Identifying Tsunami Risk in South China Sea

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Identifying Potential Tsunami Generation Sources for Hong Kong and South China Sea and Modelling Tsunami

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Identifying Tsunami Risk in South China Sea

  1. 1. Identifying Tsunami Risk in South China Sea Chiu Hon Chim (chius@hku.hk) Department of Earth Sciences The University of Hong Kong
  2. 2. Outline <ul><li>Potential Tsunami Generation Sources for Hong Kong and South China Sea </li></ul><ul><li>Modelling Tsunami </li></ul>
  3. 3. Tsunami Generation Sources (Switzer, 2006)
  4. 4. Tsunami Generation Sources (Switzer, 2006)
  5. 5. Tsunami Generating Earthquakes <ul><li>Distant </li></ul><ul><li>Regional </li></ul><ul><li>Local </li></ul>
  6. 6. 05/26/09 <ul><li>Distant </li></ul>Tsunami Generating Earthquakes
  7. 7. 05/26/09 D istant Earthquakes <ul><li>3 Times in the Past Century </li></ul><ul><li>Simple, well-understood effect on HK </li></ul>HKO A bout 0.1m 6.7 C hile 1985.03 N one reported 9.5 S umatra 2004.12 0.6 m 8.5 C hile 1960.05 0.15 m 8.2 K amchatka 1952.11 M ax Amplitude Recorded in HK E arthquake Magnitude O rigin D ate
  8. 8. 05/26/09 <ul><li>Regional </li></ul>Tsunami Generating Earthquakes
  9. 9. 05/26/09 Regional Earthquakes <ul><li>South China Sea Sub-sea Geology Mostly Unknown / Imprecise </li></ul><ul><li>Manila Trench </li></ul>Hong Kong Subduction Zone Strike-slip Faults Horst and Graben Continental Shelf
  10. 10. 05/26/09 M anila Trench <ul><li>2 - 3 E arthquakes of M>7 every 10 years due to this movement </li></ul><ul><li>Each EQ will be potentially tsunami generating </li></ul>7mm yr -1
  11. 11. 05/26/09 Regional Earthquakes <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 (0.28m) generated regionally occur as a M5.4 earthquake <ul><li>M>7 Earthquakes in the Philippines </li></ul><ul><li>M>7 Earthquakes within 500km of HK </li></ul>
  12. 12. 05/26/09 Tidal Record: 24 June 1988 Time of Earthquake HKO
  13. 13. 05/26/09 However … <ul><li>Questions remain as to whether the earthquake is the source / only source of the tidal anomaly </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>
  14. 14. 05/26/09 Tsunami Generating Earthquakes <ul><li>Local </li></ul>
  15. 15. 05/26/09 Earthquake: Local <ul><li>Things to consider </li></ul><ul><li>Direction of movement </li></ul><ul><li>How far can it move? </li></ul><ul><li>When did it last move? </li></ul>HK
  16. 16. 05/26/09 What we know <ul><li>Direction of Movement </li></ul><ul><ul><li>Mostly strike-slip type </li></ul></ul><ul><ul><li>Some horst-and-graben – vertical movement? </li></ul></ul><ul><li>How much can it move? </li></ul><ul><ul><li>Not much as we know of </li></ul></ul>
  17. 17. 05/26/09 T he 3 Large Earthquakes near HK 1918.02.13 M7.3 1604.12.29 M7 1994.09.16 M7.3 HKO 1988.06.24 M5.4 When did it last move? HKO HK
  18. 18. Summary <ul><li>Regional earthquakes most likely to generate tsunami in HK </li></ul><ul><li>Effect of distant tsunami quite well understood ( & predictable) </li></ul><ul><li>The sense of movement of seafloor during earthquake dictates whether tsunami will be generated </li></ul><ul><ul><li>Chance of local earthquake generating tsunami is remote - most are strike - slip type movements </li></ul></ul>
  19. 19. 05/26/09 Tsunami Generation Sources (Switzer, 2006)
  20. 20. 05/26/09 V olcanism in the Past <ul><li>South China Sea was volcanically active in the past – </li></ul><ul><ul><li>Young seafloor : 35 Ma - 15 Ma </li></ul></ul><ul><ul><li>Recently Small volcanic events ( Eg. Weizhou Volcanics) 36 - 33 ka </li></ul></ul>Q = Q uaternary = Past 1.8 million years Large volume of lava release could trigger tsunami HK E Q E E E N E Q Q Q N Q N E Q E E E N E Q Q Q N Q N
  21. 21. 05/26/09 Any future volcanism? <ul><li>Geothermal heat flow distribution </li></ul><ul><li>Strongest flux : Approx 140 W / m 2 and dropping </li></ul><ul><li>Pinatubo : At least 300 W / m 2 </li></ul>Heat Flow Distribution in South China Sea, After Lau (1994) Geothermal Heat Flux 20 40 60 80 100 120 140 HK
  22. 22. 05/26/09 Tsunami Generation Sources (Switzer, 2006)
  23. 23. 05/26/09 Extensional Basin HK
  24. 24. 05/26/09 Extensional Basin S ubmarine slumping: River sediments “falls” from here to here HK
  25. 25. 05/26/09 Submarine Slides / Slumps <ul><li>Accumulation of sediments </li></ul><ul><li>Sudden Jolt ( eg. Earthquakes) </li></ul><ul><li>Sediments run downslope </li></ul><ul><li>Large submarine landslide </li></ul><ul><li>Displacement of water causes tsunami </li></ul>
  26. 26. 05/26/09 Tsunami Generation Sources (Switzer, 2006)
  27. 27. 05/26/09 Bolide Impact – More often than you think? <ul><li>Bobrowsky & Rickman (Eds) </li></ul><ul><li>Published 2006 / 12 </li></ul><ul><li>Much more often – small event every 100 years </li></ul><ul><li>Largely unnoticed </li></ul><ul><ul><li>Uninhabited, eg. Siberia (Early 1900s) </li></ul></ul><ul><ul><li>Into sea </li></ul></ul>
  28. 28. 05/26/09 How likely is it? R eturn Period 10 7 10 2 10 2 10 4 S ize of Bolide, diameter in m E g. Mega-tsunami, SE Australia, ~0.5ka E g. Global tsunami, Chicxulub, Bay of Mexico, 65Ma (R ickman, 2005) Millions of Years Centuries
  29. 29. 05/26/09 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>www.ngdc.noaa.gov/mgg/fliers/95mgg01.html
  30. 30. Tsunami Risk in HK <ul><li>Most Likely Cause </li></ul><ul><ul><li>Tsunami from Distant Source </li></ul></ul><ul><ul><li>Manila Trench Earthquakes </li></ul></ul><ul><li>Possible Cause </li></ul><ul><ul><li>Asteroid Impact </li></ul></ul><ul><li>Highly Improbable Causes </li></ul><ul><ul><li>Reactivation of local faults </li></ul></ul><ul><ul><li>Renewed volcanism in South China Sea </li></ul></ul>
  31. 32. Outline <ul><li>Potential Tsunami Generation Sources for Hong Kong and South China Sea </li></ul><ul><li>Modelling Tsunami </li></ul>
  32. 33. Tsunami Characteristics <ul><li>Long period, Long wavelength wave </li></ul><ul><ul><li>Eg. 2004 Indonesian Tsunami </li></ul></ul><ul><ul><ul><li>Initial Wavelength approx 95 – 130 km </li></ul></ul></ul><ul><ul><ul><li>Wave in Indian Ocean (Deep sea) ~ 180 km, observed wave amplitude 1 – 2 m, period at about 15 min. </li></ul></ul></ul><ul><li>At open sea act as Shallow Water Wave </li></ul><ul><ul><li>When depth < ½ λ </li></ul></ul><ul><ul><li>Sinosoidal characteristics become negligible </li></ul></ul><ul><ul><li>ν = g d </li></ul></ul>
  33. 34. D istant Earthquakes T ravel time graphs Speed = gravity / depth Nearshore effect ignored HKO
  34. 35. Regional Earthquakes <ul><li>UNESCO:IOC International Oceanographic Commission </li></ul><ul><li>Worldwide sharing of modelling techniques </li></ul><ul><li>For use in governmental meteorological agencies to estimate tsunami effects </li></ul>
  35. 36. Based on Imamura (1997) <ul><li>Based on Shallow Water Theory </li></ul><ul><li>Different calculations for far field and near field tsunami </li></ul><ul><li>Relatively simple mechanism enable more accurate data to be input </li></ul>
  36. 37. Parameter used <ul><li>Gridded Bathymetry </li></ul><ul><li>Tsunami Generation Event </li></ul><ul><li>Friction? And other parameters </li></ul><ul><li>Output: Time and Water Height at shore </li></ul>
  37. 38. Inundation Results: HKO <ul><li>Source Earthquake </li></ul><ul><ul><li>Manila Trench 17˚20’ N 119˚30’E </li></ul></ul><ul><ul><li>M8, 25km Depth </li></ul></ul><ul><ul><li>Rupture: N-S </li></ul></ul><ul><ul><li>Movement: Vertical </li></ul></ul>HKO Tidal Range Runup
  38. 39. Regional & Local Earthquakes <ul><li>Complex bathymetry </li></ul><ul><li>Complex inundation and Runup mechanisms </li></ul><ul><li>Imprecise data </li></ul><ul><li>Time too short to model & forecast </li></ul>
  39. 40. Modelling Tsunami in South China Sea - General Modelling Techniques <ul><li>Identify scenario most likely to generate tsunami </li></ul><ul><ul><li>eg. M7 earthquake in N Luzon </li></ul></ul><ul><li>Identified worst case scenario </li></ul><ul><ul><li>eg. M8.5 earthquake, ½ of Manila Trench </li></ul></ul><ul><li>Predict maximum run-up when it reaches HK </li></ul><ul><li>Map areas of “inundation” </li></ul><ul><li>Assess risk to lives, structures, utilities …. </li></ul>
  40. 41. And remember … NOAA

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