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Erosion rates for Hoodoos formed in soft sediments, and implications for constraining seismic hazard

Mark W. Stirling, & F. Della Pasqua

Submitted 2014, SCEC Contribution #1960

Abstract We provide estimates of the rates of natural modification of Hoodoos formed in soft sediments at two sites in New Zealand, and assess the utility of Hoodoos for constraining seismic hazard models. One of the sites is in a humid temperate environment, and the other is in a semi-arid environment. Rates of Hoodoo modification of 30-270mm/year (equivalent to 3-27m/century) are estimated by comparison of images acquired one-to-two decades apart. The primary mode of modification is the progressive narrowing of the Hoodoo column by precipitation-induced erosion, along with consequent loss of Hoodoo height as the columns become thin and unstable. Additional damage may have been produced by earthquake shaking for specific Hoodoos, but in general this is likely to be trivial, given that: (1) Estimates of the levels of peak ground acceleration required to shake down the Hoodoos at the two sites are a factor of five to 13 greater than the peak ground accelerations predicted or measured from regional earthquakes during the time periods of observation, and; (2) a fragile Hoodoo has actually formed in the time periods of observation. The Hoodoo modification rates are rapid enough that the Hoodoos would not have existed one-to-two centuries ago, or would have been of substantially different geometries. As such, the Hoodoos are unlikely to be useful for constraining seismic hazard models for return periods greater than a few decades.

Stirling, M. W., & Della Pasqua, F. (2014). Erosion rates for Hoodoos formed in soft sediments, and implications for constraining seismic hazard. Bulletin of the Seismological Society of America, (submitted).