Test of the microtextural analysis of quartz grains of tsunami and non-tsunami deposits in Tirúa (Chile) - an unsuitable method for a valid tsunami identification

Abstract:

The tsunami caused by the 2010 Maule earthquake (M_W 8.8) significantly affected the village of Tirúa (Central Chile). In order to estimate the hazard potential of tsunami events it is essential to reliably identify and differentiate tsunami deposits from deposits of other high-energy events like storms. Recently, the microtextural analysis of quartz grain surfaces was introduced as a method to differentiate between tsunami and other deposits.

We tested the microtextural analysis method for its capability to identify tsunami deposits using paleotsunami intercalations from a bank profile of the Tirúa river. A total of 815 quartz grains of 4 river bank samples (2 tsunamigenic, 2 non-tsunamigenic) and of 3 reference samples from nearby beach, dune and river were analyzed. In order to generate a valid statistical basis even within individual grain size fractions a large number of grains was studied. Another reason was to compensate the error of the operator’s subjectivity during random picking and microtexture observation. Grain surfaces were analyzed using SEM. We detected 30 individual microtextures grouped into five microtextural families according to angularity, fresh surfaces, percussion marks, adhering particles and dissolution.

The grains from the tsunami deposits have high numbers of fresh surfaces and percussion marks. However, in comparison with the non-tsunamigenic deposits and all reference samples (beach, dune and river) the tsunamigenic deposits do not show statistically significant differences in characteristics and abundances in all microtextural families.

The homogeneity in microtextural results of all samples indicate the absence of differences between tsunamigenic, beach dune and river deposits. A distinct tsunami signature could not be identified from our microtextural analysis. Our study indicates that the microtextural analysis of quartz grains may not be a suitable method to identify tsunami deposits.