The good, the bad and the ugly - lessons from and methodologies for extracting shear-wave velocity profiles from microtremor array measurements in urban Newcastle

Abstract

We report on our experience gained in applying the multiple-mode spatially-averaged coherency method (MMSPAC) at 25 sites in Newcastle (NSW) for the purpose of determining shear-wave velocity profiles as part of an earthquake hazard study. The MMSPAC technique is logistically viable for use in urban and suburban areas, both on grass sports fields and parks, and on footpaths and roads. A set of seven recording systems designed for earthquake after-shock observations plus a team of three personnel is sufficient to survey three sites per day. The uncertainties relating to local noise sources from adjacent road traffic or from service pipes contribute to loss of low-frequency SPAC data in a way which is difficult to predict in survey design. Coherencies between individual pairs of sensors should be studied as a quality-control measure with a view to excluding noise-affected sensors prior to interpretation; useful data can still be obtained at a sites one sensor is excluded. The combined use of both SPAC data and horizontal:vertical spectral ratio (HVSR) data in inversion and interpretation is necessary in order to make effective use of low frequency data (typically 0.5 to 2 Hz at these sites) and thus resolve shear-wave velocities in bedrock below 20 to 50 m of unconsolidated sediments.