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

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

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

Not just salt - the 11 March 2011 Tohoku-oki Tsunami and the significance of geochemical proxies

Researchers most often rely on the extent of sandy deposits to assess the magnitude and extent of palaeotsunamis, assuming that the landward limit of the deposit approximates the maximum runup of the tsunami. While it has been reported in many modern examples, it is not always the case. One such example is the 11 March 2011 Tohoku-oki tsunami, which inundated c. 4.5 km inland in the Sendai plain. However, a recognisable (>0.5 cm thick) sand deposit was recorded only up to c. 2.8 km inland (~ 62% of the inundation distance). Further inland, the deposit was dominated by mud containing at least a one-grain thick sand lamina up to the inundation limit. Therefore, relying on the extent of sandy deposits may lead to a gross underestimate of the generating event, and extent of tsunami inundation. The magnitude of the 869AD Jogan earthquake, which was based on the extent of the sandy deposits reported in the Sendai Plain, was estimated at ~8.4. Data acquired after the Tohoku-oki tsunami suggest it was an underestimate. Then the question arises - how will one be able to distinguish mud deposited by a tsunami from terrestrial mud and soil? Geochemical markers might provide the answer. We reported brackish and saline ponded water up to 2.6 km inland, despite >60 mm of precipitation in the two months since the tsunami, and we also observed salt crusts on numerous paddy fields up to the limit of tsunami inundation, where the water had evaporated. Elevated concentrations of chloride (salt), sulphate, magnesium, calcium and other associated elements were measured not only in mud deposits, but also in sandy deposits, where seawater had stagnated and then evaporated. The preservation of the marine geochemical signature (mostly salt but also sulphur) is problematic in sandy deposits, mainly due to leaching and post-depositional diagenetic processes. On the other hand, it has been proven in organic-rich sediments, such as mud, and has been used as an additional proxy to identify palaeotsunami deposits. Other geochemical markers, which are specific to each source area, can also be used. Geochemical markers could be used in conjunction with other proxies, such as microfossil data (e.g. marine diatoms), which have also been found to occur beyond the limit of recognisable tsunami deposits. They can provide the clues to identify the maximum inundation limit of palaeotsunamis that extends beyond the limit of the recognisable sandy deposits. The marine geochemical signature of the 2011 Tohoku-oki tsunami was found up to the limit of tsunami inundation near the Tobu Highway, and even beyond, where the waves reached through the underpasses. As geochemical markers show a good preservation potential in fine-grained and organic-rich sediments, we could use them to estimate the inundation distance of the Jogan tsunami beyond the preserved sand layer. © American Geophysical Unio

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

New light through old windows – reaping the benefits of a palaeotsunami database.

The New Zealand palaeotsunami database was first established in 2008 and has continued to grow in a somewhat ad hoc manner since then. While ostensibly a geological database, it has grown markedly through the addition of geomorphological, archaeological, anthropological and ecological data. The main tsunami research focus in New Zealand has tended to be along those coastlines either adjacent to a local subduction zone or exposed to events generated by similar distant sources; in this case - the eastern shores of New Zealand. Examining the database, we unexpectedly identified at least two large events within the past 700 years on the western shores. Event 1 occurred between 1470 and 1510AD and Event 2 between 1320 and 1450AD. Once the probable extent of these events was defined we were able to then propose a range of potential tsunamigenic sources including non-subduction fault ruptures and several slope failure mechanisms. While numerous sites with contemporaneously-aged evidence along the eastern shores point to significant probable subduction zone tsunamis, at least one anomalously high elevation data point stands out. The ~60 masl site at Korapuke Island fits comfortably within the timing of a probable 15th century palaeotsunami but evidence from neighbouring sites along 10s of kms of coastline range from only 0-10 masl. A subsequent reassessment of the site identified no unusual characteristics that might amplify runup and no local tsunamigenic source. However, a similar nearby site has high elevation (~45 m) deposits laid down in 1996 by a waterspout and it seems increasing likely that Korapuke Island may have the first reported palaeo-waterspout deposit. This adds an interesting wrinkle to the palaeotsunami-palaeostorm deposit debate, but also reveals the value of a palaeotsunami database. There are probably similar anomalies (and errors) in the database, but without the context of related data it is unlikely that they would be exposed to such scrutiny. © 2014, American Geophysical Unio