Microtremor observations in Tamar Valley, Launceston, Tasmania: evidence of 2D resonance from observed microtremor and numerical modelling

Abstract

We use the microtremor survey method to record ambient ground vibrations in Launceston, Tasmania. The presence of the ancient Tamar Valley, in-filled with soft sediments that vary rapidly in thickness from 0 to 250m over a few hundreds meters, is thought to induce a 2D resonance pattern, amplifying the surface motions over the valley and in Launceston. We use the spatially averaged coherency (SPAC) microtremor survey method to study its potential for characterising site effects across the Tamar Valley. We present SPAC observations, recorded with increasing array size at 3 sites KPK (r1=28m), DBL (r1=50m) and RGB (r1=140m) in Launceston. Replacing the traditional spatial averaging of coherencies by temporal averaging, we compute coherencies with single pairs of sensors, oriented parallel (axial-coherency) and perpendicular (transverse-coherency) to the valley axis. Using multi-radii arrays, we analyse the impact of increasing radius on the spatial and temporal averaged coherencies recorded at all sites, to study SPAC capability to detect a 2D resonance pattern induced from the Tamar valley in Launceston.