What can seismic waves tell us about the interior of the Earth? (Photo Credit: Science Over Everything)

Good vibrations: What Earthquakes can tell us about the Earth

How scientists measure earthquakes and they learn from them

Next Generation Science Standards:

  • MS-ESS2-2. Construct an explanation based on evidence for how geoscience processes have changed Earth’s surface at varying time and spatial scales.

Key Vocabulary: earthquakes, seismologist, seismogram, seismometer, P-waves, S-waves, tectonic plates, fault,

Earthquake Triangulation Lab

Seismologists, people who study earthquakes, are continuously learning more about our planet by monitoring these phenomena. But how do researchers study earthquakes and what can they tell us about the Earth? By installing a network of instruments around the world, seismologists are able to detect and record earthquakes and gain a better understanding of the interior of the Earth.Earthquakes are one of nature’s most destructive forces. Most of the world’s large-scale earthquakes occur at or near plate boundaries, where pieces of rock billions of ton in mass collide, separate, or grind past each other. As plates slip during an earthquake, they release enormous amounts of energy, sending waves traveling through Earth’s crust.

Why do earthquakes happen in the first place? 

An earthquake is the sudden release of stored energy that has built up in rocks over time as tectonic plates move around each other. This causes a slip along a fault, a fracture or zone of fractures between two blocks of rock that allow them to move relative to one another. Energy builds up within the rocks as they get stuck trying moving past one another, stored until the force of the moving rocks overcomes the force that keeps them stuck together in the form of an earthquake.

Left: Diagram showing the stages of an earthquake. In (a) there is a un-deformed fence across the fault, but overtime as either side of the fault moves in (b), the fence bends. This is happening because the blocks of rock on either side of the fault are getting stuck together as they try to move past one another. Eventually in (c), the fault slips suddenly in the form of an earthquake, snapping the fence back to being straight. (Photo Credit: Rick Callender via Incorporated Research Institute for Seismology). Right: A 3D diagram to show a slice of the Earth in between the fences where the earthquake occurred. The hypocenter is the origin of the earthquake while epicenter is the point directly above it at the surface. (Photo Credit: Science Over Everything).

When the energy is released they form seismic waves, radiating outward from the origin of the earthquake in all directions like ripples on a pond. It is when the seismic waves reach the surface of the Earth, that we feel the ground shake. There are two types of seismic waves: P-waves (primary waves) and S-waves (secondary waves). P- and S- waves travel at different speeds with P-waves being the faster. Another important characteristic about P- and S- waves is that P-waves can travel through all types of materials but S-waves can only travel through solids (not liquids). But, how do seismologists records these seismic waves? And what can they tell us about the Earth?

Seismic data

In order to study earthquakes, seismologists must have a way of detecting and recording them. They use a very sensitive instrument that records ground motions, called a seismometer. Traditional seismometers consisted of a base with an attached frame from which a weight (mass) was suspended by a string or wire and hung over a rotating drum of paper. When the earth shook, the base with the rotating drum moved with it, while the pen that was attached to the suspended mass remained motionless recording the shaking on the moving roll of paper. What was produced on the paper was a tracing called a seismogram.

Modern seismometers are instead electronic, the relative motion between the mass and the frame generates an electrical voltage that is recorded by a computer. Seismometers use three masses to record ground motion in all directions: north/south, east/west, and up/down. By comparing recordings of a single earthquake at numerous stations, seismologists can determine an earthquake’s location and magnitude.

Left: Diagram of a traditional seismometer with a rotating drum. (Photo Credit: USGS via Wikimedia) Right: Example of a modern seismometer. (Photo Credit: Science Over Everything)

Electronic seismometers are extremely sensitive, enough to detect passing traffic, trains, high winds that shake the trees and thus their roots below the surface, and even nearby crashing surf. Since the location of these instruments is extremely important to data quality, seismologists tend to choose a location that is far away from all the potential noise sources, typically open fields out in the country and away from busy cities. To help reduce the vibrations from other sources and to get the best recording signal, seismometers are generally buried underground.

Example of a seismogram for recordings of the three different directions of ground motion for an earthquake. P-waves arrive first as they are the fastest. Photo Credit: Bramfab via Wikimedia

Researchers place multiple instruments along the seismometer: GPS, station box (with car batteries and cell modem inside), and solar panel. The GPS uses satellites orbiting the Earth to get accurate timing and location, similar to technology in your phone or car. With no outlets in the field, seismologists use car batteries for power and the solar panel uses energy from the sun to keep the batteries charged. The cell modem connects the entire station to the internet, sending data from the field back to the lab without having to visit the seismometer everytime seismologists want to download data.

Left: One of Miami University’s seismic recording stations in eastern Ohio (Photo Credit: Science Over Everything). Right: An example of a vault that Miami University uses to put the seismometer in. The vault is a plastic bucket attached to a slab of rock which is placed in concrete (to keep the vault from moving) at the bottom of the hole. The vault is then sealed up tight and buried. (Photo Credit: Science Over Everything).

What can seismologists do with the data?

The most important thing seismologists can do with the data is to use multiple seismometers to locate an earthquake and determine its magnitude. With the locations of earthquakes, seismologists can map out faults and use them to study the different plate boundaries. By placing seismometers around the world, scientists can get a clearer picture of how tectonic plates are moving. They can even detect earthquakes that occur on the opposite side of the earth and nuclear explosions.

The data from earthquakes can also give us insight into the different layers of Earth’s interior. As seismic data collecting improved, seismologists noticed that there were abrupt changes to how fast P- and S-waves were traveling at specific depths. These sharp changes were used to help map out the layers of the Earth: crust, upper mantle, mantle transition zone, lower mantle, outer core, and inner core. Each layer has different characteristics such as a different composition and density. The behavior of the waves also informed seismologists that the outer core was a liquid and the inner core was solid. Since S-waves only travel through liquids, their abrupt halt at the outer core indicated that the layer was liquid.

The IASP91 Earth Reference Model, showing S- and P- wave velocities (grey and black lines, respectively) in the Earth’s interior. You can see not only do P-waves move faster, but that S-waves are stopped at the Outer Core, meaning that the layer is in a liquid state. (Photo Credit: Iwoelbern via Wikimedia)

Global Impact

Most of the world’s most destructive earthquakes occur at plate boundaries, more specifically at subduction zones. In fact, nearly 80% of the world’s earthquakes occur on faults that make up the boundaries of the tectonic plates. Some of these devastating earthquakes can cause tsunamis if the earthquake is below the seafloor where large-scale displacements of the seafloor will displace a column of water in the form of a tsunami.

While seismologists cannot predict earthquakes, by studying where they are occurring and their history, seismologists can better understand their potential hazards and where and when a future earthquake is more likely to occur. If residents of areas that are prone to seismic activity are able to better prepare for the next big event, it could help reduce the damage and save human lives.

World map of seismicity (orange dots) with the tectonic plates (black lines). Notice how most of the seismicity follows the boundaries of the plates. Photo Credit: Rick Callender via Incorporated Research Institute for Seismology

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