Bill Stephenson (left) holding a Stephenson probe, and GNS technician David Baguley (right) at the control box for an array of soft soil probes in the Wainuiomata basin near Wellington (image: A. Ballance)
Recently retired GNS scientist Bill Stephenson began his career as a physicist, but now if you ask him to describe what he does he thinks that earthquake seismologist is the best fit. Bill is a leading expert on the effects of earthquake shaking on soft soils – soft soils behave very differently to earthquake waves than rock does, and Bill’s career was devoted to trying to understand those differences. He is the inventor of Stephenson Probe, which is a cone-shaped device containing tiny accelerometers. It can be pushed into the ground and left to transmit data about the vibrations it detects.
Bill takes Alison Ballance out to a study site in Wainuiomata near Wellington, where an array of these probes has been in place for 13 years. This array, and another in nearby Parkway, have shown how surface shaking in soft soils is very violent compared to deeper down, and the arrays have also shown how rock sides channel and amplify shaking along a valley. You can see some movies of ground motion measured during an earthquake at the Parkway array on the GNS website here – complete the search boxes with "Stephenson" and "2007".
