Presentation Transcript

1. Natural Hazards and Disasters Chapter 4 Earthquake Prediction and Mitigation
2. Predicted Earthquake Arrives on Schedule Early afternoon February 4, 1975: officials in Haicheng, China issued warning to expect large earthquake in next two days, asked people to remain outside Prediction based on Increase in small earthquakes Rise in elevation, ground tilting near fault Changes in groundwater levels, magnetic field Strange animal behavior 7:36 pm: magnitude 7.3 earthquake struck 90% of buildings damaged or destroyed 2,014 deaths, 27,500 injuries, out of 3 million people
3. Predicted Earthquake Arrives on Schedule Early 1976: officials in Tangshan, China issued forecast to expect large earthquake later that year On July 26, scientist noticed changes in electrical properties of ground, issued warning of impending earthquake Warning did not reach Tangshan before magnitude 7.6 earthquake struck Tangshan (city of 1 million) was destroyed More than 250,000 people killed 164,000 people injured
4. Predicting Earthquakes Scientists cannotpredict date and time when earthquake will strike, but do understand which regions are likely to experience earthquakes Objective is to provide reasonably reliable warning of impending earthquake http://www.usgs.gov/corecast/details.asp?ID=76
5. Earthquake Precursors Research continues to explore phenomena that warn of imminent earthquake Track fault movement to anticipate when it will break Monitor deformation with GPS (Global Positioning System) Anticipate accumulation of stress necessary to break fault loose at predictable time Stress applied at constant rate does not produce results at constant rate Far more complicated conditions include juxtaposing different rock types with different strengths, changing stress patterns after nearby earthquakes
6. Earthquake Precursors
7. Earthquake Precursors http://www.gim-international.com/issues/articles/id1365-Earthquake_Prediction_New_Findings.html
8. Earthquake Precursors Swarms of minor earthquakes or foreshocks may announce onset of fault slippage Swarms precede 1/3-1/2 of earthquakes overall but only small percentage of large earthquakes Difficult to distinguish from background activity Study of microearthquakes identifies previously unknown faults Impossible to determine in advance if a given small earthquake is foreshock or ordinary small earthquake
9. Earthquake Precursors Change in levels of radon is possible earthquake precursor Rare gas forms as part of uranium’s decay chain Forms no chemical compounds Remains trapped in rock until escapes along fracture Formation of new fractures before large earthquake may allow radon to escape closer to ground surface (and may cause drop in water table) Changes detectable as increase in radioactivity of groundwater wells
10. Earthquake Precursors Radon concentration changes in ground water prior to and after the 1966 Tashkent earthquake in the USSR Radon concentrations prior to the Songan-Pingwu earthquakes in China. The arrows indicate the time and magnitude of the earthquakes Gray 1996. A Review of Two Methods of Predicting Earthquakes. http://tc.engr.wisc.edu/UER/uer96/author3/index.html
11. Earthquake Precursors Vertical movements of bench marks along the west coast of Japan near the June 1964 Niigata earthquake. Changes in level (in centimeters) before and after the earthquake are shown on the right Gray 1996. A Review of Two Methods of Predicting Earthquakes. http://tc.engr.wisc.edu/UER/uer96/author3/index.html Change in ground elevation is possible earthquake precursor In 1975, geologists noticed rise in surface elevation near Palmdale, California Accompanied by drop in groundwater level, increase in background radiation No recent earthquakes on adjacent fault might indicate that earthquake was overdue No earthquake has occurred in time since then
12. Earthquake Precursors Zone of high fluid pressure may localize fault movement Pumping water into ground has inadvertently triggered earthquakes Addition of fluids into area of fault increases pore-fluid pressure and decreases strength of fault until it breaks Suggestion that deliberate injections into fault may trigger smaller earthquakes and relieve stress, may preempt large earthquakes Consequences are too serious to attempt Too many smaller earthquakes would be necessary to relieve stress
13. Earthquake Precursors Combination of factors monitored by UCLA team Increase in frequency of small earthquakes Clustering of small earthquakes in time and space Nearly simultaneous earthquakes over large parts of region Increase in ratio of medium-magnitude earthquakes to small-magnitude earthquakes for region Two successful predictions in 2003 Unsuccessful prediction for 2004 Still being tested
14. Early Warning Systems Large earthquake 100 km or more away could be detected in time to give useful warning Earthquake waves travel 4 km/s, take 25 seconds to travel 100 km Enough time for early warning system to shut down critical facilities
15. Prediction Consequences Short-term forecasts or warnings also raise complex political issues Possible consequences of announcing reliable prediction days in advance: Hysterical public reaction Major physical and economic damage With current state of knowledge, earthquakes are inherently unpredictable in short term
16. Earthquake Probability Can make reasonable reliable forecasts Statement of where and how frequently an event is likely to occur and how large it is likely to be Understand tectonic environment Establish record of past events
17. Forecasting Where Faults Will Move Digging steps into the trench at Tule Pond (Tyson Lagoon), Hayward fault, Fremont, CA. photo by Jennifer Adleman, USGS. Paleoseismology: study of prehistoric fault movements Expose fault by digging trench across it Study offset of sediment layers by fault movement Larger offset correlates to larger earthquake
18. Forecasting Where Faults Will Move Seismic gaps: segments of active fault that lack historic (or recent) earthquakes More likely to be location of next large earthquake 1989 Loma Prieta earthquake filled in earlier identified ‘Southern Santa Cruz Mountains gap’
19. Forecasting Where Faults Will Move Migrating earthquakes: earthquakes occur sequentially from one end of the fault to the other North Anatolian fault in Turkey runs east-west for 900 km (comparable to San Andreas fault) Earthquakes from east to west, from 1939 to 1999
20. Earthquake Frequency Knowledge of past earthquakes makes it possible to estimate recurrence interval for earthquakes of various sizes Can statistically estimate probability that earthquake of a particular size will strike a particular region within a specified time period Some faults exhibit regular patterns of activity, though often not too reliably Parkfield section of San Andreas fault: experienced a moderate earthquake every 22 years, until 38 year interval ended in 2004 Tokyo, Japan experiences strong earthquakes about every 70 years, with most recent expected in 1993 – hasn’t occurred yet
21. Earthquake Frequency North Anatolian fault in Turkey has periods of activity and inactivity Coast of northern California, Oregon, Washington and southern British Columbia experiences major earthquakes at long intervals, with last earthquake 300 years ago Paleoseismology of Reelfoot Fault in Tennessee shows three major earthquake sequences in last 2400 years, with recurrence interval of 500-1000 years
22. Earthquake Frequency Fresh appearance of Wasatch Front and other clues indicate large but infrequent earthquakes through urban areas of Utah
23. Populations at Risk Probability of where and when an earthquake will strike used to construct risk map
24. Populations at Risk Highest-risk U.S. areas include heavily populated coast of California
25. The San Francisco Bay Area Wide zone of San Andreas Fault (SAF) system includes nearly entire San Francisco Bay area SAF began to move 16 million years ago, has caused thousands of earthquakes since then Gutenberg-Richter frequency vs. magnitude relationship indicates that any 100 km long segment of SAF should release energy equivalent to One magnitude 6 earthquake every 8 years One magnitude 7 earthquake every 60 years And one magnitude 8 earthquake every 700 years Many segments overdue
26. The San Francisco Bay Area Hayward Fault splays north on east side of San Francisco Bay through Hayward, Oakland, Berkeley, Richmond Magnitude 7 earthquake on Hayward Fault could kill several thousand people, destroy 57,000 structures Rodgers Creek Fault extends north from Hayward Fault, also through rapidly growing populations Over 1836-1906: magnitude 6-7 earthquakes every 10-15 years Over 1911-1979: no magnitude 6 or higher earthquakes Over 1979-1989: four magnitude 6-7 earthquakes Currently in cluster of strong earthquakes?
27. The San Francisco Bay Area Forecast that before 2032: 21% probability of magnitude 6.7 or larger earthquake on San Andreas Fault 27% probability of magnitude 6.7 or larger earthquake on Hayward-Rodgers Creek Fault 11% probability of magnitude 6.7 or larger earthquake on Calaveras Fault Or in summary 62% probability of magnitude 6.7 or larger earthquake somewhere in San Francisco Bay area
28. The Los Angeles Area San Andreas Fault is 50 km northeast of Los Angeles Many related thrust faults cross Los Angeles basin Moderate earthquakes (like 1994 magnitude 6.7 Northridge earthquake) are likely every 10 years Far too few moderate earthquakes in 1857-2007 to account for observed strain across region Moderate earthquakes may occur in clusters
29. Minimizing Earthquake Damage Primary cause of death and damage is from collapse of buildings and other structures Aftermath includes fire, disease epidemics Load-bearing masonry walls can shake apart and collapse Bridge decks and parking garage floors can be shaken off unanchored supports External walls are loosely attached to framework and can collapse Reinforced concrete can break, leaving steel unenclosed so it can buckle and fail Weak floors, unbraced windows can not resist lateral movements
30. Structural Damage and Retrofitting Shattering windows are major source of injuries – safety glass becoming more common Older buildings have loosely-resting beams – can pull out, collapse Older houses are not bolted down – can be shaken off their foundations Overhanging parapets or external décor can be easily shaken off, fall onto sidewalk, parked cars
31. Structural Damage and Retrofitting Buildings can be constructed to withstand severe earthquake Wood-frame houses are flexible enough to bend without shattering Welded or bolted steel frames can resist collapse Retrofitting old, existing buildings can be much more expensive than building new ones at higher standard
32. Structural Damage and Retrofitting Tall buildings can collapse if upper floors are moving in one direction while ground snaps back in other direction Computer modeling shows whiplash effect on 20-story L.A. building, built according to latest codes Amount of sway depends on frequency of earthquake waves If natural vibration frequency of building matches frequency of ground shaking, resonance can greatly amplify shaking of building Tall buildings most vulnerable to lower frequencies
33. Structural Damage and Retrofitting FEMA issued new steel construction guidelines in 2001 for areas prone to earthquakes Engineers use base isolation to protect buildings from shaking ground Can be done in new construction or as part of retrofitting
34. Earthquake Preparedness Evaluate structural weaknesses in home and retrofit Walls should be anchored to floors and foundation Bolt bookcases and water heaters to walls Secure chimneys and vents with brackets Make water and gas mains flexible Consider purchasing earthquake insurance Most well-built wood-frame houses will not collapse, but may be rendered uninhabitable or worthless Extremely expensive, not part of homeowners insurance Covers only cost of house, not land
35. Earthquake Preparedness Plan what to do when earthquake strikes If indoors, stay there Usually not enough time to get out Longer-lasting earthquakes accompanied by stronger ground motions that would make it very difficult to stay mobile Lie down next to sturdy object, under table or desk, in doorway If you do leave a building, avoid elevators and run for open ground
36. By the Numbers Frequency of Building Vibration Buildings of different heights sway at different frequencies During earthquake, ground vibrates at different frequencies in response to different types of earthquake waves Buildings should avoid shaking at similar frequency as ground Short, rigid building on soft sediment often does well in earthquake Tall, flexible building on bedrock often does well in earthquake
37. Land Use Planning and Building Codes Best defense against deaths, injuries and property damage Should require structures framed in wood, steel or reinforced concrete Should forbid masonry walls of brick, concrete blocks, stone or mud that support roofs Uniform Building Code provides seismic zonation map of U.S. indicating construction standards Enforcement of building codes often issue Modern, strongly enforced construction codes  significant damage but few deaths Poor-quality construction and little enforcement of construction codes  high death tolls
38. Land Use Planning and Building Codes Zoning should strictly limit development along active faults, areas prone to landslides, on soft mud or fill Such dangerous areas could be used as parks, golf courses Many cities grew in dangerous areas before dangers were understood Increasing problem of millions living in hazardous environments
39. Case in Point Earthquake Fills a Seismic Gap: Loma Prieta Earthquake, California, 1989 Just before start of Game Three of World Series between San Francisco Giants and Oakland Athletics Jarring arrival of P wave was followed 10 seconds later by intense shaking of S wave  earthquake about 80 km away, in Santa Cruz mountains 2.25 mile stretch of upper deck of freeway in Oakland collapsed, killing 42 motorists (would have been many more in ordinary rush hour traffic, if not for World Series game) Buildings in Marina District were badly damaged, because located on soft fill
40. Case in Point Earthquake Fills a Seismic Gap: Loma Prieta Earthquake, California, 1989
41. Case in Point Earthquake Fills a Seismic Gap: Loma Prieta Earthquake, California, 1989 62 people killed, 3757 people injured, 12,000 displaced from homes 963 homes destroyed, $6 billion in property damage Offset of fault began at 18 km depth, did not reach surface Working Group on California Earthquake Probabilities had already identified fault segment as one likely to produce earthquake of magnitude 6.5 or higher between 1988-2018
42. Case in Point One in a Series of Migrating Earthquakes: Izmit Earthquake, Turkey, 1999 Slip on North Anatolian Fault, 100 km east of Istanbul, caused magnitude 7.4 earthquake, followed three months later by magnitude 7.1 earthquake Magnitude 6.4 earthquake struck in 2003 Series of six large earthquakes progressed westward on fault between 1939 and 1957 Building codes same as in U.S. but poor enforcement Tax code based on area of first floor, so favors extended second stories Many buildings collapsed in 1999
43. Case in Point A Case of Equal-Interval Earthquakes: Parkfield Earthquakes, California San Andreas Fault at Parkfield, California had generated magnitude 5.5-6.0 earthquakes at ~22 year intervals 1857, 1881, 1901, 1922, 1934, 1966 Prompted forecast of 95% probability of magnitude 6 earthquake at Parkfield between 1985 and 1993 USGS installed variety of expensive instruments Magnitude 6 earthquake finally struck – in 2004 Instrumentation recorded no clear precursor activity
44. Case in Point Devastating Fire Caused by an Earthquake: San Francisco, California, 1906 Foreshock followed by magnitude 7.8 earthquake, with shaking that lasted 45-60 seconds More than 28,000 buildings destroyed Shaking was followed by three days of fire Death toll was officially given as 700 – probably actually more than 3000
45. Case in Point Damage Depends on Building Design: Kobe Earthquake, Japan, 1995 Magnitude 7.2 earthquake caused by horizontal slippage on Nojima fault, similar to SAF Despite earthquake design and construction among best in world, more than 5000 deaths and $355 billion in damage 15 seconds of shaking at focus, reverberated for more than 100 seconds in soft sediments At least 180,000 buildings damaged Bridges, elevated highways, rail lines collapsed
46. Case in Point Collapse of Poorly Constructed Buildings: Kashmir Earthquake, Pakistan, 2005 Magnitude 7.6 earthquake struck western Himalayas 87,000 people killed, mostly by collapse of heavy masonry structures or landslides triggered by earthquake Evidence of shoddy construction practices or low-quality building materials Combination of intense shaking, enormous population, poor-quality building materials and construction Numerous aftershocks hampered rescue efforts
47. Case in Point Building Code Not Enforced: Bhuj Earthquake, India, 2001 Magnitude 7.7 earthquake struck city of Bhuj in western India, just south of collision zone between Indian plate and Asia Earthquake on blind thrust fault More than 30,000 people died, mostly killed by collapsing buildings India’s building codes similar to U.S., but not enforced – merely advisory