Lecture 16 // Earthquakes: Seismic Waves & EQ Scales

Question 1

Earthquakes

Answer 1

• Earthquakes occur when there is sudden release of strain along a fault, including plate boundaries.
• The sudden release of energy stored in deformed crust as strain is released, generating seismic waves.
• The overriding plate suddenly moves trench-ward while the subducting slab slips in the opposite direction, allowing the deformed (flexed) crust to subside.

Question 2

Elastic Rebound Theory: strain buildup

Answer 2

Long-term stress results in strain buildup; the crust is deformed (bent) and incipient fractures form.

Question 3

Elastic Rebound Theory: elastic limit / rupture

Answer 3

Eventually the crust surpasses its elastic limit – the max. amount of deformation that can be accommodated. Rupture occurs generating seismic waves as crust is displaced along fractures/faults.

Question 4

Elastic Rebound Theory: rebounds

Answer 4

With the build-up of strain released, the crust rebounds. That is, it returns to its original undeformed (non bent) state.

Question 5

Mega-thrust EQs.

Answer 5

The very largest earthquakes occur at subduction zones when strain releases along hundreds of kilometres of plate boundary.

Question 6

Focus

Answer 6

The point within the Earth where displacement first begins.
• Shallow focus (less than 50km depth)
• Intermediate focus (50 - 300 km depth)
• Deep-focus (300 - ~700 km depth)

Question 7

Epicentre

Answer 7

The point on the Earth’s surface directly above the focus.

Question 8

Seismic waves

Answer 8

Energy that is released from the deformed rock as displacement occurs. This energy radiates in all directions creating body waves.

Question 9

Body waves

Answer 9

Seismic waves emitted from the focus and radiate outwards in all directions are termed body waves.

Question 10

Surface

waves

Answer 10

When body waves reach the Earth’s surface they are converted into surface waves that travel along the Earth’s surface. It is the motion of the surface waves that tend to cause the most damage.

(referred to as L waves due to their long periods (time between wave crests). L waves are the slowest to arrive.

Question 11

P waves - primary waves

Answer 11

• P waves compress and dilate the rock as they pass (change the volume of the rock).
• P waves travel the fastest.

Question 12

S waves - secondary waves

Answer 12

• S waves shear the rock as they pass (change the shape of the rock).
• S waves travel slower than P waves.

Question 13

Rayleigh waves

Answer 13

Rayleigh waves move the ground up and down and forward to backward (rolling motion).

Question 14

Love waves

Answer 14

Love waves move the ground from side to side (shearing sideways).

Question 15

Detecting earthquake

Answer 15

Seismic waves are detected worldwide by a network of seismographs (stations). Seismometers (machines) record the shaking and this information can be displayed as a seismogram.

Question 16

Magnitude

Answer 16

Two common scales: Richter Scale & Moment Magnitude Scale

A quantitative measure that reflects the amount of energy released during rupture.
• There is no upper limit to the magnitude scale
• The largest EQ on record is the 1960 Great Chilean EQ (9.6).
• Logarithmic scale:
o 10 times the amplification of shaking for each whole
number increase on the scale.
o 31.4 times more energy is released for each whole number
increase on the scale.

 An M-6 EQ releases 31.4 times more energy than an M-5.
 An M-7 EQ releases (31.4)2 more energy than an M-5. That is 986 times more energy is released.

Question 17

Richter Scale (ML)

Answer 17

The commonly referred to Richter Scale is the Local Magnitude Scale (ML) is NOT used for all earthquakes. This scale was developed based on EQ data from the San Andreas Fault.
• Only used for small to ‘moderate’ EQs (magnitude 8 and less).
o transform-boundary EQs
o small- to moderate- subduction zone EQs

Question 18

Moment Magnitude Scale (MW)

Answer 18

The Moment Magnitude Scale (MW) was developed in the 1970s uses field data instead of seismic wave data. This scale is used for mega-thrust EQs. The subscript stands for mechanical work done when the subduction zone ruptured. Keiiti Aki of the Massachusetts Institute of Technology developed the Moment Magnitude Scale during the 1970s.

Field data required:
• area of rupture
• vertical displacement

Question 19

Frequency of Occurrence of Earthquakes since 1900

Answer 19

• On average, one great EQ occurs each year, including mega-thrust earthquakes.
• The most damaging EQs are great or major, and shallow focus.

E.g.: 2004 Sumatra EQ MW 9.2
• rupture length 1200 km
• rupture width 60 km
• vertical displacement 6m

E.g.: 2010 Port-au-Prince Haiti EQ ML 7.0
• Strike-slip, 13 km focal depth
• 200,000+ deaths (building collapse)

Question 20

Intensity

Answer 20

A qualitative measure that reflects the perception of shaking by people, and/or the amount of damage that occurred due to shaking (note: tsunami damage not included).
• Scale used is the Modified Mercalli Intensity Scale.

Earthquake intensity varies across a region and depends on numerous factors/variables:
– Magnitude (amount of energy released at the focus)
– Distance from the epicenter
– Quality of building construction (to seismic code?)
– Type of ground built upon (bedrock vs unconsolidated sediments)