Living and working just 25km above a rapidly moving plate boundary provides a compelling motivation to learn about the environment and the risks linked to this setting. There are a few large urban regions of the world where this occurs – some examples include, Tokyo, Seattle, Vancouver, Santiago and Wellington. I live in the latter, which is the capital city of New Zealand. Twenty-five kilometers beneath us the Pacific plate subducts at a velocity of about 40 mm/y. GPS data shows that the overlying Australian plate is locked to the Pacific plate, and this locked state has persisted for at least as long as the GPS network itself (20 y) and possibly longer. This situation can’t go on forever and there is a reluctant acceptance from city planners that at some point the plate boundary will slip and release the pent-up elastic stresses in a large earthquake. Historically, the largest quake we have experienced is a Magnitude 8.2 in 1855 that was located 40km NE of the city.
In this talk I will take you from the top few meters of the crust to the lithosphere-asthenosphere boundary (LAB) at a depth of 100 km, and show you how this combined view paints a picture of the complete plate boundary zone.
Our seismic imaging of the plate boundary starts in the top 30 m, where an engineering focus has gone into determining the average shear-wave speed in the top 30 m (V30). This is a key parameter for seismic hazard assessment as it is directly related to the shear modulus. We also use shear-wave reflection and gravity methods to map the basin that the city is built on, as the depth and shape of this basin control shaking from both remote and local earthquakes.
The jump from the top 30m to the mid and lower crust comes from a joint NZ-US- Japan funded seismic transect, which was carried out just north of Wellington City in 2010. This project, called SAHKE, involved 1000 seismographs on land (one every 100 m from coast to coast) and ocean-bottom seismometers offshore. A seismic ship was deployed to shoot airguns offshore while onshore a series of twelve 500 kg dynamite shots were detonated in 50m deep bore holes. This survey defined the top of the plate interface beneath Wellington City (see figure). The plate dips at an angle 10-15 degrees steeper beneath the city then starts to plunge more steeply further to the west.
Three new discoveries about plate tectonics in general arose from the SAHKE study. First, there is a huge - 7 km high - build-up of sediment on top of the plate interface at depths of 20-30 km. Second, unusually high P-wave speeds were found in the mantle of the subducted Pacific plate. Lastly, for the first time, we managed to image the LAB at a depth of about 100 km beneath Wellington, from the dynamite shots. We see a 10 km-thick channel at the LAB, which is inferred to be a focussed zone of melt and/or volatiles that provides a low-viscosity cushion for the plates to slide along, and indeed may be a necessary condition for plate tectonics to work.
In this talk I will explain what these findings mean for the origin of the Earth’s plates, the dynamics of plate tectonics, and tease out some of the clues as to what makes a major plate boundary susceptible to either large or intermediate magnitude earthquakes.