Lecture 7 – Earthquakes
• They result from the rupture of rocks along a fault.
• Energy from an earthquake is released in the form of
• They are mapped according to the epicentre; the focus is
located directly below the epicentre.
• They are measured by seismographs and compared by
• Focus is below group epicenter on the surface
• Energy emitted in all directions from the focus
• The magnitude of an earthquake is expressed as a number to one decimal place.
• This type of measurement was first developed by Richter in 1935
• The Richter Scale was a measure of the strength of a wave at a distance of 100 km from the
• Since then, more accurate methods have been developed and the Richter Scale is no longer in use
The Moment Magnitude Scale
• Today, earthquakes are measured using the Moment Magnitude scale (M).
• The scale is determined by:
o The area ruptured along a fault
o The amount of movement along the fault
o The elasticity (rigidness, how hard rock is in the crust) of the crust at the focus
• Similar to the Richter Scale, it is a logarithmic scale.
o Example: An M7 earthquake represents 10 times the amount of ground motion as an M6
earthquake. M3 to m5 is 100 times the intensity. (EXAM QUESTION)
Magnitude and Frequency of Earthquakes
• Except for very large earthquakes, the magnitude on the Moment Magnitude Scale is similar to the
• The strongest earthquake to ever occur is M9.5 in Chile in 1960. In canada, it is M8.1 in B.C. in 1949
• There are only a few M9+ earthquakes each century.
• The Modified Mercalli Intensity Scale is a qualitative (descriptive) scale based on damage to
structures and the affect on people.
• It is based on 12 categories
• Maps are produced showing the differences in Modified Mercalli intensities over broad areas
The Modified Mercalli Intensity Scale Earthquake Processes
• Earthquakes are most common at or near plate boundaries.
• Quakes can still happen at places in the middle of continents, away from boundaries.
• Motion at plate boundaries is not usually smooth or constant.
• Friction along plate boundaries exerts a force (stress) on the rocks, exerting strain or deformation
• When the stress exceeds the strength of the rocks, there is a sudden movement along a fault.
• There are two basic types of geologic faults distinguished by the direction of the displacement of rock
• Strike-slip faults
o Displacements are horizontal
• Dip-slip faults
o Displacements are vertical
• The San Andreas Fault is the best
example of this type.
• Neither rock sinks or rises relative to
the other one but move in opposite
• There are three types:
o Reverse faults, thrust faults, and normal faults
• They are comprised of two walls on an incline
defined by miners:
o Footwall (where miners place their feet)
o Hanging- wall (where miners placed their
o These are just names for each rock, so we
can say which one rises or which falls
• Reverse fault: hanging wall has moved up relative to
the footwall inclined at an angle steeper than 45
• Thrust Fault: similar to reverse fault except the
angle is 45 degrees or less
• Normal fault: the hanging wall has moved down
relative to the footwall
• In terms of activity, faults can fall into one of three categories:
- Movement during the past 11,600 years
• Potentially Active
- Movement during the past 2.6 million years
- No movement during the past 2.6 million years
• Definition: The slow movement of rock or sediment along a fracture caused by stress
• It is also referred to as fault creep.
• This can damage roads and building foundations (i.e. movement of a few cm per decade). • Along these faults, periodic sudden displacements producing earthquakes can also occur
• Some seismic waves generated by fault rupture travel within the body of the Earth and others travel
along the surface.
• Body waves:
• These include P waves and S waves
o 1. P waves
They are also called primary or compressional waves
They move fast with a push-pull motion and can travel through solids or liquids
P waves are horiztontal
o 2. S waves
They are also called secondary or
They move more slowly, in an up-
and-down motion and can only
travel through solids
• Surface Waves:
o Definition: Seismic waves that form when P
and S waves reach Earth’s surface and
then move along it
o These waves move more slowly than body
o They are responsible for damage near the
• Factors that determine the shaking people experience during an earthquake:
- Distance to epicentre
- Focal depth – how deep beneath
surface did rupture occur
- Direction of rupture – is fault running E/W
- Local soil and rock types – hardness of
- Local engineering and construction
• Seismographs record the arrival of waves to a
• Because P waves travel faster than S
waves, they appear first on a seismogram.
• Earthquake shaking decreases with distance from
Distance to the Epicentre
• The difference between the arrival times of the
first P and S waves at different locations
determine the distance to the epicentre.
• The distance to the epicentre is calculated at 3
different seismic stations
• A circle with radius equal to that distance is
drawn around the station Locating an Earthquake
• The epicentre is located where the circles intersect; this process is called triangulation.
• Seismic waves lose some of their energy before they reach the surface.
• The greater the focal depth, the less intense the shaking at the surface
• This loss of energy is referred to as attenuation
Direction of Rupture
• Earthquake energy is focused in the direction of rupture.
• This is known as directivity and contributes to increased shaking
• Radiated waves are sometimes stronger in one direction along the fault
Local Soil and Rock Types
• The local geology influences the amount of ground motion.
• Dense homogeneous crust (bedrock) can transmit earthquake energy quickly
• Seismic energy slows down in areas with heterogeneous, folded, faulted crust.
• Earthquakes in eastern North America are felt over larger areas than those in western North
• Definition: An increase in ground motion during an earthquake
• P and S waves slow as they travel through alluvial (sediment that has been deposited by a river)
sand, gravel, clay, soil, etc.
• As the waves slow, some of their energy is transferred to surface waves
• Amplification has historically enhanced
damage in San Francisco area earthquakes
• The combination of these effects results in widespread variation of the shaking felt in the vicinity of an
• Therefore, earthquakes that have the same
magnitude may have much different impacts
The Earthquake Cycle
• Definition: A hypothesis that explains
successive earthquakes on a fault
• It is based on the idea that str