3D shaking scenarios
Ground Motion Simulation of the 1962 Makarska Earthquake
One of the strongest earthquakes in Croatia is certainly the Makarska earthquake of 1962, which occurred just a few days after the beginning of a series of earthquakes along the Makarska coast (Figure 1). Specifically, the series began with an earthquake on January 7th at 11:03 local time with a magnitude ML = 5.9 with its epicenter in the Hvar channel (between the islands of Hvar and Brač). The main earthquake that occurred on January 11, 1962, at 06:05 local time with a magnitude ML = 6.1 had an epicenter somewhat closer to the mainland, and its highest intensity was rated VIII – IX °MCS. This earthquake series is one of the most significant recorded earthquake sequences in the second half of the 20th century in Croatia and the main earthquake event greatly defines the seismic hazard of the Makarska coast. An overview of the earthquake series is available at the link.
Figure 1. Epicenters of the Makarska coast earthquake series from the Croatian earthquake catalogue (Herak et al., 1996).
Given the importance of this earthquake, and the fact that it is certain that one day an earthquake of the same magnitude could occur in this area again, the aim is to estimate the expected ground motion parameters for such a scenario. To assess the expected ground motion parameters of the main Makarska earthquake, the method of physics-based waveform modeling was applied. Specifically, a deterministic simulation of low-frequency (f < 1 Hz) ground motion was performed using the SPECFEM3D Cartesian software package (Komatitsch et al., 2010) for the Central Adriatic Sea crustal model. This three-dimensional model of the Central Adriatic Sea reflects the most important geological characteristics of the studied area and was developed using available geological and geophysical data. Details about the model are available on the link.
In the simulations, the attenuation determined from the S-wave velocity model according to Olsen’s empirical relations (Olsen et al., 2003) is also included besides the 3D model. The seismic source is represented using a simplified kinematic finite fault model. In this simplified model, an uniform distribution of displacements along the entire fault plane with bilateral temporal symmetry of faulting (concentric from the initiation point, i.e., the hypocenter located in the middle of the fault plane and a depth of 10 km) is assumed. The displacement along the fault 9.7 km long and 8.8 km wide was mostly horizontal with a small reverse vertical component. This source parameterization was made following the work of Herak et al. (2000) and is shown in Figure 2.
Figure 2. Representation of the seismic source of the main Makarska earthquake of 1962 used in simulations of low-frequency ground motion.
Below are video simulations of the 1962 Makarska earthquake. Three components (E-W, N-S, Z) and the norm of the velocity norm are displayed in 5x real-time starting from the hypocentral time of the event.
Due to the proximity of the earthquake epicenter, very intense shaking in the Makarska coast begins almost simultaneously with the onset of the earthquake and lasts for about 30 seconds. In the sea, this shaking is further amplified and prolonged – intensely for over 2 minutes, and altogether for more than 4 minutes, which is the duration of this simulation. The difference in the duration of shaking on the land and at the sea primarily stems from different composition of these two units. At the sea, seismic waves remain trapped in softer sedimentary layers between the surface and the solid rock below. Consequently, there is amplification and prolongation of the duration of ground shaking. On land, which is composed of carbonate rocks with higher velocities, the wave front passes faster, and due to the strength of the rocks, the amplitudes decay faster, resulting in a shorter duration of shaking. Furthermore, between differently composed seabed and islands, there is a channeling effect of waves and energy towards the open sea, further increasing the intensity and duration of shaking in this area (see horizontal components after the first minute).
This simulation was conducted as part of the research work of PhD candidate Helena Latečki under the supervision of Professor Josip Stipčević from the Faculty of Science, University of Zagreb. This research was performed using the Advanced computing service provided by University of Zagreb University Computing Centre – SRCE.
Literature:
Herak, M., Herak, D., Markušić, S. (1996): Revision of the earthquake catalogue and seismicity of Croatia, 1908–1992. Terra Nova, 8, 86–94.
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.
Komatitsch, D., Erlebacher, G., Göddeke, D., Michéa, D. (2010): High-Order Finite-Element Seismic Wave Propagation Modeling with MPI on a Large GPU Cluster. Journal of Computational Physics, 229(20), 7692–7714. https://doi.org/10.1016/j.jcp.2010.06.024
Olsen, K.B., Day, S.M., Bradley, C.R. (2003): Estimation of (Q) for Long-Period (>2 Sec) Waves in the Los Angeles Basin. Bulletin of the Seismological Society of America, 93(2), 627–38.
Prepared by Helena Latečki
