Spectral decomposition of ground motions in New Zealand using the generalized inversion technique

To gain new insights into ground-motion phenomena in New Zealand (NZ), we apply the non-parametric generalized inversion technique (GIT) in the Fourier domain to isolate the systematic source, path, and site effects from 20 813 seismograms, recorded by 693 sensors at 439 unique locations, from 1200 shallow crustal events (Mw > 3) during the period 2000–2021. From the inverted source spectra, we derive Brune's stress parameter, ∆σ, which is found to follow a lognormal distribution with a log10 standard deviation of 0.36 or equivalently 0.83 in natural log unit. ∆σ slightly increases with focal depth and is practically independent of earthquake size (i.e. self-similar), but displays a statistically significant spatial clustering. Based on the inverted attenuation, a trilinear geometric-spreading function, and a distance-dependent quality-factor Q(f) model are found to well describe the attenuation in NZ; though a single Q(f) model is also obtained for the whole distance range:Q(f) = 149.1f0.62. Using the site response decomposed from GIT, we find that the soil classification scheme specified in NZ seismic code, NZS1170.5, has a limited capability in discerning the site-specific frequency-dependent amplification functions in comparison to a non-parametric clustering with the same number of discrete classes. The potential use of the spatial variation in source parameters from this GIT analysis in region-specific physics-based simulations is discussed.