The month of July saw the most energetic swarm of earthquakes in the region since the Maple Creek earthquake swarm of June-September 2017, the Yellowstone Volcano Observatory (YVO) reported in their monthly update released August 2, 2021.
While above average, this level of seismicity is not unprecedented, and it does not reflect the magmatic activity, YVO said.
Earthquakes at Yellowstone are dominantly caused by motion on preexisting faults and can be stimulated by increases in pore pressure due to groundwater recharge from snowmelt. If magmatic activity were the cause of the quakes, we would expect to see other indicators, like changes in deformation style or thermal/gas emissions, but no such variations were detected.
There was only one eruption of Steamboat Geyser in July 2021, so the total number of eruptions for the year is now 13. Over the past few months, the time between eruptions has increased, which may indicate that the present period of frequent eruptions is coming to a gradual close.
In July 2021, the University of Utah Seismograph Stations, responsible for the operation and analysis of the Yellowstone Seismic Network, located 1 008 earthquakes in the Yellowstone National Park region. This number is preliminary and will likely increase, since dozens more small earthquakes, from July 16, require further analysis.
This is the most earthquakes in a month since June 2017, when over 1 100 earthquakes were located.
The largest event of the past month was a minor earthquake of magnitude 3.6 located 17.7 km (11 miles) beneath Yellowstone Lake, 11.9 km (7.4 miles) SSE of Fishing Bridge at 18:45 MDT on July 16. This event was part of an energetic sequence of earthquakes in the same area that began on July 16.
July seismicity in Yellowstone was marked by seven earthquake swarms:
1) A swarm of 764 earthquakes occurred beneath Yellowstone Lake (with several small earthquakes from July 16 still requiring 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 in the magnitude 3 range and 85 in the magnitude 2 range. These numbers will be updated upon completion of the analysis. This sequence has decreased to only a few earthquakes per day but may continue into August.
2) A swarm of 40 earthquakes, ~19 km (12 miles) ENE of West Yellowstone, MT, began on July 19, with the largest event (M2.1) occurring on July 23 at 22:20 MDT.
3) A series of 34 earthquakes ~17.7 km (11 miles) NE of West Yellowstone, MT, continued into July from a swarm that began on June 19. The largest July event (magnitude 1.5) occurred at 19:12 MDT on June 30, ~18.5 km (11.5 miles) NE of West Yellowstone, MT.
4) A series of 24 earthquakes ~17.7 km NE of West Yellowstone, MT, continued up to July 3rd (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 miles) NNE of Old Faithful, Yellowstone National Park.
5) A swarm of 20 earthquakes occurred July 29 - 31 N of Norris Geyser Basin in Yellowstone National Park. The largest event (M1.9) was on July 29 at 09:31 MDT, ~4.8 km (3 miles) N of Norris Geyser Basin.
6) A small swarm of 14 earthquakes occurred July 9 - 10. The largest, M2.0, took place at 12:08 MDT on July 10, located 24.9 km (15.5 miles) S 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) NNE of Old Faithful, Yellowstone National Park.
Subsidence of Yellowstone Caldera, which has been ongoing since 2015, has paused during the summer months, reflecting seasonal groundwater recharge, YVO said.
Every summer, water from snowmelt causes the ground to swell slightly, resulting in a pause in subsidence trends or even a minor amount of uplift (less than 1 cm / fraction of an inch). In the area of Norris Geyser Basin, no significant uplift or subsidence has been detected by a nearby GPS station since the start of 2020.
No deformation associated with the energetic earthquake swarm beneath Yellowstone Lake was noted.
Featured image credit: USGS/YVO
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