USGS Information Page

USGS Information Page

Tectonic Summary

The October 30, 2020, M 7.0 earthquake offshore Samos Island, Greece, occurred as the result of normal faulting at a shallow crustal depth within the Eurasia tectonic plate in the eastern Aegean Sea. The focal mechanism solution indicates that the earthquake occurred on a moderately dipping normal fault striking either eastward or westward. This mechanism indicates north-south oriented extension that is common in the Aegean Sea. Tectonics in the region surrounding this earthquake are relatively complex; to the south, Africa (Nubia) lithosphere subducts northward beneath the Eurasia plate at the Hellenic Trench; to the east, the Anatolian microplate (part of Eurasia) moves in a general westward direction, driving right-lateral faulting along its northern boundary, the North Anatolian Fault, and left-lateral faulting in southeast Turkey. The Aegean Sea region, western Turkey, and southern Greece where the October 30 earthquake occurred overall experiences north-south extension that is driven by southward migration of the Hellenic Trench.

In western Turkey and the Aegean Sea region, historic earthquakes demonstrate a mixture of transtensional and extensional faulting focal mechanism solutions. The October 30, 2020 earthquake produced nearly pure normal faulting, and is broadly consistent with past earthquakes in the region. The location of the October 30, 2020 event is about 250 km north of the closest main plate boundary, where the Africa plate moves to the north at a rate of approximately 10 mm/yr with respect to Eurasia; this earthquake is thus considered an intraplate event.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. M7 normal faulting events typically have a fault area of 50 x 20 km.

The Aegean Sea frequently experiences moderate to large earthquakes, and the region within 250 km of the October 30, 2020 earthquake has hosted 29 other M 6+ events over the preceding 100 years. The largest instrumentally documented earthquake in the region was a M 7.7 earthquake in July 1956 between the islands of Naxos and Santorini (Thera), and southwest of the October 30, 2020 earthquake. The 1956 earthquake was the largest earthquake of the 20th century in Greece. As a result of this 1956 event, a M 7.2 aftershock occurred 13 minutes later with an accompanying large tsunami, 53 fatalities, 100 injuries, and extensive damage in Amorgos and nearby Santorini. More recently, on July 20 2017 a M 6.6 earthquake occurred southeast of the October 30, 2020 event near Bodrum, Turkey, and resulted in 2 fatalities and hundreds of injuries reported in Greece and Turkey.

USGS Information Page

Tectonic Summary

The October 19, 2020, M 7.5 earthquake southeast of Sand Point, Alaska (south of the Alaska Peninsula), occurred as the result of strike-slip faulting near the subduction zone interface between the Pacific and North America plates, either within the upper (North America) Plate or within the downgoing (Pacific) slab. The preliminary focal mechanism solution indicates rupture occurred on either a moderately dipping right-lateral strike-slip fault striking towards the NNW or on a steeply dipping left-lateral strike-slip fault striking towards the east, and therefore that this earthquake was not a thrust event on the plate interface itself. 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-Aleutian Trench 100 km to the southeast of the earthquake. This event is an aftershock of the M 7.8 earthquake that occurred on July 22, 2020.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike slip faulting events of the size of the October 19, 2020 earthquake are typically about 130×25 km (length x width).

Large earthquakes are common in the Alaska-Aleutian subduction zone. Since 1900, 8 other earthquakes M7 and larger have occurred within 250 km of the October 19, 2020 event, including the July 22, 2020 M 7.8 mainshock of this sequence. An updated aftershock forecast for the July 22, 2020 M 7.8 sequence, that includes the occurrence of this October 19th, 2020 earthquake, can be found here. The largest of these was a M8.6 earthquake on April 1, 1946, which generated a large tsunami that caused destruction and loss of life both locally on Unimak Island and more distantly at Hilo, Hawaii. 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 350 km of this event.

The majority of large earthquakes in this area are thrust events on the plate interface, unlike the strike-slip event of October 19, 2020. The stretch of the Alaska-Aleutian subduction zone beneath the Shumagin Islands has not ruptured in a historical great (M>8) earthquake. This observation led to definition of the ‘Shumagin Gap’ in the context of seismic gap theory, which proposes that fault sections that have not slipped for the longest elapsed time will be the site of future earthquakes. The interpretation that the Shumagin Gap can host extremely large earthquakes is clouded by 1) geodetic observations, which show that the Shumagin Islands and neighboring Sanak Island are above a section of the subduction interface that is very poorly coupled and storing very little elastic strain; and 2) geologic observations, which have documented little land level change and tsunami inundation since ~3,000 years ago on Simeonof Island in the Shumagins.

USGS Information Page

USGS Information Page