2007 Solomon Islands Tsunami Left Little Sand Onshore, Buried Backshore Reefs
Abstract
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.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMOS13B..06M
- Keywords:
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- 4558 Sediment transport (1862);
- 4564 Tsunamis and storm surges