Shadow zone

Part of a series on
Earthquakes
Types Mainshock Foreshock Aftershock Blind thrust Doublet Interplate Intraplate Megathrust Remotely triggered Slow Submarine Supershear Tsunami Earthquake swarm
Causes Fault movement Volcanism Induced seismicity
Characteristics Epicenter Epicentral distance Hypocenter Shadow zone Seismic waves P wave S wave
Measurement Seismometer Seismic magnitude scales Seismic intensity scales
Prediction Coordinating Committee for Earthquake Prediction Forecasting
Other topics Shear wave splitting Adams–Williamson equation Flinn–Engdahl regions Earthquake engineering Seismite Seismology
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A seismic shadow zone is an area of the

In 1913, Beno Gutenberg noticed the abrupt change in seismic velocities of the P waves and disappearance of S waves at the core-mantle boundary. Gutenberg attributed this due to a solid mantle and liquid outer core, calling it the Gutenberg discontinuity.^[4]

Seismic wave properties

The main observational constraint on identifying liquid layers and/or structures within the earth come from

The reason for this is P wave and S wave velocities are governed by different properties in the material which they travel through and the different mathematical relationships they share in each case. The three properties are: incompressibility (${\displaystyle k}$), density (${\displaystyle p}$) and rigidity (${\displaystyle u}$).^[11][14]

P wave velocity is equal to:

${\displaystyle {\sqrt {(k+{\tfrac {4}{3}}u)/p}}}$

S wave velocity is equal to:

${\displaystyle {\sqrt {u/p}}}$

S wave velocity is entirely dependent on the rigidity of the material it travels through. Liquids have zero rigidity, making the S-wave velocity zero when traveling through a liquid. Overall, S waves are

Although the core-mantle boundary casts the largest shadow zone, smaller structures, such as magma bodies, can also cast a shadow zone. For example, in 1981, Páll Einarsson conducted a seismic investigation on the Krafla Caldera in Northeast Iceland.^[16] In this study, Einarsson placed a dense array of seismometers over the caldera and recorded earthquakes that occurred. The resulting seismograms showed both an absence of S waves and/or small S wave amplitudes. Einarsson attributed these results to be caused by a magma reservoir. In this case, the magma reservoir has enough percent melt to cause S waves to be directly affected.^[16] In areas where there are no S waves being recorded, the S waves are encountering enough liquid, that no solid grains are touching.^[17] In areas where there are highly attenuated (small aptitude) S waves, there is still a precent of melt, but enough solid grains are touching where S waves can travel through the part of the magma reservoir.^[12][15][18]

Between 2014 and 2018, a geophysicist in Taiwan, Cheng-Horng Lin investigated the magma reservoir beneath the

• Seismic wave
• Ray tracing (physics)
• P wave
• S wave
• Snell's Law
• Structure of Earth
• Core-mantle boundary

References