Shadow zone

The most recognized shadow zone is due to the core-mantle boundary where P waves are refracted and S waves are stopped at the liquid outer core; however, any liquid boundary or body can create a shadow zone.

For example, magma reservoirs with a high enough percent melt can create seismic shadow zones.

[4] The main observational constraint on identifying liquid layers and/or structures within the earth come from seismology.

[6] The P waves are refracted by the liquid outer core of the Earth and are not detected between 104° and 140° (between approximately 11,570 and 15,570 km or 7,190 and 9,670 mi) from the hypocenter.

[7][8] This is due to Snell's law, where a seismic wave encounters a boundary and either refracts or reflects.

In this case, the P waves refract due to density differences and greatly reduce in velocity.

[10] The S waves cannot pass through the liquid outer core and are not detected more than 104° (approximately 11,570 km or 7,190 mi) from the epicenter.

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.

In this case, the magma reservoir has enough percent melt to cause S waves to be directly affected.

Lin attributed this finding to be due to a magma reservoir with at least 40% melt that casts an S wave shadow zone.

[19][20] However, a recent study done by National Chung Cheng University used a dense array of seismometers and only saw S wave attenuation associated with the magma reservoir.

[21] This research study investigated the cause of the S wave shadow zone Lin observed and attributed it to either a magma diapir above the subducting Philippine Sea plate.

[22][23] Determining the percent melt of a volcano could help with predictive modeling and assess current and future hazards.