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Site characterisation in north-western Turkey based on SPAC and HVSR analysis of microtremor noise

Asten, Michael W.
Askan Gündoğan, Ayşegül
Ekincioglu, E. Ezgi
Sisman, F. Nurten
Ugurhan, Beliz
The geology of the north-western Anatolia (Turkey) ranges from hard Mesozoic bedrock in mountainous areas to large sediment-filled, pull-apart basins formed by the North Anatolian Fault zone system. Duzce and Bolu city centres are located in major alluvial basins in the region, and both suffered from severe building damage during the 12 November 1999 Duzce earthquake (Mw = 7.2). In this study, a team consisting of geophysicists and civil engineers collected and interpreted passive array-based microtremor data in the cities of Bolu and Duzce, both of which are localities of urban development located on topographically flat, geologically young alluvial basins of Miocene age. Interpretation of the microtremor data under an assumption of dominant fundamental-mode Rayleigh-wave noise allowed derivation of the shear-wave velocity (Vs) profile. The depth of investigation was similar to 100m from spatially-averaged coherency (SPAC) data alone. High-frequency microtremor array data to 25 Hz allows resolution of a surface layer with Vs < 200 m/s and thickness 5m (Bolu) and 6m (Duzce). Subsequent inclusion of spectral ratios between horizontal and vertical components of microtremor data (HVSR) in the curve fitting process extends useful frequencies up to a decade lower than those for SPAC alone. This allows resolution of two interfaces of moderate Vs contrasts in soft Miocene and Eocene sediments, first, at a depth in the range 136-209 m, and second, at a depth in the range 2000 to 2200 m.