Mohorovičić discontinuity

The seismic boundary between Earth’s crust and mantle, marked by an abrupt increase in seismic-wave velocities. It is commonly called the Moho.

Seismic wave: An elastic disturbance that travels through Earth or another material, carrying energy without transporting matter overall. Changes in seismic-wave speed and paths are the main evidence for the Moho.
Andrija Mohorovičić: A Croatian geophysicist who used earthquake recordings to infer a boundary within Earth. His analysis of a 1909 earthquake established the discontinuity now bearing his name.
Earth's crust: Earth’s outermost solid rocky layer, comprising continental and oceanic crust. The Moho marks the crust’s lower boundary.
Crustal thickness: The vertical distance between Earth’s surface and the base of its crust. Mapping Moho depth provides the standard measure of crustal thickness.
Moho transition zone: A finite depth interval across which seismic properties change from crustal to mantle values. Some regions show a gradational transition rather than a single sharp Moho.
P-wave: A compressional seismic wave that can travel through solids and fluids. Its velocity typically rises across the crust–mantle boundary.
1909 Kupa Valley earthquake: An earthquake near Pokupsko, Croatia, whose recorded seismic arrivals revealed a deep velocity boundary. Its seismograms supplied the evidence behind Mohorovičić’s interpretation.
Earth's mantle: The thick rocky layer between Earth’s crust and core, deforming over geological time. The Moho marks the mantle’s upper boundary.
Receiver function: A seismological method that isolates converted seismic phases to infer structure beneath a recording station. Converted waves in receiver functions are widely used to estimate local Moho depth.
Lower crust: The deepest part of the crust, whose composition and seismic properties vary among tectonic regions. Its variable properties can blur the boundary’s seismic expression.

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