Earthquakes
Earthquakes are some of the most powerful natural forces on Earth. Though they have been recorded since ancient times, scientists first developed theories of their origin in the 17th century. Today, seismologists study plate tectonics, measure shockwaves and monitor geological activity to better understand these fascinating, sometimes catastrophic, planetary rumblings.
The geological term, ‘plate’, refers to large subsections of the Earth’s crust that border one another. Though inflexible, plates are constantly abrading against one another at edges where they meet. This movement was originally known as ‘continental drift’ when the theory first appeared in the 17th century. Subsequent research led to the name ‘plate tectonics’; ‘tectonics’ is derived from the Greek word for ‘movement’.
Plate tectonic theory states that the outer layer of the Earth is made up of a rock cover called lithosphere, which extends approximately 70-150 kilometers below the crust. The lithosphere is broken up into plates; scientists have identified nine large plates, as well as several smaller ones. These plates slide along the surface of the subsequent molten layer, known as the asthenosphere, which extends to a depth of roughly 350 kilometers. Plates move at an average rate of five centimeters per year. Earthquakes occur when plates interact with each other.
Plates border each other in three ways. With divergent edges, two plates move away from one another. In the newly created space, new lithosphere is created from basaltic flows. When divergent edges occur on land, rift valleys and lakes are formed. When they occur in the ocean, a mid-oceanic ridge is formed.
Convergent edges occur when two plates move toward one another. When they meet, one of the plates will slide underneath the other in a process called subduction. Unlike divergent edges where new lithosphere is created, sub-ducting plates destroy underlying lithosphere; the dynamic between these two forces allows the planet’s size to always remain balanced. Convergent plates are responsible for mountains, volcanic arcs and deep-sea troughs.
Occasionally plates simply pass each other without creating or destroying lithosphere. These are called transform-displacement edges.
Due to subsections of the lithosphere constantly moving, scientists estimate that millions of earthquakes occur every day. However, the vast majority of these tremors are too minor to be felt by humans.
Powerful earthquakes occur when pressure builds along plate edges, and the tension finally breaks. The majority occur at convergent or transform-displacement edges, where rocks are harden; at divergent edges, magma effusion causes rocks to soften, and thus create less of an impact. However, the vast majority of these tremors are too minor to be felt by humans.
However, earthquakes can be caused by forces other than plate tectonics. Volcanic earthquakes occur when pressure from the asthenosphere builds beneath the lithosphere. Eventually, the tension is released through a weak section of the crust and a volcanic eruption occurs. As lava or magma enters the atmosphere, it solidifies and eventually blocks the mouth of the volcano. This causes pressure to build until an explosion occurs that results in an earthquake.
Man-made operations can cause earthquakes, too. Extensive drilling, mining and explosive devices used to erect roads, bridges, dams and other large structures put undue pressure on the lithosphere, which can result in earthquakes.
An earthquake’s point of origin is called the focus. Since this location might be located miles below the crust, scientists determine location directly above the focus on the Earth’s surface. This is called the epicenter. Major earthquakes are distinguished from minor ones by both the size of the focus/epicenter and the strength of concentric shockwaves that are generated from the point of origin.
There are three types of shockwaves. P-waves, or primary waves, have high frequency and short wavelength. They move longitudinally, and create compressions in the ground that cause it to move back and forth. S-waves, or secondary waves, are similar to P-waves in their size and frequency. However, they travel transversely in all directions from the epicenter, and their speed is determined by mineral density and the presence of liquid (which they cannot travel through). Finally, L-waves, or surface waves, are created close to the epicenter. They move in a circular pattern, and can cause the Earth surface to resemble oceanic waves. L-waves are responsible for the most building damage during earthquakes.
Two scales are used to measure the severity of earthquakes. The Richter Scale measures the magnitude, or energy released by an earthquake. This reading is accomplished using an instrument called a seismograph, which measures the amplitude of shockwaves. The Mercalli Intensity Scale, on the other hand, measures the effects of an earthquake in regard to people, man-made objects and natural features.
Since earthquakes are caused by interaction between plates, they can potentially occur anywhere in the world. However, earthquakes are more common to certain regions/areas.
Roughly 81 percent of the largest earthquakes occur in the circum-Pacific Seismic Belt, also known as the Ring of Fire. This region extends across the Pacific Ocean, and encompasses coastal regions of South and North America, Asia and Oceania. The high degree of seismic activity is caused by deep ocean trenches that parallel mountain ranges throughout the belt. Notable quakes that have occurred in this region include the Japanese quake/tsunami of April 2011 that claimed nearly 16,000 lives and a series of quakes that claimed 70,000 lives in Peru in 1970.
About 17 percent of the largest earthquakes occur in the Alpide, which extends from Indonesian islands in the east to the Mediterranean/Atlantic Ocean in the west, by way of the Himalayan Mountains. Quakes that have occurred in this region include the 1968 Iran disaster that claimed 11,000 lives and two quakes that hit Turkey in 1970-71 that each claimed 1,000 lives.
Most of the remaining 2 percent of large quakes occur throughout the world. Many occur at the Mid-Atlantic Ridge, a divergent boundary that bisects the Atlantic Ocean between the Eurasian and North American plates.
Additional Resources
UPSeis – A web-based resource to help people understand various aspects of seismology. This link contains a color map of the Earth’s plates.
MSNBC – All the world’s earthquakes on a single map.
National Geographic – Video of early Earth and plate tectonics.
United States Geological Survey – A series of animations depicting various seismic activities.
USGS – A series of graphs pertaining to worldwide earthquake occurrence.
National Earthquake Information Center (NEIC) – Sponsored by the USGS, this organization compiles information about past seismic occurrences and provides resources to the scientific community, academic institutions and the general public.