Earthquakes
Effect of Earthquakes on the Society
• Earthquake is a scary
experience – if you have
ever experienced an
earthquake you know that
it may cause massive
damage and change lives
of people effected by it.
Classroom in Courtney after
an earthquake in 1946
What is an Earthquake?
• An earthquake is shaking caused by the breaking (rupture) and
subsequent displacement of rocks beneath the surface.
• In an earthquake one body of rocks moves in respect to another body
of rocks.
• Break occurs along the lines of stress on the rock. Once the stress
cannot be handled anymore by a fault (meeting point of two plates)
two sides slip causing the shaking of the ground.
Rupture Lines
• Stress is applied to a
rock due to ongoing
plate movement. This
results in a strain or
deformation of the
rock.
• Rocks can withstand
large amounts of stress
before they slip and
break
• The lines along which
the rock breaks are
known as rupture lines
or fault planes.
Rupture Surface
• This is surface along which
an earthquake occurs. As
seen on the previous slide
the rupture takes place
along an edge not at a
point.
• The surface along which it
occurs is called rupture
surface.
• The extent of a rupture
surface and the amount of
displacement depends on
the rock type and
strength; plus, the degree
to which it was stressed
before rupture took place
plays a role as well.
Aftershocks
• Once an earthquake has stopped (rupture has slipped, and rocks are
not strained anymore) there might be a series of aftershocks.
• Aftershocks are earthquakes caused by an earlier earthquake.
• During earthquakes, a rock moves along the fault line. This puts strain
on the surrounding area and causes aftershocks.
• Aftershocks are triggered by stress transfer
• Aftershocks are usually smaller than the original quake but there are
instances when they can be bigger.
• They can happen minutes, days or even years after the original quake.
Area of Aftershocks
• In the area of original
earthquake stress is eventually
reduced
• However, the adjacent areas
experience increased levels of
stress and deformation due to
slippage of ground during the
original earthquake
• The area of aftershocks can be
much greater than the area in
which earthquake occurred.
Episodic
Tremor and
Slip (ETS
zone)
• Area of slow periodic sliding along subduction
boundary.
• Does not produce recognizable earthquakes, but
does show up on seismograph
• The subducting plate has three specific zones
1) Locked zone – where there is no frequent
movement; however, infrequent large
earthquakes do occur
2) ETS zone – area that slips every so often (150250 km from the trench)
3) Continuous slip zone – area that is close to the
mantle, and due to melting of surrounding
rocks, it continuously slips into the mantle.
Shallow Earthquake
Zones
• Quakes are infrequent along the divergent
oceanic boundaries
• These earthquakes are very shallow.
• They are restricted to the narrow margin that
closely follows the fault.
• Shallow earthquakes also occur along
transform faults, such as the San Andreas
Fault in California.
Divergent and Transform
Boundary Earthquakes
• Most of the earthquakes in these
two groups are along the
transform fault.
• Divergent plates are seeping lava
to the surface, but they do not
produce earthquakes due to high
rock temperatures which makes
the rock more elastic
• Transform boundaries experience
more quakes as these are the
areas where tension builds up
due to the divergence.
Deep Earthquakes
• Along the subduction zones these
types of earthquakes occur
frequently.
• Their depth depends on the type of
subduction that is occurring.
• The deepest earthquakes are found at
ocean-continent subduction zones.
• Specifically, they are on the landward
(toward land) side of the subduction.
Convergent Boundary
Earthquakes
• Very large earthquakes occur at subduction zones
• Their locus can be measured at various depths
from very shallow to 400 km deep
• Ocean-ocean convergence produces volcanic
island chain
• Ocean-continent makes volcanic arc of mountains
(Cascades)
• Continent-continent allows for mountain build up
but not associated with major earthquakes