The Carrizo Plain in San Luis Obispo County, California, is divided by the famous San Andreas Fault as illustrated here.
Hey there! So, the big ol' Earth ain't perfect, neither is her little troublemaker friend, earthquakes. They happen every day, sure as the sun rises, but the most recent to make headlines rattled northern California.
A giant 6.9 earthquake shook things up 50 miles off Eureka, California, on March 9, as per the U.S. Geological Survey. The action was deep down – around 10 miles beneath the Pacific Ocean – where ocean and continental crust dance together to form faulty zones.
Earthquakes? They come from these deep-rooted faults - the question is what kind? Finding that out takes scientist time.
The Earth's crust is like a giant Tetris game of continental and oceanic plates, always bumping, sliding, or dancing apart. Along the infamous Ring of Fire, the seafloor dives beneath continents, building mountains, feeding volcanoes, and triggering earth-shaking adventures.
Most earthquakes originate from such fault zones. First, the ground bends, then it snaps – that's an earthquake, letting out energy along faults. Here are the culprits:
Strike-Slip
When the Earth's crust moves sideways, we got a strike-slip going. California's famous San Andreas Fault falls into this category, meandering about 600 miles from southern California to north of San Francisco. The sideways shift is due to the Pacific plate pushing northwest under the North American continent.
Dip-Slip
Up-and-down movement in earthquakes happens over dip-slip faults, either with the ground dropping (normal fault) or being pushed up (reverse fault). A normal fault occurs where the deeper part of the crust pulls away, forming valleys, while a reverse fault is the opposite.
An example of a normal fault is the 150-mile-long Wasatch Fault in Utah, driven by the Pacific plate sliding under western North America. A magnitude 7.0 quake along it, perhaps 550 years ago, dropped the ground on one side of the fault by three feet. Scientists predict more magnitude 7.0 earthquakes along the Wasatch Fault.
Oblique
Faults that combine sideways and up-and-down movements are called oblique. They create a particular earthquake type south of San Francisco, known for their wacky behavior.
Human-Induced
Humanity's not innocent in the seismic department. Pumping wastewater onto faults in deep disposal wells can trigger quakes, like those experienced in Oklahoma, Texas, and Ohio in recent years.
And when Seattle Seahawks football fans went football crazy during their Superbowl-winning playoff run this year, they triggered "Beast Quakes," detected by seismologists across the Pacific Northwest. The most intense seismic activity during a game? During a touchdown run.
Correction: The original story placed the focal center of the earthquake incorrectly, which should have been 10 miles beneath the Pacific Ocean.
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Enrichment Insights:
To better understand earthquakes and the geological changes they cause, some key concepts to remember are the three main types of fault motion: normal faults, reverse faults, and strike-slip faults.
Types of Earthquakes Based on Fault Motion:
1. Normal Faults
These involve downward movement of the hanging wall relative to the footwall along the dip of the fault due to tensional forces in the Earth's crust. Examples include the Wasatch Fault and Hurricane Fault in Utah, often causing grabens (down-dropped blocks forming valleys) and horsts (uplifted blocks forming mountains).
2. Reverse Faults
In reverse faults, the hanging wall moves upward relative to the footwall along the dip of the fault. They are associated with compressional forces, often found at convergent plate boundaries. Thrust faults, like those forming mountain ranges such as the Himalayas, are a type of reverse fault.
3. Strike-Slip Faults
Strike-slip faults involve horizontal movement along a nearly vertical fault plane, and may be either dextral (right-lateral) or sinistral (left-lateral) depending on the direction of the relative motion between the two sides of the fault. The San Andreas Fault and the North Anatolian Fault are examples of strike-slip faults.
In the Earth's crust, three main types of fault motion create different types of earthquakes: normal, reverse, and strike-slip. Normal faults result in downward movement, such as the Wasatch Fault in Utah, and often form grabens, like valleys. Conversely, reverse faults feature uplifted blocks, associated with compressional forces, and are found at convergent plate boundaries. Strike-slip faults involve horizontal movement along a vertical fault plane, like the San Andreas Fault, and may be either right-lateral or left-lateral. Additionally, human activities such as wastewater disposal and even sports events can, in some cases, trigger earthquakes.
