Lecture 6 - Earthquakes Flashcards
What do earthquakes result from?
they result from the rupture of rocks along a fault
energy from an earthquake is released in the form of seismic waves
Bonus:
mapped according to the epicenter; the focus is located directly below the epicenter
measured by seismographs and compared by the magnitude
What is the magnitude of an earthquake expressed in?
expressed as a number to one decimal place
Earthquakes are measured using?
The moment magnitude scale
Earthquakes are measured using?
The moment magnitude scale
Earthquakes are measured using?
The moment magnitude scale
The moment magnitude scale is determined by?
- The area ruptured along a fault
- The amount of movement along the fault
- the elasticity of the crust at the focus
- its a logarithmic scale
What is the modified Mercalli Intensity Scale?
- It is a qualitative scale based on damage to structures and the effect on people
- based on 12 categories
What is the process of an earthquake?
- most common at or near plate boundaries
- friction along plate boundaries exerts a force (stress) on the rocks
- when the stress exceeds the strength of the rocks, there is a sudden movement along a fault
- the movement starts at the focus and propagates in all directions, called seismic waves (faults are considered seismic sources)
Two types of faults
Strike - slip faults - displacements are horizontal (e.g. san andreas fault)
Dip-slip faults - displacements are vertical
Three types of dip-slip faults
- Reverse faults
- thrust faults
- normal faults
They are compromised of two walls on an incline defined by what miners?
- Footwall (where miners placed their feet)
- Hanging Wall (where miners placed their lanterns)
Reverse fault
The hanging wall has moved up relative to the footwall incline art an angle steeper than 45
Thrust fault
these are like reverse faults except the angle is at 45 degrees or less
In terms of activity, what three categories can faults fall into
- Active: Movement during the past 11, 600 years
Potentially active: Movement during the past 2.6 million years
-Inactive: No movement during the past 2.6 million years
What is tectonic creep?
The slow movement of rock or sediment along a fracture caused by stress
(also referred to as fault creep)
What are seismic waves
- Also called secondary or shear waves
- they move more slowly in an up and down motion and can only travel through solids
What are surface waves
Seismic waves that form when P and S waves reach earths surface and they move along it
- responsible for damage near the epicentre
Body waves include?
P waves
S waves
What is P waves
- they are also called primary and compressional waves
- they move fast with a push pull motion and can travel through solids or liquids
Which waves appear first on a seismograph and why?
P waves appear first because they travel faster than S wave
Factors that determine the shaking people experience during an earthquake include?
- Magnitude
- Distance to the epicentre
- Focal depth
- direction of rupture
- Focal soil and rock types
- Local engineering and construction practices
Where is the epicentre located?
The epicentre is located where the circles intersect. This process is called triangulation
True or false? The greater the focal depth, the less intense the shaking at the surface?
true
Reduction of energy in seismic waves is referred to as?
Attenuation
What is directivity
Directivity is the process in which earthquake energy is focused along the direction of rupture
- it contributes to increased shaling
Seismic energy slows down in areas with what type of crusts?
Heterogenous, folded and faulted crusts
Seismic energy transmits quicker in areas with which type of crust?
Dense homogenous crusts
What is amplification
An increase in ground motion during an earthquake
- P and S waves slow as they travel through alluvial sand, gravel, clay, soil, etc.
What is the earthquake cycle
- A hypothesis that explains successive earthquakes on a fault
- 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
What are the stages of an earthquake cycle?
- An inactive period
- Period where stain produces minor earthquakes
- a period of foreshocks prior to a major release of stress (this stage does not always occur)
- a period where the main-shock occurs allowing the fault to release built up stress
- a period of aftershocks with epicentres in the same general area as the mainshock
Formula for calculating the number of aftershocks on a given day
aftershocks on first day after / given day
What plate boundaries do most earthquakes occur in?
Pacific ring of fire, himalaya mountains, middle east
What North American cities are at high risk of earthquakes
Anchorage, Vancouver, Victoria, Seattle, Portland, San Fransisco, LA, Mexico city
What are Plate boundary earthquakes?
Earthquakes the occur on faults separating lithospheric plates
Name the tree types of plate boundary earthquakes
- strike - slip earthquake
- thrust earthquake
- normal fault earthquake
What are strike slip earthquakes?
- Earthquakes occur along transform faults where plates slide horizontally past one another
- common in cali (san andreas fault)
- most known for loma prieta earthquake
Explain thrust earthquakes?
Occur on faults that separate converging plates
- also called subduction earthquakes ( the strongest earthquakes on earth)
- common off the coast of B.c, washington and oregon
Define Normal Fault earthquakes
Occurs on faults associated with divergent plate boundaries
- Usually smaller than M6 and under oceans
What are intraplate earthquakes?
An earthquake on a fault in the interior of a continent, far from a plate boundary
Which earthquakes are bigger Plate boundary earthquakes or intraplate earthquakes?
Plate Boundary
What are primary effects of an earthquake?
Ground shaking, surface rupture
List the secondary effects of an earthquake
Liquefaction, land-level change, landslides, fire, tsunamis
A displacement along faults which causes cracks in the surface is referred to as?
Ground Rupture
During strong earthquakes, ______ can be produced that extend for hundreds of KM
Fault scarps
Ground rupture can uproot trees, collapse buildings and destroy bridges, tunnels and pipelines, true or false?
True
Liquefaction is the?
the transformation of water-saturated sediment from solid to liquid
When does liquefaction occur
During strong earthquakes, when water pressure becomes high enough to suspend particles of sediment within the soil.
(watery sand and silt may flow upward along fractures in the overlying solid material, this effect can cause extensive damage)
Landslides are?
Ground motion produced by an earthquake that causes rock and sediments to move downslope
examples of Natural Service Functions of Earthquakes
- faults provide pathways for downward flow of surface water
- new minerals can be found
scenic landscapes can develop in fault zones
How can human activity cause earthquakes
- the weight from water reservoirs produced by dams can create new faults
- injecting liquid waste deep in the earth can increase pressure and cause slippage along fractures
- testing nuclear weapons leads to explosions that may increase strain
What are the 5 goals found in earthquake hazard reduction programs
- improve national seismograph networks
- develop awareness of earthquake sources
- determine earthquake potential
- predict effects of earthquake on buildings
- communicate research to educate the public
Name different precursors to earthquakes?
- the pattern and frequency of earthquakes (based on foreshocks and microearthquakes)
- land level change (UPLIFT OR SUBSIDENCE MAY PRECEDE EARTHQUAKES, gps stations can recognize small changes in elevation)
- seismic gaps along faults ( areas along a fault that have not seen recent earthquakes may be more likely to experience one
- physical and chemical changes (changes in groundwater levels and chemistry may occur if rocks expand prior to an earthquake)
Steps to minimzing seismic risk
- critical facilities should be located as safely as possible
- buildings must be designed to withstand vibrations
- educate
- earthquake insurance
