March 20, 2021 Earthquake: Honshu Island, Japan

Reference #1053:

Tectonic Summary

The March 20, 2021, M 7.0 earthquake near the east coast of Honshu, Japan, occurred as the result of thrust faulting near the subduction zone interface plate boundary between the Pacific and North America plates. Moment tensor solutions indicate that slip occurred either on a moderately dipping fault striking to the south, or a moderately dipping fault striking to the north-northeast, consistent with the east-west oriented compression expected in this region. At the location of this earthquake, the Pacific plate moves approximately westward relative to the North America plate at a velocity of 70 mm/yr, subducting beneath Japan at the Japan Trench and dipping to the west beneath Japan. Note that some researchers divide this region into several microplates that together define the relative motions among the larger Pacific, North America, and Eurasia plates; these include the Okhotsk and Amur microplates local to this earthquake that are part of North America and Eurasia, respectively.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse (thrust) faulting events of the size of the March 20, 2021 earthquake are typically about 45 km long by 25 km wide in size.

The March 20 earthquake occurred in the vicinity of the rupture area of the March 11, 2011 M9.1 Tohoku earthquake. The March 2011 M9.1 earthquake was widely felt on the islands of Honshu and Hokkaido in Japan. That much larger earthquake generated a significant tsunami that led to extensive destruction along the Japanese coast and propagated throughout the Pacific Ocean basin. Approximately 16,000 people perished because of the earthquake and subsequent tsunami. The March 20 earthquake epicenter was located ~70 km west of the epicenter of the March 2011 earthquake.

In the past century, 34 earthquakes of M7 or greater have occurred within 250 km of the March 20 earthquake, including 7 earthquakes >M7 since the March 2011 M9.1 earthquake. Most recently, on February 13, 2021 a M7.1 earthquake struck ~80 km to the south of the March 20 event. The February 13 M7.1 killed at least one person and injured 185 people at Chiba, Gunma, Ibaraki, Kanagawa, Miyagi, Saitama, Tochigi and Yamagata prefectures. Landslides were reported at the Joban Expressway, Nihonmatsu and Soma. At least 900,000 households had power outages.

Reference #1250:

11 injured; Total damages USD 550 million