Why do earthquakes occur only above the brittle-ductile transition depth?

The Effect of the Brittle/Ductile Transition on Earthquake Occurrence:

Earthquakes occur when large amounts of energy are released from tectonic plate movements deep within the Earth’s crust. The energy can only be released if the rocks are sufficiently brittle to fracture. Higher temperatures and higher pressures make rocks softer and more malleable. This is why the brittle ductile transition depth or BDTD (Brittle-ductile Transition Depth) represents the lower limit for regions that are seismically active (Gao, et al. 2017, 2017). The depth of this zone is usually around 20 to 35 km below the surface. The BDTD is too ductile for rocks to fracture below and cannot therefore cause earthquakes (Gao et. al., 2017). Because magma can be found under the BDTD, the BDTD also serves as a boundary (Gao et. al. 2017, 2017). Because magma has a higher plasticity than solid rocks, it absorbs seismic energy to prevent it from creating an earthquake. Magma may also partially melt rocks around it, making them less ductile and weaker (Gao, et al. 2017).

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