Andrija Mohorovic: Seventy years ago an earthquake shook Zagreb: His analysis resulted in a fundamental discovery



On October 8, 1909, at 9 hours 59 minutes Greenwich time, Zagreb was shaken by a strong earthquake which knocked down chimneys but did little other damage.

Thus it was that Andrija Mohorovicic, director of the Zagreb Meteorological Observatory and professor at the university, began his report in 1910. He drew on this earthquake and others to document his findings that there is a seismic velocity increase with depth at approximately 50 km in the earth. He recognized that the P waves occurred in two phases and were followed by S-waves which apparently followed corresponding travel paths. Later when this was confirmed by others, the boundary between the crustal layer and the mantle below was referred to as the Discontinuity of Mohorovicic. No doubt one reason that today the discontinuity is most frequently known as the Moho, or simply as the M-discontinuity, is that many find his name difficult to pronounce (try; moe-hoar-oh-VEE-cheach).