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Introduction to Geological Disasters of Taiwan C. C. Lin Central Geological Survey Email: [email protected] Web: http://www.moeacgs.gov.tw Introduction 1. The active tectonic movement and mountain building process have resulted in high-rise mountains and strongly deformed rock formation in Taiwan. 2. People are threatened by a variety of geological hazards, such as earthquakes, landslides, land subsidence, coastal erosion, and tsunami. Among them, earthquakes and landslides are the most disastrous and extensive. 3. Various geoscientific research and monitoring programs have been proceeded to provide comprehensive information for disaster mitigation. Where do we live in? TAIWAN In the with part most of theactive most a belt active platebeing boundary volcanoes, calledwith frequent earthquakes and “Ring of Fire” … Tectonic Setting of Taiwan Part of the Circum-Pacific Seismic Belt, active tectonic movement induced frequent earthquakes Located along the convergent boundary between Philippine Sea and Eurasian plates ♦ ♦ ♦ Philippine Sea plate subducted toward N in NE Taiwan Eurasian plate subducted toward E in SE Taiwan Phil. Sea plate collided with Eurasian plate in Central E Taiwan Being the junction of Ryukyu Arc and Luzon Arc Topography Geologic setting Geologic hazards of Taiwan “Taiwan may be the place on Earth most vulnerable to natural hazards, with 73 percent of its land and population exposed to three or more hazards;＂ “More than 90 percent of the populations of Bangladesh, Nepal, the Dominican Republic, Burundi, Haiti, Taiwan, Malawi, El Salvador, and Honduras live in areas at high relative risk of death from two or more hazards;＂ --Natural Disaster Hotspots: A Global Risk Analysis, March 29, 2005, World Bank Geologic hazards of Taiwan Earthquakes ♦ ♦ causing the most severe and extensive damage the most hazardous geologic processes in Taiwan Landslides ♦ ♦ among the most wide-spread geologic hazards Landslides associated with typhoon pose fatalities and property losses Subsidence and coastal erosion ♦ ♦ occurred very slowly being deteriorated by human activities Volcanic erutpion ♦ no volcanic eruption record in history Coastal erosion Most of the coastal line is subjected to erosion, due to: ♦ ♦ ♦ ♦ Over excavation of river sediments to support the massive demand for aggregate of concrete has reduced the supply of coastal sands Land reclamation from the sea for industry parks and construction of seaports interfere the distribution the sediments along coast. Dams for water supply or debris flows control retain the sediments behind the dams. Land subsidence along the coast Land subsidence Natural compaction of sediments Excessive withdrawal of groundwater for fish farms in low-lying areas speeding up the subsidence rate Mostly occurred in coastal areas Land subsidence Cumulative subsidence since 1970s (cm) Annual subsidence rate (cm/yr) Landslides Landslides include many types of downhill earth movements, ranging from rapidly moving catastrophic rock avalanches and debris flows in mountainous regions to more slowly moving earth slides and other ground failures. Thousands of landslides can be triggered by a single severe storm or earthquake, causing spectacular damage in a short time over a wide area. Landslide distribution Slope failures Debris flows Causal factors of landslides Lithology—shale, conglomerate, slate, and schist are highly susceptible to landslide Structures—bedding, foliation,joints, fractures, cleavage, and faults etc. Topography—angle, aspect, shape of slopes Improper land-use—removal of down-slope supporting Annual precipitation (unit: 1,000 mm) Triggering factors of landslides Heavy rainfalls Earthquakes Volcanic eruption Change of water level Snow melting Landslides People living in mountainous area are endangered by slope failures and debris flows Landslide disasters Landslide disasters occurred in the surroundings of urban areas Triggered by heavy rainfalls Most of them can be avoided Case of landslide triggered by typhoon rainfalls Date: 25/08/2004 Hazard type: landslide Triggered by: Typhoon Aere Losses: ♦ ♦ Some 20 houses buried 22 deaths Area of sliding: 5.8 hectares Volume of sliding: 1 million m3 Water crisis caused by extensive landslides Extensive landslides in the catchment area of Shihmen Reservoir Large amount of sediments being carried into the reservoir, raising the turbidity of impoundment. Causing crisis of lacking water for daily use of millions people for 19 days. Cases of earthquake induced landslide (1)— Tsaolin Date: 21/09/1999 Hazard type: landslide Triggered by: earthquake Losses: ♦ ♦ 4 houses 36 deaths Area of sliding plus debris piling up: 522 hectares Volume of sliding mass: 120 million m3 Geology: Dip slope consists of sandstone overlying on thick soft shale Cross sections of Tsaolin landeslide Historical landslide events of Tsaolin Date Triggering factor 1861 earthquake（M 6-7） 12/17/1941 earthquake（M 7.1） 8/10/1942 heavy rainfalls 8/15/1979 heavy rainfalls 9/21/1999 earthquake（M 7.3） Case of earthquake induced landslide (2)— Chiufenershan Date: 21/09/1999 Triggered by: Chi-Chi earthquakes Area: 159 hectares Volume: 36 million m3 Losses: ♦ ♦ 14 houses buried 39 deaths Dip slope composed of lateMiocene sandstone and shale alternation Debris flows Case of debris flow disaster (1) Date: 03/07/2004 Hazards: floods and debris flows Triggered by: Rainstorm after Typhoon Mindulle Losses: ♦ ♦ 30 dwelling houses totally destroyed 1 death, 1 injury, 2 missing Landslide occurrence after the 1999 Chi-Chi earthquake In central Taiwan, the Chi-Chi earthquake had triggered 25,845 landslides of varied scale, with a total slide area of some 16,000 hectares, 8 times compared with landslides before earthquake. Changes of occurrence of landslides and debris flows: ♦ ♦ ♦ Increase the frequency of landslides and debris flows; Increase the magnitude of landslides and debris flows; Remarkably reducing the threshold precipitation needed to trigger landslides and debris flows ; Typhoon Toraji (30/07/2001) posed severe landslide and debris flow disaster for eastern and central Taiwan, caused 240 deaths and/or missing. Landslide Hazards Mitigation Strategies Restricting development in landslide-prone areas. Standardizing codes for excavation, construction, and grading. Protecting existing development. Utilizing monitoring and warning systems. Landslide susceptibility map First step to landslide hazard assessments Statistic methods are being developed to prepare landslide susceptibility map Active Faults & Earth Quakes the destiny of people in Taiwan 台灣地區地震分布 Historical disastrous earthquakes Last century, Earthquakes have killed some 8,000 people There were 7 earthquakes with M>7 Most of disastrous earthquakes occurred on land and were accompanied with surface ruptures Locations of disastrous earthquakes occurred in Taiwan area for the past 100 years 1999 Chi-Chi earthquake Main Shock Time: Sep. 21, 1999 Magnitude: MW 7.6, ML 7.3 Deaths : 2,494 Injuries : 11,305 Houses damaged: 107,002 Nominal property losses : US$11.5 billions Damage caused by ground ruptures Damage caused by ground motion liquefaction Active fault investigation 42 active faults being classified into 3 categories: ♦ Holocene active fault— ♦ ♦ activated in the Holocene; offset man-made structures; relate to historical earthquakes; offset the recent alluvium; show the creeping phenomena from the recent geodetic survey Pleistocene active fault— activated within 100,000 years; offset the terrace deposits. Suspect fault— by now without definite information to classify into appropriate category Geologic data (mainly paleoseismologic) needed for Seismic Hazard Assessments GEOLOGIC DATA FAULT ZONE SEGMENTATION Slip Rate Recurrence Interval Elapsed Time Displacement / Event Fault Geometry RECURRENCE MODELS Rupture Length Maximum Earthquake LONG-TERM EARTHQUAKE POTENTIAL • Hazard Model • Probability of Occurrence From Schwartz and Coppersmith (1986) Active fault research program Fault geometry ♦ ♦ ♦ Detail mapping Geophysical exploration (seismic reflection, electric resistivity) Drilling Slip rate ♦ ♦ Short term—monitoring horizontal and vertical displacement Long term—paleoseismology Recurrence interval ♦ ♦ Paleoseismology Historical records Elapsed time ♦ ♦ Paleoseismology Historical records Displacement ♦ ♦ Short term—monitoring horizontal and vertical displacement Long term—paleoseismology Surface investigation and mapping Seismic reflection Seismic reflection profile 1060 980 940 860 900 820 780 740 700 660 620 580 550 470 430 390 350 310 230 270 190 150 110 70 0 510 Distance (m) 1020 TTC-HL04P N S 0.0 37 77 0.1 123 173 224 0.2 281 338 0.3 397 459 Depth (m) 0.4 583 645 0.5 711 781 852 0.6 922 992 0.7 1066 1143 1220 0.8 1296 1373 0.9 1450 1527 1.0 Two-Way Time (second) 521 Resistivity profile across active fault 470 50 420 100 370 150 320 200 37 220 300 70 450 20 500 250 0.1 173 200 0.2 224 A A A 281 150 A 338 LR 150 397 A 459 100 m) 521 583 L 50 50 0 0 -50 -50 -100 10 1.0 470 50 420 100 31 1.5 370 150 100 316 2.0 2.5 RESISTIVITY (OHM-M) 320 200 0.3 270 250 220 300 DISTANCE (m) 1000 3.0 170 350 -100 3160 3.5 120 400 70 450 20 500 0.4 Two-Way 100 S 0.0 1935年地震地表破裂 123 200 520 0 1060 980 940 900 860 820 740 780 660 120 400 700 620 580 510 430 170 350 470 350 390 310 230 270 150 270 250 77 溼潤表土 ELEVATION (m) 190 70 520 0 S ELEVATION (m) 250 110 TTC-HL03E 0 N 550 Distance (m) 1020 TTC-HL04P N Active fault monitoring GPS High Precision Leveling Reference Point 雙冬斷層 車籠埔斷層 彰化斷層 Elevation change across active fault (1) Leveling showing a 40 mm elevation change across the Changhua active fault between the year 2002 and 2004. No significant change found across the Chelungpu fault 後甲里斷層 Elevation change across active fault (2) Reference Point The leveling survey delineates a uplifting rate of 12 mm/yr for the Tainan tableland. The eastern margin of the tableland is bordered by a westdipping back-thrust. GPS velocity field (1) GPS velocity field (2) Showing the variation of velocity field across the active faults Trenching of CLP fault S3 S2 S7 S5 S1 S3 S6 S4 S5 S2 S4 S3 Another Trench of CLP fault Before the 921 Chi-Chi earthquake, there had been 3 to 4 paleoseismic events occurred within the latest 1800 years according to the result of C14 dating. The recurrence of fault activation are calculated around 450 years. The latest one (the one before Chi-Chi) occurred between 150 to 430 years (elapsed time) before present. Combination of trench and boreholes Probablistic seismic hazard assessment Probabilistic hazard assessment has been performed using the available geologic data collected from the active faults investigation, incorporated with seismic catalogue. (map showing the PGA of 10% probability of exceedance in 50 years) . Earthquake precursors and prediction A variety of earthquake precursors are under studied, including: Measurable changes in rock ♦ ♦ ♦ ♦ ♦ ♦ Minute cracks developed in rocks, and dilation of rocks Changes in groundwater level Changes in the electrical resistivity of rocks Increases in radon gas levels of ground water Decrease in the P wave/S wave speed ratio Variation of electromagnetic intensity Seismological variations Sum up (1) 1. Landslides are a significant component of many major natural disasters, but the damage caused by landslide is often reported as a result of a triggering event, such as floods (rainstorm) and earthquakes. 2. Landslide losses are increasing in Taiwan as development expands under pressures of increasing populations. The resulting invasion of developments into hazardous areas, expansion of transportation infrastructure and deforestation of landslide-prone areas may lead to continually increasing landslide losses. Sum up (2) 3. The increase in the cost of landslide hazards can be restrained through better understanding and mapping of the hazards and improved capabilities to mitigate and respond to the hazards. 4. Landslide inventory and susceptibility maps and other data are a critical first step and are prerequisite to producing probabilistic hazard maps and risk assessments, but the susceptibility maps are not yet available in Taiwan. 5. Statistic methods are being developed to landslide susceptibility map. Sum up (3) 6. Earthquakes result in catastrophic disasters every tens years, government and general public are expecting the success in short-term prediction of earthquakes. 7. Earthquake is nothing but a physical phenomenon. The rapid progress in measuring and information handling technologies make the prediction promising. 8. A variety of research and monitoring programs on earthquake precursors are in progress in Taiwan. Sum up (4) 9. International cooperation is necessary for developing earthquake prediction technologies. 10. Before earthquake prediction becoming practical, seismic hazard assessments are the fundamental information for earthquake disaster reduction. Detailed active fault investigation and paleoseismological research are requirement for hazard assessments.
