Dynamic site characterisation of Canterbury strong motion stations using active and passive surface wave testing

Abstract

An extensive database of earthquake ground motions were recorded by the GeoNet strong motion station network in the Canterbury region during the Canterbury earthquake sequence. However, in order to comprehensively understand the records at these sites and to be able to relate these to other locations, a detailed understanding of the soil profile down to rock at these locations is needed. The aim of this research was to improve the knowledge of the soil profile characteristics at a number of the GeoNet sites across the Canterbury Plains and the general characteristics across the wider plains region.

This research was able to provide an indication of the variation of the depth to rock across the Canterbury Plains, with the depth increasing rapidly moving out from the base of the Canterbury foothills and Banks Peninsula. In Christchurch, the relatively shallow gravels and volcanic deposits that extend out under the city from the Port Hills were identified. The effect of the presence of these layers on earthquake ground motions requires further study to identify their importance.

In conjunction with previous studies, the soil profile characteristics at the GeoNet strong motion stations that were in the region where the earthquakes occurred in Canterbury were defined. This showed that apart from locations with very soft surface deposits, all sites greater than a few hundred metres from the base of the Canterbury foothills and the Banks Pensinsula should use the same soil profile classification when designing for earthquake loading. These locations has a wide range of characteristics, with a clear increase in the stiffness of the near surface soils moving inland from the coast.

This research has provided a dataset that will allow for more detailed assessment of the characteristics of the ground motions during the Canterbury earthquake sequence, will aid in the estimates of potential ground motions in future events, and will help to assess the standard methods currently used to define earthquake loading for design of the built environment.

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Technical Abstract

An extensive database of ground motion records was captured by the SMS network in the Canterbury region during the Canterbury earthquake sequence. However in order to comprehensively understand the ground motions recorded at these sites and to be able to relate these motions to other locations, a detailed understanding of the geotechnical profile at each SMS is required. This report presents an overview of the development of dynamic site characterisation metrics at a number of strong motion station (SMSs) sites across the Canterbury Plains, and wider regional site period characterisation.

This report summarises the characterisation of the shallow shear wave velocity profile (to 30 m) and site period (T0) at seven SMS locations across the Canterbury Plains, and uses site period to characterise an additional 14 SMS. According to NZS1170.5 (SNZ 2004), all SMS outside of the urban Christchurch region and greater than a few hundred metres from the Canterbury foothills and the Banks Peninsula outcrops should be classified as site subsoil class D. Moving inland from the coast, the time averaged shear wave velocity to a depth of 30 m (Vs30) increases as stiff surface gravels increasingly dominate the near surface profile.

Wider regional site characterisation using site period measurements was able to provide an indication of the variation in depth across the Canterbury Basin, and the effects of multiple impedance contrasts close to the coast. H/V spectral ratio peaks indicate site periods in the range of 5-7 seconds across much of the Canterbury Plains. Site periods decrease rapidly in the vicinity of the Canterbury foothills and the Banks Peninsula outcrops. In Christchurch, the Riccarton Gravel and the Miocene Volcanics result in a significant mode of vibration that has a much shorter period than the site period of the entire soil column down to basement rock. Further study is required to determine the impacts of these shallower impedance contrast on ground motion amplification and how to best account for it within the NZ1170.5 site classification framework.