Could tsunami risk be underestimated using core‐based reconstructions? Lessons from ground penetrating radar

Abstract

Where should we take cores for palaeotsunami research? It is generally considered that local depressions with low energy environments such as wetlands are one of the best places. However, it is also recognized that the presence or absence of palaeotsunami deposits (and their relative thickness) is highly dependent upon subsoil microtopography. In the beach ridge system of Ishinomaki Plain, Japan, several palaeotsunami deposits linked to past Japan Trench earthquakes have been reported. However, the number of palaeotsunami deposits reported at individual sites varies considerably. This study used ground penetrating radar (GPR) combined with geological evidence to better understand the relationship between palaeotopography and palaeotsunami deposit characteristics. The subsurface topography of the ~3000–4000 bp beach ridge was reconstructed using GPR data coupled with core surveys of the underlying sediments. We noted that the number (and thickness) of the palaeotsunami deposits inferred from the cores was controlled by the palaeotopography. Namely, a larger number of events and thicker palaeotsunami deposits were observed in depressions in the subsurface microtopography. We noted a total of three palaeotsunami deposits dated to between 1700 and 3000 cal bp, but they were only observed together in 11% of the core sites. This result is important for tsunami risk assessments that use the sedimentary evidence of past events because we may well be underestimating the number of tsunamis that have occurred. We suggest that GPR is an efficient and invaluable tool to help researchers identify the most appropriate places to carry out geological fieldwork in order to provide a more comprehensive understanding of past tsunami activity. Copyright © 2017 John Wiley & Sons, Ltd.

Millennial paleotsunami history at Minna Island, southern Ryukyu Islands, Japan

“…2b to identify cross-sectional subsurface structures. The data were processed by an analytical method using software (RADAN7; GSSI Inc., Nashua) referred from a report by Takeda et al (2018). Using a "Pick" module with REFLEXW software (Sandmeier Geophysical Research, Karlsruhe), strong reflectors were detected from GPR profiles.…”

Origen y distribución de depósitos de tsunami en la marisma de Chaihuín (40° S/73,5° O), Chile

“…2). La microtopografía subaérea correspondería a un factor secundario que afectaría, en menor medida, a la variación en el espesor local de las distintas capas (Takeda et al, 2018).…”

“…Las características y la preservación de un depósito de tsunami son dependientes de muchos factores, entre ellos la dirección de propagación de la ola, la forma de la costa, los rasgos topográficos, la disponibilidad de sedimento y la interferencia entre las partículas durante el transporte (Nelson et al, 1996;Goff et al, 2006Goff et al, , 2012Takeda et al, 2018). Se asume que el transporte de los sedimentos más finos (arcillas a arenas) ocurre a través del mecanismo de suspensión cuando el depósito originado presenta gradación normal.…”

Three Thousand Year Paleotsunami History of Southern Part of Japan Trench

“…Since the 2011 Tohoku-oki event, many researchers have studied paleotsunami deposits along the Paci c coasts facing the Japan Trench to reevaluate paleotsunami histories along the central to northern parts of the Japan Trench (e.g., Goto et al 2015Goto et al , 2019Inoue et al 2017;Ishimura and Miyauchi 2015;Ishizawa et al 2018Ishizawa et al , 2019Kusumoto et al 2018;Minoura et al 2013;Sawai et al 2012;Sawai et al 2015;Takada et al 2016;Takeda et al 2018;Tanigawa et al 2014a;Tanigawa et al 2014b;Watanabe et al 2014, see also Sawai 2017, accepted). For instance, two event layers for the past 6000 years were reported in a meadow of Misawa city, Shimokita Peninsula, Aomori prefecture ( Fig.…”