Geological geophysical and seismological investigations for earthquake hazard estimation in western Crete

Abstract

The purpose of the thesis is the determination of potentially seismic active sources and of the dynamic response evaluation of surface and subsurface structure at sites where the geometric and dynamic properties of the ground can strongly amplify seismic motions. A combination of methods involving the study of geology, geophysics and seismology disciplines permitting cross-comparison of techniques in a robust approach is applied to address these issues. The study area is focused in Kastelli-Kissamou and Paleohora half graben basins in northwestern and southwestern Crete that is located in one of the most seismically active parts of the Africa-Eurasia collision zone. Ground truthed geological field survey, 2D Electrical Resistivity Tomography (ERT), Horizontal to Vertical Spectra Ratio (HVSR) technique using microtremors and microseismicity study are conducted. Microseismicity study involves two different earthquake dataset acquired from a regional permanent network installed on Crete and local temporal network installed on Paleohora. 2D Electrical resistivity tomography (ERT) reveals seven faults in the territory of Kastelli-Kissamou and three faults large scale faults in the territory of Paleohora basin. HVSR technique using microtremors is applied only in the populated area of Kastelli and Paleohora basins and reveals five fault zones in Kastelli and four major fault zones in Paleohora crosscutting the densely populated areas. The effects of the surface and subsurface structure are well patterned in the horizontal to vertical spectra ratios. One amplified clear frequency, two high amplified clear frequencies, broad and flat or low amplitude HVSR peaks attributes the effects of surface and subsurface structure on seismic ground motion. The effects of soft rocks, stiff soils, thick and thin alluvial deposits, fault zones, lateral heterogeneities and discontinuities on seismic ground motion are determined. The higher ground amplification level is observed in Paleohora (A=5.7) compared to Kastelli (A=3.4). Three case studies of building vulnerability evaluation in Paleohora half-graben basin using HVSR technique and microtremors are presented. Temporal seismological network is installed in the territory of Paleohora to study the seismotectonic setting of southwestern Crete. Microseismicity using data from the permanent seismological regional network of Crete is used to compare the seismicity of the study areas.