Shadow zone

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.