Earthquakes and Earth’s Interior - The Changing Earth: Exploring Geology and Evolution - page 198

Greenland

Alaska

Compass

Canada

D

NY

United States

B

=

Seismograph stations

Mexico

=
=
=
=

B
D
NY

Berkeley
Denver
New York
Epicenter

◗

Figure 8.10 Determining the Epicenter of an Earthquake Three seismograph stations are needed to
locate the epicenter of an earthquake. The P-S time interval is plotted on a time-distance graph for each
seismograph station to determine the distance that station is from the epicenter. A circle with that radius
is drawn from each station, and the intersection of the three circles is the epicenter of the earthquake.

TABLE 8.2 Modifi ed Mercalli Intensity Scale

I Not felt except by a very few under especially favorable
circumstances.
II Felt by only a few people at rest, especially on upper fl oors
of buildings.
III Felt quite noticeably indoors, especially on upper fl oors
of buildings, but many people do not recognize it as an
earthquake. Standing automobiles may rock slightly.
IV During the day felt indoors by many, outdoors by few. At
night some awakened. Sensation like heavy truck striking
building, standing automobiles rocked noticeably.
V Felt by nearly everyone, many awakened. Some dishes,
windows, etc. broken, a few instances of cracked plaster.
Disturbance of trees, poles, and other tall objects
sometimes noticed.
VI Felt by all, many frightened and run outdoors. Some heavy
furniture moved. A few instances of fallen plaster or
damaged chimneys. Damage slight.
VII Everybody runs outdoors. Damage negligible in buildings
of good design and construction; slight to moderate in
well-built ordinary structures; considerable in poorly built

or badly designed structures; some chimneys broken.
Noticed by people driving automobiles.
VIII Damage slight in specially designed structures;
considerable in normally constructed buildings with
possible partial collapse; great in poorly built structures.
Fall of chimneys, monuments, walls. Heavy furniture
overturned. Sand and mud ejected in small amounts.
IX Damage considerable in specially designed structures.
Buildings shifted off foundations. Ground noticeably
cracked. Underground pipes broken.
X Some well-built wooden structures destroyed; most
masonry and frame structures with foundations destroyed;
ground badly cracked. Rails bent. Landsides considerable
from river banks and steep slopes. Water splashed over
river banks.
XI Few, if any (masonry) structures remain standing.
Bridges destroyed. Broad fi ssures in ground. Underground
pipelines completely out of service.
XII Damage total. Waves seen on ground surface. Objects
thrown upward into the air.

Source: U.S. Geological Survey .