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The Kupa Valley (Croatia) earthquake of 8 October 1909 – 100 years later

The Kupa Valley (Croatia) earthquake of 8 October 1909 belongs to a group of milestone events in the history of geophysics and seismology. Also known as the Kulpa Valley, Pokuplje or the Pokupsko earthquake, it is often mentioned in textbooks, encyclopedias, and overviews of history of science, as the earthquake whose seismograms provided key data for Andrija Mohorovičić’ s proof of the existence of the crust-mantle boundary which was later named after him. The earthquake occurred just at the right time and on a right place to provoke research that resulted in one of the most-cited seismological papers (Mohorovičić, 1910a, b, c) from the beginning of the last century. Our revisit of the Kupa Valley earthquake and analysis of all available data resulted in an improved instrumental location of this important historical event. Considering also the fault-plane solution – the earliest one in our FPS database – we conclude that the earthquake most probably occurred about 10 km north of Pokupsko on the steeply dipping reverse Pokupsko fault at the depth of about 14 km. The fact that epicenter lies in the NW part of the meisoseismal area suggests a unilateral rupture towards SE. Reanalyzing macroseismic data we found the maximum intensity of VIII° EMS. Pronounced elongation of isoseismals indicates strong anisotropy of attenuation in the Pokupsko epicentral area, which is the largest in the direction perpendicular to the strike of the main geological features. These new data contribute to the overall understanding of the regional tectonics, and will thus also improve hazard estimates for the Pokupsko area.

Kupa valley earthquake; Andrija Mohorovičić; seismic hazard

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