Geological investigation of palaeotsunamis in the Samoan islands: interim report and research directions

The Komandorsky seismic gap has distinctive boundaries and a length of 650 km. Its period of “seismic silence” comes close to the maximum recurrence interval for great earthquakes in the Aleutian Island Arc - the stress concentration here probably having reached the critical value. So,estimation of possible earthquake and tsunami characteristics within this gap becomes a significant problem. The closest analog of a similar gap is the area where the 2004 Sumatra-Andaman catastrophic event occurred. Thus, for the present study we used the same modeling scheme as we used for that event. It was assumed that a source length of 650 km, consisting of 9 blocks, and an earthquake with a moment magnitude MW=8.5. Several block motion scenarios were considered. The tsunami generation and propagation in the Pacific Ocean and the possible wave characteristics on near and far-field coasts were estimated. Modeling of such an event showed that the wave heights on different Pacific coasts will vary from 3 to 9 meters. A tsunami wave with a 9-meter height is capable in causing significant loss of human life and economic damage.© 2013, Tsunami Society International

GEOLOGICAL INVESTIGATION OF PALAEOTSUNAMIS IN THE SAMOAN ISLANDS: INTERIM REPORT AND RESEARCH DIRECTIONS

The September 29, 2009 Samoa Tsunami provided the opportunity to sample the sediments deposited in the Samoan Islands landscape by the tsunami. Analysing the characteristics of the sediment deposits using an established suite of diagnostic criteria, and assessing how they differ from cyclone deposits enables the identification and dating of similar events in the geologic record. This helps to better understand the long-term frequency and likely magnitude of these events. Here we report on a pilot palaeotsunami field-sampling investigation carried out in 2010 at selected sites on Upolu and Savaii Islands in the Independent State of Samoa, and on Ta’u Island in American Samoa. We present empirical stratigraphic data for the investigated sites, and we demonstrate the existence of high energy marine inundation deposits at some of these sites which were laid down by past tsunamis and/or cyclones. We review and discuss the analytical outcomes, as well as summarise the overarching directions of this research. We propose that there is a need for this study to continue and for such studies to be carried out in other islands in the Pacific. By doing this, we can build on the sparse palaeotsunami database in the region, thereby helping to improve our understanding of the long-term frequency, impact distribution, and likely magnitude of these events. Further, we can start assessing their likely sources and the long-term risk these hazards pose to coastal cities and communities in the Pacific

Magnetic anisotropies for tsunami deposits: application to the 3.11

Tsunami deposits consist of well-sorted fine sand intercalating with non-marine black organic mud. It is difficult to reveal a transport direction of the deposit if the deposit showed no sedimentary fabrics,such as ripples. The proxy of anisotropy of magnetic susceptibility (AMS) appears to be a promising tool for the study of flow fabrics in recent-tsunami deposits such as Sumatra tsunami (Wassmer et al. 2010). The AMS fabric might allow us to reconstruct transport directions of unconsolidated tsunami sediments during emplacement because AMS provides a cryptic alignment of ferromagnetic and paramagnetic minerals. Such cryptic minerals, such as magnetite or phyllosilicate minerals, would behave as a different emplacement mode in a different hydrodynamic condition. In the AMS fabrics of volcanic rocks, there are large discrepancies between the magnetic lineation and the framework-forming silicate linear fabric. This suggests that the uncorroborated use of bulk AMS to detect flow fabric in tsunami deposits has risks. In this article, we show that the anisotropy of anhysteretic remanent magnetization (AARM) may resolve the difficulties. The combination of inundation eye-witness, SEM, AMS, and AARM confirms the flow pattern of recentand paleo-tsunami deposits from the geoslicer sampleing at Rikuzen-Takata city, Japan during 2011, 11th March Tohoku tsunami. We determined if the sandy deposits are of tsunami from these magnetic anisotropies. © 2013, Japan Geoscience Union

Modern and possible paleotsunami deposits in Samenoura, Sanriku Coast, and their relation to tsunami source mechanisms

Samenoura is situated in the bay head of a small inlet on the Pacific coast of Oshika Peninsula, one of the nearest places to the epicenter of the 2011 Tohoku-oki Earthquake. According to the Joint Survey Group, wave heights were measured at more than 20 m near the coastline. This area was severely damaged as a result of both co-seismic subsidence and tsunami inundation. We carried out field surveys of the Tohoku-oki and paleotsunami deposits at Samenoura in March, May and October 2013. Sandy deposits laid down by the Tohoku-oki tsunami were up to 20 cm thick at locations with an elevation greater than 10 m, and were several cm thick within the forest higher up. The tsunami deposit also contained numerous shell fragments and foraminifera. Although some possible sources of the tsunami deposits can be attributed to narrow sandy beaches near the study area, the deposition of such a thick sandy deposit is more or less enigmatic, considering the steep Ria-type coastal topography.Using a gouge auger and geoslicer, we found at least two sand layers intercalated within muddy sediments. A volcanic ash layer, which corresponds to the AD 915 Towada-a tephra, was also identified from a horizon between these sand layers. The underlying sand layer was most probably laid down by the 869 Jogan earthquake tsunami, one of the large-scale events known to have affected the region. Previous studies of the Jogan tsunami have proposed several possible source models that involve an interplate thrust earthquake. Given that the local bathymetry and topography of Samenoura Bay may be sensitive to the waveform of a large-scale tsunami, paleotsunami deposits found from this area may be the key to determining the source mechanisms of events on the Sanriku Coast.In this presentation, the possible correlation of the sandy deposits with known paleotsunami events based on detailed radiocarbon dating is discussed. The hydrodynamic character and processes of tsunami sediment erosion and deposition in Samenoura Bay are analyzed using numerical modeling of both interplate and outer-rise earthquake scenarios.Copyright on Japan Geoscience Union Meeting, 2014

The Australian Tsunami Database - a review.

There has been a significant increase in the number of peer-reviewed publications, critical reviews and searchable web-based databases, since the first substantial tsunami database for Australia was published in 2007. This review represents a complete reorganization and restructuring of previous work coupled with the addition of new data that takes the number of events from 57 (including 2 erroneous events) to 145. Several significant errors have been corrected including mistaken run-up heights for the event of 19 August 1977, Sumba Island, Indonesia, that suggested it was the largest tsunami in Australia's history. The largest historical event in the database is now the 17 July 2006, Java, Indonesia, tsunami that had a run-up height of 7.90 m at Steep Point, Western Australia. Although estimated wave heights of 40 ft (approximate to 13 m) were noted for the 8 April 1911 event at Warrnambool, Victoria, no run-up data were provided. One of the more interesting findings has been the occurrence of at least 11 deaths, albeit for events that are generally poorly defined. Data gathered during the construction of this database were rigorously reviewed and as such several previous palaeotsunami entries have been removed and other potentially new ones discarded. The reasons for inclusion or exclusion of data are discussed, and it is acknowledged that while there has been an almost three-fold increase in the number of entries the database is still incomplete. With this in mind the database architecture has been brought in line with others in the region with the ultimate goal of merging them all in order to provide a larger, interrogatable and updatable data set. In essence, the goal is to enhance our understanding of the national and regional tsunami hazard (and risk) and to move towards an open-source database. © 2014, SAGE Publications