Seismology

Seismology is the scientific study of earthquakes and the way waves move through planets. It helps us understand how earthquakes happen and what effects they can have on the environment, like causing big waves in the ocean called tsunamis.

Scientists who study seismology are called seismologists. They use special tools called seismographs to record how the Earth moves during an earthquake, creating a picture called a seismogram. This helps us learn about past earthquakes and prepare for future ones.

Seismology also looks at other things that make the ground shake, such as volcanoes, moving plates in the Earth's crust, glaciers, rivers, and even explosions. This wide study helps us understand our planet better.

History

Ancient and classical eras

People have been interested in earthquakes for a very long time. Early ideas about why earthquakes happen can be found in old writings by Thales of Miletus (around 585 BCE), Anaximenes of Miletus (around 550 BCE), Aristotle (around 340 BCE), and Zhang Heng (132 CE).

In 132 CE, Zhang Heng from China’s Han dynasty made the first known seismoscope, an early tool to detect earthquakes.

Beginnings of modern science

In the 1600s, Athanasius Kircher thought earthquakes were caused by movement of fire inside the Earth. Others like Martin Lister and Nicolas Lemery believed they were caused by chemical explosions inside the Earth.

The big Lisbon earthquake of 1755 encouraged scientists in Europe to study earthquakes more closely. John Michell discovered that earthquakes come from movements deep inside the Earth.

In 1839, after several earthquakes near Comrie in Scotland, a group in the United Kingdom worked on better ways to detect earthquakes. This led to the creation of an early seismometer by James David Forbes in 1842.

Robert Mallet began modern seismology in the mid-1800s and even created the word “seismology.” He is often called the “Father of Seismology.”

In 1889, Ernst von Rebeur-Paschwitz recorded an earthquake in Japan from Germany, showing that earthquakes could be detected far away.

Andrija Mohorovičić discovered in 1909 that there is a boundary called the “Moho” between the Earth’s outer layer and its deeper part.

Harry Fielding Reid developed the “elastic rebound theory” in 1910 after studying the 1906 San Francisco earthquake. This theory helps us understand how earthquakes happen.

By the 1960s, the theory of plate tectonics brought together many ideas about why earthquakes occur.

Types of seismic wave

Main article: Seismic wave

Seismic waves are special kinds of waves that move through solid or liquid materials. They can be split into three main groups: body waves, surface waves, and normal modes.

Body waves travel through the inside of materials. There are two types of body waves: P waves and S waves. P waves are the fastest and move by squeezing and stretching the material they pass through. S waves are slower and move by sliding side to side. S waves can only travel through solids, not liquids.

Surface waves move along the Earth's surface. They are slower than body waves but can cause stronger shaking because they lose energy more slowly. There are two kinds of surface waves: Rayleigh waves, which move in both squeeze and slide motions, and Love waves, which only slide side to side.

Normal modes happen when a very big earthquake makes the whole Earth vibrate like a bell. These vibrations can be felt for weeks after the earthquake and help scientists learn about the deep structure of our planet. We first noticed these modes in the 1960s after some of the largest earthquakes ever recorded.

Earthquakes

Main articles: Earthquake and Lists of earthquakes

People have been studying earthquakes scientifically since a big one hit Lisbon in 1755. Since then, many big earthquakes have helped scientists learn more about how the Earth moves. Important earthquakes include those in Basilicata in 1857, San Francisco in 1906, Alaska in 1964, Sumatra-Andaman in 2004, and the Great East Japan earthquake in 2011. These events helped shape what we now know as seismology.

Controlled seismic sources

Detection of seismic waves

Installation for a temporary seismic station, north Iceland highland.

Seismometers are tools that sense and record the Earth’s movements caused by waves. They can be placed on the ground, in small pits, deep holes, or even underwater. When all the parts that record these waves are together, it is called a seismograph. Many seismographs around the world work together to watch and study earthquakes and other things that make the ground shake. Finding earthquakes quickly helps give warnings for tsunami waves, because the waves from earthquakes travel much faster than tsunami waves.

Seismometers also pick up signals that are not from earthquakes. These can come from explosions, both big and small, or from everyday noises like wind or things people do. They also record constant signals from the ocean, like waves hitting the shore or moving ice, and even events high in the sky, such as space rocks hitting the Earth. One big reason we have many seismographs around the world is to help watch for and learn about nuclear testing.

Mapping Earth's interior

Main article: Earth's interior

Seismic waves help us learn about the inside of our planet without harming it. These waves move through Earth’s layers and give scientists clues about what’s inside. One important discovery was that Earth’s outer core is liquid. This was found because certain waves called S waves cannot go through liquids, creating a gap or “shadow” where these waves don’t reach.

By studying many readings from tools called seismometers, scientists have created detailed maps of Earth’s mantle. These maps show big patterns, like slow-moving areas near where the mantle meets the core.

Seismology and society

Earthquake prediction

Main article: Earthquake prediction

Sometimes people argue about whether scientists can predict earthquakes. This happened after a big earthquake in Italy in 2009. Some scientists were questioned about whether they gave clear information about the risks before the earthquake happened.

When we have records of past earthquakes, we can try to guess when and where future ones might happen. But it’s tricky because old records might not tell the whole story or might be hard to understand.

Engineering seismology

Engineering seismology is about using knowledge of earthquakes to help build safer structures. It connects studying the Earth with building designs. This includes looking at past earthquakes to understand what might happen in the future and studying how the ground moves during earthquakes to prepare for similar events. This information helps engineers design buildings that can withstand shaking.

Tools

Seismologists use special tools to study earthquakes and the movements of the Earth. These tools can collect a lot of information, and there are systems to help organize and understand the data. Some of these systems include CUSP (Caltech-USGS Seismic Processing), RadExPro seismic software, and SeisComP3.