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

The Kathmandu Valley — the most developed and populated region in Nepal – is seismically active and has experienced recurring destructive earthquakes. The first was recorded around year 1255, and the much more recent Gorkha earthquake in 2015 registered magnitude 7.8. Unfortunately, there is little recorded strong-motion data for seismic activity here, and geophysical information in the Valley is also sparse, limiting efforts to model seismic hazards. It is known that these kinds of basins can strongly amplify seismic waves, which reverberate within such softer sediments resting on harder crystalline rock. Many experts believe that Kathmandu is at high risk because of its location on top of this basin.

Yet efforts to build geophysical knowledge of the area have been hampered by the lack of an open database compiling in situ geophysical tests, borehole records and geo-technical laboratory data. In a new paper, LML External Fellow Max Werner and colleagues offer new data in the 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). This database contains data from groundwater wells and boreholes originally commissioned for research and commercial purposes. In the paper, the researchers review the quality assessment and harmonization process conducted on the data set and examine several other data components. These include information on the variation of shear-wave velocity measurements and geotechnical parameters with depth and elevation in the Valley, the current understanding of the Valley sediment/bedrock topography, and new geological cross sections.

The authors suggest that this database should help fill some gaps in existing knowledge, although many areas of data are still sparsely covered and require intensive further examination. In particular, they note, while the SAFER/GEO-591 presents a large amount of new and historic data, most of the available mechanical, physical, and index measurements relate to the top 35 meters of the basin sediments. This is not surprising as most data were originally collected for construction purposes. However, more data will be needed to support useful 3D modelling of the basin for analyses of seismic hazard.

The paper is available here.

Additional links:

A recent news release by Bristol University highlights some of the engineering work.

An interesting related video.

 

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