Estimates of site periods from mHVSR analysis of IRIS temporary seismograph networks

Abstract

The effects of soil stratigraphy and sedimentary basins on earthquake ground shaking are well-established. These effects can be represented using a range of site metrics, with the focus of this paper being the fundamental period of vibration of soil above bedrock (T0). T0 has been used to differentiate between sub-soil site class C and D in NZS1170.5 and in the seismic microzonation of sedimentary basins as a proxy for basin depth. An increasingly common method to estimate site period is via identification of peaks in the Horizonal-to-Vertical Spectral Ratio of microtremor (mHVSR) data. Several recent studies in Aotearoa New Zealand and internationally have focused on the temporary deployment of sensors to characterise sedimentary basins on local and regional scales. This paper leverages a dataset of ambient vibration recordings from several studies collected using instrumentation borrowed from the Incorporated Research Institutions for Seismology (IRIS). Broadband seismometers have been deployed at 785 locations across the country. Data was publicly available at 311 sites and suitable for mHVSR analysis at 113 sites. New estimates for site period are presented herein for sites across the country, including Southland, Otago, along the Alpine Fault, and throughout the Taupō Volcanic Zone. All provide insight into sedimentary basin and earthquake ground shaking.