Earthquakes

Reading Smith Ch 6

Tectonic Hazards Tectonic: forces of ‘earth-building’ –Volcanoes –Earthquakes Very much connected –plate tectonics –most earthquakes on plate boundaries, fault lines driven by plate movements.

Earthquakes Sudden crustal movements Usually shallow km deep

Earthquakes Slow build-up of strain –driven by mantle convection? Sudden release as crust moves: earthquake Seismic waves radiate outwards

Earthquakes Most damaging earthquakes are shallow –>40 km depth

San Fernando EQ 1971 Richter M 6.4 Depth: 13 km

Tangshan EQ 1976 At shallow depth Killed 250,000 to 750,000

Earthquakes Hypocentre: –underground centre of earthquake Epicentre –the point on the surface above the hypocentre

Earthquake Magnitude Richter Scale Moment Magnitude Mercalli Scale

Richter Scale a.k.a. Local magnitude Energy of seismic waves 100 km from source =logarithm of ground motion in micrometres 100km from epicentre Scale is logarithmic –M7 is 10x motion of M6

Richter Scale >M 5.5 for a major disaster Most Eqs are small Most EQ energy released in the Big Ones

Moment Magnitude Surface area of fault displaced <M6.5 use Richter >M6.5 use Moment magnitude

Kobe EQ 1995 M6.9 Strong shaking on soft soils, landfill Dense urban area nearby –Older homes with heavy clay-tile roofs not earthquake proof

Problems with these scales Richter & Moment Magnitude don’t measure shaking and overall impact, just seismic energy Can only be used if ground movement, epicentre, fault area is known Useless for “historic” quakes

Modified Mercalli Scale MM scale: intensity of damage Roman numerals MM: I = not felt at all MM: XII= widespread destruction

Seismic Waves Vertical stress, short wave period –‘P’ or primary waves –‘S’ or secondary waves –Moment Magnitude measures this Horizontal stress, long wave period –Love Waves –Rayleigh waves –Richter scale /local magnitude measures this

P Waves compression-dilation 8km/sec in solid & liquid

S Waves Sine waves 4km/sec in solid only can be destructive

Love & Rayleigh Waves Horizontal shaking at 90 degrees to wavefront Most damaging to buildings

Other issues Ground acceleration Wave frequency

Mexico City EQ 1985 High rise buildings collapse especially if in storey range Taller and shorter buildings less damaged EQ has just the right resonance to tear certain buildings apart

Small buildings Resonant to short wave periods P&S waves knock them down

Tall buildings Resonant to long wave periods Love & Rayleigh waves knock them down

Effects of Local Topography Soft bouncy sediments amplify wave effects –ash –alluvium –landfill Mexico City EQ 1985, San Salvador EQ 1986: cities on bouncy sediments

Secondary Effects Soil failure Slope failure Tsunamis

Soil Liquifaction Wet soil can liquify Loses load-bearing properties Richmond BC: –If Richter M6.5 EQ, high-rises may fall like dominoes in Fraser delta sediments

Lateral Spread On shallow slope soil moves sideways

Ground Oscillation Soil blocks rotate and surface crumples

Loss of Bearing Strength Buildings sink into soil

Flow failure On steeper slopes, slope collapses Can cause damage over extensive area, long distance

Landslides, Rock & Snow Avalanches EQ triggers slope failure

Huascaran Mountains, Peru m high wave of rock & mud Travelled m/sec buried Yungay & Ranrahirca Debris 10 m deep, Killed 18,000 in 4 minutes

Tsunami a.k.a. “tidal waves” Large shallow submarine EQ triggers shockwave in sea Difficult to spot out at sea Moves fast

Tsunami Wave height & speed increase as wave moves into shallow coastal water Begins with water drawdown Then violent oscillation for hours

Human Vulnerability Many EQ zones well-known but densely inhabited Populations often in coastal zone, valley lands –vulnerable to Tsunamis, debris flows –cities built on wobbly sediments

Human Vulnerability Increased by poor construction techniques –not designed for horizontal shear –failure to avoid vulnerable sites, soil conditions –failure to avoid vulnerable zones

Maharashtra 1993

Timing Deathtoll and damage rises if timing is unfortunate –1923 Tokyo Bay EQ struck at lunchtime, starts fires, 130,000 die –1976 Tangshan EQ struck at night, people trapped indoors

Trigger Earthquakes by crustal loading –build a large dam –storm surge in cyclone conditions 1923 Tokyo Bay EQ

Prediction Easier to predict where an EQ will happen than when –spotting “locked” areas surrounded by zones of movement Forecasting not advanced –West: relies on science Failed to spot Kobe 1995, Northridge 1994 –China: relies on culture (more accurate) Failed to spot Tangshan 1976