Activity 2 Flashcards
Earthquakes
Definition
An Earthquake is shaking caused by the rupture of rocks in the subsurface
Elastic Deformation
Definition
When a rock is subjected to stress, it will experience strain and deform elastically
What happens when the rock can no longer accommodate deformation?
Definition
At some point, though, the rock will rupture (break) along a surface that we refer to as the fault plane
Rupture + fault plane relationship
Definition
- The rupture’s surface area on the fault plane determines the earthquake’s magnitude (size).
- Different types of faults occur in different plate tectonic settings
Epicentre
Definition
point on the ground surface directly above the hypocentre.
The hypocentre (or focus)
Definition
where the rupture occurs
Seismometer
- …device to record earthquakes
- A basic seismometer consists of a frame, a suspended weight, a pen, and a rotating drum (outcome paper looks like a lie detector)
- Almost all seismometers are based on the principle of inertia, where a suspended mass tends to remain still when the ground moves.
Modern seismometers record motion in three directions:
Definition
- Z (up and Down)
- E (east-west motion)
- N (north-south) motion
The first waves recorded by a seismometer are…
- P waves
- …followed by S waves and then the surface waves (Love and Rayleigh)
- Rather than recording seismic waves on a drum, signals are sent electronically to monitoring stations.
Magnitude - quantitative
Measuring Earthquakes
the amount of energy an earthquake releases (quantitative measurement)
Most known magnitude scale
Richter magnitude
Most used magnitude scale
- The Moment Magnitude
- …based on the properties of the earthquake derived from all of the waveforms recorded during the event.
- scale is logarithmic - amount of energy released on each level on the magnitude scale being 32 times greater than the previous and the ground motion amplitude increasing by 10 times
EX: a magnitude 9 earthquake would release 32 times more energy and cause motion 10 times more than that of a magnitude 8 earthquake
Intensity - qualitative
Measuring Earthquakes
a qualitative measurement of the degree of shaking experienced by people and infrastructure at the surface
Variations in intensity:
- DISTANCE: Intensity decreases with increasing distance from the earthquake epicentre.
- COMPOSITION: Intensity can also vary depending on what the ground surface is composed of.
Types of waves & motion caused:
Intensity - qualitative measurment
- Surface waves cause the greatest shaking during an earthquake
- Raleigh waves cause vertical motion, rolling like ocean waves
- Love waves cause horizontal motion. Both are around 10% slower than S waves
- Surface waves cause damage to buildings during an earthquake
Modified Mercalli Intensity Scale (MMI)
Intensity
- It is composed of intensity levels ranging from shaking not felt by most of the population (MMI of I) to catastrophic earthquake experiences (MMI X+)
- The lower ranges of MMI reflect how people experience earthquakes (MMI I to MMI VI)
- MMVII and above reflect structural damage, and we assume that by this level, the entire population would be in a state of panic.
Earthquakes are located by considering…
Locating Earthquakes
the arrival times of P and S waves at seismic stations.
Which wave comes before the other (P/S)?
- P waves arrive first, with S waves arriving second
- With increasing distance away from an Earthquake, the difference in the arrival time between P and S ways will increase.
Circles to compute distance from S and P arrival times
- Using the distances calculated from the P and S arrival times, circles are drawn around each station, with the distance used as the circle’s radius.
- The circles represent the potential points (along the circumference of the circles) relative to each station of the earthquake’s epicentre
- If you have three or more stations, the intersection point of these circles represents the actual location of the epicentre
Vancouver is located on…
Megathrust Vancouver
- the western edge of the North American Tectonic Plate
- A spreading ridge is generating material along the Juan de Fuca Plate that is being slowly subducted below the continent
- This area is called the Cascadia Subduction Zone
What happens when a plate experiences friction?
- Plate frequently “sticks” with pressure slowly building as new material continues to be added at the ridge
- Eventually overcomes, and the plate moves suddenly and catastrophically, generating a major earthquake.
- These Earthquakes are called Megathrust Earthquakes and have a magnitude of 9 or above
How does a megathrust earthquake produce a tsunami?
- A Tsunami is generated as rocks rupture under the ocean floor, creating a “cliff” called a fault scarp that displaces the water above it, causing a bulge at the ocean surface.
- This bulge collapses and forms two tsunami waves moving in opposite directions
