Geostructural complexity and passive seismic surveys: a geostatistical analysis in the Kathmandu basin

I n this study a set of 39 single station passive seismic surveys conducted in the Kathmandu basin(Nepal), based on the horizontal to vertical spectral ratio methodology (HVSR), is analyzed bymeans of a geostatistical approach. The Kathmandu basin is characterized by a heterogeneoussedimentary cover and by a complex geostructural setting, inducing high spatial variability of thebedrock depth. In relation to the complex geological setting, the interpretation and analysis ofHVSR data are challenging, both from the perspective of bedrock depth analysis as well as ofseismic site effects detection. In order to maximize the broad range of information available, theHVSR data are analyzed according to a geostatistical approach. First, the spatial continuitystructure of HVSR data is analyzed and interpreted taking into consideration the geological settingand available stratigraphic and seismic information. In addition, we test the possibility to integratethe analysis with potential auxiliary variables, derived from geomorphometric variables andconsidering the distance from outcropping bedrock. The explorative geostatistical analysisconfirms the complexity of the geo-structural setting of the area. Finally, a mapping of HVSRresonance periods, with the evaluation of interpolation uncertainty, is obtained by means ofordinary kriging interpolation. The resulting map, even if characterized by a large interpolationsupport, is congruent with the geo-structural setting and the main lineaments of the area. Theadopted approach is particularly useful in the context of micro-zonation studies based on HVSRmethodology conducted in historical urban areas. Moreover, this work contributes to the geostructuralknowledge of the deep structure of the Kathmandu basin.