This post is part of the GeoConnect³d blog.
The Croatian part of the Pannonian basin system is a tectonically very active area. The unfortunate reminder are the recent destructive earthquakes in Petrinja (29th of December 2020, M=6.2 on Richter scale; https://www.hgi-cgs.hr/potres-u-petrinji-preliminarno-izvjesce/) and Zagreb (22nd of March 2020, M=5.5; Markušić et al., 2020). However, this is not the first strong earthquake close to Petrinja and Zagreb area. On 10th of October 1909, a strong earthquake (VIII ° MSC scale) hit the area of Pokuplje (http://www.gfz.hr/sobe-en/discontinuity.htm). Despite immeasurable human life and material damage, it also shed light on the Earth’s interior. The Mohorovičić discontinuity is going to be discovered!
Andrija Mohorovičić was a Croatian geophysicist born in Volovsko, a small place on the Adriatic Sea coast (Dinarides- Adria plate). After graduating in mathematics and physics in Prague, Mohorovičić first moved to Zagreb and Osijek town (Pannonian basin system), and then to Bakar (Adria plate) to become a teacher and professor (Skoko&Mokrović, 1982).
Finally, he moved to Zagreb, where he will find his most significant discovery, with a little help of two factors. The first one is a freshly acquired seismograph for the Zagreb station, and second, the proximity to the hypocentral area (Herak&Herak, 2010). Moreover, Zagreb and the surrounding area is situated in the complex contact zone of Pannonian basin system with Dinarides and Alps (van Gelder et al. 2015). The ongoing tectonic inversion, caused by Adria (Africa) motion towards the European plate units, strongly influences the area’s structural framework, causing reverse and strike-slip faulting. One of the examples is the Pokupsko fault, in which zone the earthquake happened in October 1909.
The 1909 earthquake was observed on many seismic stations all around Europe. Despite limited communicational options in that time (no email or video call), Mohorovičić managed to collect data from 41 stations (Herak&Herak, 2010). After analyzing the data, he concluded that individual phases of the seismic waves do not reach the station at distances above 700 km, and some of the stations were reached by two longitudinal waves (http://www.gfz.hr/sobe-en/discontinuity.htm). Mohorovičić came up with the answer: “both kinds of waves…are of the same type, that differs one from another only because they reach the surface of the Earth by different paths…” (Mohorovičić, 1910). Thus, based on the seismic wave’s analysis, Mohorovičić concluded that Earth interior is not homogeneous and it is composed of discontinuities. In the next few decades, his discovery was confirmed by other studies (Rothé, 1924). Eventually, in honor of Mohorovičić’s pioneer work, the sharp increase in seismic waves velocity from the Earth’s crust to the mantle is named Mohorovičić (Moho) discontinuity.
Finally, what can we conclude and learn from this short story?
Some of the essential preconditions for discovery are simple. Basically, human curiosity, data sharing, thinking out of the box spiced with consistent work, and a little bit of luck can lead to new insights into nature. Most importantly, science can help us learn from a bad situation like an earthquake and, consequently, prepare us for future challenges!
Marko Špelić, Marko Budić, Ivan Mišur, Radovan Filjak & Tomislav Kurečić
HGI-CSG – Croatian Geological Survey
Herak, D. Herak, M. (2010): The Kupa Valley (Croatia) earthquake of 8 October 1909 – 100 years later. Seismological research latters, 81, 1, 30-36.
Kapularić, J., Šumanovac, F., Markušić, S. (2019): Crustal structure of the northern Dinarides and southwestern part of the Pannonian basin inferred from local earthquake tomography. Swiss Journal of Geosciences, 112, 1, 181-198.
Markušić, S., Stanko, D., Korbar, T., Belić, N., Penava, D., Kordić, B. (2020): The Zagreb (Croatia) M5.5 earthquake on 22 March 2020. 10 (7), 252.
Rothé, E. (1924): Sur la propagation des ondes séismiques au voisinage de l’épidentre. Préliminiares continues et trajets a refraction, UGGI, Section de Seismologie Série A, Trav. Sci. Fasc, 1, Paris.
Skoko, D., Mokrović, J. (1982): Andrija Mohorovičić. Školska knjiga, Zagreb, 147.
Stipčević, J., Tkalčić, H., Herak, M., Markušić, S., Herak, D. (2011): Crustal and uppermost mantle structure beneath the External Dinarides, Croatia, determined from teleseismic receiver function. Geophsical Journal International, 185 (3), 1103-1119.
van Gelder, I. E., Matenco, L, Willingshofer, E., Tomljenović, B., Andriessen P.A.M., Ducea, M.N., Baniest, A., Gruić, A. (2015): The tectonic evolution of a critical segment of the Dinarides -Alps connection: Kinematic and geochronological inferences from the Medvednica Mountains, NE Croatia. Tectonics, 34 (9), 1952-1978.
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