Seismic and accelerometric network

Figure 1. Locations of seismic stations in Dalmatia. Stations marked with red triangles were set up as part of Component 1 of the CRONOS project: stations HR01A – HR12A were operating October 2022 – October 2024, while stations JANJ, TRNV, PTNK, STRV, MRCN snd VTLJ were deployed in August 2024. Blue triangles indicate existing seismic stations (seismographs) of the Croatian Seismograph Network (permanent and temporary stations) maintained by the Department of Geophysics.

The operation of the temporary seismic stations is monitored from Zagreb, and in case of recorded problems, they are visited and difficulties are resolved. Data from the stations are stored on a server at the Geophysical Department of the Faculty of Science and with Norwegian partners at the University of Bergen and are open to all member institutions of the AdriaArray initiative.

Since seismometers are very sensitive instruments (they can detect strong earthquakes even on the opposite side of our planet) and data is transmitted to the main server in real time, it is important that they are located in relatively quiet locations without too much background noise, with available mobile internet network signal and access to electricity to enable uninterrupted power supply to all devices. Also, in selecting locations, great importance was given to local soil conditions, with the requirement that the seismometer be placed on solid rock. Such choice of locations has enabled easy installation of instruments and ensured their smooth operation with an optimal ratio of the quality of recorded earthquake signals and device protection. An example of a location that meets these conditions is the Franciscan monastery on the island of Visovac (Figure 2) in Krka National Park, where the temporary seismic station HR03A is located.

Figure 2. The island of Visovac (a) and the Franciscan monastery (b) in Krka National Park where the temporary seismic station HR03A is located. Installation of the seismic station inside the monastery with a thermal insulation box around the instrument and a protective box for the remaining devices (c) and analysis of background noise for this location (d).

Other examples of locations that meet these conditions are regional schools in smaller places (e.g., station HR01A), basements of buildings owned by local government (e.g., HR02A).

An example of how the newly installed temporary network has raised the quality of earthquake recordings in the Dalmatia region is the record of a local Dalmatian earthquake, local magnitude ML4.3. The depicted earthquake occurred on 23 September 2023 at 17:01, with an epicentre 10 km southwest of Metković (42.912N; 17.587E) at a depth of 4 km (Figure 3). The epicentre of the earthquake was just 8 km from the CRONOS station HR12A and 23 km from station DF06 on the island of Mljet of the temporary seismic network Du-Net (DN) set up for the HRZZ DuFAULT research project. Since the epicentre of the earthquake was between these two stations, Figure 4 shows a textbook example of the propagation of a seismic wave recorded at the northern (CRONOS) and southern (Du-Net) stations.

Figure 3. Layout of seismic stations in the CRONOS (blue triangles) and DuFault (green triangles) projects. The red star marks the epicentre of the local magnitude ML4.3 earthquake that occurred on 23 September 2023 at 17:01, with an epicentre 10 km southwest of Metković.

Figure 4. Records of the vertical component of ground motion velocity of the earthquake near Metković (Figure 3) at seismic stations of the CRONOS project (HRXX label) in Northern and Central Dalmatia and the DuFAULT project (DFXX label) in the wider Dubrovnik area. Stations are arranged by the distance of the earthquake location from the seismic stations.

We have already written about distant earthquakes and how they were recorded at our stations, for example, the earthquake in Turkey in February 2023.

CRONOS in International Projects

AdriaArray is a European initiative with one of its goals being to cover the Adriatic microplate and its active edges in the central Mediterranean with a dense network of seismic stations (Figure 5) to better understand the causes of active tectonics and volcanism and improve seismic hazard analysis in the area.

Figure 5. Seismic networks and stations within the international initiative AdriaArray for studying the geodynamics of the Adriatic microplate.

Earthquakes are a global concern and do not respect national borders. To truly understand their patterns and behaviour, we need data from all parts of the world. Therefore, open data are of utmost importance; they enable scientists and researchers from different countries to collaborate, promote transparency, and encourage innovation.

By collaborating with colleagues from different countries, we can collect data from various seismic networks, helping us build a more comprehensive understanding of earthquake dynamics. Sharing knowledge and resources on an international level is not just an option – it is a necessity in the field of seismology.

Through global collaboration, we can improve early warning systems, enhance earthquake preparedness, and ultimately save lives. When seismologists from different countries join forces, they bring their unique expertise and perspectives, resulting in more reliable and accurate earthquake research.

Acknowledgements

The research within the CRONOS project would not have been possible without the immense help of the local community who assisted us in finding and securing locations and the necessary infrastructure for our seismological stations.

Prepared by Dr. Dinko Šindija