Magnetic fingerprint of tsunami-induced deposits in the Ixtapa-Zihuatanejo Area, Western Mexico

摘要

The Pacific coast of Mexico has repeatedly been exposed to destructive tsunamis. Recent studies have shown that rock magnetic methods can be a promising approach for identification of tsunami- or storm-induced deposits. We present new rock magnetic and anisotropy of magnetic susceptibility (AMS) results in order to distinguish tsunami deposits in the Ixtapa-Zihuatanejo area. The sampled, 80 cm-deep sequence is characterized by the presence of two anomalous sand beds within fine-grained coastal deposits. The lower bed is probably associated with the 14 March 1979 Petatlan earthquake (M_w = 7.6), whereas the second one formed during the 21 September 1985 Mexico earthquake (M_w = 8.1). Rock mag netic experiments discovered significant variations within the analysed sequence. Thermomagnetic curves reveal two types of behaviour: one in the upper part of the sequence, after the occurrence of the first tsunami, and the other in the lower part of the sequence, during that event and below. Analysis of hysteresis parameter ratios in a Day plot also allows us to distinguish two kinds of behaviour. The samples associated with the second tsunami plot in the pseudo-single-domain area. In contrast, specimens associated with the first tsunami and the time between both tsunamis display a very different trend, which can be ascribed to the production of a considerable amount of superparamagnetic grains, which might be due to pedpgenic processes after the first tsunami. The studied profile is characterized by a sedimentary fabric with almost vertical minimum principal susceptibilities. The maximum susceptibility axis shows a declination angle D = 27°, suggesting a NNE flow direction which is the same for both tsunamis and normal currents. Standard AMS parameters display a significant enhancement within the transitional zone between both tsunamis. The study of rock magnetic parameters may represent a useful tool for the identification and understanding of tsunami deposits.