Lecture 4 Earthquakes Part 2

Earthquakes Part 2

What are seismic belts?

Zones where earthquakes happen fairly frequently.

Where are earthquakes most commonly found?

At plate boundaries

• Divergent

• Convergent

• Transform

Can earthquakes occur away from plate boundaries?

Yes

→ rift zones

→ intraplate earthquakes

What are intraplate earthquakes?

Earthquakes occuring within a tectonic plate, away from it’s boundaries.

How do human activities induce earthquakes?

1. Weight from reservoirs (withdrawal)

2. Oil and gas drilling that extracts salty groundwater

3. Disposal wells injecting waste water in the ground.

→ human activities changing water pressure beneath surface.

→ stable water pressure underground keeps rock layers intact

→ M>3.0

T/F All earthquakes occur at plate boundaries,

False

What types of faults occur at mid-ocean divergent boundaries?

→ normal faults

→ transform faults

What types of earthquakes do faults at mid-ocean divergent boundaries often generate?

shallow focus earthquakes

Why do earthquakes at mid-ocean divergent boundaries generally have little societal impact?

→ occur far from population centers

What type of earthwaukes are common along transform boundaries?

Shallow focus earthquakes.

T/F Populations near continental transform boundaries are unsafe.

True.

What earthquakes are common at convergent boundaries?

Shallow, intermediate, and deep-focus earthquakes.

What generates megathrust (M>8.6) earthquakes?

Shallow focus and large slip areas

What is a Wadati-Benioff zone?

Sloping band of seismicity tracing a subducting lithospheric plate.

→ includes shallow, intermediate, and deep-focus earthquakes.

→ up to 660 km deep

What percent of seismic energy occurs in the interior of continents?

5%.

Where do intraplate earthquakes commonly occur?

Along ancient, pre-existing faults.

→ Usually shallow focus

T/F: Wide open fissures from ground ruptures are uncommon

True

Strike slip faults can ____ buildings, roads, and foundations

Crack

Normal and Reverse Faults can ______ ________ apart infrastructure

vertically tear

When do landslides occur and where are they common?

Soil and rock tumble / flow downslope

→ common in areas with weak substrate and frequent seismic activity

When does liquefaction occur?

When tightly packed wet sand, silt, or clay are shaken into a slurry.

→ Building foundations sink into slurry or buildings tip over.

What are the variables of shaking severity?

• Earthquake Magnitude

• Distance from Focus

• Strength of Substrate

Seismic waves create unique ground motion, what are they?

P-Waves → push ground up and down

S-Waves → shear the ground side to side

L-Waves → make ground move like snake

R-Waves → make ground undulate in waves

Most damaging waves

L and R waves

Resonance

Period at which a building sways.

→ increases a building’s sway amplitude by adding energy

Resonance Disaster

When a building’s resonant periods match the ground’s resonant period.

How do earthquakes cause fires?

Ground shaking can topple lamps, stoves, and furnances.

Seismic waves can damage power lines, pipelines, and fuel tanks.

How do firestorms erupt after earthquakes?

• sparks catch on rubble, flames spread due to fuel splills

• firestorm erupts as hot air rises and draws in air that fans the flames.

How do tsunamis and disease happen after earthquakes?

• Tsunamis caused by earthquakes along ocean plate boundaries displacing ocean water, waves move fast as jet plane, 100s km wide and tall plateaus of water

• Disease due to ruptured water and sewer lines post earthquake and cutoff transportation of food and medicine.

2010 Haiti Catastrophe resulted in 250,000 casualties because

• poor construction on sediment substrate → liquefaction

• inadequate emergency services

• poor initial response

Can earthquakes be predicted? Short term? Long term?

No, short term predictions impossible

Yes, predictions over decades / centuries possible using probabilistic risk assessments.

What factors are considered in long-term earthquake risk assessments?

Location

Magnitude

Time

Chance/Probability

What does past earthquake activity suggest about future earthquakes?

Regions with earthquakes today are likely to have future earthquakes

How does earthquake frequency change with magnitude?

• earthquakes of a given magnitude occur 10 times more frequently than earthquakes of 1 magnitude higher.

How many more times Mw 4 earthquakes are there than M_w 5? M_w 6?

10, 100

Paleoseismicity

• geologic record of prehistoric seismicity

• used to determine when previous earthquakes happened

What does paleoseismic data include?

• disrupted rock beds

• buried pond beds disrupted by liquefaction

• sedimentary beds containing sand volcanoes

• asymmetrical tree rings.

What is a seismic gap?

• Portions of an active fault with no major earthquakes for a long time.

Explanantions for seismic gaps?

• perhaps long “sticking” period between slip

• perhaps fault creep

Fault creep

slow slippage instead of sudden seismic pulse.

Seismic gaps may represent areas of greater risk for large magnitude events, unless

a creep is occurring.

What was the magnitude of the 1700 Cascadia megathrust earthquake?

Approximately MW 8.7–9.2.

How large was the area that slipped during the 1700 Cascadia earthquake?

About 1,000 km long and 200 km wide, with a maximum slip of 30 m.

What is the estimated probability of an MW 8.2 or greater Cascadia earthquake occurring in the next 50 years?

Approximately 40%.

Which tectonic plates are involved in the Cascadia subduction zone?

The Juan de Fuca Plate subducts beneath the North American Plate.

How is the Pacific Northwest preparing for future Cascadia earthquakes?

By creating hazard maps, updating building codes, and retrofitting infrastructure.

Why are buildings vulnerable during earthquakes?

A: Because of shearing, g-forces, and inertia.

: If the ground suddenly moves eastward, which direction does a building shear?

Westward, due to inertia

What happens to a building when the ground suddenly moves upward?

Stress on support columns increases, and the building temporarily weighs more.

What happens when the ground suddenly moves downward?

Inertia temporarily elevates the upper floors, which then land on support columns with increased stress.

How to prevent earthquake casualties on a community scale?

• plan for earthquake emergency

• ensure personnel, equipment, and supplies are ready

• have earthquake drills

How to prevent earthquake casualties on a personal scale?

• secure non structural components to walls

• know how to shut off gas and electricity

• install fire extinguishers

• know how to locate family members

• have earthquake drills

T/F: Megathrust earthquakes typically occur in subduction zones

True

Wadati-Benioff seismic zone is associted with

a. divergent plate boundaries

b. transform plate boundaries

c. convergent plate boundaries

d. continental rifts

c. convergent plate boundaries

Megathrust earthquakes typically occur along

a. Transform plate boundaries

b. divergent plate boundaries

c. mid ocean ridges

d. oceanic spreading centres

e. convergent plate boundaries

e. convergent plate boundaries

Which province in Canada has the largest number of major earthquakes Mw=7.8

a. Ontario

b. Quebec

c. British Columbia

c. British Columbia

Which of the following provinces in Canada has the least hazards in terms of seismic activity?

a. Yukon

b. Nunavut

c. Manitoba

d. British Columbia

e. Alberta

c. Manitoba

T/F: Ontario is seismically less hazardous than Quebec

True

Which of the following cities in Ontario has the highest recorded seismic activity?

a. Windsor

b. London

c. Thunder Bay

d. Ottawa

e. Sault St. Marie

d. Ottawa

Which of the following cities in Canada is prone to impact of megathrust earthquakes?

a. Ottawa

b. Saskatoon

c. Vancouver

d. Winnipeg

e. Halifax

c. Vancouver