Earthquakes learning goals
1. Where do earthquakes happen worldwide?
They happen near plate boundaries and intraplate.
2. How frequently do earthquakes happen?
Earthquakes happen very frequently, small ones are very common and are often un felt while
large ones are rare.
3. Types of tectonic plate boundaries
Divergent play boundaries is where the plates move apart leading to tension from stretching
and this causes smallish earthquakes, Convergent plate boundries, there are 2 kinds; subduction
zone and continental collision. Continental collision is where continental plates collide and
subduction zone is where the ocean subducts under a continental plate or another ocean plate.
Also there are transform plate boundaries which are plates that move past each other resulting
in a shearing motion.
4. Types of earthquakes that occur at these plate boundaries
For convergent plate boundaries the earthquakes can be extensively small or very large.
Subduction zones are where the largest earthquakes occur. For divergent plate boundaries only
smallish earthquakes occur. Transform plate boundaries produce many earthquakes from their
shearing motion they are moderate to large but not as big as convergent margins.
5. Describe how the Earth builds, stores, and releases energy in earthquakes (elastic rebound)
The earth builds up energy as elastic stresses build up as rock deforms slightly overtime, the
energy is stored as the elasticity of the fault strains and then the energy is released when the
elastic stresses exceed what the fault can bear this causes rocks along fault to spring back to
undeformed state which is known as elastic rebound.
6. Understand that stress causes strain, and differences between plastic, elastic, and brittle
Stress causes strain because the stress strains the fault to bend in one way or another, Plastic
deformation is like putty it is where the material strains in response to stress but permanent
also no energy is stored in plastic deformation and so it does not bounce back when the stress is
removed. Elastic deformation has relatively small stress and because of these small stresses it is
not permanent because the materials shape is restored when force is removed due to the
stored energy and this released energy can pass as waves. In Brittle deformation the material
does store elastic energy but at some point the material will break this results in catastrophic
release of energy, Brittle deformation also includes repeated breaking of pre-existing weak
surfaces (faults) 7. Describe how a fault slips in an earthquake and why shaking and damage are not always
greatest at the epicenter
Faults are weak surfaces (weaker than surrounding rock) they break repeatedly and may
accumulate hundreds of km of slip over millions of years. Faulting occurs when the friction along
he boundary line may temporarily slow down the rough lithospheric plates moving past each
other and this braking action exerts forces on the rocks near the plate boundaries and as a result
the rocks undergo strain or deformation and when the stress on them exceeds their breaking
point the rock suddenly moves along a fault which is how a fault slip occurs. Shaking and
damage are not always the greatest at the epicentre because when a rupture begins at the
hypocenter and travels away, the rupture propagates away from the hypocenter at about 2-3
km/sec and shaking is greater in the direction the rupture travels.
8. Describe the different types of seismic waves and how they move through the Earth
The 2 categories of seismic waves are body waves and surface waves. Body waves travel inside
the earth while surface waves travel along boundaries between materials. There are 2 types of
Body waves which are P and S waves. P waves move by compression and extension of the solid
which is similar to a sound wave it is the fastest type of seismic wave and particles move the
same way the wave propagates. S waves move by shearing distortion of the solid, Slower than P
wave, it is about 3.5 km per sec as compared to the P waves 6km per sec. S waves also cannot
pass through fluids while P waves can. Surface waves require an interface to move such as
ground-air, water-air, mantle-liquid outer core. Surface waves are slower than body waves but
cause more damage. The 2 types of surface waves are the Rayleigh wave which is a vertical and
horizontal motion parallel to the wave direction (like an ocean wave) and then there is a Love
wave which has horizontal movement perpendicular to wave travel direction.
9. Understand the principle behind early warning systems, and know how much warning time they
The principle behind early warning system is that through the use of S-P lag times and it would
also be based on the principle that radio waves travel faster than seismic waves. It would work
by having a network from of seismometers which would sense the first motion from a large
earthquake which would then send a warning to the city and critical facilities the warning time
would be from as short as 15 seconds to as long as 1 min
10. Describe how an earthquake is recorded and how we locate the epicenter
An earthquake is recorded by a seismograph which analyzes its S-P lag time to figure out the
distance of the epicenter of the earthquake from the seismograph. The exact epicenter can be
located by using 3 or more seismographs at various locations that are apart from one another to
triangulate where the epicenter is which works by creating a radius around each seismograph
and the point where the 3 radii touch is where the epicentre is located.
11. Predict how local ground conditions will affect the duration and amplitude of shaking Local ground conditions will affect the duration and amplitude of the shaking because if the
local ground conditions are unconsolidated sediment or if the ground has high water content
the seismic waves would slow which would cause some of the P and S Wave`s forward directed
energy to be transferred to surface waves which would amplify the amount of ground motion
because surface waves cause more shaking.
12. Compare and contrast the meanings and uses of magnitude and intensity scales
Magnitude indicates how much energy was released and Intensity is how strong the ground
motion is at the felt location. Magnitude scales indirectly estimate the magnitude they use