The Mohorovičić discontinuity, commonly called the Moho, is the boundary separating the Earth’s crust from the underlying mantle. It is defined by a sudden increase in the velocity of seismic waves as they move from the less dense crustal rock to the denser mantle material beneath it. Understanding the depth and nature of the Moho provides fundamental insights into the planet’s layered structure and the differing compositions of the crust and the mantle. The discontinuity is a global feature, varying significantly from around 5 to 10 kilometers beneath oceanic crust to an average of 30 to 50 kilometers under continental crust. The Moho’s existence and depth were first revealed through the careful analysis of earthquake recordings.
The Seismic Event of 1909
The discovery of the Moho was made possible by the 1909 Kulpa Valley earthquake in Croatia. This moderate seismic event, with an epicenter approximately 39 kilometers southeast of Zagreb, provided the necessary data. Croatian seismologist Andrija Mohorovičić, director of the meteorological observatory in Zagreb, meticulously collected and analyzed seismograms from this earthquake. His focus on local earthquake data was a departure from the common practice of the time, which favored more distant events.
The proximity of the earthquake’s focus, combined with seismic recordings from multiple European stations, created an ideal dataset for observing variations in wave travel times. Mohorovičić gathered records from over 40 stations, some located relatively close to the epicenter, giving him a high-resolution view of how seismic waves traveled through the local crust. This innovative use of regional records allowed him to plot the travel times of waves with unprecedented precision.
Identifying Dual Seismic Waves
Mohorovičić’s discovery centered on his observation of primary waves (P-waves), which are compressional waves that travel fastest through the Earth. Analyzing the seismograms, he noted that at a certain distance from the epicenter, two distinct sets of P-waves arrived at the recording stations. One set followed a direct path through the crust, corresponding to the expected slower velocity of crustal rock. The second set, however, arrived significantly earlier than the first at stations beyond about 200 kilometers from the epicenter.
This early arrival demonstrated that the second wave set must have traveled a different, faster route. Mohorovičić deduced that these faster waves had been refracted, or bent, at a distinct boundary beneath the crust. The waves traveled down to this boundary, moved horizontally along the top of the faster, denser material below, and then refracted back up to the surface. The P-wave velocity just below this boundary was approximately 7.75 kilometers per second, a marked increase from the 5.68 kilometers per second velocity measured within the crust. This jump in speed defined the Mohorovičić discontinuity.
Calculating Depth via Refraction
The existence of the dual P-waves—the slower, direct crustal wave and the faster, refracted mantle wave—provided the necessary information for a geometric calculation of the boundary’s depth. Mohorovičić used the observed difference in arrival times between these two wave types at various distances from the epicenter. He realized that the travel time difference was directly related to the depth of the interface causing the refraction.
By knowing the velocities of the P-waves in both the upper layer (crust) and the lower layer (mantle), and the distance at which the refracted wave began to arrive first, he applied principles of basic geometry. The relationship between the time, distance, and velocities allowed him to solve for the unknown depth. Mohorovičić’s initial calculation from the Kulpa Valley data placed the depth of this discontinuity at approximately 54 kilometers. This geometrically derived value offered the first physical evidence of a layered Earth structure separated by an abrupt change in rock properties.