Estimation of Shear Wave Velocity (Vs) Using Earthquake Horizontal-to-Vertical Spectral Ratio (EHVSR) Method and Cianjur 2022 Aftershock Data

Abstract

A magnitude 5.6 earthquake that rocked Cianjur, West Java, Indonesia, on November 21, 2022, damaged infrastructure and claimed numerous lives. Due to signs of correlated faults in the region, the earthquake occurred at a depth of roughly 10 km. The study uses the Earthquake Horizontal to Vertical Spectral Ratio (EHVSR) approach to characterize the subsurface structure of the Cianjur area. EHVSR curves from the noise, S-wave, and coda-wave recording sections were generated by analyzing aftershock recordings from 19 measuring stations. The S-wave section displays results that suggest a thicker soft soil layer with a larger amplitude and a lower dominant frequency when compared to these sections. Nonetheless, the noise recording section curve and the coda wave section curve share a similar form. This resemblance suggests that the waves resonate at almost the same frequency or geological structure and flow similarly in the medium. Data on each measuring station's S-wave component's EHVSR curve is collected. A shear wave velocity (VS) profile is obtained by inverting the EHVSR curve. Although there is a small layer of soft soil in some places, Cianjur has a comparatively thick hard soil and soft rock structure, according to the results of the EHVSR curve inversion. The material is still somewhat variable at depths of 30 and 100 meters, showing signs of a less compacted layer beyond 500 meters; however, the material becomes more homogeneous and compacted. At a depth of 1000 m, an increase in VS indicates a denser and harder layer, possibly bedrock. Based on the classification, the study area has medium to hard soil.