Shadow zone

Earthquakes send out waves. These waves move through the ground.

Some waves can go through solid ground. Other waves cannot pass through liquid. The middle of Earth has a liquid part. This liquid part stops some waves. This creates a shadow zone.

In a shadow zone, tools cannot feel the waves. This happens because the waves hit the liquid. The liquid blocks the waves from passing. This helps us see inside the Earth. We can learn about the hidden parts.

When an earthquake happens, it sends out waves. These are called seismic waves. Two main types move through the Earth. P waves are the first waves to arrive. S waves are the second waves.

Scientists use tools to find these waves. They look for them at different spots on Earth. Sometimes, the tools cannot find the waves. This area is called a shadow zone.

Shadow zones happen because of liquid layers. The Earth has a liquid outer core. S waves cannot pass through liquid. This creates a large S wave shadow zone. P waves can pass through liquid, but they bend. This bending is called refraction. This creates a P wave shadow zone too.

Other things can make shadow zones. A magma reservoir can do this. Magma is melted rock. If a reservoir has enough melt, it stops S waves. This helps scientists study volcanoes. It can help them know if a volcano might erupt. Knowing how much melt is inside is very useful.

A seismic shadow zone is a special area on Earth's surface. In this area, seismographs cannot detect certain earthquake waves. These waves are called body waves. There are two main types called P waves and S waves.

P waves are the primary waves. They move in the same direction that the wave travels. S waves are secondary waves. They move in a different way by traveling perpendicular to the wave. Scientists use these waves to study the inside of our planet. Knowing where waves go helps us find hidden layers.

Shadow zones happen because of how waves interact with different materials. When an earthquake occurs, waves spread out from the hypocenter. The Earth has a solid crust, mantle, and inner core. However, the outer core is entirely liquid.

S waves depend on the rigidity of a material. Liquids have zero rigidity. This means S waves cannot pass through the liquid outer core at all. This creates an S wave shadow zone beyond 104 degrees from the earthquake. P waves can pass through liquid, but they change speed. They bend when they hit the core-mantle boundary. This bending is called refraction. This creates a P wave shadow zone between 104 and 140 degrees.

Scientists first learned about these layers by watching waves. In 1906, a geologist named Richard Oldham studied seismograms. He saw that some stations did not record direct S waves. These stations were about 120 degrees away from the earthquake. In 1913, Beno Gutenberg noticed more changes. He saw P waves change speed quickly at the core-mantle boundary. He also saw S waves disappear there. He called this the Gutenberg discontinuity. These discoveries proved the outer core was liquid.

Shadow zones do not only happen at the Earth's core. Smaller liquid areas like magma reservoirs can cause them too. In 1981, Páll Einarsson studied the Krafla Caldera in Iceland. He used many seismometers to record earthquakes there. He found that S waves were missing or very small. This happened because of a magma reservoir. Between 2014 and 2018, Cheng-Horng Lin studied Taiwan. He looked at the Tatun Volcanic Group. He found that P waves were delayed. He also saw that S waves were missing. He believed a magma reservoir with 40% melt caused this.

Studying these zones helps us understand volcanoes. It can help us predict if a volcano might erupt. This is because of something called the rheological lockup. This is the amount of melted rock needed for an eruption. Scientists look at how much melt is in a volcano. In 2021, researchers studied Mt. Etna in Italy. They saw some areas had no S waves. Other areas had very small S waves. In 2014, a study of an eruption showed it might happen with 30 to 70% melt.

This information helps keep people safe near volcanoes.

A seismic shadow zone is a specific area on the Earth's surface where seismographs cannot detect certain earthquake waves. These waves are known as body waves, which travel through the interior of the planet. When an earthquake occurs, these waves radiate outward in a spherical pattern from the earthquake's hypocenter. Scientists study these waves to understand the hidden structures deep beneath our feet. The presence of a shadow zone indicates that the waves have encountered a liquid layer or a different type of structure.

There are two primary types of body waves: P waves and S waves. P waves, or primary waves, are compressional waves that move in the same direction as the wave propagates. S waves, or secondary waves, are transverse waves that move perpendicular to the direction of the wave. The speed and movement of these waves depend on three physical properties: density, incompressibility, and rigidity. P wave velocity is determined by these three factors, while S wave velocity depends entirely on the rigidity of the material.

Shadow zones occur because of how these waves interact with different materials, such as the Earth's layers. The Earth is composed of the crust, the mantle, the inner core, and the outer core. While the crust, mantle, and inner core are typically solid, the outer core is entirely liquid. Liquids have zero rigidity, which means they cannot support shear stress. Because S waves are shear waves, their velocity becomes zero when they hit a liquid. This causes the S wave shadow zone, where no direct S waves are detected more than 104 degrees from the epicenter.

P waves behave differently because they are compressional waves. When P waves hit the liquid outer core, they undergo refraction, which is the bending of waves at a boundary. This bending happens because of density differences and a great reduction in velocity, following Snell's law. This creates a P wave shadow zone located between 104 and 140 degrees from the hypocenter. This distance represents approximately 11,570 to 15,570 km from the source. However, some P waves can still be detected in this zone as PKP waves if they refract through the outer core.

Our understanding of these zones comes from important historical discoveries in seismology. In 1906, geologist Richard Oldham observed seismograms from various earthquakes. He noticed that certain seismic stations, specifically those 120 degrees away from the hypocenter, did not record direct S waves. Later, in 1913, Beno Gutenberg identified an abrupt change in P wave velocities at the core-mantle boundary. He also noted the disappearance of S waves at this boundary. Gutenberg called this the Gutenberg discontinuity, and his work helped prove the outer core is liquid.

Shadow zones are not limited to the Earth's core; they can also be caused by smaller liquid structures like magma reservoirs. In 1981, Páll Einarsson studied the Krafla Caldera in Northeast Iceland using a dense array of seismometers. He found an absence or very small amplitude of S waves, which he attributed to a magma reservoir. Similarly, between 2014 and 2018, Cheng-Horng Lin investigated the Tatun Volcanic Group in Taiwan. He observed P wave delays and an absence of S waves, suggesting a magma reservoir with at least 40% melt.

Studying these zones is vital for understanding volcanic activity and predicting hazards. The amount of liquid in a volcano can determine its eruptibility through a concept called rheological lockup. This refers to the specific percent crystal fraction required for a volcano to erupt. For example, a study of Mt. Etna in Italy in 2021 showed varying S wave patterns above its magma chamber. A 2014 study of an eruption there suggested that eruptions could be triggered when melt levels are between 30% and 70%. By measuring these shadow zones, scientists can better model future volcanic risks.