Physical and chemical alteration of onshore tsunami deposits - implications for inverse modelling tsunami deposits

This study uses a time series of three years to document the early-diagenetic changes in siliciclastic onshore tsunami sediments at La Trinchera (Central Chile) that were deposited by the 2010 Maule tsunami. The sediments are rich in heavy minerals with magnetite representing up to 76% of the original deposit. The taphonomy proceeded in three stages. First, the thickness was reduced by 20% within the first months because of compaction and erosion by runoff. Second, vegetation returned and bioturbation destroyed the sedimentary structures and shrouded the basal contact. The thickness was reduced further to 20-40% of its initial thickness. The mean grain size relatively 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 iron reduction set in and dissolved up to 57% of the magnetite. Thickness, grain size and bulk density are the three sediment specific parameters required to calculate flow parameters of past tsunamis by means of inverse modelling. We numerically tested the sensitivity of inverse modelling to the taphonomic changes documented at La Trinchera and found that the calculated flow velocity decreased by 60% (4.2 m/s in 2010; 2.5 m/s in 2013). We conclude that diagenetic changes to the thickness, composition and grain size distribution of tsunami deposits may occur on very short time scales, i.e. in a few years. In our case these changes lead to an underestimation of the magnitude of the event. In general, any study attempting to invert the parameters of a past tsunami from its deposits needs to consider and quantify changes to the main physical properties of the deposit in order to arrive at a realistic estimation of past tsunami magnitudes.