OS13B-01 INVITED
A Decade of Tsunami-Deposit Studies in the Russian Far East – Opportunities and Challenges
Pioneering tsunami-deposit studies by I. Melekestseev and T. Pinegina led to an extensive (and ongoing) field campaign on Kamchatka and the Kuril Islands over the last ten years. This work has included large teams of Russian and American scientists and students, funded primarily by NSF Earth Sciences and Biocomplexity, the Russian Foundation for Basic Research, and institutes of the Russian Academy of Sciences. This region has rich history of 20th-century tsunamis, so most localities have a historical deposit to compare with paleotsunami deposits. In the time frame of our campaign alone, there have been two large tsunamis generated in the Kuril-Kamchatka subduction zone (Kronotsky 1997; middle Kurils 2006). Moreover, because the volcanic arc associated with the subduction zone is also very active, volcanic ash layers (tephra) serve as important correlation and dating tools for historic and pre-historic deposits. Some accomplishments of our work include: 1) quantification of (paleo)tsunami frequency north of the active volcanic arc, in the SW Bering Sea, supporting presence of a plate boundary in this region; 2) ongoing quantification of (paleo)tsunami frequency at more than 30 localities along 1000 km of coastline; 3) documentation of runup for important historical tsunamis in unpopulated regions (e.g., 1737, 1923, 1952, 1969, 1971, 1997, 2006); 4) modeling of tsunami source regions to attempt matches with runup patterns, illustrating cases of source- region heterogeneity (1952, 1997, 2006?]; 5) generating a benchmark case for modeling (paleo)tsunami sediment transport. Challenges in these studies have included (not counting remote access and bears): 1) correlation of deposits over any distance – from within a single profile to inter-locality (sometimes easy, commonly difficult); 2) paleotopographic (and paleobathymetric) reconstruction – the former is more critical because more likely to have changed over centuries to millennia; 3) given the above challenges, it is difficult to develop methods for statistical treatment of the field data – e.g., how many tsunamis of what size in how many years?; 4) access to good bathymetry and topography (and then digitizing it). This presentation will focus on challenges, which are common to all paleo-tsunami studies, and on ongoing research. Moreover, we plan to offer new sets of benchmark data for tsunami-sediment studies from the 2006 middle Kurils tsunami.
OS13B-02 INVITED
Some Uncertainties in the Inversion of Sediment Deposits to an Incident Tsunami Waveform
This presentation will present a numerical inversion of the deposit data from a recent and pre-historic tsunami using a coupled hydrodynamic-sediment transport model. The hydrodynamic model uses an established nearshore wave evolution scheme, and includes breaking and bottom friction dissipation. The sediment transport module includes both bed and suspended loads, and is driven by a Shields-type bottom flux condition. Multiple grain sizes can be approximated. Through an iterative technique, an incident tsunami time series is created which represents an optimum match with the recorded deposit data. The resulting incident time series is dependent on a large number of assumed parameters. For the physical beach these include (but are not limited to): pre-tsunami beach profile, regions of erodible bottom, and initial distribution of the sediment source. The bottom type is also an important parameter, and will be discussed in terms of a bottom friction coefficient. The modeling itself also brings a level of uncertainty to the inversion. Most significant is the method used to entrain sediment; namely its applicability to a highly unsteady flow. All of these parameters will be varied, and the impact measured. It is the goal of this work to assist in the identification of the most important variables that can be measured in the field, as well as modeling aspects that should be improved.
OS13B-03
Tsunami Deposits on Simeulue Island, Indonesia—A tale of two tsunamis
As tsunami deposits become more widely used for evaluating tsunami risk, it has become increasingly valuable to improve the ability to interpret deposits to determine tsunami characteristics such as size and flow speed. A team of U.S. and Indonesian scientists went to Simeulue Island 125 km east of Sumatra in April 2005 to learn more about the relation between tsunami deposition and flow. Busong, on the southeast coast of Simeulue Island, was inundated twice in a three-months period by tsunamis. The 26 December 2004 tsunami inundated 130 m inland to an elevation of approximately 4 m. The 28 March 2005 tsunami inundated less than 100 m to an elevation of approximately 2 m. Both tsunamis created deposits that were observed to be an amalgamated 20- cm thick, predominately fine to medium sand overlying a sandy soil. The contact between 2004 and 2005 tsunami deposits is at 13 cm above the top of the sandy soil and is clearly marked by vegetation that grew on the 2004 deposit in the 3 months between tsunamis. Grass roots are present in the upper half of the 2004 deposit and absent both below that level and in the 2005 deposit. We analyzed the fine-scale sedimentary structures and vertical variation in grain size of the deposits to search for diagnostic criteria for unequivocally identifying deposits formed by multiple tsunamis. At Busung, we expected there to be differences between each tsunami's deposits because the tsunami height, period, and direction of the 2004 and 2005 tsunamis were different. Both the 2004 and 2005 deposits were predominately normally graded, although each had inversely graded and massive sections. Faint laminations, which became more defined in a peel of the deposit, were discontinuous and predominately quasi-parallel. Knowing where the contact between the two tsunamis was, subtle sedimentary differences were identified that may be used to tell that it is composed of two separate tsunamis. We will present quantitative analyses of the variations in the deposits that arise from the different flow speeds, direction of inundation, and wave heights of the 2004 and 2005 tsunamis and assess whether it is possible to apply the criteria we develop to interpret whether paleotsunami deposits were formed by multiple tsunamis. http://walrus.wr.usgs.gov/news/reports.html
OS13B-04
Tephrochronologic Dating and Correlation of Historic and Prehistoric Tsunami Deposits in South-Central Alaska
Volcanic Ash deposits can be used to identify, date and correlate tsunami deposits in the Cook Inlet and Prince William Sound areas of south-central Alaska. The 1964 Good Friday Earthquake (M9.2) produced a major regional tsunami that affected the entire coast of south-central Alaska, and coeval submarine landslides in Prince William Sound generated local tsunamis more than 30 m high. The 1964 tsunami deposits on Augustine Island in Cook Inlet are partly buried by tephras and pyroclastic flows from recent eruptions and record an 8-m-high wave. Tsunami deposits recording a volcanic tsunami produced by an eruption at Augustine Volcano in 1883 indicate initial wave heights of more than 18 m, and are overlain by coeval 1883 tephra and also Katmai 1912 tephra at multiple sites on Augustine Island, as well as at Nanwalek and Homer 80-100 km to the north. Newly discovered tsunami deposits recording the penultimate great earthquake on the subduction zone beneath south- central Alaska are dated to ca. 950 yr BP near Valdez. The tsunami deposits are overlain by a mafic tephra from the Wrangell area erupted ca. 750 yr BP. The tephra and new dates on the tsunami deposit help refine the age of the late major earthquake on the subduction zone beneath south-central Alaska, as there is wide range of radiocarbon dates associated with broadly coeval episodes of local subsidence identified at sites in Prince William Sound and Cook Inlet. A regionally extensive Augustine tephra erupted ca. 800 yr BP occurs in sediments deposited after the subsidence at several sites around Cook Inlet, and further helps refine the age of the penultimate great earthquake in south-central Alaska. An older tsunami deposit located near Homer is dated to ca. 3400 yr BP and occurs above tephra from Redoubt Volcano, and records an older subduction zone earthquake. A still older tsunami deposit dated to 3600 yr BP directly underlies the Redoubt tephra, and may have been generated by a huge eruption at Redoubt Volcano that sent a large debris avalanche into Cook Inlet at that time.
OS13B-05
Results of Coring Survey Indicate Eastern Mediterranean Paleotsunami Events
The upper shelf of high-energy coastlines is a dynamic and changing environment, and therefore has not traditionally been viewed as a potential location for finding preserved in situ tsunami deposits. A unique shell-rich 2nd century AD deposit that was discovered a meter below the seabed outside the ancient harbor of Caesarea, Israel in 2002, has been interpreted as paleotsunami deposit. Prior to this discovery, there was no recorded physical evidence substantiating any of the 3 tsunami events mentioned in written texts. Our finding introduced the possibility that the demise of the ancient harbor may have included tsunami-related damage. It also raised the possibility that an in situ tsunamigenic layer might have been preserved along the shoreline and to deeper depths. With the goal of investigating this layer, 1 to 3 meter length cores were collected along shore-parallel (at the -15 m contour) and shore-perpendicular (from -10 to -30 meters depth) transects. The coarse-sand environment required the use of a novel submerged diver-operated pneumatic percussion coring device. The cores were analyzed using a variety of proxies (micropaleontology, sedimentology, geochemistry) to identify mixing indicators between terrestrial, beach zone, shelf, and deep sea. The results demonstrate that 1) the horizon is natural and not anthropogenic in origin, 2) fine resolution particle size distribution analysis proved useful for differentiating between normal storm cycles and tsunami events, 3) subtle micropaleontological indicators of rare-occuring, environmentally significant species were present in the tsunamigenic horizons, and 3) not one, but multiple tsunamigenic horizons are preserved offshore from Caesarea. This paper will discuss these findings and methods, and their potential for better finding, understanding and identifying paleotsunami deposits.
OS13B-06 INVITED
2007 Solomon Islands Tsunami Left Little Sand Onshore, Buried Backshore Reefs
In many places struck by the 2007 Solomon Islands tsunami, little onshore record of the tsunami's passage remains yet considerable sediment was transported offshore. This sediment represents an ecological hazard in tropical regions because of its potential for burying coral reefs. At Nusa Agana, a 50 m-wide, 2 m-high barrier island ~36 km N of the epicenter, flow depths recorded by debris wrapped around tree trunks did not exceed 50 cm—the sedimentary record on land was similarly small at ~2 cm thick. Nevertheless, the "outer" coastline of the island was stripped of sediment and the "inner" coastline filled with enough sediment to bury coral reefs to an extent that only soft corals at the top of the reef survive. The source of the sediment appears to be a mixture of sand from both the outer and inner beach, suggesting that scour occurred at both these locations. Perhaps because of the island's low relief, Nusa Agana acted less as a barrier to flow and more as a topographic high; sediment cover thinned over the high and selectively infilled the topographic low of the lagoon. At Tapurai, ~55 km ENE of the epicenter, the tsunami left a layer of coral rubble 20-30 cm thick and moved basalt boulders up to 1 m in diameter more than 100 m inland. The tsunami here reached flow depths of more than 8 m and swept N-SW across fan-shaped Tapurai, piling coral rubble mixed from offshore reefs and the modern beach onto farm fields before striking a basalt cliff behind the town and deflecting SW, carrying basalt debris with it before exiting through the town's harbor. The sediment leaves a vivid account of the passage of the wave, progressing from a solely coral rubble deposit to a mixed basalt-coral deposit and thinning downflow as sediment supply waned. Where the tsunami washed completely over islands, the side facing the waves is typically stripped of sediment, whereas the lee side shows a well developed scarp, suggesting that at least some tsunami scarps are formed during backflow and not during initial tsunami attack. At Nusa Agana, no scarp was visible one month after the event on the outer coast, but a 40 cm high scarp remained visible on the inner coast. Similarly, at Tapurai no scarp was visible on the coast first struck by the tsunami, but a well-developed scarp 50 cm high remained where the tsunami flowed offshore. Although relatively small, the 2007 Solomon Islands tsunami provides an unusual test for ideas about how tsunamis affect coastlines, and demonstrates that in at least some cases most of the sediment deposition (and ecological damage) are offshore.
OS13B-07
Sediment Transport, Mixing, and Erosion by an Impact Generated Tsunami: Gulf of Carpentaria, Australia
The Gulf of Carpentaria contains two impact crater candidates, the 18 km Tabban and 12 km Kanmare craters. We have identified an impact ejecta layer in cores from the Gulf of Carpentaria containing probable shocked quartz, magnetic iron oxide impact spherules with a bimodal size distribution, vitreous Ca phosphate with a few percent Na2O and MgO(whitlockite?), lithified glauconite microfossil casts with partial calcite rims, and other impact ejecta. The quartz grains have at least 3 different orientations of closely spaced linear fractures only a few micrometers apart (probable planar deformation features). As we have imaged these planar features using a scanning electron microscope, the shocked nature of the grains must be confirmed by measuring crystallographic directions on conventional thin sections. We found impact ejecta at the deepest depth of our sampling in six different cores (Table 1). The ejecta layer extends up into the top 2 cm of every core. However, the thickness of the layer in cm (Obs) is much greater than that predicted by simple air fall models(1) (Calc) of ejecta thickness as a function of kilometers from the nearest crater candidate (Dist). The concentration of impact ejecta is much too high to explain the layer thicknesses by bioturbation of a formerly thin layer of ejecta. Thus, we interpret these layers as a megatsunami deposit from the impact event that formed Tabban and Kanmare craters. As supporting evidence, core MD31 (also from the Gulf of Carpentaria) has 14C ages in the top 70 cm that do not increase uniformly with increasing depth, but instead fluctuate in a random manner(2). Although the dominant ostracod assemblage is marine, MD32 has a large percentage of reworked lacustrine fossils and broken shell in the top 38 cm(3). Many Holocene marine sequences from the deepest part of the Gulf of Carpentaria contain reworked lacustrine fossils(4). Because the Gulf of Carpentaria was a lake until around 10,400 yr B.P., the mixture of fossil types is suggestive of erosion and redeposition by a tsunami or other means. Table 1. Impact Ejecta in Deep Sea Cores Latitude Long. Core Calc Obs Dist -16.850 139.885 VC01 232 >245 31 -16.501 139.890 VC23 052 >69 69 -15.659 138.010 BC48 7.4 >20 151 -12.313 138.979 MD32 0.3 >37 471 -10.789 138.719 MD29 0.1 >32 641 -9.8395 135.348 GC04 0.1 >61 846 References. 1. G. S. Collins, H. J. Melosh, R. A. Marcus, Meteoritics and Planetary Science (2004). 2. J. M. Reeves, A. R. Chivas, A. Garcia, P. D. Deckker, Palaeogeography, Palaeoclimatology, Palaeoecology 246, 163 (2007). 3. J. M. Reeves, A. R. Chivas, S. Holt, M. J. J. Couapel, B. G. Jones, Quaternary International , (in press). 4. P. De Deckker, Palaeogeography, Palaeoclimatology, Palaeoecology 62, 463 (1988).
OS13B-08 INVITED
Characterization of Recent Extreme Wave Deposits, SE Hawaii
Deposits formed by extreme waves, such as those from large swell, storms, or tsunamis, leave a sedimentary record that provides clues to the type and characteristics of the depositional event. The island of Hawaii has been impacted by both storms and tsunamis over the last century and is an ideal location to study deposits produced by such events. Recent basalt flows along the southeastern coast of the island of Hawaii are mantled by several types of extreme wave deposits. These deposits can be characterized as: 1) Scattered gravel fields and occasional thin sand sheets that extend up to several hundred meters inland. The gravel ranges in size up to nearly 4 m (a-axis) and consists of both angular (common) and rounded (rare) basalt fragments with occasional marine debris such as coral and shell material. The gravel field deposits are attributed to deposition from the locally generated 1975 Kalapana tsunami. 2) Shore-parallel ridges composed mostly of basalt sand and/or gravel with variable amounts of carbonate detritus. The ridges range in elevation from about 1 to 3 m and are tens of m wide. They occur either on elevated basalt platforms (1.0-6.5 m high) where the ridges are set back from the cliff edge and separated from the edge by a narrow, sediment-free zone, or, as a supratidal extension of a pocket beach. No major tsunamis have impacted this area since November 1975 and there is photographic evidence that indicates the ridges have formed since the 1975 tsunami and therefore are the product of wave deposition during storms or long distance swell events. Results from this study provide useful information in differentiating between storm and tsunami deposits in coarse clastic sediment.