Mojave Desert, California magnitude 6.4 earthquake (10:33 AM, 4 July 2019)

James St. John
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Mojave Desert, California magnitude 6.4 earthquake (10:33 AM, 4 July 2019)
Mojave Desert, California magnitude 6.4 earthquake (10:33 AM, 4 July 2019)

This is the seismogram for the Little Thumb Creek seismic station in Yellowstone, Wyoming. The station is north-northwest of the West Thumb Geyser Basin. The prominent noise at the bottom of the graph was caused by shock waves from a magnitude 6.4 earthquake that hit California's Mojave Desert at 10:33 AM local time on 4 July 2019. The shock waves first arrived at the seismic station at 11:36 AM, Yellowstone time, three minutes after the quake hit. The United States Geological Survey reports that the earthquake's hypocenter was at about 8.7 kilometers depth. The epicenter is at 35° 42' 18" North latitude, 117° 30' 29" West longitude.

This earthquake hit just north of the Mojave Block's northern boundary in southern California. The Mojave Block is a large, wedge-shaped area bordered by the Garlock Fault Zone to the north and the San Andreas Fault Zone to the south. The area has many active transform faults (strike-slip faults/transcurrent faults). See info. & illustrations at: scedc.caltech.edu/significant/mojave.html and web.gps.caltech.edu/~jstock/Ge136-2012/Memoir195Chapter02...
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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: July 4, 2019
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