Within the fractured Deccan Volcanic Province of western India, the Palghar earthquake swarm region has experienced repeated seismic activity since 2018 and provides an opportunity to investigate seasonal variations in site-response characteristics and shallow subsurface properties. In this study, we applied the diffused-regime earthquake-derived Horizontal-to-Vertical Spectral Ratio (EHVSR), Site-to-Reference Spectral Ratio (SSR), and EHVSR inversion techniques to earthquake recordings from six broadband seismic stations to evaluate seasonal changes in site response and shallow shear-wave velocity (Vs) structure within the upper ~ 250 m. The analyses were performed separately for pre-monsoon (February-June) and monsoon/post-monsoon (July-December) periods. For robustness, bootstrap analysis was performed to evaluate the stability and uncertainty of the EHVSR estimates, with 95% confidence intervals (CI95), while a source-effect analysis was carried out to assess the influence of source characteristics and wavefield composition. The EHVSR results reveal well-defined resonance frequencies (f₀) at each station, but with generally small seasonal variations across most stations, while the spatial variations ranging from approximately 6 to 15 Hz (across the region). In contrast, amplification factors (A₀), site vulnerability indices (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">${\text{K}}_{\text{g}}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mtext>K</mtext> <mtext>g</mtext> </msub> </math></EquationSource> </InlineEquation>), inversion-derived Vs models, and VP/VS ratios exhibit pronounced temporal variability. SSR analyses produce resonance frequencies broadly comparable to those obtained from EHVSR, but with stronger and sharper amplification peaks. SSR-derived amplification factors range from approximately 2.6 to 6.3 and generally exceed the corresponding EHVSR amplitudes. The fair agreement between EHVSR and SSR-derived resonance frequencies supports the robustness of the identified site-response characteristics (mainly f0). The inversion results indicate subtle reductions in shallow Vs and increases in VP/VS ratios at several stations during the monsoon/post-monsoon period, with some of the most pronounced variations observed at KAWA, located closest to the swarm nucleation zone. This study supports the hypothesis of possible hydrological influences associated with monsoonal recharge and fluid circulation within fractured basaltic formations, as similarly suggested by regional studies. The results provide new constraints on the seasonal sub-surface characterization in the Palghar swarm region and demonstrate the usefulness of integrated EHVSR, SSR, and inversion approaches for, seismic microzonation, and long-term monitoring of fluid-driven earthquake sequences in western India.

Monitoring hydrologically influenced seismic processes in Palghar region using Horizontal-to-Vertical Spectral Ratio (HVSR) analysis

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发表日期：

2026年8月4日

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Srikanth Murarishetti,
V. Pavan Kumar,
K. Sivaram,
M. Shekar,
Prakash Kumar

摘要

Within the fractured Deccan Volcanic Province of western India, the Palghar earthquake swarm region has experienced repeated seismic activity since 2018 and provides an opportunity to investigate seasonal variations in site-response characteristics and shallow subsurface properties. In this study, we applied the diffused-regime earthquake-derived Horizontal-to-Vertical Spectral Ratio (EHVSR), Site-to-Reference Spectral Ratio (SSR), and EHVSR inversion techniques to earthquake recordings from six broadband seismic stations to evaluate seasonal changes in site response and shallow shear-wave velocity (Vs) structure within the upper ~ 250 m. The analyses were performed separately for pre-monsoon (February-June) and monsoon/post-monsoon (July-December) periods. For robustness, bootstrap analysis was performed to evaluate the stability and uncertainty of the EHVSR estimates, with 95% confidence intervals (CI95), while a source-effect analysis was carried out to assess the influence of source characteristics and wavefield composition. The EHVSR results reveal well-defined resonance frequencies (f₀) at each station, but with generally small seasonal variations across most stations, while the spatial variations ranging from approximately 6 to 15 Hz (across the region). In contrast, amplification factors (A₀), site vulnerability indices ( ${\text{K}}_{\text{g}}$ $K_{g}$ ), inversion-derived Vs models, and V_P/V_S ratios exhibit pronounced temporal variability. SSR analyses produce resonance frequencies broadly comparable to those obtained from EHVSR, but with stronger and sharper amplification peaks. SSR-derived amplification factors range from approximately 2.6 to 6.3 and generally exceed the corresponding EHVSR amplitudes. The fair agreement between EHVSR and SSR-derived resonance frequencies supports the robustness of the identified site-response characteristics (mainly f₀). The inversion results indicate subtle reductions in shallow Vs and increases in V_P/V_S ratios at several stations during the monsoon/post-monsoon period, with some of the most pronounced variations observed at KAWA, located closest to the swarm nucleation zone. This study supports the hypothesis of possible hydrological influences associated with monsoonal recharge and fluid circulation within fractured basaltic formations, as similarly suggested by regional studies. The results provide new constraints on the seasonal sub-surface characterization in the Palghar swarm region and demonstrate the usefulness of integrated EHVSR, SSR, and inversion approaches for, seismic microzonation, and long-term monitoring of fluid-driven earthquake sequences in western India.