It was also the largest earthquake in 2020 by magnitude. However, research suggests that the seismic gap, contrary to speculation that it might be a relatively "safe" seismic zone in the arc, may pose a threat similar to that of the rest of the Aleutian subduction zone. Previously, it was thought that the subducting plate in the Shumagin Gap was poorly coupled to the overriding crust, quietly slipping and preventing large quakes. The earthquake struck east of the Shumagin Gap, a 125 mile-wide (200 km) seismic gap in the subduction zone which hasn't ruptured with a large quake for at least 100 years. On average, the rate at which these plates converge is about 64 mm/year. This convergent boundary is one of the most active in the world, and was the location of the M w9.2 1964 Alaska earthquake the largest recorded in North America, and the second largest in the world. The earthquake on July 22, 2020, was the result of thrust faulting on the Aleutian subduction zone where the Pacific plate subducts underneath the North American plate, forming the Aleutian Trench and Arc. The United States Geological Survey have since considered the two earthquakes as foreshocks to the M w 8.2 in 2021. It was followed by another aftershock of M w 7.6 in October the same year. In July 2020, the Aleutian Subduction Zone was the source of an M w 7.8 earthquake which struck the same region south of the Alaska Peninsula. Rupture of this earthquake did not reach into the Shumagin segment. The 1938 earthquake ruptured east of the Shumagin segment for a length of approximately 300 km. An unusually weak tsunami was generated because the earthquake occurred at a deep depth of 35 km. While the earthquake generated an ocean-wide tsunami, the maximum wave height was only measured at 0.3 meters. It was felt with a maximum Modified Mercalli intensity of VI ( Strong), causing limited damage in the relatively unpopulated region of the Alaska Peninsula. It had an estimated magnitude of 8.2 on the moment magnitude scale, and had an epicenter 40 km west of the epicenter of the 2021 earthquake. The 1938 Semidi earthquake occurred on November 10 at 20:18 UTC. That proposal also highlighted the potential for a tsunami accompanying an earthquake on the gap. Two smaller earthquakes shook near the seismic gap in 19. The most recent events is thought to have occurred in 1788, which were a pair of large events, and in 1854. In 1971, the Shumagin segment drew the attention of seismologists as it was proposed as a seismic gap because no major seismic activity have occurred in recent times. This segment lies between the Unimak and Semidi segments which had ruptured in earthquakes in 19 respectively. South of the Alaska Peninsula is a segment of the Aleutian subduction zone known as the Shumagin segment, named after the nearby Shumagin Islands. This megathrust fault has been the source of many large earthquakes including the 1964 Alaskan earthquake that registered a magnitude 9.2 and remains the second largest earthquake in recorded history. Off the coast of Alaska lies the Aleutian subduction zone–a 2,500 mile long convergent plate boundary where the Pacific Plate subducts under the North American Plate at a rate of 6–7 cm/yr. A tsunami warning was rescinded and no large waves were observed because the earthquake took place 32 km beneath the seafloor deep enough that vertical uplift of the seafloor was limited therefore displacing smaller amounts of water. The limited impact from this earthquake was attributed to it occurring offshore from the sparsely-populated Alaska Peninsula. In Perryville, the closest populated area to the quake, cracks appeared in drywalls and on the ground. No casualties or serious damage were reported in the aftermath of the mainshock, as well as some of the strong aftershocks. It was also the strongest earthquake globally since the 2015 Illapel earthquake, tying the 2017 Chiapas earthquake and 2018 Fiji earthquake. This was the largest earthquake in the United States since the 1965 Rat Islands earthquake, and the 7th largest earthquake in US history. The mainshock was followed by a number of aftershocks, including three that were of magnitude 5.9, 6.1 and 6.9 respectively. A tsunami warning was issued by the National Oceanic and Atmospheric Administration (NOAA) but later cancelled. The large megathrust earthquake had a moment magnitude ( M w) of 8.2 according to the United States Geological Survey (USGS).
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