Local Site Effects During the Orkney Earthquake of 5 August 2014

Abstract

Site response analysis is conducted at 37 seismic stations located in the Gauteng, North West and Free State provinces in South Africa, using the Nakamura H/V spectral ratio technique on records of the Orkney 5 August 2014 earthquake. The earthquake, of magnitude ML = 5.5, led to the unfortunate death of one person and damage of more than 600 houses. Intensity data collected soon after the event showed that the effects of the earthquake appeared to vary significantly across the region. This motivated the authors to conduct a more detailed investigation of the effects of site conditions on seismic station records in the region. Resonance frequency values obtained from the H/V ratios were observed to vary strongly across the region and also within seismic station clusters. Similar behaviour was observed with the peak amplitude of the ratios at the resonance frequency, except for the Johannesburg area whose results showed a relatively simple shape of the ratios implying less complex velocity structure. All the H/V ratios exhibit dominant peaks at resonant frequencies that varied between 0.5 and 35 Hz. The average observed resonant frequency was f = 7.9 Hz. The amplitude of the dominant peaks also varied strongly from 1.66 to 11.69, with only two sites exhibiting maximum peaks with amplitude smaller than 2. These results serve as a strong motivation or justification for the on-going microzonation studies in South Africa, where a detailed study of the velocity structure will be used to obtain reliable information on site amplification and resonance.