SKS splitting analysis for the area of central and northern Dinarides

Seismic anisotropy refers to the directional dependence of seismic wave propagation within the Earth’s subsurface. Upper mantle anisotropy is primarily attributed to the lattice-preferred orientation (LPO) of olivine, an abundant mineral in the mantle, while anisotropy in the crust is mostly due to stress-aligned cracks. Mantle flow at depth appears to preferentially orient olivine crystals within upper mantle rocks – generally aligned parallel to the direction of flow.  One of the most common indicators of seismic anisotropy is shear wave splitting that occurs for seismic body-wave phases, for example, SKS phases. These core refracted phases from teleseismic events are sensitive to the orientation of olivine crystals and they split into two phases with different velocities while traveling through the anisotropic medium. The splitting beneath a seismic station is characterized by two splitting parameters: ϕ (polarization direction of the faster phase) and δt (delay time between the arrival of the two phases). Measuring fast directions on several stations in an area can give information about current mantle dynamics.

In the region of study, the Dinarides, we find complex tectonic interactions within the broader Mediterranean area. This mountain chain lies in a convergent plate boundary zone, with the Adriatic microplate positioned between the African and Eurasian plates. The tectonic complexities are a result of the Adria plate’s role, which underthrusts the Dinarides towards the northeast and collides with the European plate in the north, leading to active subduction processes. The subduction process beneath the Dinarides was active from the early Cretaceous to the Paleogene, and today it is only active in the southern part where Adria subducts beneath the Hellenides. Currently, the Adria plate is moving toward north/northwest with a counterclockwise rotation around a pole located in the western Alps. These tectonic interactions have left traces in the form of seismic anisotropy.

Tomographic studies conducted on the region (Fig. 1.) detected a low-angle high-velocity slab reaching a depth of about 200 km in the southern External Dinarides, Albanides, and northern Hellenides. No such slab is detected northwest along the Adriatic coast which means there is a slab gap in the northern and central Dinarides, probably due to a recent slab break-off. In this part of the Dinarides, a low-velocity zone was shown to be present which supports the idea of a slab gap. While neighboring mountain ranges like the Apennines, Alps, and Hellenides have been extensively studied using these methods, the Dinarides and the Croatian side of the Adriatic Sea have received less attention.

Fig. 1. Tomographic results at 150 km of depth taken from Salimbeni et al., 2022. A low-velocity zone, a slab gap, is visible in the northern and central Dinarides (circled).

This study uses 21 stations from a Croatian national seismic network and 7 stations from a Z3 network of the AlpArray project which covered most of Croatia, excluding the southernmost stations. This study aims to address the slab gap while also studying the anisotropy of a wider Croatian region and drawing comparisons with prior research conducted in this area. The SKS splitting analysis was done using the SplitRacer software for the MATLAB environment. SplitRacer is designed to process large data sets and can be operated using a graphical user interface or a newer automated extension. In this study, a newer extension was used on the data and the process was fully automated. The shear-wave splitting analysis is based on the minimum transverse energy method after applying the inverse splitting operator in a grid search for the delay time and fast polarization direction ϕ. Teleseismic events that occurred between January 2010 and December 2022, with M ≥ 6.0 and epicentral distance range 82°-120°, were used. Fig. 2 shows the directions of the fast axis along with their averages, with their length representing delay time.

Fig. 2. Results of the splitting analysis with directions of the fast axis along with their averages and length representing delay time.


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