The SAFER geodatabase for the Kathmandu Valley: Geotechnical and geological variability

Abstract

The Kathmandu Valley is within a seismically active region with only few recorded strong-motion data. Geophysical information in the Valley is also sparse. In addition, the absence of an open database which compiles in situ geophysical tests, borehole records, and geotechnical laboratory data is affecting the advancement of knowledge in the region. This article presents SAFER/GEO-591 database, named after the Engineering and Physical Science Research Council (EPSRC)-funded project Seismic Safety and Resilience of Schools in Nepal (SAFER). SAFER/GEO-591 contains data from groundwater wells and boreholes originally commissioned for research and commercial purposes. This work describes (1) the quality assessment and harmonization process conducted on the dataset, (2) the variation of shear-wave velocity ( VS) measurements and geotechnical parameters with depth and elevation in the Valley, (3) the current understanding of the Valley sediment/bedrock topography, and finally (4) new geological cross sections. A companion article presents an updated VS30 map across the Valley based on the contributions of this article. The database can be downloaded from the University of Bristol repository via DOI: https://doi.org/10.5523/bris.3gjcvx51lnpuv269xsa1yrb0rw

Mapping long-period soil resonances in the Kathmandu basin using microtremors

“…In fact, as reported in Paudyal et al (2013), the detected HVSR peaks are referred to seismic reflectors located in the sedimentary sequence, hundreds of meters above the effective bedrock surface. This is also highlighted by the study of Gilder et al (2020), where a preliminary analysis of an extended stratigraphic database confirms that the "bedrock" surface detected by Paudyal et al (2012) is much shallower than the expected depth of lithological bedrock. Paudyal et al (2012Paudyal et al ( , 2013 interpret the detected seismic reflectors as a seismic bedrock and not as the geological bedrock.…”

“…The geo-structural setting of Kathmandu basin is complex, both regarding the bedrock, strongly deformed during the orogeny, as well as concerning the thick sequence of sediments filling the Kathmandu basin (e.g., Suresh et al 2018;Gilder et al 2020). The Kathmandu valley is an intermountain basin, developed within a Nappe structure located in the Lesser Himalaya belt, once occupied by a lake until the Late Pleistocene.…”

“…Unfortunately, the deep stratigraphy and the effective bedrock top morphology is still not well known. Moreover, most of the deep boreholes (e.g., Gilder et al 2020) have been conducted for ground water purposes by means of a non-core drilling, and consequently the stratigraphic records have a high level of uncertainty and potential interpretative ambiguities. Even the seismic analyses conducted by means of passive methods (e.g., Molnar et al 2017;Poovarodom et al 2017;Sandron et al 2019) for deriving Vs depth profiles have a limited areal coverage and often a shallow investigation depth.…”

The SAFER geodatabase for the Kathmandu valley: Bayesian kriging for data-scarce regions

“…For comparison purposes, in Figure 11, the two contour maps of shear wave velocity obtained through BRK ( Figure 7b) and USGS estimates conditioned to the measurements (Figure 10b) are compared with the geology maps given in the companion paper Gilder et al (2020). The first map, originally developed by Yoshida and Igarashi (1984), provides information on the geology inferred from geomorphology (Figure 11a and b).…”

Empirical seismic fragility models for Nepalese school buildings

“…Therefore, it is problematic: (1) to evaluate the extent of the area affected by ground shaking and (2) to estimate the variation of the seismic excitation within the area (McGowan et al 2017). In addition, the limited knowledge on the country's geology (Gilder et al 2020) and the lack of representative Ground Motion Prediction Equations (GMPEs) for the region (Bajaj and Anbazhagan 2019) result in large uncertainties on shake maps. In the last five years, the United States Geological Survey (USGS) has released a set of shake maps for the Gorkha sequence, progressively updated to include more advanced studies (McGowan et al 2017).…”