RAPID/Collaborative Research: Advanced Site Characterization of Key Ground Motion and Ground Failure Case Histories Resulting from the Mw7.8 Kaikoura, New Zealand, Earthquake

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

A large and widely-damaging earthquake has not occurred in the U.S. for over 20 years. However, advancements have continued to be made in earthquake engineering by observing the effects of large earthquakes in countries that have similar seismic design standards and well-constructed infrastructure. Documenting the effects of earthquakes in a country like New Zealand, which has rigorous building codes and similar infrastructure to the U.S., provides the greatest opportunities for learning lessons that will be directly transferable to our country. The Mw7.8 Kaikoura earthquake provides a unique opportunity to evaluate in considerable depth the effects of earthquake shaking on diverse types of infrastructure, in cities of different population densities, underlain by various types of soil profiles, subjected to a wide range of shaking intensities. These circumstances provide an invaluable opportunity to study ground shaking amplification caused by site, basin and topographic effects, and cases of liquefaction-induced ground failure, that will be directly applicable to U.S. cities such as Los Angeles, San Francisco, Seattle and Salt Lake City. Urgency is required because the PIs currently have access to some key facilities, such as the CentrePort of Wellington. After they have rebuilt the port, for example, there will no longer be access and the opportunity to characterize the ground as it was immediately after the earthquake will be lost . Moreover, New Zealand researchers are rapidly mobilizing to carry out initial studies at key sites. In order to combine resources and expertise, the project will need to also mobilize quickly. The end-goal of this project is to benefit society at-large through increased resiliency, sustainability and affordability of civil infrastructure via improved seismic design. 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. In short, the broader impacts of this work stretch far beyond the borders of New Zealand and will positively impact seismic hazard practices in the U.S. and abroad. The intellectual merits of the research plan include an improved understanding of: (1) ground motion stratigraphy, basin and topographic effects in complex geotechnical settings; (2) liquefaction response of gravelly soils; (3) the influence of non-liquefiable capping layers on liquefaction damage potential; and (4) the response of buildings and critical port infrastructure on liquefiable soil. Of particular interest are the much-higher-than-expected long period ground motions recorded on natural and reclaimed soil sites 60 km from the fault rupture in the capital city of Wellington. It is hypothesized that highly variable topography, both above and below the valley floor, resulted in complex interactions between stratigraphy, basin and topographic effects, amplifying ground motions. Advanced site characterization is needed in the form of deep shear wave velocity (Vs) profiling and topographic array studies to investigate these phenomena. Without this information, it will be impossible to place observations of structural damage and ground failure caused by the Kaikoura earthquake in context. Wellington is the perfect test bed for this study, because abruptly changing topography allows all of these ground motion effects to be studied on a remarkably small scale of several kilometers. Advanced site characterization is also needed at Wellington's port facility, where surprisingly severe liquefaction damage occurred under moderate shaking (PGA values of 0.13 - 0.24g) from this relatively distant earthquake In addition to documenting the damage to wharves and shipping cranes due to liquefaction-induced lateral spread movements, there are rare incidents of gravel liquefaction to document at CentrePort Other important ground failure sites on the South Island should also be characterized rapidly to add valuable, perishable data to the U.S. soil liquefaction triggering and lateral spreading empirical databases. Advanced site characterization studies are planned at key case history sites in Wellington and the South Island using a combination of: (1) Single-station horizontal-to-vertical spectral ratio noise measurements for investigating fundamental site periods, (2) Combined active-source and ambient-wavefield surface wave testing for developing deep Vs profiles; (3) Cone penetration tests for revealing detailed site stratigraphy; and (4) Direct-push crosshole tests for high-resolution Vs and Vp profiles. Collecting and analyzing these data will help to better understand valuable lessons-learned that are directly transferrable to improved seismic design practices in the U.S.