Oaxaca, Mexico magnitude 7.4 earthquake (10:29 AM, 23 June 2020)

James St. John
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Oaxaca, Mexico magnitude 7.4 earthquake (10:29 AM, 23 June 2020)
Oaxaca, Mexico magnitude 7.4 earthquake (10:29 AM, 23 June 2020)

This is a seismogram for the Little Thumb Creek seismic station in Yellowstone, Wyoming. The prominent noise in the 09:30 line near the bottom of the graph (click on the image once or twice to zoom in) was caused by shock waves from a magnitude 7.4 earthquake that hit Oaxaca State in southern Mexico at 10:29 AM, local time, on 23 June 2020. Shock waves arrived at the seismic station at 9:35 AM, Yellowstone time, ~6 minutes after the quake hit.

The Mexican earthquake occurred along a subduction zone - such settings experience the most powerful tectonic quakes on Earth. In the far-eastern Pacific Ocean, the Cocos Plate, composed entirely of oceanic lithosphere, subducts beneath the North American Plate. The subduction zone's surface expression is a deep seafloor trough called the Middle America Trench.

See info. at:
earthquake.usgs.gov/earthquakes/eventpage/us6000ah9t/exec...
and
en.wikipedia.org/wiki/2020_Oaxaca_earthquake
and
en.wikipedia.org/wiki/Cocos_Plate
and
en.wikipedia.org/wiki/Middle_America_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: June 23, 2020
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