Earthquakes

Question 1

Two scales for measuring earthquakes

Answer 1

Moment-Magnitude scale
◦ Mercalli Scale

Question 2

Focus

Answer 2

The precise location below the earth’s surface where the
earthquake starts is called the focus

Question 3

Epicenter

Answer 3

The epicenter is the geographic location directly above the
focus. Since the epicenter is on the earth’s surface and can be
shown on a map, it is the way earthquake locations are usually
reported in the media

Question 4

Earthquakes can strike anywhere, but historical data shows that they occur
in distinct locations, concentrated in three large zones. These are

Answer 4

-The submerged mid-Atlantic ridge.
-The Alpide earthquake belt extending from Java to Sumatra, through the Himalayas, the Mediterranean, and into the Atlantic
-The circum-Pacific seismic belt along the rim of the Pacific Ocean

Question 5

Convergent boundaries

Answer 5

adjacent plates move toward each
other

Question 6

Divergent boundaries

Answer 6

adjacent plates move away from each
other

Question 7

Transform boundaries

Answer 7

adjacent plates move sideways with
respect to each other

Question 8

elastic rebound

Answer 8

Often, the two adjacent plates will smoothly pass by each other as they move. In some
cases, however, the boundary (called a fault line) will lock due to excess friction between
the plates. Over time, stresses will build up until this friction is overcome, and the stress is
released.

Question 9

Seismic waves

Answer 9

Seismic waves are how the energy released during elastic rebound is transferred outward from the
earthquake focus. Physically, seismic waves are deformations of rocks and sediments as energy passes
through them.

Question 10

P waves

Answer 10

P-waves: The “P” stands for primary - these waves travel fastest and therefore arrive first at locations
away from the earthquake focus.
P-Waves are also called compressional waves because as they pass through the Earth materials are
squeezed (compressed). It is possible for P-waves to pass through solids, liquids, or gasses. In fact,
sound waves are a form of P-wave that passes through the atmosphere

Question 11

S waves

Answer 11

S-waves: The “S” stands for secondary - these waves travel more slowly than P-waves and arrive at
distant locations after the P-waves. The “S” can also stand for shear. Shear waves cause the material
transmitting them to sway back-and-forth or up-and-down (a so-called shearing motion).
S-waves can only travel through solids because fluids (liquids and gases) continuously deform when
shear (sideways stress) is applied with no mechanical energy being stored. S-waves generally have a
much larger amplitude (displacement or movement) than P-waves, so S-waves cause much more
damage

Question 12

Seismographs

Answer 12

Seismograms can be used to find the location of an earthquake epicente

Question 13

How to find the epicenter

Answer 13

Actually finding the actual location of the epicenter requires a procedure called triangulation. As suggested by the “tri” in
triangulation, one needs three seismograms from three different recording stations to locate an epicenter. If you draw
circles on a map around three different seismographs where the radius of each circle is the distance from that station to
the earthquake, the intersection of those three circles is the epicenter.

Question 14

magnitude

Answer 14

One way of measuring the ‘size’ or significance of an earthquake is by calculating its
magnitude. Magnitude is a single value measure of the total energy released by the earthquake. The
scale is referred to as the moment magnitude scale (MMS)

Question 15

MMS

Answer 15

The MMS is a logarithmic scale in which each increase of one ‘point’ represents 10 times the amplitude
(displacement) of ground motion. The amount of energy to create ground motion is not linearly related
to the amount of movement, so an increase of one ‘point’ on the MMS represents 32 times the energy
released by the earthquake

Question 16

Intensity

Answer 16

Another way of measuring
earthquake size/significance is intensity. The
intensity is a number (written as a Roman
numeral to distinguish it from the moment
magnitude value) which describes the amount
of ground motion and the effects on people and
structures

Question 17

Intensity scale

Answer 17

The most common intensity scale is called the
Modified Mercalli scale. The intensity value will
vary by location, unlike the moment magnitude
scale which reports just one number for a
particular earthquake

Question 18

Affects intensity of eathquakes

Answer 18

magnitude: The longer the rupture and the greater the stored elastic stress per unit length along the
rupture, the greater the total energy released. The total amount of energy released by the earthquake
largely determines the amount of shaking.
distance: All waves, including seismic waves, diminish in intensity as they spread outward and travel
through the ground. Earthquake shaking decreases in intensity with increasing distance from the
earthquake focus.
directivity: Seismic waves tend to travel more effectively in the same direction as the progress of the
rupture along the earthquake fault.
local conditions: The type of soil and/or bedrock can significantly affect the type of shaking
experienced at a location. A seismic wave passing through soft materials will travel slower, but with
greater amplitude. Greater amplitude waves mean larger distances of ground motion and greater
damage to buildings. An example of this effect occurred in the Marina district of San Francisco during
the 1989 earthquake. Although the epicenter was 100 kilometers from San Francisco, some sites on soft
landfill materials experienced very damaging shaking