Seismic hazard
Seismic hazard assessment is based on knowledge of the seismicity of the area and its surroundings, its internal structure and local soil conditions. It tells us about the probability of exceeding the selected value (which can cause an undesirable outcome: for example, severe damage) of a certain size (for example, maximum/peak horizontal ground acceleration) in a certain period of time (usually 50 years). Seismic hazard is the basis for determining seismic risk in an area, along with vulnerability and exposure. The goal is for all buildings – modern and cultural heritage, existing and future – to be earthquake resistant. In order to achieve this, it is necessary to apply the latest knowledge on seismic hazards in spatial planning and construction.
The results that we will get during the CRONOS project by applying various methods, which will include different maps, will be used directly in the assessment of seismic hazard in the study area. The purpose of this project is to publish the results of our research, a new seismic hazard map, and provide access to the data to all interested parties in order to contribute to the safety of Croatian citizens.
Shake maps for the city of Sinj (Phase 1)
Republic of Croatia is located in a seismically active area (especially its coastal part together with the northwestern continental part), where thousands of earthquakes are recorded annually. On average, four of them are strong enough to cause damage. Although earthquakes cannot be prevented, seismic hazard can be assessed and mitigation plans can be made accordingly. A high-quality seismic hazard assessment, the inclusion of the necessary measures in national and local legislation and the education of citizens, creates an environment for minimizing the effects of future earthquakes, both on people and on material property.
That is precisely why the main goal of the CRONOS project is the development and modernization of seismic hazard assessment in Croatia. As part of the above, it is planned to prepare all input parameters for a quality seismic hazard assessment and the assessment of the same for the selected area. The area of the City of Sinj was selected as a “case study”, and the majority of the results will refer to that area.
Seismic hazard calculation procedures were developed for the needs of civil engineers. Initially, seismic hazard was calculated for macroseismic intensity (Cornell, 1968) because macroseismic intensity, by definition, describes the effects of an earthquake and can be directly applied in risk assessment. The transition from intensity to peak acceleration occurs, among other things, because macroseismic intensity is a pseudophysical quantity (Peruzza, 1996), while other ground motion parameters, acceleration, speed and displacement, are physical quantities that can be directly used in design.
In order to assess the seismic hazard, it is necessary to have a good knowledge of the geological characteristics of the wider researched area, which includes knowledge of the position, slope and extension of active faults and the modes of faulting on individual faults. A complete and unified catalog of earthquakes with reliable locations of epicenters and magnitudes is prerequisite. In addition, it is necessary to define ground motion prediction equations that describe the movement of the ground surface during an earthquake, and a number of other parameters, as well as to perform a detailed seismic microzoning of the researched area.
For the needs of the local public, local governmental units and civil protection bodies, it is necessary to know the parameters of the potential ground shaking in the case of an earthquake (as a rule, the peak ground acceleration), so that they can improve the response to the consequences after the earthquake occurs, and reduce them.
Therefore, shake maps were calculated for three earthquake scenarios, which are based on three earthquakes that occurred, whose parameters were taken from the Croatian earthquake catalog (the continuously updated version of the earthquake catalog for Croatia published in Herak et al. (1996), Markušić et al. (1998) , Ivančić et al. (2002, 2006 and 2018)). These are: the strongest earthquake in the catalog, with an epicentral distance of about 165 km from the City of Sinj and a magnitude of MW = 7.1 (Markušić et al., 2017), which occurred in 1667 in the sea in front of Dubrovnik, and two strongest instrumentally recorded earthquakes in the vicinity of the City of Sinj , which occurred in 1962 in Makarska (MW = 6.2) and in 1996 in Ston (MW = 6.0) at epicentral distances of 55 and 130 km (Figure 1).
Figure 1. Locations of three considered earthquake scenarios regarding location
of the City of Sinj.
Data on soil types and Vs30 velocities were obtained on the basis of research carried out as part of the CRONOS project (Figure 2).
Figure 2. Map of Vs30 values for the City of Sinj area.
Shake maps for the area of the City of Sinj, for considered earthquake scenarios (Figures 3-5), were calculated using the OpenQuake software package (Pagani et al., 2014; Silva et al., 2014), whose development and maintenance is led by the Global Earthquake Model Foundation ( GEM).
Figure 3. Shake map for the City of Sinj area for the case of an earthquake scenario based on the earthquake that occurred in Dubrovnik in 1667 (MW = 7.1).
Peak ground acceleration (PGA) values are shown in units of g.
Figure 4. Shake map for the City of Sinj area for the case of an earthquake scenario based on the earthquake that occurred in Makarska in 1962 (MW = 6.2).
Peak ground acceleration (PGA) values are shown in units of g.
Figure 5. Shake map for the City of Sinj area for the case of an earthquake scenario based on the earthquake that occurred in Ston in 1996 (MW = 6.0).
Peak ground acceleration (PGA) values are shown in units of g.
The calculated shake maps for the City of Sinj area (Figures 3-5) for the considered three earthquake scenarios define the areas of increased seismic hazard, and provide important information for the preparation of further urban plans and the implementation of earthquake resistant construction. Further research will go in the direction of calculating the parameters necessary for the assessment of seismic risk.
These results raise awareness of the danger of earthquakes and increase preparedness for earthquakes, both for local authorities and the general public.
Seismic hazard maps (Phase 2)
This phase is still in preparation. The results will be available on this website.
References:
Cornell, C.A. (1968): Engineering seismic risk analysis. Bulletin of the Seismological Society of America, 58, 1583 -1606.
Herak, M., Herak, D., Markušić, S. (1996): Revision of the earthquake catalogue and seismicity of Croatia, 1908-1992. Terra Nova, 8, 86-94.
Ivančić, I., Herak, D., Markušić, S., Sović, I., Herak, M. (2002): Seismicity of Croatia in the period 1997–2001. Geofizika, 18–19, 17–29.
Ivančić, I., Herak, D., Markušić, S., Sović, I., Herak, M. (2006): Seismicity of Croatia in the period 2002–2005. Geofizika, 23(2), 87–103.
Ivančić, I., Herak, D., Herak, M., Allegretti, I., Fiket, T., Kuk, K., Markušić, S., Prevolnik, S., Sović, I., Dasović, I., Stipčević, J. (2018): Seismicity of Croatia in the period 2006–2015. Geofizika, 35(1), 69–98, https://doi.org/10.15233/gfz.2018.35.2
Markušić, S., Herak, D., Ivančić, I., Sović, I., Herak, M., Prelogović, E. (1998): Seismicity of Croatia in the period 1993-1996 and the Ston-Slano earthquake of 1996. Geofizika, 15, 83–101.
Markušić, S., Ivančić, I., Sović, I. (2017): The 1667 Dubrovnik earthquake – some new insights. Studia Geophysica et Geodaetica, 61/3, 587-600. DOI: 10.1007/s11200-016-1065-4.
Pagani, M., Monell, D., Weatherill, G., Danciu, L., Crowley, H., Silva, V., Henshaw, P., Butler, L., Nastasi, M., Panzeri, L., Simionato, M., Vigano, D. (2014): OpenQuake Engine: An Open Hazard (and Risk) Software for the Global Earthquake Model. Seismological Research Letters, 85/3, 692-702. DOI: 10.1785/0220130087.
Peruzza, L. (1996): Attenuating intensities. Annali di Geofisica, 39, 1079-1093.
Silva, V., Crowley, H., Pagani, M., Monelli, D., Pinho, R. (2014): Development of the OpenQuake engine, the Global Earthquake Model’s open-source software for seismic risk assessment. Natural Hazards, 72(3), 1409-1427. DOI: 10.1007/s11069-013-0618-x.
