Download Earthquakes

Earthquakes
(‫)الزالزل‬
Roughly 1,000 damaging earthquakes occur each year
What is an earthquake?
An earthquake is the vibration of Earth
produced by the rapid release of energy
Energy radiates in all directions from its
source, the focus
Energy moving outward from the focus of an
earthquake travels in the form of seismic
waves
Seismographs record the event
Earthquakes and faults
Earthquakes are associated with faults
Movement along a fault produces
earthquake and waves
Motion along faults can be explained by plate
tectonics
Most earthquakes result from movements along large fractures called faults
The San Andreas fault zone separates two great sections of the crust
of earth, the North American plate and the Pacific plate
What causes the rock to beak?
Stress- a force of directed pressure acting on a rock
Types of stress:
– Compressional
– Tensional
– Shear
Strain-the result of stress of deformation
– Elastic deformation- when stresses are removed, rock returns to original
shape
– Plastic deformation-permanent deformation, when stresses are removed,
rock stays bent
– Rupture-breakage and fracturing of the rock, causing an earthquake
– Brittle materials break during elastic deformation
Elastic rebound
Mechanism for Earthquake's explained by
H. Reid
– Rocks on sides of fault are deformed by
tectonic forces
– Rocks bend and store elastic energy
– Frictional resistance holding the rocks together
is overcome by tectonic forces
Elastic rebound
Earthquake mechanism
– Slips starts at the weakest point (the
focus) occurs
– Earthquakes occur as the deformed
rock “springs back” to its original
shape (elastic rebound)
Elastic rebound. As rock is
deformed it bends, storing
elastic energy. Once the
rock is strained beyond its
breaking point it rupture,
releasing the store-up
energy in the form of earth
waves.
Aftershocks and
foreshocks
The change in stress that follows
a mainshock creates smaller
earthquakes called aftershocks
Foreshocks- small earthquakes
may occur before a large one
Red dots show location of
aftershocks formed by 3
earthquakes in Missouri
and Tennessee in 1811/1812
Strike Slip Fault Quake - Japan
Strike Slip Fault Quake - California
Normal Fault Quake - Nevada
San Andreas: An active
earthquake zone
San Andreas is the most studied fault system in the
world
Displacement occurs along discrete segments 100
to 200 kilometers long
Most segments slip every 100-200 years producing
large earthquakes
Some portions exhibit slow, gradual displacement
known as fault creep
Fence offset by the 1906 San
Francisco earthquake
Seismology
Seismometers - instruments that record
seismic waves
Records the movement of Earth in relation to
a stationary mass on a rotating drum or
magnetic tape
The first seismograph was developed by the
Chinese almost 2000 years ago
The study of earthquake waves, seismology, dates back to the Chinese almost 2000 years ago
Modern seismographs, instruments that record seismic waves, are not unlike those
used by the early Chinese
A seismograph designed to
record vertical ground motion
Types of seismic waves
Surface waves
– Complex motion (up-and-down and side-toside)
– Causes damage to structures during an
earthquake
– High amplitude and low velocity
– Longest periods (interval between crests)
– Termed long, or L waves
– Slowest
Surface waves
Surface waves travel along the outer part of Earth
Types of seismic waves
Body waves
– Travel through Earth’s interior
– Two types based on mode of travel
– Primary (P) waves
Primary, Push-pull motion
Travel through solids, liquids & gases
Travel fastest of the seismic waves
– Secondary (S) waves
Secondary, shaking, shear, side-to-side
Moves at right angles to their direction of
travel
Travels only through solids
Body waves
Body waves travel through the interior of Earth