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Natural DisastersEarthquakes & Their Damages

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San Francisco - 1906

• M = 7.8; 3,000 killed

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What is an Earthquake?

•Ground shaking usually accompanied by sudden movement on a fault

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What Causes Earthquakes?

• Elastic Rebound Theory – movement on 2 sides of a fault leads to bending

of rock until it snaps to release the bending strain

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Stress vs. Strain

• Stress – forces imposed on a rock

• Strain – change in shape of a rock in response to the imposed tress

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Types of Strain

• Elastic– Rocks revert to their original shape when force is relieved

• Plastic– Rocks permanently change shape

• Brittle– Rocks fracture or break

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Stress vs. Strain

•With increasing stress, a rock deforms elastically, then plastically, before ultimately fracturing

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Where Do Earthquakes Occur?

•Faults – fractures in Earth’s crust where rocks on one side move past those on the other side

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Types of Faults

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Fault Terminology

•Hanging wall

– Fault block above the fault – Fault block below the fault

•Footwall

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Normal Fault

•Hanging wall moves down relative to footwall

• Exhibits lengthening or extension of the crust

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Reverse Fault

•Hanging wall moves up relative to footwall• Exhibits shortening or compression of crust• Have dips > 45° • If dip’s < 45°, then thrust fault

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Strike-Slip Fault

•Horizontal displacement

– Right lateral – As you face the fault, opposite side of plate moves right

• Types of Strike Slip:

– Left lateral – opposite side of plate moves left

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San Andreas Fault

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What Type of Fault is This?

Reverse Fault

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What Type of Fault?

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Normal Fault

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Type of Fault?

Thrust Fault

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Measuring Earthquakes

•Seismograph– An instrument that

measures Earth’s movement

•Seismogram– Records seismic waves

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Locating Earthquakes

•Focus (hypocenter)– Location within the Earth where movement originates

•Epicenter– Point on Earth’s surface directly above the focus

•Energy radiates in all directions from the focus •Energy is in the form of “seismic waves”

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Seismic Waves

•Types of Seismic Waves– P-waves (primary)

• Push-pull motion• Travel fastest• Travel through solids and liquids

– S-waves (secondary)• Side-to-side motion• Slower than p-waves• Travel through solids only

– Surface waves (Love)• Travel on Earth’s surface• Slowest• Causes the most damage

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Step 1 – Locating Epicenter

•Measure the distance between P- and S-waves–This is the time difference in arrival times

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Step 2 - Locating Epicenter

•Plot the time difference on y-axis (time interval)•Trace plot to blue line to determine distance to epicenter

•The greater the difference, the farther the epicenter

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Step 3 - Locating Epicenter

•Measure the distance around each seismic station

•Epicenter may be located anywhere on that line

•Minimum of 3 seismic stations needed to determine epicenter

Triangulation

200 km

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Measuring Earthquakes

•Magnitude - measure of the strength of an earthquake or the amount of energy released

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Determining Magnitude

•Measure amplitude of the strongest wave

•Amplitude is the height on paper

•Plot distance between p- and s-wave

•Plot amplitude

•Connect plots to determine magnitude

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Mercalli Intensity Scale

Intensity – measures two things:1. how strongly people feel the shaking2. The severity of the damage

Higher Roman #’s depict greater intensities

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Intensity Map

•Typically, intensity decreases as you move away from epicenter

•Distribution not perfectly circular from epicenter

– Elongate parallel to fault– Intensities increase over

loose wet muds• i.e. near epicenter

intensity = 8• Oakland intensity = 9

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Loma Prieta, 1989

• October 17th, 5:04 p.m.• San Francisco Giants vs. Oakland A’s • Also called “Earthquake Series”

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Loma Prieta, 1989

• 7.1 M; 63 dead; >13,000 injured, • >1,000 homes destroyed, >23,000

damaged• $5.9 billion damage

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Liquefaction

•Occurs when water-saturated ground is reorganized due to violent shaking

•Sediment collapses, expels water, and causes ground to subside

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Northridge, 1994

• January 17th, 4:30 a.m.• 6.7 M, 67 deaths• Blind thrust – 11 miles

deep

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Damage Control

• Collapsed structures– Reinforcement

Northridge Meadows

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Damage Control

• Collapsed Structures– Reinforce garages &

windows

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Damage Control

• Bridges collapse

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Steel Reinforced Concrete

Steel sheathing to reinforce columns

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Damage Control

• Houses slip off foundations– Anchor foundations

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Damage Control

• Taller Buildings– Anchor together– Build far apart

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Damage Control

• Safeguarding your home