Earthquakes

Question 1

Plate tectonics

Answer 1

movement of a series of plates which comprise the crust of the earth in the upper mantle. Could be between 5-10km thick in the ocean and up to 100 km thick on the continents.

Question 2

Divergent Plates

Answer 2

divergent plates are moving apart from each other forced by magma being pushed up from the mantle through a crack in the earth and the injection of material pushes the two plates apart. Example: Red Sea is narrow as it is a young divergent margin. It will progressively get wider over time.

Question 3

Convergent Plates

Answer 3

instead of two plates being created (like in divergent), they are being (a) consumed or (b) colliding. Example: Juan de Fuca Plate that dives beneath North America from the Pacific along the West Coast, producing earthquakes of great magnitudes. Usually caused by subduction

Question 4

Subduction

Answer 4

Process of subduction is when one plate is diving below another

Oceanic-Continental Convergence: Oceanic plate (composed of iron and magnesium) dives below the continental plate (silica based) because the oceanic rock is denser and therefore heavier

Oceanic-Oceanic Convergence: the cooler plate is denser and therefore dives below the hotter, less dense plate

Continental-Continental Convergence: convergence of two continental plates of the same density doesn’t allow for one to be subducted easily therefore the plates rise and form mountain ranges

Question 5

Transform Plates

Answer 5

here, plates are neither created nor destroyed; they simply slide by one another along the transform boundary (or strike-slip fault). Example: the San Andreas fault that is responsible for the numerous earthquakes in California

Question 6

Which plates cause the most powerful earthquakes?

Answer 6

Convergent are the most powerful earthquakes. Subduction of plates involves a tremendous amount of friction creating highly destructive earthquakes

Question 7

What is the pacific ring of fire?

Answer 7

Where large disruptive earthquakes and volcanic eruptions occur

Most of the large earthquakes in this area occur when an oceanic plate subducts under a continental plate

All of the M9 earthquakes in this area have generated tsunamis

Question 8

Explain Seismic Hazard

Answer 8

Areas of high hazard are usually plate margins, as well as highly populated areas

The risk is higher in developing countries as their population is always increasing

With earthquakes, we always talk in probabilities

Question 9

Types of Faults

Answer 9

Normal faults (vertical): a normal fault is the result of tensional forces (e.g. rifting)

Reverse/thrust faults: reverse and thrust faults are caused by a horizontal compression

Strike-Slip Fault: Two pieces of crust are sliding against each other (associated with transform plate boundaries). Can be either left-lateral (sinistal) or right-lateral (dextral) according to their movement. We always use the upper block as a reference.

Question 10

What is elastic rebound theory?

Answer 10

As rocks on opposite side of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.

Question 11

What is particular about Cascadia?

Answer 11

This area is seismically quiet `because the plates are stuck

Question 12

Richter Magnitudes

Answer 12

Logarithmic scale

Measures the amplitude of the ground shaking

Small earthquakes are more frequent than larger ones

Question 13

Moment Magnitude

Answer 13

Moment magnitude is a measure of 3 things
○ Area of slip
○ Amount of slip
○ Length of fault

Question 14

Mercalli index

Answer 14

Measures the effect of the earthquake on structures such as building

The value of the scale depends on factors such as proximity and depth of the earthquake

Question 15

The San Andreas Fault

Answer 15

Although some people think San Francisco is ‘falling’ into the Pacific Ocean, part of the city is actually already part of the Pacific plate

The San Andreas Fault is a right-lateral strike-slip or transform fault, relative motion to the Pacific plate. Rocks on either side of the fault are different and the older the rocks, the greater the displacement

Question 16

Lessons learned from the 1906 earthquake

Answer 16

Big earthquakes can be followed by decades of seismic quiet

Earthquakes the size of the 1906 event appear to occur every several hundred years

In the short term, SF is most at risk from an event of magnitude 6-7

Question 17

Impacts of Earthquakes

Answer 17

Liquefaction:
o Wet, unsolidated soils and sediments are highly vulnerable
o Under shaking, the ground simply flows
o Landfills and harbour areas are at risk

Tsunamis:
o Tsunamis are ocean waves caused by earthquakes, landslides, etc.
o They can be devastating at great distances from the epicenter

Aftershocks:
o Normally occur after a major earthquake
o There may be many thousands of aftershock events over the space of months or even years
o Although their magnitudes generally decrease with time, aftershocks have potential to cause significant damage to already weakened materials (e.g. rocks, soils, buildings, power and gas lines)

Landslides:
o The ground vibrations and severe shaking associated with an earthquake can induce landslides in mountainous areas

Fires:
o The ground shaking will rupture power and gas lines
o Damage to water mains prevents or hinders fire fighting efforts

Personal loss: human element