Offshore Japan magnitude 7.1 earthquake (4:42 PM, 8 August 2024) 2

This seismogram is from the Santo Domingo seismic station in Venezuela. The noise is from a magnitude 7.1 earthquake that hit just offshore from southern Kyushu Island, Japan. The quake occurred at 4:42 PM, local time, on 8 August 2024. The epicenter was about 19 kilometers northeast of (offshore from) the town of Nichinan, Japan. The hypocenter was between 20 and 30 kilometers deep. Shaking resulted from thrust faulting along a north-northast to south-southwest striking fault zone. This is the seventh magnitude 7+ earthquake of 2024.

This was a subduction zone earthquake. Such settings have one tectonic plate composed of oceanic lithosphere diving beneath another tectonic plate. In this case, the Philippines Plate is subducting westward beneath the Eurasian Plate (for tectonic lumpers; for tectonic splitters, the overriding plate is the Amur Plate / Okinawa Plate boundary area) . The surface expression of the subduction zone is a deep seafloor trough called the Ryuku Trench.
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Info. at:
earthquake.usgs.gov/earthquakes/eventpage/us6000nith/exec...
[An event-specific summary is at the bottom of that web page.]
and
en.wikipedia.org/wiki/Ryukyu_Trench
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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.

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Taken on: August 8, 2024

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