This map shows the epicenters of about 1200 earthquakes in Westmorland, California during the week of 30 September to 7 October 2020. The largest event was a magnitude 4.9 quake in the late afternoon of 30 September. All other events range in magnitude from the 1s to the 4s.
Geographically and temporally clustered earthquakes are called swarms. This one has a rhomb-shaped distribution. It began about 2.5 hours before the M=4.9 quake. Seismicity has significantly slowed as of 7 October.
The Westmorland, California area is known as the Brawley Seismic Zone, a small region of extensional tectonics between two transform faults: the San Andreas Fault to the north and the Imperial Fault to the south. This is an unusual example of a mid-ocean ridge on land. The extensional area and transform faults are part of the mid-ocean ridge system in the Gulf of California, in Mexico.
See info. at:
www.usgs.gov/center-news/earthquake-forecast-westmorland-...
and
en.wikipedia.org/wiki/Brawley_Seismic_Zone
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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.
