Chapter 2
Earthquakes
2.1 Introduction
Earthquakes result from the rupture of rocks along a fault
Fault a fracture in the earth’s crust
Seismic Waves the energy formed when rocks on opposite sides of the fault move
Earthquake Magnitude
Epicentre the point on the surface directly above the fault rupture
Richter Scale a decimal number on a scale that represents the size or magnitude of a
quake
See Figure 2.3 on pg 33
Today, the most common unit to measure quakes is Moment Magnitude (Mw), which is
determined:
o From an estimate of the area that ruptured along a fault plane during the quake
o The amount of movement or slippage along the fault
o The rigidity of the rocks near the focus
Magnitude and Frequency of Earthquakes
o See Table 2.3 on page 34
Earthquake Intensity
The intensity depends on magnitude, distance from epicenter, and the nature of the ground
at the site
Modified Mercalli Intensity Scale
o Measures the degree to which an earthquake affects people, property, and the
ground
o Usually shown on maps
o Based on newspaper articles, reports of damage assessment teams, and
questionnaires to residents
2.2 Earthquake Processes
Process of Faulting
Compared to sliding two rough boards past each other two lithospheric plates move past
each other
This rupture produces vibrational energy called seismic waves
Fault Types
o Strike-Slip Faults: displacements are mainly horizontal
o Dip-Slip Faults: displacements are mainly vertical
Classified as a reverse fault or a normal fault (depending on how the earth’s
material moves)
Can also be called thrust faults, which are mostly located in deep-sea
trenches, where they are low-angle reverse faults
o See Figure 2.12 on pg 40 Tectonic Creep
o Gradual movement along a fault without accompanying felt earthquakes
o Can slowly damage roads, buildings, etc
Seismic Waves
o P Waves compressional or primary waves
Faster than S waves
Can travel through solids, liquids, or gases.
o S Waves shear or secondary waves
Travel through solids
Produce back-and-forth motion
o Surface Waves
Form when P and S waves reach the surface
o See Figure 2.15 on pg 42
2.3 Earthquake Shaking
Important factors that determine the shaking people experience:
1. Earthquake’s magnitude
2. A person’s distance from the focus
3. Local soil and rock conditions
Depth of Focus
Seismic waves lose some energy before reaching the surface (AKA: attenuation)
The deeper the source, the less intense the shaking
Local Geological Conditions
Dense granitic and metamorphic rocks of the Canadian Shield transmit energy efficiently
Heterogeneous, folded, and faulted crust transmit energy rapidly away from epicenters
Seismic Waves move slower through unconsolidated sediment and material with high water
content
Material amplification increases the amount of ground motion experienced in an
earthquake
Local geologic structures can also amplify shaking
o Example: synclines and fault-bounded sedimentary basins (See figure 2.2 on pg 46)
2.4 The Earthquake Cycle
Elastic strain drops abruptly after an earthquake and then slowly accumulates before the
next event
o See figure 2.25 on pg 49
Elastic strain temporary deformation
If the strain continues, deformed material eventually ruptures
Stages
1. Long period of Inactivity along fault segment
2. Accumulated elastic strain produces small earthquakes 3. Foreshocks (small to moderate sized ear
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