3D shaking scenarios
Ground Motion Simulation of the 2021 Central Adriatic Earthquake
Significant seismicity in the Central Adriatic Sea has been noted in a number of publications, indicating the complexity of the tectonics within the Adriatic microplate. Adriatic microplate or Adria is nearly closed basin comprising Meso-Cenozoic continental block that represents the foreland of the Appenines and Dinarides-Hellenides mountains. Current deformation in this region is a result of the combined motions of the African Plate, Eurasian Plate, Anatolian block and Hellenic Arc (Figure 1a). Consequently, seismicity is conditioned by the sole movements of the Adriatic microplate and the resistance of the Dinarides to those movements. Most of the seismic activity occurrs offshore (Herak et al., 2005) and in the last few decades several series of earthquakes (Figure 1b) have been detected and analyzed, most notably:
1) 1986, ML = 5.0 series in the open sea;
2) 1988, ML = 5.3 series near the island of Palagruža;
3) 2003, ML = 5.5 series near the island of Jabuka;
4) 2021, ML = 5.5 series near the island of Vis.
The most recent earthquake series in 2021 implies anew higher seismic potential than what was previously assumed and opens questions regarding present-day tectonic stress distribution within the Adria microplate in general.
Figure 1. a) Tectonic sketch of Adria and surrounding regions. Shadowed area indicates the inferred extension of Adria continental block (Piccardi et al., 2011); b) Seismicity of the studied area. Epicenters of the earthquakes from the Croatian Earthquake Catalogue (Herak et al., 1996) are shown for years 1600 – 2022.
On March 27, 2021, a Mw = 5.2 (ML = 5.5) earthquake hit Central Adriatic Sea close to the island of Vis (Croatia; Fig. 1). The earthquake was felt in central Dalmatia (Croatia) and many central-southern Italian regions (from Ancona to Foggia). The fault-plane solution of the mainshock inicates that the event occured on a reverse fault at shallow depth (less than 10 km; Figure 2). By the end of the year 2022, more than 1000 aftershocks of varying magnitude have been located in the Croatian Earthquake Catalogue (Herak et al., 1996).
Figure 2. Mainshock epicenter (white star) from the Croatian Earthquake Catalogue. Stations (triangles) and several recorded seismograms (filtered between 0.01 and 8.0 Hz) used in this study.
To evaluate the expected ground motion parameters of the event, we make use of physics-based waveform modelling and simulate the earthquake using a new 3D crustal model for the central Adriatic Sea. The model was build using available geological and geophysical data. It honors surface topography and bathymetry and reflects most important geological features necessary to generate wave resonance effects that impact the duration and amplitude of the shaking. Details about the model are available on the link.
We use the software package SPECFEM3D Cartesian (Komatitsch et al., 2010) to deterministically simulate low-frequency ground motion (f < 1 Hz) for the new crustal model with attenuation scaled from the S-wave velocity model following Olsen’s empirical relations (Olsen et al., 2003). Source is represented as a point source with parametrization FPS1 shown in Figure 3. We plan on simulating the waveforms using other descriptions of the source in order to investigate how the source parametrization affects final results.
Figure 3. Low-frequency (0.01-0.5 Hz) recorded (black) and simulated (red) data for the Central Adriatic Sea earthquake on several seismic stations. Source is represented as a point source with parametrization FPS1 (Herak, M., 2021, personal communication) described in the left part of the figure.
Below are shown the video simulations of the Central Adriatic Sea for three components (E-W, N-S, Z) and the norm of the velocity in 5x real-time starting from the hypocentral time of the event. It takes seismic waves around 4s to reach Palagruža (Croatia), 7s to Sušac (Croatia) and Isola Pianosa (Italy), 10s to Italy shore (Gargano region) and 20s to Croatia shore (cental and southern Dalmatia region).
Although preliminary, results indicate how model structure impacts ground motion, especially in terms of amplification and duration. For instance, in the sedimentary basins within Adria itself, between Croatian islands and Foggia region, waves remain trapped due to the smaller velocities compared to the neighbouring carbonate structures. This results in amplification of amplitudes and prolongation of the overall shaking duration – whilst most continental parts shake for 1 minute, the seafloor and Foggia region remain shaking for almost 4 minutes!
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 and in collaboration with Dr. Irene Molinari from the National Institute of Geophysics and Volcanology (INGV), Italy. For the purposes of this research, resources from the ADA-Bologna computer cluster maintained by INGV in Bologna were also utilized.
Literature:
Herak, D., Herak, M., Prelogović, E., Markušić, S., Markulin, Ž. (2005): Jabuka island (Central Adriatic Sea) earthquakes of 2003. Tectonophysics, 398(3–4), 167-180. https://doi.org/10.1016/j.tecto.2005.01.007
Herak, M., Herak, D., Markušić, S. (1996): Revision of the earthquake catalogue and seismicity of Croatia, 1908–1992. Terra Nova, 8, 86–94.
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.
Piccardi, L, Sani, F., Moratti, G., Cunningham, D., Vittori, E. (2011): Present-day geodynamics of the circum-Adriatic region: An overview. Journal of Geodynamics, 51(2-3), 81–89. https://doi.org/10.1016/j.jog.2010.09.002.
Prepared by Helena Latečki
