Abstract
Seismic hazard and risk analyses are increasingly tapping into the previously underused resource of local weak-motion records. This is facilitating the development of local- or even application-specific models for the characterisation of earthquake ground motion. In turn, this offers the opportunity to derive non- or partially non-ergodic models and significantly reduce bias and uncertainty. However, weak-motion data, while carrying important information about local earthquake source, path and site effects, are susceptible to noise. We show that high-frequency noise has a record-, or region-specific, impact on pseudo-spectral acceleration (PSA). This impact depends on the shape of the records’ Fourier amplitude spectrum (FAS): PSA from moderately to highly damped ‘soil’ records (e.g. Groningen, the Netherlands) is much less susceptible to high-frequency noise than PSA from weakly damped ‘rock’ records (e.g. Eastern North America). We make use of simulated ground motion records to develop a parametric model for the lower usable period of PSA (Tmin). The model accounts for the impact of high-frequency noise on PSA, conditional on easily measured parameters characterising the shape of a record’s FAS. We then present a workflow, describing processing undertaken for records of induced seismicity from the Groningen gas field. The workflow includes the definition of maximum and minimum usable frequencies and periods of FAS and PSA, respectively. As part of the workflow, we present an approach that considers multiple estimates of Tmin. These include the parametric model and, additionally, record-specific hybrid simulations that artificially extend or modify time series’ FAS beyond the noise floor to assess subsequent impacts on PSA.
Highlights
Ground motion models (GMMs) are a key component of a variety of scientific and engineering products, from seismic hazard and risk analyses, to shakemaps and magnitude scales
The use of modern instrumentation, with broadband flat sensitivity, high-resolution dataloggers and reliable data transmission means that the main constraint on the usability of an earthquake ground motion record lies with background noise
Our simulations show that pseudo-spectral acceleration (PSA) from noisy weak-motion records, as present in many ground motion databases such as those for induced seismicity, is susceptible to high-frequency noise
Summary
Ground motion models (GMMs) are a key component of a variety of scientific and engineering products, from seismic hazard and risk analyses, to shakemaps and magnitude scales. PSA at T = 0.01 s (often assumed equivalent to PGA) is typically related to ground motions at 20–30 Hz or lower, and nowhere near the 100 Hz implied by the reciprocal of the oscillator period This was explored in detail by Douglas and Boore (2011), who concluded, through simulations of moderate to large events, that contaminating records with high-frequency noise had a negligible impact on their response spectra. In such cases, the degree to which short oscillator period PSA is driven by longer period motions reduces and the conclusions of Douglas and Boore (2011) may, not be transferable to weak-motion data It is the aim of this study to investigate the impact of high-frequency noise on the response spectrum and propose a robust workflow for defining the usable bandwidth of both FAS and PSA from weak-motion records. By virtue of the small magnitude of the events, the recordings contain small-amplitude motions, with as-recorded horizontal PGA values ranging from 0.068 cm/s2 (7 × 10−5g) to 108.68 cm/s2 (0.11 g)
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