Download Lesson 7-2 - TeacherWeb

Earthquake
Measurement
7-2
What You Will Learn
To find an earthquake’s epicenter, you must
triangulate by using data from three or more
seismometers.
Magnitude is a measure of an earthquake’s
strength.
The intensity of an earthquake depends on
four major factors.
Studying Earthquakes

Scientists use instruments called
seismometers, or seismographs, to record
seismic waves.

Seismometers record the vibrations of P waves,
S waves, and surface waves.

Seismometers also record the time it takes for
waves to arrive at a seismometer station.
Studying Earthquakes, continued
 Seismometers
create a tracing of
earthquake motion called a seismogram.
 Seismograms
help to locate the
earthquakes epicenter, the point on
Earth’s surface directly above the
earthquake’s starting point.
Studying Earthquakes, continued
 The
earthquake’s starting point inside
Earth is called the focus.
 The
epicenter is directly above the
focus.
Studying Earthquakes, continued

The lag time between the arrival of P waves and
S waves tells scientists how far the waves have
traveled.

Scientists draw a circle around a seismometer
station that has a radius equal to the distance
the waves have traveled.

Scientists draw circles around three
seismometer stations and find the point of
intersection.
Studying Earthquakes, continued
 The
point at which
all circles intersect
is the epicenter.
This process of
locating the
epicenter is called
triangulation.
Earthquake Magnitude
 Magnitude
is the measure on an
earthquake’s strength.
 The
greater the magnitude, the stronger
the earthquake.
 In
the past, the Richter scale was used to
describe earthquake strength. Now,
scientists use the magnitude moment
scale.
Earthquake Magnitude, continued
 The
Richter Scale measures ground
motion from an earthquake and adjusts for
distance to find an earthquake’s
magnitude.
 Richter-scale
 Each
values range from 0-9.
increase of one number represents a
tenfold increase in strength.
Earthquake Magnitude, continued
 The
magnitude moment scale is a more
accurate measure of earthquake strength.
 Magnitude
moment (Mw) represents the:
• size of the area of the fault that moves
• average distance moved by fault blocks, and
• rigidity of rocks in the fault zone.
Earthquake Intensity
 An
earthquake’s intensity is the effect of
the earthquake on people.
 The
Modified Mercalli scale describes
earthquake intensity.
 Intensity
ranges from barely noticeable to
total destruction of an area.
Earthquake Intensity, continued
Earthquake Intensity, continued
 Earthquake
intensity maps show the level
of intensity expected in different areas that
experience the same earthquake.
 Data
from past earthquakes are used to
create earthquake intensity maps.
The Effects of Earthquakes
 Effects
of earthquakes can vary over a
wide area.
 Effects
depend on the size of the
earthquake.
 Effects
also depend on three other factors:
distance from the epicenter, local geology,
and type of construction in the area.
The Effects of Earthquakes,
continued
Distance from the Epicenter
 The
total energy in a seismic wave stays
relatively constant as the wave travels.
 Seismic
waves grow increasingly larger as
they move away from the epicenter.
The Effects of Earthquakes,
continued
 As
seismic waves grow larger, the amount
of energy at any one point decreases.
 Therefore,
an earthquake is less
destructive to areas that are farther away
from the epicenter.
The Effects of Earthquakes,
continued
Local Geology
 The
amount of damage caused by an
earthquake depends on the material
through which seismic waves travel.
 Seismic
waves are particularly dangerous
when they travel through water-saturated
soil or sediment.
The Effects of Earthquakes,
continued

When seismic waves
shake watersaturated sediment or
soils, sediment grains
lose contact with each
other and are
surrounded by water.
 This process is called
liquefaction.
The Effects of Earthquakes,
continued
 Liquefaction
 Liquefaction
can intensify ground shaking.
can also cause the ground to
settle, which can cause structures to tilt or
collapse.
The Effects of Earthquakes,
continued
Earthquake-Resistant Construction

Brick and concrete structures are easily
damaged by earthquakes.

Wood and steel structures are more flexible and
less likely to be damaged.

Shorter buildings, on strong, anchored
foundations are also less likely to be damaged.