Assessing Tsunami Hazard for Hong Kong: INQUA 2007

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Presentation given during the 2007 INQUA Congress in Cairns, Abstract published under Quaternary International Vol 167-168, Supplement 1, Pp. 69

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Assessing Tsunami Hazard for Hong Kong: INQUA 2007

  1. 1. A ssessing Tsunami Hazards for Hong Kong Chiu Hon-Chim Adam Switzer Department of Earth Sciences The University of Hong Kong Abstract No. 0312, On Volume Page 69
  2. 2. S outh China Sea H ong Kong *
  3. 3. South China Sea is a high-risk region <ul><li>Large p opulation </li></ul><ul><li>A historical record of tsunami </li></ul><ul><li>Geological features not well studied </li></ul>
  4. 4. S C hina Sea H ong Kong * H ong Kong Pop. 7,000,000 South China Coast P op. > 50,000,000 Internet Imagery
  5. 5. H istorical (Enigmatic) Tsunami Records
  6. 6. <ul><li>C hinese meterological records are well preserved in imperial correspondence, especially during the Qing dynasty (1644-1911) </li></ul><ul><li>However, limitations exists because of </li></ul><ul><ul><li>L ack of scientific knowledge </li></ul></ul><ul><ul><li>The records only report onshore events if it involves casualities or crop failure </li></ul></ul>The 1781 Event
  7. 7. T he 178 1 Event <ul><li>Soloviev and Go (1964) (translated from Chinese to Russian then to English) </li></ul><ul><ul><li>“ 1782, May 22 (1682, December ?) </li></ul></ul><ul><ul><ul><li>“ An earthquake which affected all of Taiwan Island … </li></ul></ul></ul><ul><ul><ul><li>“ accompanied by tsunami waves … </li></ul></ul></ul><ul><ul><ul><li>“ ran onto the coast … in an east - west direction … </li></ul></ul></ul><ul><ul><ul><li>“ flooded for a distance of over 120 km … </li></ul></ul></ul><ul><ul><ul><li>“ More than 40,000 residents died … </li></ul></ul></ul><ul><ul><ul><li>“ Anpin … was washed away… ” </li></ul></ul></ul>http://nping.tncg.gov.tw
  8. 8. However .. <ul><li>In Chinese records … </li></ul><ul><ul><li>Chen et al. (1830) “Taiwan visit records” </li></ul></ul><ul><ul><ul><li>May 1781 </li></ul></ul></ul><ul><ul><ul><ul><li>A sudden thundering noise from the sea on a sunny day </li></ul></ul></ul></ul><ul><ul><ul><ul><li>A wall of water stacked up to dozens of feet </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Flooded all houses and people climbed up trees </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Water retreated very soon </li></ul></ul></ul></ul><ul><ul><ul><ul><li>The land was covered by fish and shrimps </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Just 1 woman dead </li></ul></ul></ul></ul>
  9. 9. <ul><li>Hsu H. (1983) “Source materials on the history of natural disasters in Qing Taiwan” </li></ul><ul><ul><ul><li>9 August 1782 </li></ul></ul></ul><ul><ul><ul><ul><li>Earthquake felt throughout Taiwan </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Suddenly all wind subsided, and water rose ~ 10 ft </li></ul></ul></ul></ul>However .. (2)
  10. 10. What’s going on? Tsunami 1792 (Source unknown) Earthquake Generated Tsunami 1782 Earthquake Generated Tsunami 1781 http://www.personal.psu.edu/faculty/a/c/ach13/Asia/China/ChinaImages.htm
  11. 11. Summary <ul><li>There was probably one earthquake generated tsunami in either 1781 or 1782 </li></ul><ul><ul><li>Possible double entry due to conversion error between Chinese Imperial Calendar and Julian </li></ul></ul><ul><ul><li>Number of deaths may be heavily underreported to the Central Government </li></ul></ul><ul><li>The 1792 event was probably one freak wave / offshore earthquake generated tsunami </li></ul><ul><ul><li>May also be the same event but dubious </li></ul></ul>
  12. 12. Geological Features in South China Sea
  13. 13. Subduction Zone Strike-slip Faults Inactive Horst and Graben Continental Shelf Red River Fault H ong Kong * Chiu, After various sources
  14. 14. Tsunamigenic hot spot #1 The Manila Trench Subduction Zone Strike-slip Faults Horst and Graben Continental Shelf Red River Fault H ong Kong *
  15. 15. H ong Kong * 7mm yr -1
  16. 16. <ul><li>M>7 Earthquakes in the Philippines </li></ul><ul><li>M>7 Earthquakes within 500km of HK </li></ul>USGS Records 2 - 3 E arthquakes of M>7 every 10 years due to this movement Each offshore earthquake will be potentially tsunami generating
  17. 17. Tsunamigenic hot spot #2 Vietnam coast Subduction Zone Strike-slip Faults Horst and Graben Continental Shelf Red River Fault H ong Kong *
  18. 18. A steep bathymetry in offshore Vietnam Red River Fault H ong Kong *
  19. 19. Prominent feature in SRTMs USGS Records Morton 2001
  20. 20. Potential sediment slides <ul><li>Vietnam shelf is heavily sediment laden and seismically active? Is there any evidence of large scale slope failures? </li></ul>
  21. 21. Quantifying Risk
  22. 22. Tsunami Generation Mechanisms Switzer 2006
  23. 23. Total Tsunami Risk <ul><li>Owing to the varied nature of tsunami generation mechanisms, tsunami risk must be considered the sum of the risks posed by different mechanisms </li></ul>
  24. 24. South China Sea <ul><li>Earthquake </li></ul><ul><ul><li>2 - 3 Manila Trench e arthquakes of M>7 every 10 years </li></ul></ul><ul><ul><li>Local earthquakes in Chinese Mainland extremely rare </li></ul></ul>
  25. 25. South China Sea <ul><li>Earthquake </li></ul><ul><ul><li>2 - 3 Manila Trench e arthquakes of M>7 every 10 years </li></ul></ul><ul><ul><li>Local earthquakes in Chinese Mainland extremely rare </li></ul></ul><ul><li>Submarine Slumping </li></ul><ul><ul><li>Possibly evident on the Vietnamese Coast </li></ul></ul><ul><ul><li>No reliable record exists and little is known about other areas. </li></ul></ul>
  26. 26. South China Sea <ul><li>Earthquake </li></ul><ul><ul><li>2 - 3 Manila Trench e arthquakes of M>7 every 10 years </li></ul></ul><ul><ul><li>Local earthquakes in Chinese Mainland extremely rare </li></ul></ul><ul><li>Submarine Slumping </li></ul><ul><ul><li>Possibly evident on the Vietnamese Coast </li></ul></ul><ul><ul><li>No reliable record exists and little is known about other areas. </li></ul></ul><ul><li>Bolide Impacts </li></ul>
  27. 27. Bolide impact…How likely is it? R eturn Period 10 7 10 2 10 2 10 4 S ize of Bolide, diameter in m E g. Possible Mega-tsunami, SE Australia, ~0.5ka E g. Global tsunami, Chicxulub, Bay of Mexico, 65Ma (R ickman, 2005) Millions of Years Centuries
  28. 28. The Earth <ul><li>Total surface area: 5 x 10 8 sq. km </li></ul><ul><li>South China Sea: 3.5 x 10 6 sq. km </li></ul><ul><li>(0.7% of Earth) </li></ul><ul><li>Impact on SCS: </li></ul><ul><li>1 per 14,200 years? </li></ul>http://www.ngdc.noaa.gov/mgg/fliers/95mgg01.html
  29. 29. Earthquakes near HK <ul><li>Only some earthquakes are potentially tsunami generating </li></ul><ul><ul><li>Subduction related (vertical component) </li></ul></ul><ul><ul><li>Vertical Faults </li></ul></ul>H owever, the only documented tsunami in Hong Kong (0.28m) was generated regionally by a M5.4 earthquake in 1988 <ul><li>M>7 Earthquakes in the Philippines </li></ul><ul><li>M>7 Earthquakes within 500km of HK </li></ul>
  30. 30. Tidal Record: 24 June 1988 Time of Earthquake Hong Kong Observatory
  31. 31. However … <ul><li>Tidal anomaly: Earthquake induced tsunami? </li></ul><ul><ul><li>Timing is right </li></ul></ul><ul><ul><li>Magnitude only M5.4 </li></ul></ul><ul><ul><li>Any other contributing factors? </li></ul></ul>
  32. 32. Conclusions <ul><li>Historical records good sources of past tsunami occurrences </li></ul><ul><li>Tsunami risk determination requires a multitude of data types </li></ul><ul><li>Accurate risk determination dependent on correct identification of generation mechanisms. </li></ul>

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