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Tectonic Summary

The August 9th, 2020 M 5.1 earthquake near Sparta, North Carolina, occurred as a result of oblique-reverse faulting in the upper crust of the North American plate. Focal mechanism solutions for the event indicate rupture occurred on a moderately dipping fault either striking to the northwest or south. This earthquake occurred in the interior of the North American plate. Such mid-plate earthquakes are known as intraplate earthquakes and are generally less common than interplate earthquakes that happen near tectonic plate boundaries. This earthquake was preceded by at least four small foreshocks ranging from M 2.1-2.6, beginning about 25 hours prior to the mainshock.

Large earthquakes are relatively uncommon in the region directly surrounding the August 9th M5.1 earthquake. Moderately damaging earthquakes strike the inland Carolinas every few decades, and smaller earthquakes are felt about once each year or two. In the 20th century, one earthquake M5 and larger occurred within 100 km to this August 9th events, a M5.2 in the Great Smoky Mountains in 1916. The largest recent earthquake to impact the east coast was the M5.8 Mineral Virginia earthquake on August 23rd, 2011, roughly 300 km to the northeast of this August 9th earthquake. The Mineral Virginia earthquake was felt widely across the east coast and caused slight damage.

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Tectonic Summary

The July 22, 2020, M 7.8 earthquake southeast of Perryville, Alaska (south of the Alaska Peninsula), occurred as the result of thrust faulting on or near the subduction zone interface between the Pacific and North America plates. The preliminary focal mechanism solution indicates rupture occurred on a fault dipping either shallowly to the northwest, or steeply to the southeast. The location, mechanism and depth – and the large size of the event – are all consistent with slip occurring on the subduction zone interface between the two plates. At the location of this event, the Pacific plate converges with North America to the northwest at a rate of about 64 mm/yr, subducting at the Alaska-Aleutians trench 125 km to the southeast of the earthquake.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust faulting events of the size of the July 22, 2020 earthquake are typically about 120×50 km (length x width).

Large earthquakes are common in the Alaska-Aleutian subduction zone. Since 1900, 6 other earthquakes M7 and larger have occurred within 250 km of the July 22, 2020 event. The largest of these was a M8.2 earthquake on November 10, 1938, which occurred in an almost identical location as this July 22, 2020 earthquake. The Alaska-Aleutian Trench also hosted the second largest earthquake recorded by modern seismic instrumentation, the M9.2 March 27 1964 earthquake, which ruptured to within about 250 km of this event, at the eastern end of the 1938 rupture area. The 1938 event produced a small tsunami that was recorded both locally and in Hilo, Hawaii. The remote location of the earthquake resulted in little impact to people and infrastructure.

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The July 17, 2020, M 7.0 Papua New Guinea earthquake occurred as a result of oblique normal faulting at an intermediate depth, approximately 80 km beneath eastern Papua New Guinea, near the northern edge of the Australia plate. Focal mechanism solutions indicate that rupture occurred either on a steeply dipping fault striking to the northwest, or a moderately dipping fault striking to the northeast. At the location of the earthquake, the Australia plate moves to the north relative to the Pacific plate at a velocity of about 73 mm/yr. Earthquakes in this geographical region are generally associated with the large-scale convergence of these two major plates and with the complex interactions of several associated microplates, most notably the South Bismarck plate, the Solomon Sea microplate, and the Woodlark plate. The depth of the July 17th earthquake implies that it occurred in subducted or foundered lithosphere.

Papua New Guinea experiences a high rate of seismic activity, with 64 earthquakes occurring of M 6 and larger within 250 km of this event in the past 50 years, 8 of which were M7 or larger. Most of the previous seismicity has occurred to the north of this event. Recently, on July 17th, 2019, a M7.1 earthquake occurred 175 km to the northeast of this July 17th event at a depth of 146 km and destroyed at least 130 homes. A M7.4 earthquake on February 8th, 1987, 200km north of this event caused landslides that resulted in 3 fatalities.

Earthquakes like the July 17th event, with focal depths between 70 and 300 km are commonly termed “intermediate-depth” earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distances from their epicenters.

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