TAPHONOMIC CHANGES IN TSUNAMI DEPOSITS PRESERVED FROM THE 1960 CHILE EARTHQUAKE IN DIFFERENT COASTAL ENVIRONMENTS

Paleotsunami studies rely upon finding preserved tsunami deposits to determine the extent of inundation and to estimate paleoearthquake parameters by reverse-modeling events from grain size characteristics.

This study presents a quantitative analysis and comparison of taphonomic changes in a 20-cm thick tsunami deposit from the 1960 Chile tsunami (Mw 9.5) preserved in three different coastal environments in Nigue Sur, Chile. Previous studies have confirmed that post-deposition preservation potential of tsunami deposits depends on complex processes that can differ in intensity between locations. Factors that affect preservation include: initial deposit thickness, mixing depth, erosion, anthropogenic modification, earthquake-induced uplift or subsidence, vegetation, and geomorphic environment.

Core samples with buried tsunami deposits collected from grassland, pasture, and forest sites were analyzed for grain size distribution using laser diffraction, and organic matter content using Simultaneous Thermal Analysis (STA) and Loss-On-Ignition (LOI). Organic content and mud content was assumed to be negligible immediately after deposition. Therefore, well-preserved deposits were defined as having uniform grain size distribution, low mud content, and low organic matter content from infiltration through the upper or lower contacts, while poorly-preserved deposits were defined as having non-uniform grain size distribution, higher mud content, and higher organic matter content.

Samples show that in the grassland environment the 1960 tsunami deposit is well-preserved in terms of thickness and grain size, but poorly-preserved in the forest environment. Most locations surveyed in the pasture environment showed poor preservation, but in some places the tsunami deposit was undisturbed and moderately well-preserved. The results obtained in this study help identify how the long-term preservation potential of tsunami deposits compares in different environmental settings and provides insight into why some environments poorly preserve deposits.