Download 19.2 Seismic Waves and Earth`s Interior Seismometer (seismograph

19.2 Seismic Waves and Earth’s Interior
Seismometer (seismograph) – a sensitive instrument that can detect vibrations of the Earth’s crust. It consists of
a frame that is anchored to the ground, a suspended mass, a writing instrument, and
a rotating drum covered with paper.
The readout that is obtained from a seismometer is called a seismogram. This is an example of one:
Lag Time
The difference between the first P-wave arrival and the first S-wave arrival is known as Lag Time. This
information can be used to determine the distance to the epicenter.
By recording the path that seismic waves travel as they go thru the Earth it can be determined what the Earth’s
internal structure is like. The diagrams and information on page 536-537 in your text explains this in detail.
Seismic waves can also be used to
identify features inside of the Earth
(ie. slabs and plumes)
The diagram and information
on page 538 explains this in
detail.
Slabs – portions of lithospheric
plates that are being subducted
Plumes – regions where hot mantle
material is rising (ex. hot spots)
P-waves – can travel thru solids, liquids, and gases. Can travel thru the entire Earth.
-- they will travel the fastest thru more rigid material (more dense material) (solids)
-- they will travel the slowest thru less rigid material (liquids – Earth’s outer core)
S-waves – can travel only thru solids. Cannot travel thru the Earth’s outer core (it is a liquid)
-- they will travel the fastest thru more rigid material (more dense material)
(very dense solids) (faster thru granite rock and slower thru sand)
Surface waves – found only on the surface. They are produced when the P-waves and S-waves reach the
surface. They cause the ground to move sideways & up and down like ocean waves.
They usually cause the most destruction because they cause the most movement of
the ground, and take the longest time to pass. Last waves to be recorded (slowest).
19.3 Measuring and Locating Earthquakes
Seismologist – a person who studies earthquakes.
There are several methods for describing the size of an earthquake.
Richter Scale – Developed in 1935 by a seismologist named Charles Richter. It is an older numerical rating
system that measures the energy (magnitude) of the largest seismic waves from an
earthquake. The height of these waves as they are recorded is known as their amplitude.
An 8.0 earthquake is 32 times more powerful than a 7.0 earthquake.
Moment Magnitude Scale – A newer more accurate numerical scale that also measures the magnitude of an
earthquake. In addition to measuring the energy released by an earthquake it takes into
account the size of the fault rupture, the amount of movement along the fault, and the
rock’s stiffness. The strongest rating ever was a 9.5 in Chile in the year 1960.
Modified Mercalli Scale – A scale that describes the amount of damage (intensity) that is caused by an
earthquake. It rates the types of damage and other effects of an earthquake as noted by
observers during and after its occurrence. It uses Roman numerals I – XII to designate
the degree of intensity. The higher the numeral – the worse the damage. You are
responsible for knowing the damage for I , VI , and XII. This information can be found
on the chart in your textbook on page 540.
The depth of an earthquake focus
Earthquakes can be classified as being shallow, intermediate, or deep focus earthquakes.
More Rigid
Less Rigid
Deep-focus earthquakes – This type of earthquake generally produces smaller vibrations at the epicenter than
shallow-focus earthquakes. The reason for this is that the rocks at this depth are
less rigid (more flexible). They do not have the ability to store a lot of energy.
Also the seismic waves must travel a great distance to reach the surface so they
will lose a lot of their energy.
Shallow-focus earthquakes – This type of earthquake generally produces larger vibrations at the epicenter than
a deep-focus earthquake. The reason for this is that the rocks at this depth are
more rigid (not flexible). They have the ability to store a lot of energy before
breaking, also the seismic waves do not have to travel very far to reach the surface
so they will retain a lot of their energy. Usually destructive earthquakes.
Seismic Belts – The majority (but not all) of the world’s earthquakes occur along narrow belts that separate
large regions with little or no seismic activity. There are 3 major seismic belts:
Circum-Pacific Belt – Found around the Pacific Ocean. (80% of all earthquakes occur here)
Mediterranean-Asian Belt – Found running through the Mediterranean Sea region and southern Asia.
(15 % of all earthquakes occur here)
Mid-Atlantic Ridge – Found running down the middle of the Atlantic Ocean.
Locating an Earthquake
Time-distance graph
showing the average travel
times for P- and S-waves.
The farther away a
seismograph is from the
focus of an earthquake, the
longer the interval between
the arrivals of the P- and Swaves.
•
(3) Three seismograph
stations are needed to locate
the epicenter of an
earthquake
•
You determine the distance
to the epicenter by using the
above graph. A circle is then
drawn where the radius
equals the distance to the
epicenter.
•
The intersection of the
circles locates the epicenter