An earthquake happens when two slabs of the ground abruptly move past one another.
The fault or fault plane is the surface where they slip. The hypocenter is the point beneath the earth’s surface where the earthquake begins, while the epicenter is the location directly above it on the earth’s surface.
Types of Earthquakes
Foreshocks are small earthquakes that happen before a larger earthquake. The main earthquake is called the mainshock. Earthquakes that happen after a large earthquake are called aftershocks. (Related: What is the Difference Between an Earthquake Swarm and Aftershocks)
Earthquakes are recorded using a machine called a seismograph. The seismograph measures the difference between the stationary and moving parts of the device to determine the duration, time, and magnitude of the earthquake. The vertical axis records ground displacement and the horizontal axis measures time.
The seismograph is securely fastened to the ground so that when the earth shakes, everything on the seismograph shakes except for the mass on the spring.
The size of an earthquake is recorded as a magnitude using the Richter Scale. The Richter Scale measures earthquakes on a logarithmic scale.
Each year, there are about 14,000 recorded earthquakes that are magnitude 4 or higher.
Table: Worldwide Earthquakes 2000–2019
Largest Recorded Earthquake in the World
The largest recorded earthquake in the world was a 9.5 magnitude earthquake in Chile on May 22, 1960.
Earthquakes in the United States
Alask is the U.S. state that experiences the most earthquakes each year. A magnitude 7 earthquake occurs virtually every year in Alaska, and a magnitude 8 or higher earthquake occurs every 14 years on average.
California was the second most active state for number of earthquakes until 2014. An increase in seismicity in Oklahoma in 2014 has the state now ranked second for number of earthquakes. In Oklahoma, there were 585 earthquakes of magnitude 3 or larger in 2014, and roughly 200 in California.
Only four states experienced no earthquakes between 1975 and 1995: Florida, Iowa, North Dakota, and Wisconsin.
Earthquake Counts by State 2010-2015 (M3+)
Largest Earthquake in the United States
On Friday March 28, 1964 UTC, a magnitude 9.2 earthquake rocked Prince William Sound, Alaska, making it the greatest recorded earthquake in the United States.
San Andreas Fault
The San Andreas fault is actually a fault zone made up of multiple segments, rather than a single continuous fault. At any time, movement may occur along any of the zone’s numerous fault segments.
The San Andreas fault system stretches for more than 1300 kilometers (800 miles) and is as deep as 16 kilometers (10 miles) in some places.
The San Andreas fault has moved 0.59 inches per year (1.5 centimeters per year) over the last 23 million years.
Most Seismically Active Areas on Earth
There are three major zones where earthquakes are more likely to happen: the Circum-Pacific belt, the Alpide Belt, and the mid-Atlantic Ridge belt.
The “Ring of Fire,” also known as the Circum-Pacific belt, is an earthquake-prone zone that surrounds the Pacific Ocean, accounting for over 90% of all earthquakes worldwide.
The belt runs along tectonic plate boundaries, where plates of predominantly oceanic crust descend (or subduct) beneath another plate. Slip between plates and rupture within plates produce earthquakes in these subduction zones.
The world’s largest earthquake, the 9.5 magnitude Chilean earthquake in 1960 occurred in this zone. The largest earthquake in the United States, the 1964 9.2 Alaskan tremor that occurred in the Prince William Sound region is also part of the Ring of Fire.
The Alpide belt (which stretches from the Mediterranean region eastward to Turkey, Iran, and northern India) is the next most seismically active region (5-6 percent of earthquakes).
The submerged mid-Atlantic Ridge belt is the area that designates the separation of two tectonic plates (a divergent plate boundary). The majority of the mid-Atlantic Ridge is submerged and far from human settlement, although Iceland, which stands directly above the ridge, has seen earthquakes as strong as M6.9.
East African Rift System
The East African Rift System is a 50-60 km (31-37 mile) wide zone of active volcanics and faulting in eastern Africa that stretches north-south for more than 3000 km (1864 miles) from Ethiopia in the north to Zambezi in the south.
The East African Rift System is a rare example of an active continental rift zone, in which a continental plate attempts to split into two plates that are moving away from one another.
Earthquakes and Icequakes in Antarctica
Antarctica does experience earthquakes although not as frequently as other continents. Antarctica did experience an 8.1 magnitude earthquake in 1998 in the Balleny Islands region.
On Antarctica, the Global Seismographic Network (GSN) has only four stations. This means that smaller magnitude earthquakes may go unnoticed.
Antarctica more frequently experiences icequakes which is the movement within the ice sheet.
A tsunami is a displaced sea wave created by an underwater earthquake or landslide (typically initiated by an earthquake). Tsunamis can be incredibly destructive and cause death.
The 1964 9.2 magnitude earthquake in Alaska triggered a tsunami that devastated many towns along the Gulf of Alaska. Waves reached a height of 67 meters at Valdez Inlet. 122 people lost their lives as a result of this tsunami.
Is There Such a Thing as Earthquake Weather?
There is a statistically equal distribution of earthquakes in cold, hot, wet, and other weather conditions.
The moon also experiences seismic activity. Known as moonquakes, they are less frequently and occur at smaller magnitudes than earthquakes.