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RAPID: Deep Shear Wave Velocity Profiling for Seismic Characterization of Christchurch, NZ — Reliably Merging Large Active-Source and Passive-Wavefield Surface Wave Methods

Principal Investigator: Brady R. Cox, University of Texas at Austin

In 2010-2011, the city of Christchurch, New Zealand was devastated by a series of powerful earthquakes, the most destructive being the 22 February 2011 Mw6.2 Christchurch Earthquake. During this event, the seismic demands imposed on the built environment at many locations in the city were higher than engineering design levels, causing severe structural damage and collapse, especially within the central business district (CBD). Ultimately, the Christchurch Earthquake resulted in 181 casualties, thousands of injuries, and widespread soil liquefaction that caused billions of dollars in damage to buildings, homes and infrastructure. The entire CBD was cordoned-off following this event and remains closed to the public today (October 2012), while an estimated 1000 structures are being demolished. A network of 19 seismic recording stations in the greater Christchurch area captured an extensive and unique set of ground motions (GM) during the 2010-2011 earthquakes. Potentially, these GM can be used for back-analyses aimed at understanding the spatial variability of the ground shaking (particularly site and basin effects), followed by accurate forward-estimates aimed at quantifying the amplitude and frequency content of future design GM. However, detailed GM analyses cannot presently be conducted because no information exists on the shear wave velocity (Vs) structure of the greater-than-400-m deep interlayered sand and gravel deposits that underlie Christchurch.

The thrust of this Rapid Response Research (RAPID) grant is to conduct deep (>400 m) Vs profiling at 12-15 key sites in Christchurch, New Zealand to aid in important seismic GM response analyses. This information is needed rapidly, as plans for reconstruction of the CBD are proceeding quickly and the proposed testing will be significantly complicated (if not prohibited) once reconstruction begins in earnest in early-to-mid 2013. The only way to economically and rapidly obtain Vs estimates to these great depths is through non-intrusive surface wave testing. However, there is currently a great deal of uncertainty involved in the passive-wavefield techniques most commonly utilized for deep Vs profiling. Therefore, a unique study will be conducted to compare and merge data from large active-source and passive-wavefield surface wave methods over an extended frequency/wavelength range, which will allow robust determination of data uncertainty and relative bias. The active-source surface wave measurements will be conducted using one of the large and unique NEES@UTexas mobile, servo-hydraulic shakers and up to 48, 1-Hz geophones, while passive-wavefield data will be collected using intermediate- and large-diameter circular sensor arrays composed of 10 broadband seismometers. This research will triple the available comparisons between large active-source and passive-wavefield surface wave methods utilized for deep Vs profiling. These comparisons are needed before confidence in utilizing passive-wavefield methods independently can be achieved. Therefore, the intellectual merits of this work include: (a) the collection and interpretation of a one-of-a-kind dataset that can be used for evaluating the reliability involved with merging large active-source and passive-wavefield surface wave methods for deep Vs profiling, and (b) the advancement in accurate ground motion prediction for deep sedimentary basins made possible by these deep Vs profiles through analysis of a unique set of damaging GM records from multiple seismic events. Progress made on both of these issues will directly impact earthquake engineering studies in the US, New Zealand, and throughout the world.

The broader impacts of this work are many, as these research efforts will impact Christchurch society at large through development of robust seismic design GM that will help to mitigate such extreme losses in future earthquakes. Furthermore, the additional understanding of site response in deep sediments garnered from analyzing the strong GM recorded in the Canterbury Earthquake Sequence will benefit seismically active areas of the U.S. underlain by deep sedimentary basins, such as Los Angeles and Seattle. This work will also serve to strengthen international research collaborations between the U.S. and New Zealand, and will provide U.S. graduate students with rewarding international travel experiences that will serve to balance their technical education and expose them to the globally-connected problems that still exist in earthquake engineering.