Untitled Flashcards Set

How do we find out where an earthquake

happened?

New Vocabulary

Richter scale

magnitude

amplitude

moment magnitude scale

modified Mercalli scale

Review Vocabulary

plot: to mark or note on a map

Earthquake Magnitude and Intensity

More than 1 million earthquakes occur each

year, but news accounts report on only the

largest ones.

Scientists have developed several methods

for describing the size of an earthquake.

Richter Scale

The Richter scale, devised by a geologist named

Charles Richter, is a numerical rating system

that measures the magnitude of an earthquake.

Magnitude is the measure of the energy

released during an earthquake.

The numbers in the Richter scale are

determined by the height, called the amplitude,

of the largest seismic wave.

Earthquake Magnitude and Intensity

Moment magnitude scale

The moment magnitude scale is a rating

scale of the energy released by an

earthquake, taking into account the size of

the fault rupture, the amount of movement

along the fault, and the rocks’ stiffness.

Earthquake Magnitude and Intensity

Modified Mercalli scale

The modified Mercalli scale is used to measure

earthquake intensity on a scale from I to XII. The

higher the number, the greater the damage the

earthquake has caused.

The intensity of an earthquake depends primarily

on the amplitude of the surface waves generated.

Maximum intensity values are observed in the

region near the epicenter; Mercalli values

decrease to I at distances far from the epicenter.

Earthquake Magnitude and Intensity

Depth of focus

One factor that determines the intensity of

an earthquake is the depth of its focus.

Earthquakes are classified as shallow,

intermediate, or deep, depending on the

location of the focus.

Earthquake Magnitude and Intensity

Shallow-focus earthquakes are the most

damaging.

Earthquake Magnitude and Intensity

Locating an Earthquake

The location of an earthquake’s epicenter,

as well as the time of occurrence, can be

determined using seismograms and

travel-time curves.

Distance to an

earthquake

Seismologists

determine the

distance to an

earthquake’s

epicenter by

measuring the

separation on any

seismogram and

identifying that same

separation time on the

travel-time graph.

Locating an Earthquake

To locate the epicenter

of an earthquake,

scientists identify the

seismic stations on a

map, and draw a circle

with the radius of

distance to the epicenter

from each station.

The point where all the

circles intersect is the

epicenter.

Locating an Earthquake

Time of an earthquake

Seismologists can use a seismogram to gain

information about the exact time that an

earthquake occurred at the focus.

The time can be determined by using a table

similar to a travel-time graph.

Locating an Earthquake

Seismic Belts

The majority of the world’s earthquakes occur

along narrow seismic belts that separate large

regions with little or no seismic activity.

Seismic Belts

The locations of most earthquakes correspond

closely with tectonic plate boundaries.