Large Asymmetric Vertical Accelerations in Recent Earthquakes Caused by Elastic Flapping Effect

Abstract:

Ground motion records are critical for seismic hazard assessment and seismic design of buildings and infrastructures. Large ground motions (>1g) with asymmetric vertical accelerations (AsVA) have been observed in a number of strong motion stations in recent earthquakes. However, it is not clear whether all the observed AsVAs reflect actual ground shaking or the interaction of a building structure and underlying soil ground. Here we investigate the cause of large AsVAs recorded at several seismic stations in New Zealand during the 2011 Mw6.2 Christchurch earthquake. We first define three metrics and quantify the degree of waveform asymmetry in all available records from nearby M>3 earthquakes. Histograms of the metrics show greater waveform asymmetry for larger accelerations which is consistent with the prediction of an elastic flapping effect (Goto et al., Scientific Reports, 2019) – that is the elastic bouncing of a foundation slab on which the sensor is installed. Finite element models are used to examine the occurrence of the elastic flapping effect for the Mw6.2 mainshock and Mw5.6 aftershock of the 2011 Christchurch earthquake. The parameters of numerical models are constrained by the site investigation. Numerical simulations closely reproduce large AsVA recorded at stations HVSC, PRPC, and CCCC, suggesting that these ground motion records were contaminated by the elastic flapping effect. Seismic sensors located near the corner of a concrete slab, such as those at HVSC, PRPC, and CCCC, are shown to be more prone to the elastic flapping effect. Our results further suggest that artificial recording of large AsVA due to the elastic flapping effect can be mitigated if a seismic sensor is placed closer to the center of a foundation slab. The analytical procedure presented in this study may be useful in identifying the occurrence of AsVA elsewhere and in assessing whether AsVA are caused by the elastic flapping effect.