Presentation on theme: "Earthquakes Kobe, Japan 1995 – 5000 deaths"— Presentation transcript:
1 Earthquakes Kobe, Japan 1995 – 5000 deaths It is estimated that there are500,000 detectable earthquakesin the world each year.100,000 of those can be felt,and 100 of them cause damage.The world's deadliest recordedearthquake occurred in 1557 incentral China. It struck a regionwhere most people lived in cavescarved from soft rock. Thesedwellings collapsed during theearthquake, killing an estimated830,000 people.In 1976 another deadly earthquakestruck in Tangshan, China, wheremore than 250,000 people werekilled.
5 Seismic risk map for the U.S. Colors on this map show the level of horizontal shaking that have a 1-in-10 chance of being exceeded in a 50-year period. Shaking is expressed as a percentage of g (g is the acceleration of a falling object due to gravity).
6 Turkey, August 17, 1999…in 40 seconds $20 billion in losses – 10% of Gross National Product
7 Listening Team – Turkey, 1999 63,000 damaged buildings, 18,000 deaths, 50,000 injuries,120,000 families without housing
8 Earthquakes happenevery day, every hoursomewhere in the world.
9 95% of seismicityin Hawaii is due tovolcanism –magma movement.Other 5% is due totectonic forceson theseafloor
16 Earthquakes and Plate Margins Circum-Pacific seismic belt – 81% of worlds largest quakesAlpide Belt – 17% of largest quakesMid-Atlantic Ridge
17 Elastic rebound as a cause of earthquakes Slow storage of elastic energyover time (deformation)2. Forces holding rocks together areovercome (displacement by elasticrebound)Foreshocks – build up of strainAftershocks – Continued strain releaseLandforms – scarps, linear featuresdiverted
18 Locating an earthquake Energy is first released at focus, as slippage.This exerts strain along the fault, producing moreslippage elsewhere – this propagates as energy
19 Benioff Zone beneath the Tonga Trench Earthquake focidefine the subductingslab.
20 Surface waves (Long Waves) Two types of wave groups generated –Surface waves (Long Waves)Travel along outer layer of crust at the surface causing ground rolllike a water wave and lateral shifting…travel slowly and generatethe most damageBody waves (two types)Primary waves (P) – compress and pull (dilate) rocks in thedirection of movement, involves changing the volume and shape ofmaterial….solids, liquids and gases resist compression and will spring back.Thus they propagate the waves forward. P waves travel through all types of matterFastest waveSecondary waves (S) – motion is 90 degrees to direction ofpropagation (up and down), involves only changing the shape of transmittingmedia…fluid and gas do not resist shape change hence they will not spring backand will not transmit the wave forward. S waves travel only through solids.Second fastest wave
21 S Wave – solids only, second fastest Seismic body wavesP Wave - all types of matter, fastestS Wave – solids only, second fastest
26 Refraction and reflection of seismic body waves Increased density allows wave to travel faster…causing slow refraction (bending)Refraction also happens suddenly when wave crosses density front.Waves also reflect off density interfaces.Wave frontWave ray
27 Seismic shadow zones - Measuring Earths Interior P-wave refraction createsa shadow zoneS-wave propagationcreates a shadow zoneP waveshadow zoneS waveshadow zoneNetwork of epicenters aroundEarth’s surface defines theinterior zones
29 Changes in the seismic velocity of P and S waves mark discontinuities at 100 km (the low velocity zone – LVZ), 400 km (base of the upper mantle), 670 km (top of the lower mantle), 2900 km (top of the core), and 4800 km (inner core boundary).
31 400 discontinuity - broad depression of the surface (green and blue) under most parts of the Pacific and Indian Oceans. Large elevations in the surface (red and yellow) under continental regions such as Eurasia, North America, Australia, Antarctica and parts of Africa. In general 400 km discontinuity correlates well continents and ocean basins.670 km discontinuity - very different structure from 400 km surface. Notable features: deep depression in western Pacific, Tonga, and South America. Basin depths 25 km from average position of the surface; consistent with effect of subduction in these regions.2900 discontinuity - at core-mantle boundary reveals broad basin under Indian Ocean and eastern Eurasia; western Africa, southern Indian Ocean, Australia, and western and central Pacific have a high topography related to thermally buoyant regions on core surface
33 Richter Magnitude – M = log X/T + Y (correction factor) Energy of seismic wave is a function of both amplitude (X) and duration (T).M = log X/T + Y (correction factor)
34 Table 13.3 – Frequency of Occurrence of Earthquakes since 1900 DescriptorRichter MagnitudeAverage AnnuallyGreat8 and higher1Major18Strong120Moderate800Light6,200 (estimated)Minor49,000 (estimated)Very Minor< 3.0Magnitude 2 - 3: about 1,000 per day Magnitude 1 - 2: about 8,000 per day
36 Modified Mercalli Intensity Scale Table 13.2Modified Mercalli Intensity ScaleI. Not felt except by a very few under especially favorable conditions.II. Felt only by a few persons at rest, especially on upper floors of buildings.III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.