Unit 2: Earthquakes
Describe the global distribution of earthquakes and how often quakes of various
o Earthquakes tend to happen near plate boundaries
Plate = lithosphere = crust + top of upper mantle. Strong layer, about
100 km thick.
Plates float on the asthenosphere
Oceanic plates: fast moving (cm’s/year), young (less than 200
MY), created at mid-ocean ridges, destroyed at subduction zones
Continental plates: old (buoyant, not subducted), slower
moving (mm to cm)
Relative motion of these plates results in…
Mid-ocean ridge spreading center: tension
Transform fault: shearing , slide past
Collision boundaries/subduction zones: compression
Understand the different types of faulting at different plate boundaries, and which
plate boundaries produce the largest quakes
1. Divergent plate boundaries: Plates move apart due to tension (stretching),
2. Convergent plate boundaries: Due to compressive forces
Continental collision: Continental plates collide, tends to form mountains
Oceanic + continental: oceanic plate is subducted under
continental -> volcanic activity
Oceanic + oceanic: once plate subducted under another
Convergent boundaries cause small to very large quakes.
Subduction zones cause the greatest quakes – located in Japan, Mexico,
3. Transform plate boundaries: Plates move sideways past eachother, leads to
Many moderate to large quakes, but not as
large as convergent margins
San Andreas fault, Queen Charlotte fault
4. Intraplate earthquakes: not at plate boundaries, can
Occur along ancient fault lines/plate
boundaries which have been reactivated
Waves can travel far without getting smaller.
Many found in Ontario and Quebec.
Plate Boundaries near UBC
1. Queen Charlotte fault – north, near Vancouver island
2. Cascadia subduction zone: Juan de Fuca plate and North American plate
Divergent margin formed between Juan de Fuca and Pacific Plate
3. San Andreas fault much further, south. Fastest plate on earth = Nazca plate
1. Creates mostly shallow quakes, biggest magnitudes are deep
In BC, most earthquakes (minor) happen around Vancouver and Queen Charlotte
Islands. Some intraplates near Alberta.
Quakes follow factor of 10 rule -> minor ones common, occur 10 times as less for every
increase in magnitude
Describe how the Earth builds, stores, and releases energy in earthquakes (elastic
o Types of stress: compressional, tensional, shear = depends on direction of force
and orientation of surface
Pressure: compressional stress that is the same in all directions (Cup
example). NOT STRAIN.
Changes an object’s volume, not its shape
Stress can involve forces with different magnitudes -> changes shape.
Force per area.
Change of shape under stress = strain
Understand concepts of (1) stress causing strain and (2) plastic versus brittle
o Responses to stress
Elastic deformation: relatively small stress, smalls trains but not
permanent since material bounces back. Energy can pass as waves.
Plastic deformation: like putty, material strains but does not bounce
Brittle deformation: material stores elastic energy but will eventually
break. Catastrophic release.
o Cold rock is brittle = upper crust. Can break if stress is large.
o Hot rock is ductile = most of Earth’s interior
Elastic Rebound Theory
o A pre-existing fault is locked by friction
BUT, elastic plates (“block”) on each side move slowly relative to each
other (mm to cm per year)
This leads to the deformation of the blocks
Since they are elastic, shear stress gradually
Earthquake starts at hypocenter/focus.
At this points, two blocks slide past
In most cases, sliding stops and
earthquake is tiny
Rupture occurs when elastic stresses
exceed friction However, in a large quake, a large part of the fault breaks and the
blocks move relative to each other. Elastic strain and shear stress
o At a depth, fault zones are ductile and not brittle. Relative motion of plates here
is steady, so no quakes.
Describe how the rupture propagates from the focus and why shaking and damage
are not necessarily greatest at the epicenter
Faults are weak surfaces, weaker than surrounding rock
o They break repeatedly and may accumulate 100s of km of “slip” over millions of
o Rupture begins at hypocenter and travels two ways
Maximum slip is usually NOT at the hypocenter
The rupture propagates away at 2-3 km/sec
Shaking is greatest in the direction the rupture travels!
Foreshocks sometimes formed, aftershocks always, aftershocks decrease as 1/time
o Biggest is usually 1 magnitude smaller than mainshock, however, distribution of
SIZE does not vary
o Aftershocks form because earthquake stress changes can affect probability on
Describe the different types of seismic waves and how they move through the Earth
o When elastic energy of rocks is released, energy goes to breaking rocks,
o A tiny fraction of total energy causes seismic waves which can travel far from
Body waves: travel inside the Earth
o P wave (Primary)
Compression and extension of a solid – like a sound wave.
Fastest type of seismic wave, about 6 km/s in continental crust.
o S wave (Secondary)
Shearing distortion of the solid
Particle moves perpendicular to direction that the wave propagates
Slower than P wave, about 3.5 km/s. Cannot pass through
Surface waves: require an interface – ground-air, water-air, mantle-li