One-dimensional velocity structure modeling of the Earth's crust in the northwestern Dinarides

Abstract

Abstract. The studied area of the northwestern (NW) Dinarides is located in the northeastern (NE) corner of the Adriatic microplate and is bordered by the Adriatic foreland, the Southern Alps, and the Pannonian basin. Its complex crustal structure is the result of interactions among different tectonic units, the most important of which are the Eurasian plate and the Adriatic microplate. Despite numerous seismic studies in this tectonically complex area, there is still a need for a detailed, small-scale study focusing mainly on the upper, brittle part of the crust. In this work, we investigated the velocity structure of the crust with one-dimensional (1-D) simultaneous hypocenter–velocity inversion using routinely picked P- and S-wave arrival times. Most of the models computed in the combined P and S inversion converged to a stable solution in the depth range between 0 and 26 km. We further evaluated the inversion results with hypocenter shift tests, high- and low-velocity tests, and relocations. This helped us to select the best performing velocity model for the entire study area. Based on these results and the seismicity distribution, we divided the study area into three subregions, reselected earthquakes and stations, and performed the combined P and S inversion for each subregion separately to gain better insight into the crustal structure. In the eastern subregion, the P velocities in the upper 8 km of the crust are lower compared to the regional velocities and the velocities of the other two subregions. The P velocities between 8 and 23 km depth are otherwise very similar for all three models. Conversely, the S velocities between 2 and 23 km depth are highest in the eastern subregion. The NW and southwestern (SW) subregions are very similar in terms of the crustal structure between 0 and 23 km depth, with slightly higher P velocities and lower S velocities in the SW subregion. High vP/vS values were obtained for the layers between 0 and 4 km depth. Below that, no major deviations of vP/vS in the regional model from the value of 1.73 are observed, but in each subregion we can clearly distinguish two zones separated by a decrease in vP/vS at 16 km depth. Compared to the model currently used by the Slovenian Environment Agency to locate earthquakes, the obtained velocity models show higher velocities and agree very well with some of the previous studies. In addition to the general structural implications and the potential to improve the results of seismic tomography, the new 1-D P and S velocity models can also be used for reliable routine earthquake location and for detecting systematic travel time errors in seismological bulletins.

Seismogenic depth and seismic coupling estimation in the transition zone between Alps, Dinarides and Pannonian Basin for the new Slovenian seismic hazard model

“…Seismogenic depth analysis for earthquakes in Slovenia (Rajh et al, 2017;Rajh and Gosar, 2018) identified at least two distinct areas with relatively shallow or deep earthquake foci. Upon close examination and as suggested already by several papers (Stipčević et al, 2020;Rajh et al, 2022), one can observe that the depth distribution of earthquakes approximately follows the varying Moho topography. This has an impact on the seismogenic depth of seismic sources.…”

“…Seismogenic depth determined from the analysis of the earthquake catalogues depends greatly on the velocity model used to locate earthquakes. As Rajh et al (2022) and Rajh (2022) demonstrated, an improved velocity model in our study area can have a profound effect on earthquake locations, especially their depths. The earthquake hypocentres relocated with the new models were on average shallower by about 2-3 km.…”

“…As we are using mainly seismological data in seismic hazard analysis and sometimes infer some parameters from geophysical data, this can affect the results. Recently, Rajh et al (2022) inverted jointly for hypocentre parameters and 1-D P-and S-wave velocity models with station corrections. A joint inversion for 3-D P-and S-wave velocity models from LET (Rajh, 2022) improved the earthquake locations even further and provided additional insight into the upper crustal structure.…”

“…Recently, Rajh et al (2022) inverted jointly for hypocentre parameters and 1-D P-and S-wave velocity models with station corrections. A joint inversion for 3-D P-and S-wave velocity models from LET (Rajh, 2022) improved the earthquake locations even further and provided additional insight into the upper crustal structure. Both studies showed that the hypocentres located in previous studies were located too deep.…”