At the beginning of 1962, one of the most important earthquake sequence in the second half of the 20th century began in Croatia – the one in the area of Makarska. The sequence began with an earthquake on 7 January at 11:03 with an event if local magnitude ML = 5.9 – its epicenter was in the Hvar channel, between the islands of Hvar and Brač, and the focus was at a depth of 12 km. The earthquake was devastating with the highest intensity of VIII MCS (Mercalli-Cancani-Sieberg scale). Although there was damage in Makarska itself, the villages in the area of Tučepi and Podgora suffered the most, while two people lost their lives (Hrstić, 2012). Damage was established as far as Mostar and Čapljina, Pelješac and Korčula, Lastovo and Brač. The earthquake caused numerous landslides and rockfalls. Numerous aftershocks followed, but it turned out that they were only earthquakes that preceded the main earthquake that occurred on 11 January 1962 at 06:05 local magnitude ML= 6.1. Its epicenter was somewhat closer to the mainland, and the hypocentre was at 14 km of depth. Its greatest intensity was estimeted as VIII – IX MCS, and it took the life of a young man. Once again, the villages of Podgorje and Biokovo suffered the most (Herak and Herak, 2012; Hrstić, 2012). It was so strong that it was observed on ships at sea (Herak and Herak, 2012 and references therein). The strongest aftershocks occurred a few hours later, on 11 January 1962 at 11:02, of ML = 5.0 and on January 21, 1962 at 3:51 ML = 5.1. After the mainshock, people from the epicentral area were evacuated. In the area of the Makarska district, the earthquake irreparably damaged 2819 residential buildings, while 9023 residential buildings and 130 public buildings suffered light to heavy damage (Hrstić, 2012).
Only one seismic station was operating in Croatia at that time – it was Wiechert type of mechanical seismograph in Zagreb, and the nearest seismological station was located in Titograd, today’s Podgorica, in Montenegro (Herak et al., 2001). This enabled the location of only stronger foreshocks and aftershocks, and limited seismological analyses significantly as well as the reliability of certain earthquake parameters – especially the depth of the earthquake. The Zagreb seismic station recorded 63 earthquakes in this series (Figure 1; Herak et al., 2001) – unfortunately, the vast majority of (very) weak earthquakes were not recorded. Most of the subsequent earthquakes had their epicentre under the sea, between Brač, Hvar and the mainland. Only in the first week since the start of the series, more than 100 earthquakes were felt.
The focal mechanisms were calculated for the three strongest earthquakes of this series, and all of them suggest a fault with a large inclination, probably oriented roughly like the longer axes of the islands of Brač and Hvar (ZNZ–IJI). The slip was mostly horizontal, along the fault, but it also had a small reverse vertical component. These solutions are in agreement with today’s knowledge of local tectonics (see Herak and Herak, 2012 and Herak et al., 2001). The cause of the earthquake is the movement of the Adriatic microplate towards the northeast and its collision with the Eurasian tectonic plate – in this long process the Dinarides were created.
According to the research presented in the work of Herak et al. (2001), the mainshock probably caused a very small tsunami. The results of the numerical modelling of sea level change correspond very well to the record of the tide gauge in Split. This numerical experiment showed that the small vertical component of displacement on the earthquake fault (about 15 cm at the bottom), with the relatively small magnitude of the earthquake to generate a tsunami, was sufficient to cause a small tsunami of several centimetres traveling at an average speed of 80 km/h.
This series of earthquakes, with the mainshock of magnitude 6.1, clearly shows how strong earthquakes we can expect in this area and what we must be prepared and adapted to them. It largely defines the earthquake hazard (seismic hazard) in this area. You can read the description of the impact of this seismic sequence on the Makarska coast, the reactions to it and its long-term consequences in the work of Ivan Hrstić (2012). The Makarska earthquakes were also the subject of an excellent documentary by screenwriter and director Arsen Oremović Nesreća: Zemlja drhti (Part 2)*.
* Available at hrti.hrt.hr
Prepared by Iva Dasović
References
Herak, M., Orlić, M., Kunovec Varga, M. (2001): Did the Makarska earthquake of 1962 generate a tsunami in the central Adriatic archipelago?, Journal of Geodynamics 31, 71–86.
Herak, D., Herak, M. (2012): Seizmičnost i potrersna opasnost na Makarskom primorju, Makarsko primorje danas, Institut društvenih znanosti Ivo Pilar, Zagreb, 265–276. link (in Croatian)
Hrstić, I. (2012): Zbivanja na Makarskom primorju tijekom i nakon potresa 1962. godine, Makarsko primorje danas, Institut društvenih znanosti Ivo Pilar, Zagreb, 279–296. link (in Croatian)
