Crustal structure of the northern Dinarides and southwestern part of the Pannonian basin inferred from local earthquake tomography

We present the results of local earthquake tomography (LET) analysis to investigate the crust and uppermost mantle structure in the northern Dinarides and southwestern Pannonian basin. Datasets of P-wave travel times are inverted to recover a three-dimensional P-wave velocity model of the survey area. Two data subsets were used in this study: (1) data recorded on 15 temporary seismic stations, which were deployed in Croatia in the framework of ALPASS-DIPS project, and (2) travel time datasets from the Croatian Seismological Survey and ORFEUS databases. The data enabled to achieve a resolution of less than a hundred kilometres in horizontal directions and a few kilometres in vertical direction in the area with good ray coverage, as is documented by the resolution tests. Velocity variations are computed on a grid using the three-dimensional nonlinear tomographic inversion method. Our study provides the first crustal three-dimensional seismic modelof the studied area, and it is correlated with previous results in the survey area allowing us to infer the main crustal structures with high confidence.The velocity model reveals crustal thickening beneath the Dinarides and significant crustal thinning beneath the Pannonian basin. The Moho surface was determined on the basis of the highest velocity gradients in the vertical cross-sections. We find relatively high velocities below the northern Dinarides at shallow depths (<10km), and low velocities caused by deep local depressions in the Pannonian basin. A very pronounced high-velocity body is present in the transitional part between the Dinarides and the Pannonian basin at a depth range of 5-15km. The strong velocity increase at depth of about 20km indicates that the Dinaridic crust could be interpreted as two-layered, while the Pannonian crust is probably one-layered.