Source Characteristics, Site Effects, and Path Attenuation from Spectral Analysis of Strong-Motion Recordings in the 2016 Kaikoura Earthquake Sequence

This study used the spectral inversion technique to investigate the source characteristics, site effects, and path attenuation of S-wave Fourier amplitude spectra recorded in the M-w 7.8 Kaikoura, New Zealand, earthquake sequence. For this purpose, we assembled a dataset comprising 2445 strong-motion recordings, obtained at 126 GeoNet stations, of 148 earthquakes in this sequence with local magnitudes of 4.02-6.16. The seismic moment, corner frequency, and stress drop for each earthquake were estimated based on the inverted source spectra. Investigation of the characteristics of these source parameters revealed the following. (1) The seismic moment is inversely proportional to the cube of the corner frequency, implying that the Kaikoura earthquake sequence within the investigated magnitude range followed self-similarity. (2) The stress drop varied predominantly from 0.2 to 6.0 MPa with a logarithmic mean of 1.25 MPa, and it did not show any evidence of dependence on magnitude size and focal depth. Investigation of the features of the site-response functions of the 126 strong-motion stations inverted in this study revealed the following. (1) Linear relationships between the site amplification and site characteristic parameters V-S30 and T-site at each frequency point of 0.5-3.0 Hz were regressed with correlation coefficients of 0.5-0.8, indicating that site amplification correlates moderately with both parameters. (2) Site amplification in the Wellington region exhibited strong correlation with the geological and geotechnical characterization, for example, the depth of the soil deposit. (3) Strong site amplification appeared considerable in the Christchurch Central Business District. The inverted S-wave quality factor was regressed to 130.87f(1.07) at frequencies of 0.5-20.0 Hz. No significant regional dependence of anelastic attenuation was identified in the study region.