Earthquakes Reading Smith Ch 6 Tectonic Hazards Tectonic

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 0 -700 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 100 km from epicentre • Scale is logarithmic – M 7 is 10 x motion of M 6

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 • <M 6. 5 use Richter • >M 6. 5 use Moment magnitude

Kobe EQ 1995 • M 6. 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 • 8 km/sec in solid & liquid

S Waves • Sine waves • 4 km/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 22 -25 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 M 6. 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 1970 • • • 30 m high wave of rock & mud Travelled 70 -100 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