Basement Topography of Kathmandu Valley by Array Microtremor Observations

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

Earthquake vulnerability of Kathmandu valley is considerably high, due to the active tectonics and local fragile geological conditions. The valley is approximately 30 km in the east–west and 25 km in the north–south directions. It is an intermontane basin with young soft fluvio-lacustrine sediments with thickness variation from place to place and surrounded by mountains on all sides. The 2015 Gorkha earthquake (Mw 7.8) revealed effects of soil sediments on ground motion characteristics as the horizontal components during the main shock exhibit long dominant period component in the range of about 4 to 5 second. This unusual long period wave could possibly be resulted from the soft soil layer. The effects of thick soil sediments on ground motion characteristics have long been recognized from major earthquakes, however, past studies on this issue are still quite primitive. As the site conditions especially at deeper level are very important, this research aims to explore such information for Kathmandu valley by microtremor observation. Array microtremor tests were conducted for 30 sites by the Centerless Circular Array technique (CCA) which has been developed to provide accurate phase velocity up to very long wavelength comparing with conventional microtremor methods. Different sized array arrangements were set with radius ranging from 1 to 120 m. The derived phase velocity dispersion curves from the observations provide long detectable wavelength and sufficient accuracy for inversion of shear wave velocity profile of sedimentary sites down to 500-1,000 m. The depths of basement were clearly identified from the level with shear wave velocity greater than 2,000 m/s. The inferred depth of basement ranges from 10 m at rock outcrop or basin edge to about 600 m in the central of the studied area. The basement topography derived by this study is in good agreement with the gravity survey from the past research.

Presenting Author: Nakhorn Poovarodom

Authors