Shallow V<inf>S</inf> imaging of the Groningen area from joint inversion of multimode surface waves and H/V spectral ratios

Abstract

The Groningen gas field in the northern Netherlands is subject to production-induced earthquakes and has quickly become one of the seismologically best-instrumented areas on Earth. Accurate quantification of seismic hazard from potential future earthquakes requires accurate shallow velocity structure for ground-motion prediction. Toward this end, we present a shear-wave velocity model developed through the joint inversion of multimode Love- and Rayleigh-wave dispersion curves (DCs) and H/Vspectral ratio (HVSR) measurements. We obtain local DCs from azimuthally averaged frequency-time analysis of the cross correlation of the ambient seismic field (ASF) between pairs of stations. HVSR is measured at each station from the directional energy density, that is, the autocorrelation of the ASF for all components. We simultaneously fit these observables at each station of the dense Loppersum array to infer a 1D velocity model from the surface to a depth of ∼900 m. In the frequency range considered (∼1-7 Hz), Rayleigh-wave DCs show high modal complexity, which makes clear identification of the modes challenging and leads us to downweight their contribution to the result. Fundamentaland higher-mode Love-wave dispersion is much clearer. We find good agreement between our model and independently derived models of shallow structure, which validates our approach and supports the value of HVSR analysis as a tool to map subsurface properties.