File:Offshore Japan magnitude 6.8 earthquake (10-27 AM, 1 May 2021).jpg

English: This seismogram is from the Willy Bob seismic station on Barbuda Island in the Caribbean. The noise represents shock waves from a magnitude 6.8 earthquake that hit just offshore from Honshu Island, Japan at 10:27 AM, local time, on 1 May 2021. The quake's magnitude has been assigned values ranging from 6.6 to 6.9 by different seismic reporting agencies. The epicenter was about 40 kilometers southeast of the town of Ishinomaki, Japan. The hypocenter was at about 47 kilometers depth.

This was a subduction zone earthquake. Subduction zones have tectonic plates composed of oceanic lithosphere diving beneath another tectonic plate. In this case, the Pacific Plate is subducting ~westward under the Okhotsk Plate along the Japan Trench.

See info. at: earthquake.usgs.gov/earthquakes/eventpage/us7000dz5t/exec... and en.wikipedia.org/wiki/Japan_Trench and en.wikipedia.org/wiki/Okhotsk_Plate

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An earthquake is a natural shaking or vibrating of the Earth caused by sudden fault movement and a rapid release of energy. Earthquake activity is called "seismicity". The study of earthquakes is called "seismology". The actual underground location of an earthquake is the hypocenter, or focus. The site at the Earth's surface, directly above the hypocenter, is the epicenter. Minor earthquakes may occur before a major event - such small quakes are called foreshocks. Minor to major quakes after a major event are aftershocks.

Most earthquakes occur at or near tectonic plate boundaries, such as subduction zones, mid-ocean ridges, collision zones, and transform plate boundaries. They also occur at hotspots - large subsurface mantle plumes (Examples: Hawaii, Yellowstone, Iceland, Afar).

Earthquakes generate four types of shock waves: P-waves, S-waves, Love waves, and Rayleigh waves. P-waves and S-waves are body waves - they travel through solid rocks. Love waves and Rayleigh waves travel only at the surface - they are surface waves. P-waves are push-pull waves that travel quickly and cause little damage. S-waves are up-and-down waves (like flicking a rope) that travel slowly and cause significant damage. Love waves are side-to-side surface waves, like a slithering snake. Rayleigh waves are rotational surface waves, somewhat like ripples from tossing a pebble into a pond.

Earthquakes are associated with many specific hazards, such as ground shaking, ground rupturing, subsidence (sinking), uplift (rising), tsunamis, landslides, fires, and liquefaction.

Some famous major earthquakes in history include: Shensi, China in 1556; Lisbon, Portugal in 1755; New Madrid, Missouri in 1811-1812; San Francisco, California in 1906; Anchorage, Alaska in 1964; and Loma Prieta, California in 1989.