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Seismic Site Characterization and Basin Depth Estimation Using Seismic Ambient Noises: An Example From the Hetauda Dun Valley, Nepal

Description: 

Subsurface geometry, particularly the depth to bedrock, is important for site-specific seismic hazard studies, as basin geometry and local geology plays an important role in the altering and amplification of seismic waves. In this study, we characterize site response within Hetauda Dun Valley by utilizing the seismic ambient noises measured at ninety-two points. The horizontal to vertical spectral ratio (HVSR) and the centerless circular array techniques are used to estimate fundamental frequency and shear wave velocities respectively.

The HVSR data reveal a wide variation on fundamental frequency spanning 0.55 Hz to 7 Hz. The central part of the valley is characterized by low fundamental frequency (<1 Hz), whereas northwestern part of the valley exhibits relatively higher fundamental frequency. The lowest frequency is found in and around Chaughada area, characterized by fine grained sand, silt and clay, where the urbanization is rapidly but haphazardly expanding. In accordance with the guidelines of the National Earthquake Hazard Reduction Program, two site classes, site Class C and site Class D, are identified based on the time-averaged shear wave velocity up to a depth of 30 m. The northeastern part of the valley falls in the site class D and the remaining area lies on site class C. This is consistent with the known response of the northeastern part of the valley, which is comparatively less stiff than the western part, making it more susceptible to damage during an earthquake. The shear wave velocity models indicate variation in basin depth from 11 m to 281 m. The eastern part of the valley, including Chaughada, is deeper than the western part. Two distinct N-S trending troughs of low shear wave velocity are observed, the first east of Chaughada and second through the Hetauda Industrial Area. We hypothesize these are paleo-river channels.

Session: Geophysics in a Changing World: Monitoring Applications from Seismology and Beyond [Poster]

Type: Poster

Date: 4/15/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Deepak

Student Presenter: No

Invited Presentation: 

Poster Number: 45

Authors