (12 miles) east-east of West Yellowstone, MT, began on July 19, and the largest event (M2.1) occurred on July 23 at 22:20 MDT.

3) A series of 34 earthquakes ~17.7 km (11 mi) northeast of West Yellowstone, MT, continued into July after a swarm that began on June 19. The largest July event (magnitude 1.5) occurred at 19:12 MDT on June 30 at ~18.5 km (11.5 mi) northeast of West Yellowstone, MT.

4) A series of 24 earthquakes at ~17.7 km northeast of West Yellowstone, MT, continued through July 3 (MDT) from a swarm that began June 9. The largest July event (magnitude 2.6) occurred on July 3 at 07:31 MDT, ~23.3 km (14.5 mi) northeast of Old Faithful, Yellowstone National Park.

5) A swarm of 20 earthquakes occurred July 29 – 31 north of Norris Geyser Basin in Yellowstone National Park. The largest event (M1.9) occurred on July 29 at 09:31 MDT, ~4.8 km (3 miles) north of Norris Geyser Basin.

6) A small swarm of 14 earthquakes occurred July 9 – 10. The largest, M2.0, occurred at 12:08 MDT on July 10, 24.9 km (15.5 mi) south of Old Faithful, Yellowstone National Park.

7) A small swarm of 12 earthquakes occurred July 10 – 15. The largest was M2.7 at 11:35 MDT on July 12, 24.1 km (15 miles) northeast of Old Faithful, Yellowstone National Park.

Yellowstone’s caldera subsidence, which has been ongoing since 2015, has paused during the summer months, reflecting seasonal groundwater recharge, YVO said.

Each summer, water from snowmelt causes the ground to swell slightly, resulting in a suspension of subsidence trends or even a slight rise (less than 1 cm/fraction of an inch). In the Norris Geyser Basin area, a nearby GPS station has not recorded significant ground uplift or subsidence since early 2020.

No deformation associated with a vigorous earthquake swarm beneath Yellowstone Lake has been noted.

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