Seismology facts for kids
Seismology is the study of earthquakes and the waves they create. In Ancient Greek "seismos" means "earthquake," and "-logia" means "the study of." So, simply put, seismology is the study of earthquakes!
Seismology also includes studying:
- Tsunamis: Giant waves in the ocean often caused by underwater earthquakes.
- Volcanoes: Their activity can cause shaking.
- Plate Tectonics: The movement of the large pieces of Earth's outer shell that causes most earthquakes.
- Glaciers and Rivers: Even these can cause small vibrations.
- Ocean Microseisms: Tiny, constant shaking caused by ocean waves.
- Artificial Sources: Like controlled explosions used for research.
There's even a related field called Paleoseismology that uses clues from geology (the study of rocks and Earth's history) to figure out where and when earthquakes happened long, long ago.
When a seismologist records the Earth's movement over time, it's called a seismogram. The tool that makes this recording is called a seismograph.
Seismology is a constantly evolving science that helps us understand the dynamic processes happening deep beneath our feet and how they affect the world we live in.
Contents
History
People have been curious about earthquakes for a very long time!
- Ancient Times: Thinkers like Thales of Miletus, Anaximenes, and Aristotle in Ancient Greece, and Zhang Heng in China, wondered what caused earthquakes.
- 132 CE: Zhang Heng of China created the first known seismoscope. This was a clever device that could tell you when an earthquake happened and even roughly which direction it came from, even if you didn't feel it yourself!
- 17th Century: Some early ideas about earthquake causes included fire moving inside the Earth (Athanasius Kircher) or chemical explosions underground (Martin Lister and ]]Nicolas Lemery]]).
- 1755: A very large earthquake hit Lisbon, Portugal. This event really made scientists in Europe want to understand earthquakes better.
- Mid-1700s: Scientists like John Bevis and John Michell started studying earthquakes more scientifically. Michell figured out that earthquakes start deep inside the Earth and are caused by large blocks of rock shifting miles below the surface, sending out waves of movement.
- 1839: After some earthquakes in Scotland, scientists in the United Kingdom decided they needed better tools to detect them.
- 1842: James David Forbes created one of the first modern seismometers. It used a pendulum and a pencil to record ground movement on paper.
- From 1857: Robert Mallet is considered the "Father of Seismology." He did experiments using explosives to create seismic waves and was the first person to use the word "seismology."
- 1889: Ernst von Rebeur-Paschwitz recorded the first teleseismic signal – that's when a seismograph records an earthquake that happened very, very far away (like an earthquake in Japan recorded in Germany!).
- Late 1800s - Early 1900s: Scientists started using seismic waves to figure out what the inside of the Earth is made of.
- 1897: Emil Wiechert used calculations to suggest Earth has a rocky mantle around an iron core.
- 1906: Richard Dixon Oldham looked at seismograms and saw different types of waves arriving at different times. This gave the first clear evidence that Earth has a central core.
- 1909: Andrija Mohorovičić discovered a boundary deep inside Earth where seismic waves suddenly change speed. This boundary is now called the Mohorovičić discontinuity, or "Moho," and it's the line between Earth's crust and the layer below it, the mantle.
- 1910: After studying the big 1906 San Francisco earthquake, Harry Fielding Reid developed the "elastic rebound theory." This theory explains how stress builds up in rocks along faults and is suddenly released during an earthquake, causing the ground to snap back like a stretched rubber band. This is still a key idea in understanding earthquakes today.
- 1920s: Scientists continued to study earthquakes and their aftershocks using seismographs.
- 1926: Harold Jeffreys used seismic waves to figure out that the Earth's outer core is liquid.
- 1937: Inge Lehmann, a Danish seismologist, discovered that inside the liquid outer core, there is a solid inner core.
- 1950s: Scientists like Michael S. Longuet-Higgins studied the tiny, constant seismic vibrations caused by ocean waves.
- 1960s: With better instruments and studies of huge earthquakes like the 1960 Valdivia earthquake in Chile and the 1964 Alaska earthquake, scientists developed the theory of plate tectonics. This theory explains why earthquakes and volcanoes happen where they do – at the edges of giant moving plates that make up Earth's outer shell.
Types of weismic waves
When an earthquake happens, it sends out different types of energy waves. Think of them like different kinds of ripples.
There are two main groups of seismic waves:
Body waves
These waves travel through the inside of the Earth.
- P Waves (Primary Waves): These are the fastest seismic waves, so they arrive first at a seismograph. They are compressional waves, meaning they push and pull the rock in the same direction the wave is traveling, like sound waves. P waves can travel through solids, liquids, and gases.
- S Waves (Secondary Waves): These waves are slower than P waves and arrive second. They are shear waves, meaning they shake the rock back and forth perpendicular to the direction the wave is traveling, like shaking a rope. S waves can only travel through solids, not liquids or gases. This fact was key to discovering that Earth's outer core is liquid!
Surface waves
These waves travel along the surface of the Earth, like ripples on the surface of water. They are slower than body waves but often cause the most shaking and damage during an earthquake because their energy stays closer to the surface.
- Rayleigh Waves:These waves have a rolling motion, like ocean waves, combining both up-and-down and back-and-forth movement.
- Love Waves: These waves have a side-to-side shaking motion.
For very, very large earthquakes, the whole Earth can actually ring like a bell! This ringing is a mix of different vibrations called normal modes, which can last for a long time after the earthquake. Studying these helps us understand the deep structure of the Earth.
How seismologists study Earth
Seismologists use the different speeds and behaviors of seismic waves to learn about our planet.
- Mapping Earth's Interior: Since P waves and S waves travel at different speeds through different materials (solid rock, liquid metal), seismologists can use the arrival times of these waves at many different seismographs around the world to create maps of what's inside the Earth. This is how we know about the crust, mantle, liquid outer core, and solid inner core! It's like using an echo to figure out what's in a dark room.
- Finding Earthquakes: By looking at the time difference between when the P waves and S waves arrive at several seismograph stations, seismologists can figure out where an earthquake happened and how deep it was.
- Studying Other Sources: Seismographs are so sensitive they can pick up shaking from many things besides natural earthquakes, like volcanic activity, large landslides, or even human-caused events like controlled explosions used for exploring for resources underground.
Seismology and society
Seismology is important for helping us understand and prepare for earthquakes.
Seismologists work to understand where earthquakes are likely to happen and how strong they might be over long periods (like decades or centuries). This information is used by engineers to design buildings and bridges that can withstand shaking. While scientists can forecast the likelihood of earthquakes in certain areas, predicting the exact time, location, and size of a specific earthquake is currently not possible.
Engineering seismology is a special area of seismology that focuses on providing information to engineers. They study past earthquakes and how the ground shakes to help engineers build safer structures in earthquake-prone areas.
Because seismic waves travel much faster than tsunami waves, detecting a large underwater earthquake quickly using seismographs allows scientists to issue tsunami warnings to coastal areas, giving people time to get to safety.
Tools seismologists use
The main tool is the seismograph, which includes a sensor called a seismometer that detects ground motion and a recording system. These instruments are placed all over the world, sometimes in special quiet vaults or even underwater, to constantly listen to the Earth's movements.
Because seismographs collect so much data, seismologists use powerful computer systems and special software to process and analyze it.
Notable seismologists
Many brilliant scientists have contributed to seismology throughout history, including those mentioned earlier like Zhang Heng, Robert Mallet, Richard Dixon Oldham, Andrija Mohorovičić, Harry Fielding Reid, and Inge Lehmann. Their discoveries helped build our understanding of earthquakes and the Earth's interior.
Related pages
Images for kids
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Seismogram records showing the three components of ground motion. The red line marks the first arrival of P-waves; the green line, the later arrival of S-waves.
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Installation for a temporary seismic station, north Iceland highland.
See also
In Spanish: Sismología para niños
