Post-depositional alteration of onshore tsunami deposits - Implications for the reconstruction of past events

Sediments provide information on parameters governing their deposition. However, this information gets blurred by post-depositional processes. Here, we document post-depositional alteration processes that affected an onshore tsunami deposit from its deposition in February 2010 until 2014. Post-depositional changes proceeded in three stages. First, the thickness was reduced by 20% within the first months because of erosion by runoff. Second, vegetation returned and bioturbation destroyed the sedimentary structures and obliterated the basal contact. The thickness was reduced further to 20-40% of its initial thickness. The mean grain size decreased as silt and mud from the enclosing soil horizons were mixed into the deposit. Finally, as post-seismic subsidence together with damming by a coastal dune transformed the coastal plain into a wetland, aqueous microbial magnetite reduction set in and dissolved up to 57% of the magnetite. Inverse models are used to reconstruct onshore tsunami flow parameters in order to quantify the intensity of ancient events. Thickness, grain size and bulk density are input parameters to any model. Thus, post-depositional processes affect the reliability of the modeled flow parameters. Of the three documented processes, the post-depositional alteration of the grain size has the highest influence, followed by the reduction of the thickness. The change of the bulk density hardly had any effect. Our sensitivity analysis shows that the calculated flow velocity decreased by 60% from 4.2 m/s for the pristine sediments to 2.5 m/s for the altered deposits. Consequently, inverse models will significantly underestimate tsunami intensity because of the alteration of the deposits.