SOURCES, GROUND MOTION AND STRUCTURAL RESPONSE CHARACTERISTICS IN WELLINGTON OF THE 2013 COOK STRAIT EARTHQUAKES

SUMMARY The Cook Strait earthquake sequence occurred in a region of known high seismicity. However, this was the strongest shaking felt in decades for the Wellington region and the top of the South Island. The location and size of the earthquake meant that the ground shaking was of rather short duration and moderate intensity, except for the epicentral region of the Lake Grassmere earthquake where a PGA of 0.7g was recorded, and for part of the Wellington foreshore where up to 0.2g was recorded in both earthquakes. The level of shaking in terms of response spectra was, in general, moderate except for very high “spiked” response at particular Wellington sites (WNKS and VUWS) at periods of 0.4-0.5 seconds. Amplification and polarization in the NE-SW direction at approximately ~1 s period at many Wellington stations is likely due to basin resonance effects, whereas dominant polarization in the NW-SE direction at shorter periods is consistent with a directivity effect, and is particularly evident in the Lake Grassmere earthquake. The earthquakes were not only a real-life test on the level of preparedness for the population but also on the behaviour of recently-built structures in the Wellington region that had not yet experienced a moderate earthquake. The ability to measure, analyse and understand the intensity and characteristics of the ground shaking coupled with well-documented damage to the buildings and building array recordings will hopefully foster collaboration across earthquake engineering disciplines.