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Crustal structure at the contact of the northern Dinarides and southwestern Pannonian basin from local earthquake tomography

The aim of this research was to determine the crustal structures on the contact area of the northern Dinarides and the southwestern part of the Pannonian basin, based on the P-wave velocity model, obtained using the seismic tomography method. Events used in this study were recorded on 15 temporary seismic stations which were deployed in Croatia (in period November 2005 – May 2007) within the ALPASS- DIPS project and several Croatian and Slovenian permanent seismic stations. 12 temporary seismic stations within ALPASS-DIPS project were deployed along the Alp07 profile which was oriented nearly perpendicular to the Dinarides, 2 stations were situated in Istria and one was in the northern part of Croatia (Šumanovac et al., 2016). First arrivals from 70 earthquakes were used in analysis. The area of study is characterized with complex structures, so travel-time tomography technique was used to obtain a three-dimensional (3-D) velocity model. Detailed models can be provided using local earthquake tomography, but depth of model is limited with the maximum hypocentral depths. In the investigated area, hypocentres are located in the crust. The deepest earthquakes used in analysis, occurred at depth of 30 km. For inverse problem, in order to determine Vp, code designed to solve the coupled velocity- hypocentre problem in three- dimensional model subdivided into layers and nodes, which was provided by Kool et al., 2006, was used. The code provides implementation of the multistage fast-marching method for calculating arrival times and ray paths of multiple phases in complex layered media (Rawlinson & Sambridge, 2004a, b). The results obtained by local earthquake tomography were compared with 2D velocity model on the Alp07 profile based on active seismic refraction data and receiver functions (Šumanovac et al., 2009 and 2016). The velocity model on the part of the profile which is sufficiently covered with seismic rays, is comparable with the model determined along the Alp07 profile. The shape of the Moho discontinuity is similar with the shape of the refraction model, but the Moho depths are 10 to 15 km larger. The greatest Moho depth of 45-50 km is under the Dinarides. Structure of the upper crust is characterized by strong lateral and vertical velocity changes. There is positive velocity anomaly (≈ 6 km/s) in Sava depression and negative velocity anomalies in the Karlovac (4.5-5.5 km/s) and Drava depression (≈ 5.5 km/s) areas. Discrepancy from the refraction model exists in Istria where smaller velocities were obtained with earthquake tomography method. In the lower crust, seismic velocities are in range 6.0-7.2 km/s with lower and more uniform velocities in the Pannonian crust.

local earthquake tomography ; Dinarides ; Pannonian Basin

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