Slide 1

Andrija Mohorovicic was a Croatian meteorologist who realized he could make a more immediate effect by studying seismology. In 1906, he ordered a seismograph from Emil Weichert. In 1909, he ordered two more, just in time for a large earthquake near his hometown! Mohorovicic collaborated with colleagues around the European continent and asked them to send him their seismographic results from the earthquake. He hypothesised that the waves would have a linear relationship with the distance from the hypocenter of the earthquake, as pictured in Figure 1.

Figure 1. Taken from https://www.iris.edu/hq/inclass/animation/layers_of_the_earthwhat_is_the_moho

If the center of the earth is uniform, then the seismic waves should have arrived as Mohorovicic predicted. So, his colleagues' results, illustrated in Figure 2, surprised him: waves from distant seismic stations arrived sooner than they should have and some stations recorded two sets of P-waves and S-waves that arrived at different times!

Figure 2. Taken from https://www.iris.edu/hq/inclass/animation/layers_of_the_earthwhat_is_the_moho

Thus, using Snell's Law, Mohorovicic concluded that there was a reflective boundary with the lower layer consisting of a faster material. Today, we call the layers on either side of this boundary the crust and the mantle. The boundary is referred to as the Mohorovicic Discontinuity, or simply the Moho.

A. Mohorovicic