The Moho’s structure in West Bulgaria obtained from receiver function

In September 2005 Geophysical Institute of Bulgarian Academy of Sciences after procurement procedure selected the Refraction Technology, Inc. to upgrade the existing National Operative System for Seismological Information (NOTSSI) to a modern digital seismological network. At the beginning of December 2005 all the equipment supplied were installed on seismological stations and acquisition and processing software was operating in the data center. The network became operational on 08.12.2005. The Bulgarian Seismological Network was equipped with broad-band sensors and digital acquisition systems. It enabled application of modern techniques of analysis of the velocity structure in Bulgaria. This study presents one of the first results from application of the receiver function technique. The Receiver functions were computed using scripts written on Seismic Handler program by Sodoudi F. The Western part of Bulgaria is characterized by mountains, river valleys and small fields between the mountains. Two stations of the network Musomishte (MMB) and Krupnik (KKB) were chosen in south-west of Bulgaria and also station Vitosha (VTS) which is close to Sofia and known as the station with lowest noise. These sites are located in areas of complex tectonic structures manifesting high seismic activity during recent years. As starting models we used shear wave velocity models for the territory of Bulgaria, obtained in Raykova R, 2004. For the study were used earthquakes in epicentral range 35 - 90o and with a magnitude more than 5,5 – 6 also with clear P-onset. All earthquakes from the end of 2005 to the summer of 2009 were used and a good azimutal covering was reached. From the seismic survey and gravimetric measurements is determined a Moho depth between 30 km and 50 km. The crust is shallower in the north-eastern part of the country and thicker in the south-western part. The obtained results show ticker crust than expected Earth crust. This can be an effect of reflections/refractions on the object close to the station. They show only the main trends of the Moho depth for the whole country but not local effects in some part, beneath the mountains for example. Further detailization of the structure of the Moho boundary could be done after estimation of receiver functions for other stations of the network. The results show also very good the faults close to stations KKB and VTS.