Rapid Post-Earthquake Microtremor Measurements for Site Amplification and Shear Wave Velocity Profiling in Kathmandu, Nepal

Abstract

This paper presents application of microtremor (ambient vibration) and surface wave field techniques for post-earthquake geotechnical reconnaissance purposes in Kathmandu, Nepal. Horizontal-to-vertical spectral ratios (HVSR) are computed from microtremor recordings at 16 individual measurement locations to obtain an estimate of fundamental frequency (site period) of the subsurface soils. A combination of active- and passive-source surface wave array testing was accomplished at five key sites including Kathmandu's Durbar Square and Airport. Joint inversion of each site's higher frequency dispersion and lower frequency HVSR data sets provides an estimate of subsurface material stiffness [i.e., shear wave velocity ( V S) depth profiles]. Direct comparison of our V S profiling at Kathmandu Durbar Square and that accomplished by downhole V S and/or standard penetration testing (SPT) profiling yield similar results. Classification of the five sites based on average V S, site period, and/or basin depth is presented. There is little differentiation in these site classification designations amongst the five sites, which does not capture significant differences in observed earthquake damage.

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

“…Similarly, Gautam and Chamlagain (2016) provided distributions of spectral acceleration and calculation of the predominant period, assigning a V S of 700 m/s at the base of 49 borehole logs to 30 m. Alternative methods for the derivation of V S have been employed, with numerous studies providing micro-zonation of the Kathmandu Valley through microtremor and horizontal-to-vertical spectral ratio (HVSR) methods (e.g. Molnar et al, 2017;Paudyal et al, 2012Paudyal et al, , 2013Pokhrel et al, 2019;Poovarodom et al, 2017;Tallett-Williams et al, 2016). Pagliaroli et al (2018) used a mix of HVSR curves and 2D circular array microtremor measurements to estimate the depths to impedance contrast and attain velocity profiles for 1D and 2D site response analysis.…”

“…The Japan International Cooperation Agency (JICA) (2002) produced five boreholes with P-S logging measurements to 30 m, which many researchers have used as a source of V S data (e.g. Gautam and Chamlagain, 2016; Gilder et al, 2018; Molnar et al, 2017). Gautam (2017) presented a correlation between standard penetration test blow count (SPT-N) and V S using these boreholes and some additional industry data.…”

Overview of the 2015 Gorkha, Nepal, Earthquake and the Earthquake Spectra Special Issue

“…The paper by Asimaki et al (2017) suggests that nonlinear site effects played a dominant role in shaping the amplitude and frequency content of ground shaking on soil and that the soft basin soils caused systematic amplification relative to the rock site in the low frequency range (0.1–2.5 Hz) and de-amplification of higher frequencies ( > 2.5 Hz). Molnar et al (2017) presents an application of microtremor (ambient vibration) and surface wave field techniques to produce site periods and shear wave velocities for five sites in Kathmandu including Kathmandu's Durbar Square and the Airport; however, the resulting classification of the five sites does not capture significant differences in observed earthquake damage. Another site response analysis study by Poovarodom et al (2017) compares a one-dimensional (1-D) deep-velocity model of soil strata with array microtremor observations close to six seismic stations in Kathmandu Valley.…”

Estimation of Vs30 and site classification of Bhaktapur District using Microtremor Array Measurement

“…Microtremor or ambient vibrations are de ned as vibrations with a displacement amplitude of roughly 0.1 to 1 micron and a velocity amplitude of 0.001 to 0.01 cm/s ( Okada & Suto, 2003;Choobbasti et al, 2015;Rezaei & Choobbasti, 2017) induced by both naturally occurring phenomena such as sea waves, earth's rotation and anthropogenic phenomena such as tra c movement and manufacturing machinery. By the measurement of such vibration one can predict ground response (Pandey, 2000;Paudyal et al, 2012), site ampli cation factor (Lu et al, 1992;Molnar et al, 2017), site classi cations (Kanlı et al, 2006;Sulistiawan et al, 2018), liquefaction (Choobbasti et al, 2015b;Huang & Tseng, 2002), and soil pro les (Cho et al, 2021;Karagoz et al, 2015;Ozep et al, 2004;Ridwan et al, 2014).…”