Seattle Fault Zone poses significantly higher risks than previously thought

May 31, 2012 – SEATTLE – The Seattle fault zone, a series of active-east-west trending thrust faults, poses seismic threat to the Puget Sound region. The 900-930 AD rupture is the only known large earthquake along the Seattle Fault, making geological records of prehistoric events the only clues to the earthquake potential of the fault. While a graduate student at the University of Washington, Maria Arcos looked at tsunami and debris flow deposits – both evidence of a paleo-quake – in the coastal marsh at Gorst, Washington. She also identified evidence of at least three meters of uplift that preceded a tsunami, which was followed by a sandy debris flow from Gorst Creek, and suggests that the 900-930 AD earthquake covered a greater geographic area than previous fault interpretations. The revised height and width of deformation caused by the quake may influence current interpretations of the Seattle fault’s structure. This study found a minimum of three meters of uplift at Gorst, which is double the amount of previous fault models for the same location. A broader zone of deformation, says Arcos, may indicate either a wider zone of slip along the dip of the fault, a shallower dip or splay faults farther to the south. –Physics

contribution by Emanni