17: Earthquake Hazard Flashcards
Where are earthquakes found? Why?
Most earthquakes are found at plate margins, aka, the boundary where two tectonic plates meet on Earth
This is because friction and faulting are most intense.
Over 80% of all earthquakes occur in the Pacific Ring of Fire, which is characterized by subduction zones.
What causes earthquakes?
Along a fault, large lasses of rock are moving past each other pushed by tectonic forces. In doing so, they can become locked due to friction.
- Stress builds up to a critical point (10’s-100’s yrs)
- the accumulated stress > friction
- Brittle failure / rupture along faults (seconds)
- EARTHQUAKE!
What is the focus or hypocenter of an earthquake? What is the epicenter?
Focus:
- Originating point of an earthquake on the fault plane
Epicenter:
- The point on the surface of the earth directly above the focus. Greatest concentration of energy here and further u are, the lower the magnitude.
What are body waves? What can they be categorized as?
What are surface waves? What do they include?
Body waves:
- transmit energy through the earth’s interior in all directions from the earthquake’s focus.
- Can be divided into P-waves (primary) and S-waves (secondary)
Surface waves:
- transit energy along earth’s surface, causing the surface to vibrate.
- These include Love and Rayleigh waves.
What are P-waves (primary)? What do they travel through? How past are they?
P-waves (compressional waves):
- energy is compressional waves that alternate between compression and dilation (the material is pushed parallel to its travel path)
- travel through SOLIDS AND LIQUIDS and into the earth
- 5km/s
What are S-waves (secondary)? What do they travel through? How past are they?
S-waves (shear waves):
- energy is up and down motion that pushes material at right angles (up and down) to its travel path
- travel through SOLIDS ONLY and into the earth
- 2-3km/s
What are Love-waves? What do they travel through? How past are they?
L-waves (Love waves):
- they make the surface of the ground shear sideways, like a moving snake
- They travel through SOLIDS ONLY but only on the surface of the earth
- slightly slower than S waves
What are Rayleigh waves? What do they travel through? How past are they?
Rayleigh waves:
- they produce a rolling motion on the ground (like ripples in a pond, they have a vertical and horizontal component of motion in the direction of wave propagation)
- They travel through SOLIDS AND LIQUIDS but only on the surface of the earth
- slightly slower than Love waves
What waves are more destructive? Why?
Surface waves (Love and Rayleigh) are much more destructive than body waves (Primary and Secondary).
more destructive because:
- Travel slower (take longer to pass)
- produce more ground movement (higher amplitude)
- complex motion
What is a seismometer? Seismograph? Seismogram?
Seismometer:
- the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring
Seismograph:
- instruments that measures the ground motion in 1 direction. To understand 3-D ground motion, they’re usually build in clusters of 3 to record the 3 principal directions of movement
Seismograms:
- The record produced by seismographs
How do you locate the epicenter of an earthquake?
- Use the seismograms to determine the arrival time of the P-wave (first jump)
- Determine the arrival time of the S-wave (second jump)
- Calculate the lag time between the 2 waves
- Plot the time on the y-axis of the time vs distance graph
- Draw a line to the S-P line then draw down from that to determine the distance (radius)
- Draw a circle with determined radius around geophone
- repeat this 2 more times
- Earthquake is in the spot where all 3 circles meet (think middle of a Ven-diagram)
What are the 2 systems we use to measure an earthquake?
The size of an earthquake is the main factor in destructiveness. 2 ways we can estimate it are intensity and magnitude:
intensity:
- subjective measure based on damage and people’s reaction.
- Measured using the Modified Mercalli Intensity Scale
Magnitude:
- quantitative measure of the total amount of energy released at its source using the Richter magnitude scale
- works on a base 10 logarithmic scale that converts the largest recorded seismic wave to a numerical magnitude
(aka, each magnitude increases represents a 10 fold increase in amplitude)
- magnitude 4 can be felt
What are the 4 main factors influencing the intensity of an earthquake?
- total energy released at focus
- distance of focus to the surface
- underlying geology in an area
(solid bedrock will inhibit large damage while thick soils settle, increasing the potential for damage) - Building construction and population density
