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Tectonic Summary

The March 31, 2020, M 6.5 earthquake west of Challis, Idaho (about 120 km northeast of Boise), occurred as the result of complex strike slip faulting within the shallow crust of the North America plate. Preliminary focal mechanism solutions for the event, which describe the style of faulting in an earthquake, indicate slip likely occurred on a steeply dipping fault striking either east-west (right-lateral) or north-south (left-lateral). This earthquake occurred within the Intermountain Seismic Belt, a prominent zone of recorded seismicity in the Intermountain West, and is within the western part of the Centennial Tectonic Belt, an area of southwest-northeast extension north of the Snake River Plain. The quake is about 16 km north-northeast of the Sawtooth fault, a 60-km-long normal fault that extends along the eastern base of the Sawtooth Range.

Historic seismicity in the immediate vicinity of the March 31 earthquake is sparse; no earthquakes of M5+ have occurred within 50 km of this event over the past 50 years, and the most notable historic seismicity in the region occurred about 100 km to the east on the Lost River fault zone. This was the site of the 1983 M6.9 Borah Peak earthquake (October 28, 1983), which was followed by five other M 5+ events over the following year, and most recently a M5.0 earthquake in January 2015, about 60 km to the east of today’s event. The March 31, 2020 event is the largest in Idaho since the Borah Peak earthquake. That event killed 2 in Challis, and resulted in over $12M in damage in the Challis-Mackay area. As of 1 hour after this earthquake, two aftershocks (M 4.6, M 3.6) have been located by the USGS, both to the south of the M 6.5 event.

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Tectonic Summary

The March 26, 2020 M 5.0 west Texas earthquake occurred as the result of shallow normal faulting about 40 km west of the town of Mentone, Texas. The earthquake occurred within the interior of the North America plate, far from any plate boundaries. The preliminary focal mechanism solution for the earthquake indicates rupture occurred on a moderately dipping fault plane striking roughly east-west. The earthquake location aligns with old faults in the basement and is nearly 75 km east of currently mapped north-south oriented Quaternary active faults. Further analysis and possibly fieldwork are required to associate this earthquake with an existing or unmapped fault.

On Aug 1, 1975, a magnitude 4.8 earthquake occurred approximately 30 km to the south of this earthquake; however, a detailed history of small (less than magnitude 3) earthquakes in this region is not well know because the region was not well covered by seismometers until recent years.

Locations across the central and eastern United States (CEUS) have been experiencing an increase in the number of earthquakes over the past two decades. Scientific studies have linked most of this increase to human (induced) activity, predominantly related to wastewater injection in deep disposal wells in several locations. However, other mechanisms such as fluid withdrawal, enhanced oil recovery, or hydraulic fracturing processes can also result in induced earthquakes. In addition, regions with frequent induced events may also be subject to damaging earthquakes that would have occurred independently of human activity.

Making a strong scientific case for a causative link between a particular human activity and a particular sequence of earthquakes typically involves special studies devoted specifically to the question. Such investigations usually address the process by which the suspected triggering activity might have significantly altered stresses in the rocks at the earthquake source, and they commonly address the ways in which the characteristics of the suspected human triggered earthquakes differ from the characteristics of natural earthquakes in the region.

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Tectonic Summary

The March 25, 2020, M 7.5 earthquake east of the Kuril Islands occurred as a result of shallow reverse faulting within the Pacific plate directly adjacent to (or beneath) the Kuril trench, where the Pacific plate subducts beneath the Okhotsk microplate. The Okhotsk microplate is itself part of the larger North America plate. Focal mechanism solutions indicate that rupture occurred on either a northwest- or southeast-striking, moderately dipping reverse fault. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting. In this region, the Pacific plate moves northwest with respect to the Okhotsk plate at a velocity of about 86 mm/yr (~80 mm/yr with respect to North America).

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the March 25, 2020, earthquake are typically about 80×40 km (length x width).

Large and great earthquakes are common in this region. Four other M 7 and larger events have occurred within 250 km of today’s earthquake over the preceding 120 years; all were west of the oceanic trench, on or near the subduction zone interface in the region. Three M 8 and larger events have occurred within 500 km of today’s earthquake; a M 8.3 megathrust event about 425 km to the south in November 2006; a M 8.1 normal faulting earthquake just east of the trench adjacent to the November 2006 earthquake; and the M 9.0 November 1952 Kamchatka megathrust earthquake, about 430 km to the north. Damage in relation to these events was almost exclusively tsunami-related. In January 2009, a M 7.4 reverse faulting earthquake occurred at a depth of about 35 km just to the east of the oceanic trench, 300 km to the south of the March 25, 2020 earthquake. The location, depth and mechanism of that event make it most analogous to today’s earthquake. The 2009 event is not known to have caused damage, or to have generated a significant tsunami.

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Tectonic Summary

The March 18, 2020 M5.7 earthquake north of Magna, Utah (just west of Salt Lake City) occurred as the result of normal faulting in the shallow crust of the North America plate. The focal mechanism solution for the earthquake indicates slip occurred on a moderately dipping fault striking either to the northwest, or to the south-southeast. The style, location, and depth of slip are consistent with an earthquake on the complex Wasatch fault system. This earthquake is located in the Intermountain seismic belt, a prominent north-south-trending zone of recorded seismicity in the Intermountain West, including the Wasatch Front urban corridor. The ISB region is 1000 km east of the primary North America–Pacific plate boundary which runs along the west coast of the United States.

The Wasatch Front, encompassing Salt Lake City, has experienced infrequent, moderate-to-large earthquakes in the past. There are 26 documented M5+ earthquakes within 250 km of the March 18, 2020 event in the combined University of Utah Seismograph Stations and USGS earthquake catalog, which stretches back to the late 19th century. The largest recorded earthquake was a M6.6 earthquake in March 1934, in Hansel Valley on the north shore of the Great Salt Lake. In September 1962, a M5.0 earthquake occurred in a very similar location to today’s M 5.7 event, with strong shaking observed locally. Geologic investigations of the Wasatch fault indicate that large (M ~7) earthquakes occur about every 1300 years near Salt Lake City, with the most recent large earthquake about 1400 years ago.

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