Abstract
The Himalayan collision zone experiences many seismic activities with large earthquakes occurring at certain time intervals. The damming of the proto-Bagmati River as a result of rapid mountain-building processes created a lake in the Kathmandu Valley that eventually dried out, leaving thick unconsolidated lacustrine deposits. Previous studies have shown that the sediments are ~600 m thick in the center. A location in a seismically active region, and the possible amplification of seismic waves due to thick sediments, have made Kathmandu Valley seismically vulnerable. It has suffered devastation due to earthquakes several times in the past. The development of the Kathmandu Valley into the largest urban agglomerate in Nepal has exposed a large population to seismic hazards. This vulnerability was apparent during the Gorkha Earthquake (Mw7.8) on April 25, 2015, when the main shock and ensuing aftershocks claimed more than 1700 lives and nearly 13% of buildings inside the valley were completely damaged. Preparing safe and up-to-date building codes to reduce seismic risk requires a thorough study of ground motion amplification. Characterizing subsurface velocity structure is a step toward achieving that goal. We used the records from an array of strong-motion accelerometers installed by Hokkaido University and Tribhuvan University to construct 1-D velocity models of station sites by forward modeling of low-frequency S-waves. Filtered records (0.1–0.5 Hz) from one of the accelerometers installed at a rock site during a moderate-sized (mb4.9) earthquake on August 30, 2013, and three moderate-sized (Mw5.1, Mw5.1, and Mw5.5) aftershocks of the 2015 Gorkha Earthquake were used as input motion for modeling of low-frequency S-waves. We consulted available geological maps, cross-sections, and borehole data as the basis for initial models for the sediment sites. This study shows that the basin has an undulating topography and sediment sites have deposits of varying thicknesses, from 155 to 440 m. These models also show high velocity contrast at the bedrock depth which results in significant wave amplification.Graphical abstract.
Highlights
The collision of the Eurasian and Indian tectonic plates forms an active plate boundary with many seismic events
The initial 1-D velocity models were constructed based on available geological data, borehole logs, and cross-sections
We examined the horizontal-to-vertical spectral ratios (H/V ratios) of earthquake ground motion to verify the adjusted velocity models estimated by forward modeling of the observed long-period S-wave
Summary
The collision of the Eurasian and Indian tectonic plates forms an active plate boundary with many seismic events. The Nepal Himalaya regularly experiences small seismic activities, and large earthquakes occur over certain time intervals (Sapkota et al 2013). An earthquake with seemingly no effect above hard ground can be felt as a strong tremor and cause severe damage in areas above soft or unconsolidated sediments due to the amplification of seismic waves. The Kathmandu Valley, along with a large part of central and eastern Nepal, suffered heavy loss of life and property due to the mainshock and ensuing aftershocks of the 2015 Gorkha Earthquake. The Kathmandu Valley had 1739 casualties, and about 13% of buildings were estimated to have been completely damaged. A location in a seismically active region, and the presence of thick sediments that amplify seismic waves, have made the Kathmandu Valley a seismically vulnerable region. The increasing tendency for haphazard building construction without proper engineering considerations has added to the potential for catastrophe
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