Earthquakes - ANSWER They result from the rupture of rocks along a fault.
Energy from an earthquake is released in the form of seismic waves. They are
mapped according to the epicentre; the focus is located directly below the
epicentre. They are measured by seismographs and compared by magnitude
Earthquake magnitude - ANSWER 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 epicentre. Since then, more accurate
methods have been developed and the Richter scale is no longer in use
The moment magnitude scale - ANSWER The scale is determined by:
1. The area ruptured along a fault
2. The amount of movement along the fault
3. The elasticity of the crust at the focus (strength)
Similar to the Richter Scale, it is a logarithmic scale. Example: An M7
earthquake represents 10 times the amount of ground motion as M6 earthquake
Magnitude and frequency of earthquakes - ANSWER Except for very large
earthquakes, the magnitude on the Moment Magnitude Scale is similar to the
Richter Scale. 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.
Modified Mercalli intensity scale - ANSWER A qualitative earthquake
measurement scale based on damage to structures and the affect on people. It is
based on 12 categories.
Earthquake processes - ANSWER Earthquakes are most common at or near
plate 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. The movement (or rupture) starts at the focus
and propagates in all directions, called seismic waves. Thus, faults are
considered seismic sources. Identifying faults is necessary to evaluate the risk of
an earthquake in a given area. Not all faults reach the Earth's surface. Blind
faults are located below the surface
Two types of faults - ANSWER 1. Strike-slip faults
2. Dip-slip faults
Strike-slip faults - ANSWER Displacements are horizontal (ex; San Andreas)
Dip-slip faults - ANSWER Displacements are vertical. They are comprised of
two walls on an incline defined by miners:
1. Footwall (where miners place their feet)
2. Hanging-wall (where miners placed their lanterns)
Three types of dip-slip faults - ANSWER 1. Reverse fault
2. Thrust fault
3. Normal fault
Reverse fault - ANSWER The hanging-wall has moved up relative to the
footwall inclined at an angle steeper than 45 degrees
Thrust fault - ANSWER These are similar to reverse faults except the angle is
45 degrees or less
Normal fault - ANSWER The hanging-wall has moved down relative to the
footwall.
(its hanging, gravity wants to go down)
Fault activity - ANSWER 1. Active; Movement during the past 11 600 years
2. Potentially Active; Movement during the past 2.6 million years
3. Inactive; No movement during the past 2.6 million years
Tectonic creep - ANSWER 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 (movement of a few cm per decade). Along
,these faults, periodic sudden displacements producing minor earthquakes can
also occur
Seismic waves - ANSWER 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
P waves - ANSWER They are also called primary or compressional waves.
They move fast with a push-pull motion and can travel through solids or liquids
S waves - ANSWER They are also called secondary or shear waves. They
move more slowly, in an up-and-down motion and can only travel through solid
Surface waves - ANSWER Seismic waves that form when P and S waves
reach Earth's surface and then move along it. These waves move more slowly
than body waves. Surface waves are responsible for damage near the epicentre
Factors that determine earthquake shaking - ANSWER 1. Magnitude
2. Distance to the epicentre
3. Focal depth
4. Direction of the rupture
5. Local soil and rock types
6. Local engineering and construction practices
Earthquake shaking - ANSWER Seismographs record the arrival of waves to a
recording station. Because P waves travel faster than S waves, they appear first
on a seismogram. Earthquake shaking decreases with distance from epicentre
Distance to the epicentre - ANSWER 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 the epicentre - ANSWER The epicentre is located where the circles
intersect; this process is called triangulation
Focal depth - ANSWER Seismic waves become less intense as they spread
outward toward the surface. Therefore, the greater the focal depth, the less
, intense the shaking at the surface. This reduction of energy is referred to as
attenuation
Direction of rupture - ANSWER 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 - ANSWER The local geology influences the amount
of ground motion. Dense homogenous crust 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 America
Amplification - ANSWER An increase in ground motion during an
earthquake. Has historically enhanced damage in San Francisco area
earthquakes
Alluvial - ANSWER Material deposited by water. P and S waves slow as they
travel through alluvial sand, gravel, clay, soil, etc. As the waves slow, some of
their energy is transferred to surface waves
Shake maps - ANSWER The combination of all of these effects results in
widespread variation of the shaking felt in the vicinity of an earthquake.
Therefore, two earthquakes that have the same magnitude can have very
different impacts
The earthquake cycle - ANSWER A hypothesis that explains successive
earthquakes on a fault. It is based on the idea that strain drops abruptly after an
earthquake and then slowly accumulates until the next earthquake. As stress
continues to increase, the deformed material will eventually rupture
Stage of the earthquake cycle - ANSWER 1. An inactive period
2. A period where strain produces minor earthquakes
3, A period of foreshock prior to a major release of stress (this stage does not
always occur)
4. A period where the mainshock occurs allowing the fault to release built-up
stress
Energy from an earthquake is released in the form of seismic waves. They are
mapped according to the epicentre; the focus is located directly below the
epicentre. They are measured by seismographs and compared by magnitude
Earthquake magnitude - ANSWER 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 epicentre. Since then, more accurate
methods have been developed and the Richter scale is no longer in use
The moment magnitude scale - ANSWER The scale is determined by:
1. The area ruptured along a fault
2. The amount of movement along the fault
3. The elasticity of the crust at the focus (strength)
Similar to the Richter Scale, it is a logarithmic scale. Example: An M7
earthquake represents 10 times the amount of ground motion as M6 earthquake
Magnitude and frequency of earthquakes - ANSWER Except for very large
earthquakes, the magnitude on the Moment Magnitude Scale is similar to the
Richter Scale. 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.
Modified Mercalli intensity scale - ANSWER A qualitative earthquake
measurement scale based on damage to structures and the affect on people. It is
based on 12 categories.
Earthquake processes - ANSWER Earthquakes are most common at or near
plate 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. The movement (or rupture) starts at the focus
and propagates in all directions, called seismic waves. Thus, faults are
considered seismic sources. Identifying faults is necessary to evaluate the risk of
an earthquake in a given area. Not all faults reach the Earth's surface. Blind
faults are located below the surface
Two types of faults - ANSWER 1. Strike-slip faults
2. Dip-slip faults
Strike-slip faults - ANSWER Displacements are horizontal (ex; San Andreas)
Dip-slip faults - ANSWER Displacements are vertical. They are comprised of
two walls on an incline defined by miners:
1. Footwall (where miners place their feet)
2. Hanging-wall (where miners placed their lanterns)
Three types of dip-slip faults - ANSWER 1. Reverse fault
2. Thrust fault
3. Normal fault
Reverse fault - ANSWER The hanging-wall has moved up relative to the
footwall inclined at an angle steeper than 45 degrees
Thrust fault - ANSWER These are similar to reverse faults except the angle is
45 degrees or less
Normal fault - ANSWER The hanging-wall has moved down relative to the
footwall.
(its hanging, gravity wants to go down)
Fault activity - ANSWER 1. Active; Movement during the past 11 600 years
2. Potentially Active; Movement during the past 2.6 million years
3. Inactive; No movement during the past 2.6 million years
Tectonic creep - ANSWER 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 (movement of a few cm per decade). Along
,these faults, periodic sudden displacements producing minor earthquakes can
also occur
Seismic waves - ANSWER 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
P waves - ANSWER They are also called primary or compressional waves.
They move fast with a push-pull motion and can travel through solids or liquids
S waves - ANSWER They are also called secondary or shear waves. They
move more slowly, in an up-and-down motion and can only travel through solid
Surface waves - ANSWER Seismic waves that form when P and S waves
reach Earth's surface and then move along it. These waves move more slowly
than body waves. Surface waves are responsible for damage near the epicentre
Factors that determine earthquake shaking - ANSWER 1. Magnitude
2. Distance to the epicentre
3. Focal depth
4. Direction of the rupture
5. Local soil and rock types
6. Local engineering and construction practices
Earthquake shaking - ANSWER Seismographs record the arrival of waves to a
recording station. Because P waves travel faster than S waves, they appear first
on a seismogram. Earthquake shaking decreases with distance from epicentre
Distance to the epicentre - ANSWER 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 the epicentre - ANSWER The epicentre is located where the circles
intersect; this process is called triangulation
Focal depth - ANSWER Seismic waves become less intense as they spread
outward toward the surface. Therefore, the greater the focal depth, the less
, intense the shaking at the surface. This reduction of energy is referred to as
attenuation
Direction of rupture - ANSWER 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 - ANSWER The local geology influences the amount
of ground motion. Dense homogenous crust 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 America
Amplification - ANSWER An increase in ground motion during an
earthquake. Has historically enhanced damage in San Francisco area
earthquakes
Alluvial - ANSWER Material deposited by water. P and S waves slow as they
travel through alluvial sand, gravel, clay, soil, etc. As the waves slow, some of
their energy is transferred to surface waves
Shake maps - ANSWER The combination of all of these effects results in
widespread variation of the shaking felt in the vicinity of an earthquake.
Therefore, two earthquakes that have the same magnitude can have very
different impacts
The earthquake cycle - ANSWER A hypothesis that explains successive
earthquakes on a fault. It is based on the idea that strain drops abruptly after an
earthquake and then slowly accumulates until the next earthquake. As stress
continues to increase, the deformed material will eventually rupture
Stage of the earthquake cycle - ANSWER 1. An inactive period
2. A period where strain produces minor earthquakes
3, A period of foreshock prior to a major release of stress (this stage does not
always occur)
4. A period where the mainshock occurs allowing the fault to release built-up
stress
