Natural Disasters Midterm 2

N.E. pacific plates

North America and Pacific Plates

N.E. pacific faults

Queen Charlotte and Sam Andreas Transform Faults (Right Lateral)

Juan de Fuca plate system is made up of

Juan de Fuca, Explorer, and Gorda plates

What type of margins are involved in the JdF system?

Spreading, Subduction, Transform

Where are the largest and most frequent EQs in Canada

West Coast

JdF and Pacific plate spreading rate

Intermediate

Seismicity of JdF Ridge

Frequent small, shallow, EQs. Occur in swarms

Nootka fault

Fault that separates JdF and Explorer plates

Queen Charlotte Fault

Caused largest recorded EQ (8.1 magnitude) in Canada

Cascadia Subduction Zone (CSZ)

JdF, Gorda, Explorer, subduction under NA plate at 4cm/year. Lacks Trench as shallow angle and high sedimentation

Crustal EQs

Occur in both oceanic and continental plates, from surface to 30km deep, up to magnitude 7.5, smaller in oceanic crust

How many crustal EQs were recoded between 1985-2002?

7800, most too small to feel

Intra-slab EQ

• Watadi-Benioff EQs

• Within subducting plate (up to 100km)

• Most frequent/damaging EQs

Nisqually EQ, 2001

Mag 6.8

~55km (too deep for crust)

Inter-plate EQs

Occur on the shear interface between subducting and over-riding plates

Repeating Sediment Sequence

Peat-Mud-Sand

What does Peat show?

Deposition when marsh was above the tideline

What does sand show?

Drop in coastline,tsunami deposited the sand

What does mud show?

Deposition of mud in marsh below tideline, gradual coastal uplift comes and repeats cycle

Drowned Forests

Areas of dead cedar trees found along west coast of Washington and Oregon

Interpretation of drowned forests

Abrupt subsidence of shorelines drowns trees, dendrochronology dates the drowning to 1700

Inter-seismic period

Elastic deformation builds in over-riding plate between EQs. Toe of plate was dragged down, coast uplift, crustal shortening

Co-seismic period

Motion during EQ, toe jumps up (1-5m) rupturing seabed and initiating tsunami, 1-2m coastal subsidence, 10-20m crustal extension

Geodetic Shortening

Coast moving inland relative to GPS reference

Seismology

Study of EQs

Seismometer

Instruments that detect vibrations in the earth

Seismograph

Instrument that records vibrations detected by seismometer

Wave Properties: Function of position

Time fixed snapshot

• Amplitude (A) - maximum value

• Wavelength (lambda) - distance of one wave cycle

• Velocity (V) - speed a point on a wave moves

Wave properties: function of time

Wave at fixed position

• Period (T) - time of one cycle

• Frequency (1/T) - number of cycles per second

• V = D/T

Body Waves

Go through earths interior

Surface Waves

Go along surface of earth

Types of body waves

Compressional waves and shear waves

Types of Surface Waves

Rayleigh waves and love waves

Compressional Waves

P waves, like a hammer blow, compression, think sound waves, also called primary waves, parallel to direction wave propagates. Can go through all states of matter, fast

Shear waves

S waves, glancing hammer blow, secondary waves, particle motion perpendicular to prop direction, velocity increases with density

What do body waves show?

They reflect and refract at internal boundaries, maps structure

Rayleigh waves

Particle motion is retrograde elliptic, decreases w depth. Long wavelength (Lr), ground roll, only through solids, slow

Love waves

Horizontal back and forth motion perpendicular to propagation. (Lq waves), transverse, building damage, only through solids.

Intensity

Local EQ effects, what people feel, damage done. Depends on magnitude, distance to hypocenter, shaking duration, rock type

Magnitude

Attempt to measure EQ nrg release, logarithmic scale

Types of magnitude scales

Richter, surface wave, body wave (logarithmic)

Moment magnitude (based on measurements of properties at source)

Richter Magnitude (Ml)

Largest wave f=0.5-10Hz

Only used today for small local EQs

Surface Wave Magnitude (Ms)

Rayleigh wave amplitude at f~0.05Hz

Not good for EQs above 50km deep

Body wave magnitude (Mb)

Direct P wave at f=1-10Hz

Saturation

EQ gets too big, increase is missed

Site response

Most fatalities due to shaking, increases are due to soil liquification, amplification, resonance

Soil liquefaction

Intense shaking of water saturated sandy soil increases water pressure causing grains to lose contact and act as a thick liquid. Sinking occurs like quicksand

Amplification

Low velocity, low density layers increase amplitude

Resonance

Fraction of trapped seismic energy that builds up. In phase

In Phase

Waves add constructively

What are the vast majority of earthquake casualties caused by?

Building collapse

Required Safety Level for EQ mitigation

➢No major damage in minor EQ
➢No structural damage in moderate EQ
➢No collapse in largest expected EQ

Building Considerations: Site Selection

➢Avoid slopes, soft soil/fill
➢Don’t have different
materials (different
shaking characteristics)
under one foundation

Building Considerations: Foundation

➢Bolt structure to
foundation to avoid slip in
horizontal shaking,
“walking” off foundation in
vertical shaking
Shifted off foundation
Earthquake slope failure

Building Considerations: Good Materials

➢Wood — very good, light and
flexible
➢Steel — good, strong in tension,
can fail in compression
➢Grass hut — very good (if roof
attached)

Building Considerations: Bad Materials

➢Masonry (brick or stone) —
poor, heavy and weak in
horizontal shaking
➢Concrete — poor
➢Adobe — poor, heavy and weak

Building Considerations: Walls

➢Must be securely attached
(bolts, brackets) to floor
and ceiling/roof
➢Strengthened by adding
plywood or concrete
sheets between columns
(shear wall bracing) or
angled beams between
columns (cross bracing)
➢Reinforce masonry walls
with steel bars

Building Considerations: Base Isolation

➢Devices on ground or within structure to absorb EQ
energy
➢Use wheels, ball bearing, shock absorbers, rubber
doughnuts, etc. to isolate building from shaking

Building Considerations: Chimney

➢Weak point for many houses
➢Must be secured to building
➢Plywood on attic floor around
chimney prevents bricks through
ceiling.

Things to prepare for EQs (personal)

• SAfe spaces

• Drills

• Learn First Aid

• Prepare Emergency Kit (water, food, first aid kit, meds, shelter, toiletries, flashlght, radio, etc etc)

How to prepare home for EQ

• Know how to turn off gas/water/electric

• Put breakables on low shelves

• Secure cupboards closed and secure tall furniture to wall

• Secure anything else that could fall

What to do during EQ: Indoors

● Get under a sturdy desk/table and hang on
● Second best: stand in archway/corner
● Avoid windows, shelves

What to do during EQ: In high rise

● Get under cover
● Avoid windows,
outside walls
● Do not use elevators

What to do during EQ: Driving

➢Pull car to side of road and stop
➢Avoid overpasses/bridges/power lines
➢Remain in car until shaking stops

What to do during EQ: In a crowded public place

● Do not rush for doors
(danger of trampling)

What to do during EQ: Outdoors

➢Get in open area away from tress, buildings,
walls and power lines

Things to do after EQ

• Stay Calm

• Check for fires, gas & water leaks

• Check building for damage

• Listen to radio

• No telephones or toilets

• check supplies

• avoid waterfront

• be prepared for aftershocks

San Andreas Fault

• Right Lateral Transform Fault

• Boundary
  between Pacific
  and North
  American Plates

• 1200 km

Locked Sections

Produce Large EQs

Creeping sections

Release energy overtime by small movements or constant motion

San Andreas Fault Notable EQs

• 1906 San Francisco EQ

• 1989 Loma Prieta EQ

• 1964 Alaska EQ