Development of multi-channel analysis of surface waves (MASW) for characterizing the internal structure of active fault zones as a predictive method of identifying the distribution of ground deformation

Technical Abstract

Canterbury and Otago are both part of the plate boundary deformation zone. Within this zone management of seismic hazards needs to be based on accurate identification of the potential fault damage zone. The thrust fault bounding the Taieri Ridge in Macraes Flat, Central Otago, is concealed by outwash slopes of Quaternary sediments and by loess. Together with the lack of exhumed structures and anthropogenic land use, this makes the fault’s location difficult to pinpoint even within 10’s of meters. In the case of such buried faults the application of non-invasive geophysical survey methods is often desirable.

A multi-channel analysis of surface waves (MASW) survey records dispersive Rayleigh waves. The fundamental mode wave velocities are plotted against their frequencies to produce a fundamental mode dispersion curve. The curves for each record are then inverted to determine s-wave velocity with depth and the resulting 1D inversions are interpolated to construct a 2D shear wave velocity profile. This pseudo-section defines a fault zone and acts as a proxy for changes in rock mass properties. The MASW shallow seismic survey method has recently been used to image a buried fault zone in Torlesse greywacke under a gravel overburden at Dalethorpe Canterbury. The results suggest that the technique is sensitive to lateral variations in shear wave velocity due to subsurface structure (Duffy et al., 2007).

The structural sensitivity of the technique is confirmed by application of MASW at the Taieri Ridge. High Resolution Reflection data (Kilner et al., 2007 ) was reprocessed using MASW in order to constrain the location of the fault, which had proved invisible to standard processing of the data. The results are promising and appear to show the high velocity Otago Schist juxtaposed against the low velocity Tertiary and Quaternary units. Trenching is planned, which should confirm the MASW results. Processing of the Taieri Ridge dataset has further led to initial development of a novel and extremely promising surface wave imaging technique termed Swept Frequency Imaging (SFI). The SFI image created of the Taieri Ridge seismic line appears to show the detailed structure of the fault zone, which predictably includes considerable deformation extending forward of the fault into the softer units overlying the unconformity. The unconformity itself is also imaged and bending moment thrusts can be seen offsetting the surface of the unconformity. Minor shears imaged using SFI appear to correlate with structures observed in the first trench. The SFI image of the Taieri Ridge fault is also expected to be confirmed by excavation of a second trench. The vertical scale of the image is indeterminate but should be possible to determine if appropriate software is developed. SFI is an exciting opportunity that awaits further development.