Passive seismic experiment at the contact of the Dinarides and Pannonian Basin (ALPASS-DIPS) – deployment and characteristic receiver functions

A group of active source seismic surveys have been performed in central Europe (CELEBRATION 2000, SUDETES 2003 and ALP 2002), covering wide area between Baltic and Adriatic sea. The main aim of these surveys was to determine more precisely the structure of the Earth’s crust and upper mantle. Due to great problems related to shot-points, active seismic surveys have been more and more replaced by passive seismic experiments. The ALPASS-DIPS (Alpine Lithosphere and Upper Mantel PASsive Seismic Monitoring-DInarides-Pannonian Segment) is a continuation of the active ALP 2002 project, which covers a wide area of NW Dinarides, transition zone towards the Pannonian basin, and SW part of the Pannonian basin. Most of the temporary seismic stations, which are denominated Cro_01 to Cro_12, were deployed along the profile Alp07 permitting comparison and amendment of the active and passive seismic methods. Profile Alp07 stretches from Istra to the Drava river at Hungarian–Croatian border in a WSW–ENE direction. It is oriented approximately perpendicular to the Dinarides and the main faults in the Adriatic region. It stretches from the edge of the Adriatic microplate through the northern part of the Dinarides. The profile also crosses a wide ophiolite zone, which is divided into the narrow Dinaridic ophiolite zone and much wider Sava-Vardar zone, and terminates at eastern part of the Tisia block in the Pannonian basin.
   Seismic modelling, both inverse and forward, was performed on the data gathered along Alp07 profile. The velocity model shows that the Moho depth is the greatest in the area of the Dinarides, reaching about 40 km and is shallowest (20–30 km) in the Pannonian basin. On the basis of seismic modelling, as well as gravity modelling, three types of crust were defined along the profile: the Dinaridic and the Pannonian crusts that are separated by a wide Transition zone. The Dinaridic crust is two-layered, while the Pannonian crust can be seen as a unique layer characterized by low seismic velocities and densities.
   The data recorded within passive seismic project ALPASS-DIPS were processed using P-receiver function method, based on converted P-to-S phase. Analysis of receiver functions shows three types of seismograms: Dinaridic, Transitional and Pannonian. Pannonian type can be represented with data analysed at station CBP4M which belongs to the Pannonian crust. Transitional type can be seen at station Cro_07 because it is located in a Transition zone, while Dinaridic type can be observed at station Cro_03, and belongs to Dinaridic crust. Three major litospheric discontinuities can be defined at the Dinaridic type and the Transitional type, while the Pannonian type reveals only two discontinuities. To validate these results, receiver function modelling was performed. The main velocity contrast under the station Cro_03 is at the 42 km depth, which can be interpreted as Mohorovičić discontinuity. The upper crust is characterised by rather low velocities, but good agreement of the calculated and observed receiver functions could be obtained only with high-velocity layer at a depth between 3 and 5 km. The existence of high-velocity layer in the upper crust at the south western end of Alp07 profile was already indicated in the P-wave velocity model obtained within active seismic experiment. The modelling revealed that under the station Cro_07 upper crust is characterised with low velocities and a strong velocity contrast at the mid-crustal boundary, resulting in high amplitude of the second peak. In the Pannonian part of the profile, modelling confirmed that crust can be considered as single layered. Pannonian type model with sedimentary layer and one-layered crust can fit very well the observed data, and based on active seismic data analysis, it corresponds very well with the unique-layer interpretation of Pannonian crust.