Chapter 2 – Earthquakes
• Large differences in the number of casualties and damage from earthquakes
o Depth of the shifting tectonic plates relative to the surface
o Obedience of zoning restrictions (creating of zoning restrictions)
o How and where structures are built
2.1 – Introduction to Earthquakes
• Earthquakes result from the rupture of rocks along a fault (a fracture in the Earth’s
o Rocks on opposite sides of the fault move suddenly and energy is released
in the form of seismic waves.
o Epicentre – point on the surface of Earth directly above the fault rupture
o Focus (Hypocentre) – location of the initial rupture along the fault,
directly below the epicenter
• Seismologists – scientists who study earthquakes
• Intensity – measure of the effects of a quake on people and structures
• Seismograph – device used to record earthquake ground motions
• Kyoo Wadati devised the first quantitative magnitude scale; 1935 further
developed by Charles F. Richter (Richter Scale).
• Bodywave scale – based on the strength of a pwave that travels through earth.
Used to measure deep earthquakes.
• Surfacewave scale – based on earthquake waves that travel along the Earth’s
• Moment Magnitude – Estimate of the area that ruptured along a fault plane during
an earthquake, the amount of movement/slippage along the fault, and the rigidity
of the rocks near the focus. Most commonly used method.
• Modified Mercalli Intensity Scale – measures the degree to which an earthquake
affects people, property and the ground.
o 12 categories on this scale – assigned Roman Numerals (Refer to page
o Primary Damage – comes from injuries and deaths caused by failed
buildings, direct damage that is physical or mechanical. Many deaths
result in the first 12 hours from being crushed by falling structures.
o Second/Tertiary Damage – activity that can be triggered following the
earthquake contributing to further damage includes; landslides, tsunami,
fire, flood, exposure, disease and stress.
• Earthquake intensities are commonly shown on maps Conventional Modified
Mercalli Intensity Maps. (see page 36). o Use of instrumental intensity (direct measurements of ground motion
during a quake from seismograph stations) to produce shake maps, which
shows both perceived shaking and potential damage.
2.2 – Earthquake Processes
Process of Faulting (Fault Rupture)
• Friction between lithospheric plates moving past each other are slowed by friction
along their boundaries – this braking action exerts force on rocks near the plate
• Rocks undergo strain/deformation – when stress on rocks exceeds breaking point
(referred to as strength) – the rocks suddenly move along a fault.
• Rupture starts at the focus and propagates up, down, and laterally along the fault
plan during the earthquake.
• Rupture produces vibrational energy – seismic waves. Faults are therefore seismic
• Strikeslip faults – a fault that displaces rocks laterally (horizontally) with rarely
any vertical movement.
• Fault below the feet is the Footwall and the fault above the heads is the hanging
• Dipslip faults – a fault that displaces rocks vertically. There are three types. (see
o Reverse Fault – The hangerwall has moved up relative to the footwall
along a plane inclined at an angle steeper than 45 degrees.
o Thrust Fault – Similar to reverse, except the angle of the fault plan is 45
degress or less.
o Normal Fault – dip slip fault in which the hanging wall has moved
downward relative to the footwall
• Fault is considered inactive if it has not moved greater than 2.6 million years
• Fault considered active if it has moved during the past 11 600 years
• Fault is considered potentially active if it has moved during the past 2.6 million
• Tectonic Creep (Fault Creep) – Gradual movement along a fault without
accompanying felt earthquakes.
Seismic Waves (page 41 and 42)
• Body Wave – travels outward form the focus of an earthquake through the interior
of the earth. They include Pwaves and Swaves. o P Waves (Compressional or primary) – Travel through any type of
material, velocity much higher in solids than liquids. They may produce
sound when reaching the Earth’s surface. Fastest wave. Travels from the
hypocenter of an earthquake by compressing and extending rock and
fluids along its path.
o S Waves (Shear or secondary) – Travel only through solid materials.
Travels in a snakelike fashion from the hypocenter.
• Surface Wave – travels along the Earth’s surface and move along it, close to the
epicenter where it causes the most damage. Include LoveWaves and Rayleigh
o Lovewave – causes horizontal shaking that is especially damaging to the
foundation. Characterized by a transverse snakelike form.
o Rayleighwave – travels with an elliptical motion at earth’s surface.
2.3 – Earthquake Shaking
• Four factors determining shaking magnitude
1. Earthquake magnitude
2. Distance to the epicenter and focal depth
• Closer the epicenter greater shaking
• Greater focal depth – less intense shaking
3. Direction of the rupture
4. Local soil and rock conditions (intervening conditions)
• Amplification – as P and S waves slow, some of their forward directed
energy is transferred to surface waves – increasing ground motion.
• Geology, surface material, groundwater, vegetation, slope, topography.
2.4 – The Earthquake Cycle
• Earthquake Cycle – explanation of successive earthquakes; drop in elastic
strain after an earthquake and the gradual accumulation of strain leading to
the next quake.
o Strain – deformation resulting from stress
o Elastic Strain – temporary deformation resulting from stress
• Stages of the Earthquake Cycle
1. Long period of inactivity along a fault segment
2. Accumulated elastic strain producing small earthquakes
3. Foreshocks (precursors) occur hours or days before a large earthquake.
4. Mainshock. Followed by potential aftershock between a couple
minutes to a year or more after the mainshock.
2.5 – Geographic Regions at Risk from Earthquakes
• High risk earthquake areas;
o Pacific Ring of Fire – high risk of earthquakes for countries, which border
the Pacific Ocean.
o Areas surrounding the Himalayan mountain ranges and some Middle
Eastern areas. Plate Boundary Earthquakes
• Earthquakes, which occur on faults separating lithospheric plates. Three types –
Strike slip, thrust, normal.
• StrikeSlip – Horizontal (lateral) movement as plates shift passed one another and
grind against each other. Common on transform plate boundaries (plates that shift
laterally past each other).
• Thrust Earthquakes (Subduction earthquakes) – Plate slides underneath another
plate. Largest on Earth. Displace the seafloor upward and laterally over –
ultimately leading to a Tsunami. Common with convergent plate boundaries
(plate that slides underneath another or subducts).
• Normal Fault – Common at divergent plate boundaries (plates that pull away
from each other and are filled by magma). Mostly located in oceans.
Factors Contributing to Vulnerability During Earthquakes
Construction material: quality, availability and cost
• Building design: including key items in structural integrity such as doorways,
windows, roof lines and layout of rooms
• Conditions that affect secondary and nonstructural damage: notably water mains,
fuel, fireplaces and chimneys, furnishings, ornamentation, storage of loose items
• Upkeep and renovation (or the absence of such): since over time structural
strength tends to decline through deterioration of all of the above items
• Site and situation: especially the stability of foundations and proximity to slopes
and other structures that may fail
• Intraplate Earthquakes – Occur ‘within’ lithospheric plates rather than at their
boundaries. They are far more dangerous than boundary earthquakes.
• Recurrence Interval – The time between large earthquakes
2.6 – Effects of Earthquakes and Linkages With Other Natural
1. Shaking and Ground Rupture
a. Resonance – Frequency of the shaking matches the natural vibrational
frequency of the building
b. Fault Scarp – ground rupture commonly producing a low cliff that extends
for hundreds of meters or kilometers along a fault.
2. Liquefaction – Changing of water saturated loose sediment from a solid