The Use of the Multi-channel Analysis of Surface Waves (MASW) Method as an Initial Estimator of Liquefaction Susceptibility in Greymouth, New Zealand

摘要

Combined analysis of the geomorphic evolution of Greymouth with Multi-channel Analysis of Surface Waves (MASW) provides new insight into the geotechnical implications of reclamation work. The MASW method utilises the frequency dependent velocity (dispersion) of planar Rayleigh waves created by a seismic source as a way of assessing the stiffness of the subsurface material. The surface wave is inverted to calculate a shear wave velocity (Park et al., 1999). Once corrected, these shear-wave (Vs) velocities can be used to obtain a factor of safety for liquefaction susceptibility based on a design earthquake.The primary study site was the township of Greymouth, on the West Coast of New Zealand’s South Island. Greymouth is built on geologically young (Holocene-age) deposits of beach and river sands and gravels, and estuarine and lagoonal silts (Dowrick et al., 2004). Greymouth is also in a tectonically active region, with the high seismic hazard imposed by the Alpine Fault and other nearby faults, along with the age and type of sediment, mean the probability of liquefaction occurring is high particularly for the low-lying areas around the estuary and coastline. Repeated mapping over 150 years shows that the geomorphology of the Greymouth Township has been heavily modified during that timeframe, with both anthropogenic and natural processes developing the land into its current form. Identification of changes in the landscape was based on historical maps for the area and interpreting them to be either anthropogenic or natural changes, such as reclamation work or removal of material through natural events. This study focuses on the effect that anthropogenic and natural geomorphic processes have on the stiffness of subsurface material and its liquefaction susceptibility for three different design earthquake events. Areas of natural ground and areas of reclaimed land, with differing ages, were investigated through the use of the MASW method, allowing an initial estimation of the relationship between landscape modification and liquefaction susceptibility. The susceptibility to liquefaction of these different materials is important to critical infrastructure, such as the St. John Ambulance Building and Greymouth Aerodrome, which must remain functional following an earthquake. Areas of early reclamation at the Greymouth Aerodrome site have factors of safety less than 1 and will liquefy in most plausible earthquake scenarios, although the majority of the runway has a high factor of safety and should resist liquefaction. The land west of the St. John’s building has slightly to moderately positive factors of safety. Other areas have factors of safety that reflect the different geology and reclamation history.