Tsunami Erosion

This study applies heavy mineral analysis to the Storegga tsunami deposit across a range of locations (Whale Firth, Maggie's Kettle Loch and Scatsta Voe) in Shetland (Scotland). The usefulness of this proxy is tested in the identification and characterization of these palaeotsunami units. Furthermore, provenance relationships are established based on the mineralogical content of tsunami deposits and their potential source. Finally, the capability of identifying different phases of tsunami inundation in an 8200 years old tsunami deposit is attempted. Our results show that, overall, tsunamigenic samples presented a clear dominance of garnets and amphiboles. While Whale Firth presented a more balanced distribution between these two mineral groups, in Maggie's Kettle Loch and Scatsta Voe the tsunamigenic samples are dominated by amphiboles (N 90% of transparent heavy minerals). Focusing on the two dominant heavy minerals (garnets and amphiboles) and their vertical variation, one could observe that garnets mimic the heavy mineral concentration variabilityhigher values at the base and decreasing values to the top. This effect of concentration of the heaviest of the heavy minerals assemblage presents similarities with the formation of beach placer deposits. In fact, based on the heavy mineral vertical variation of the tsunami deposits in Maggie's Kettle Loch, Scatsta Voe and Whale Firth it is possible to conclude that hornblende (most likely amphibole of the assemblage) has the lowest concentration factor indicating that its transport process is more efficient and consequently most of its particles eventually may have moved offshore in the backwash phase of the tsunami. Furthermore, the more platy shape of amphiboles also favours a slower deposition. The opposite can be observed for garnets, which require more energy to be transported (i.e. they are more difficult to entrain by the tsunami waves) and tend to be more easily preserved in the formation of a tsunamigenic (placer) deposit. The work presented here is of particular relevance for future high resolution sedimentological studies aiming to distinguish different inundation phases of the Storegga tsunami, and assess the degree of preservation of these deposits, especially considering the specific geomorphological and stratigraphic depositional setting of Scotland.

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This study applies heavy mineral analysis to the Storegga tsunami deposit across a range of locations (Whale Firth, Maggie's Kettle Loch and Scatsta Voe) in Shetland (Scotland). The usefulness of this proxy is tested in the identification and characterization of these palaeotsunami units. Furthermore, provenance relationships are established based on the mineralogical content of tsunami deposits and their potential source. Finally, the capability of identifying different phases of tsunami inundation in an 8200 years old tsunami deposit is attempted. Our results show that, overall, tsunamigenic samples presented a clear dominance of garnets and amphiboles. While Whale Firth presented a more balanced distribution between these two mineral groups, in Maggie's Kettle Loch and Scatsta Voe the tsunamigenic samples are dominated by amphiboles (N 90% of transparent heavy minerals). Focusing on the two dominant heavy minerals (garnets and amphiboles) and their vertical variation, one could observe that garnets mimic the heavy mineral concentration variabilityhigher values at the base and decreasing values to the top. This effect of concentration of the heaviest of the heavy minerals assemblage presents similarities with the formation of beach placer deposits. In fact, based on the heavy mineral vertical variation of the tsunami deposits in Maggie's Kettle Loch, Scatsta Voe and Whale Firth it is possible to conclude that hornblende (most likely amphibole of the assemblage) has the lowest concentration factor indicating that its transport process is more efficient and consequently most of its particles eventually may have moved offshore in the backwash phase of the tsunami. Furthermore, the more platy shape of amphiboles also favours a slower deposition. The opposite can be observed for garnets, which require more energy to be transported (i.e. they are more difficult to entrain by the tsunami waves) and tend to be more easily preserved in the formation of a tsunamigenic (placer) deposit. The work presented here is of particular relevance for future high resolution sedimentological studies aiming to distinguish different inundation phases of the Storegga tsunami, and assess the degree of preservation of these deposits, especially considering the specific geomorphological and stratigraphic depositional setting of Scotland.

As a result of the tsunami generated by the Pacific Offshore Earthquake on March 11, 2011 that occurred along the coast of the Tohoku region of Japan, most of the windbreak forests widely distributed along the coast were uprooted and swept away. Particularly, most of pine trees planted over a 500 m wide tract of land on the Sendai city coast of Miyagi prefecture were swept away and scattered inland more than 3 km. In this research, pine trees scattered in the region is measured by GIS analysis through aerial photographs taken immediately after the tsunami, showing the characteristic flow patterns of the tsunami by plotting the axis directions of the trees and the shelter effect of the vegetation. Most of the driftwood, reaching a total number of 21,054,is distributed 1 km to 3 km away from the coastline. Several cases show that sheltering effects of the pine trees prevent houses located behind it from collapsing.

Case studies of recent tsunami impacts have proven to be extremely useful in understanding the geologic processes involved during inundation and return flow, and refining the criteria used to identify paleotsunami deposits in the geologic record. Here, we report on erosion, deposition and associated landscape change resulting from the March 11, 2011 Tohoku-oki tsunami along a nearly 4.5 km shore-normal transect on the coastal plain near Sendai, Japan. The study area on the broad, low-relief prograding coastal Sendai plain comprised a sand beach backed by~3 m high sand dunes and a forest, a wetland, the Teizan canal, agricultural rice fields, buildings and roads. Field observations focused on measurements of tsunami flow characteristics (height and direction), mapping of erosion features and assessing sediment deposition based on shallow trenches at 50-100 m spacing. Recorded tsunami inundation heights reached up to about 11 m above mean sea level within the first 500 m from the shoreline and then ranged between 3 and 5 m for the next 2 km, gradually decreasing to about 3 m close to the inundation limit. The tsunami deposit generally thinned landward from an average maximum~30 cm thick sand deposit in the coastal forest to a thin mud drape several mm thick near the inundation limit. A discontinuous sand-dominated sheet was prevalent to about 2800 m from the shoreline where mud content then gradually increased further landward eventually resulting in a mud-dominated deposit ranging from 3.5 cm to a few mm thickness. The overall thinning and fining of the deposit was often interrupted by localized features that led to complex sedimentological relationships over short distances. Satellite imagery taken on 14 March 2011, 3 days after the Tohoku-oki Tsunami shows prominent foreshore incisions with 100 s + meters spacing alongshore, a foredune ridge that underwent severe erosion and development of a prominent shore-parallel elongated scour depression. Our field survey in early May 2011 revealed that the foreshore recovered quickly with rapid post-tsunami sediment deposition from incident waves, whereas the dune-ridge complex had undergone only minor re-working from eolian processes.

Case studies of recent tsunami impacts have proven to be extremely useful in understanding the geologic processes involved during inundation and return flow, and refining the criteria used to identify paleotsunami deposits in the geologic record. Here, we report on erosion, deposition and associated landscape change resulting from the March 11, 2011 Tohoku-oki tsunami along a nearly 4.5 km shore-normal transect on the coastal plain near Sendai, Japan. The study area on the broad, low-relief prograding coastal Sendai plain comprised a sand beach backed by~3 m high sand dunes and a forest, a wetland, the Teizan canal, agricultural rice fields, buildings and roads. Field observations focused on measurements of tsunami flow characteristics (height and direction), mapping of erosion features and assessing sediment deposition based on shallow trenches at 50-100 m spacing. Recorded tsunami inundation heights reached up to about 11 m above mean sea level within the first 500 m from the shoreline and then ranged between 3 and 5 m for the next 2 km, gradually decreasing to about 3 m close to the inundation limit. The tsunami deposit generally thinned landward from an average maximum~30 cm thick sand deposit in the coastal forest to a thin mud drape several mm thick near the inundation limit. A discontinuous sand-dominated sheet was prevalent to about 2800 m from the shoreline where mud content then gradually increased further landward eventually resulting in a mud-dominated deposit ranging from 3.5 cm to a few mm thickness. The overall thinning and fining of the deposit was often interrupted by localized features that led to complex sedimentological relationships over short distances. Satellite imagery taken on 14 March 2011, 3 days after the Tohoku-oki Tsunami shows prominent foreshore incisions with 100 s + meters spacing alongshore, a foredune ridge that underwent severe erosion and development of a prominent shore-parallel elongated scour depression. Our field survey in early May 2011 revealed that the foreshore recovered quickly with rapid post-tsunami sediment deposition from incident waves, whereas the dune-ridge complex had undergone only minor re-working from eolian processes.

Case studies of recent tsunami impacts have proven to be extremely useful in understanding the geologic processes involved during inundation and return flow, and refining the criteria used to identify paleotsunami deposits in the geologic record. Here, we report on erosion, deposition and associated landscape change resulting from the March 11, 2011 Tohoku-oki tsunami along a nearly 4.5 km shore-normal transect on the coastal plain near Sendai, Japan. The study area on the broad, low-relief prograding coastal Sendai plain comprised a sand beach backed by~3 m high sand dunes and a forest, a wetland, the Teizan canal, agricultural rice fields, buildings and roads. Field observations focused on measurements of tsunami flow characteristics (height and direction), mapping of erosion features and assessing sediment deposition based on shallow trenches at 50-100 m spacing. Recorded tsunami inundation heights reached up to about 11 m above mean sea level within the first 500 m from the shoreline and then ranged between 3 and 5 m for the next 2 km, gradually decreasing to about 3 m close to the inundation limit. The tsunami deposit generally thinned landward from an average maximum~30 cm thick sand deposit in the coastal forest to a thin mud drape several mm thick near the inundation limit. A discontinuous sand-dominated sheet was prevalent to about 2800 m from the shoreline where mud content then gradually increased further landward eventually resulting in a mud-dominated deposit ranging from 3.5 cm to a few mm thickness. The overall thinning and fining of the deposit was often interrupted by localized features that led to complex sedimentological relationships over short distances. Satellite imagery taken on 14 March 2011, 3 days after the Tohoku-oki Tsunami shows prominent foreshore incisions with 100 s + meters spacing alongshore, a foredune ridge that underwent severe erosion and development of a prominent shore-parallel elongated scour depression. Our field survey in early May 2011 revealed that the foreshore recovered quickly with rapid post-tsunami sediment deposition from incident waves, whereas the dune-ridge complex had undergone only minor re-working from eolian processes.