Seismic Sleuths

Earthquakes

A Teacher’s Package for Grades 7-12

Unit Three (continued)

3.3a Modified Mercalli Intensity Scale

Intensity Value & Description

I. Not felt except by a very few under especially favorable 
circumstances.

II. Felt only by a few persons at rest, especially on upper 
floors of buildings. Delicately suspended objects may swing.

III. Felt quite noticeably indoors, especially on upper floors of 
buildings, but many people do not recognize it as an earthquake. 
Standing automobiles may rock slightly. Vibration like passing of 
truck. Duration estimated.

IV. During the day felt indoors by many, outdoors by few. At 
night some awakened. Dishes, windows, doors disturbed; 
walls make creaking sound. Sensation like heavy truck striking 
building. Standing automobiles rocked noticeably.

V. Felt by nearly everyone, many awakened. Some dishes, 
windows, and so on broken; cracked plaster in a few places; 
unstable objects overturned. Disturbances of trees, poles, and 
other tall objects sometimes noticed. Pendulum clocks may 
stop.

VI. Felt by all, many frightened and run outdoors. Some heavy 
furniture moved; a few instances of fallen plaster and damaged 
chimneys. Damage slight.

VII. Everybody runs outdoors. Damage negligible in buildings 
of good design and construction; slight to moderate damage in 
well-built ordinary structures; considerable in poorly built or 
badly designed structures; some chimneys broken. Noticed by 
persons driving cars.

VIII. Damage slight in specially designed structures; considerable in 
ordinary substantial buildings, with partial collapse; great in poorly built 
structures. Panel walls thrown out of frame structure. Chimneys, factory 
stacks, columns, monuments, walls may fall. Heavy furniture overturned. 
Sand and mud ejected in small amounts. Changes in well water. Persons 
driving cars disturbed.

IX. Damage considerable in specially designed structures; well-designed 
frame structures thrown out of plumb; great in substantial buildings, with 
partial 
collapse. Buildings shifted off foundations. Ground cracked conspicuously. 
Underground pipes broken.

X. Some well-built wooden structures destroyed; most masonry and frame 
structures destroyed with foundations; ground badly cracked. Rails bent. 
Landslides considerable from river banks and steep slopes. Shifted 
sand and mud. Water splashed, slopped over banks.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. 
Broad fissures in ground. Underground pipelines completely out of service. 
Earth slumps and land slips in soft ground. Rails bent greatly.

XII. Damage total. Waves seen on ground surface. Lines of sight and 
level distorted. Objects thrown into the air.

When an earthquake occurs, we often hear news 
reporters describing it in terms of magnitude. Perhaps 
the most common question at a news conference is 
“What was the magnitude of the quake?” In addition 
to calculating the magnitude of an earthquake, 
however, we can describe the effect it had at a 
particular location by measuring its intensity. 
Magnitude and intensity are both measures of an 
earthquake, but they describe different characteristics. 
Each measurement has its uses.

Magnitude is a measurement of the amplitude of the 
earthquake waves, which is related to the amount of 
energy the earthquake releases. Magnitude is 
calculated from the size of the earthquake waves 
arriving at a seismic station. The most commonly used 
scale for magnitude is the Richter scale, developed by 
Charles Richter in 1935. The Richter scale is a 
logarithmic measurement of the maximum wave 
amplitude recorded at a seismograph station, corrected 
for distance from the epicenter. It is theoretically 
open-ended, although the largest quakes recorded in 
this century had a magnitude of 8.9. Each whole 
number increase in Richter magnitude indicates an 
increase in the severity of the ground shaking by a 
factor of 10. Thus a magnitude 6 earthquake will pro- 
duce shaking 10 times more severe than that produced 
by a magnitude 5 earthquake. Magnitude can also be 
related to the amount of energy an earthquake 
releases. Each whole number increase in Richter 
magnitude indicates an increase in the amount of 
energy released by a factor of roughly 30. Thus a 
magnitude 6 earthquake releases about 30 times more 
energy than a magnitude 5 earthquake, and roughly 
900 times as much as a magnitude 4 earthquake. 
These factors are seldom described correctly in news 
accounts.

Each earthquake has a single magnitude that is independent of the location of 
the observer. If a magnitude 
6.0 event strikes some location in the South Pacific, 
then that event will be described as a magnitude 6.0
by observers all over the world. Because magnitude is 
independent of observer location, it is a convenient 
measure to use in reporting the occurrence of an 
earthquake. No matter where it happened, or where 
you are, you get a feeling for the relative size of the 
earthquake by simply knowing its magnitude. This is 
why the press is so quick to report this number.

Intensity is a measure of the effect that the vibration 
had on natural and human-made structures. The most 
common measurement of intensity is the Modified 
Mercalli Intensity scale, originally developed in 1902 
by Giuseppi Mercalli, an Italian geologist. Wood and 
Neumann adapted it to “modern” conditions in 1931. 
The intensity scale ranges from I, the lowest 
perceptible intensity, to XII, the greatest intensity. 
Intensity is a function of many variables, including 
magnitude, depth of the earthquake, distance from the 
earthquake, local geological conditions, and local 
construction practices. Generally speaking, the 
intensity felt at a given location will increase with 
increasing magnitude, decreasing depth, decreasing 
distance from the earthquake, and a decrease in the 
quality of construction. If an earthquake is shallow, its 
intensity will be greater. If it affects an area built on 
soft sediments, such as landfills or sedimentary basins, 
the intensity will also be greater. A single quake will 
produce a range of intensities that typically decrease 
with increasing distance from the earthquake. An 
isoseismal map illustrates this range.

Intensity is more useful than magnitude as a measure 
of the impact that an earthquake had at any given 
location. Consequently, it is important to the 
professionals who establish building codes and 
insurance rates. If the maximum expected intensity in 
a given area is VII, for example, the building codes 
should specify construction practices that make 
buildings able to withstand this intensity, and property 
insurance rates will probably be high. If the maximum 
expected intensity is only III, the area can relax its 
building codes, and property insurance rates will be 
moderate.

3.3b Wattsville Map

3.3b Wattsville Map (key)

3.3c KWAT Television Script

Jake Wilde: “We interrupt our regularly scheduled programming on KWAT to bring 
you a special bulletin. This 
is KWAT news anchor Jake Wilde, and moments ago the town of Wattsville was 
shaken by a strong earthquake. 
Residents in the KWAT broadcast area are invited to call our emergency response 
number, 324- KWAT, and give 
us your name, your location, and a brief summary of what you experienced during 
the quake. Stay tuned for the 
latest reports of what your neighbors saw and felt. To report your observations, 
again, call 324-KWAT. We have 
caller number 1 on the line.”

Caller 1: “Hi, this is Charles from the hospital. Everyone ran outdoors and we 
only had moderate damage thanks 
to a well-built building.”

Caller 2: “Hello my name is Roy, and I’m calling from RQB Ranch. Everything that 
wasn’t nailed down in our 
tack room got turned upside down by the shaking. Some plaster is cracked too.”

Caller 3: “Hi, this is Bob at Long Valley Mercantile, and we have a mess here. 
When the quake struck, it 
knocked over all of our displays, and broke windows out of the store. All the 
trees and poles were moving.”

Caller 4: “Hi Jake, I’m Jane calling in from West Side Subdivision. Damage was 
slight here—only some plaster 
shaken off the walls and the heavy furniture moved around.”

Caller 5: “Hi. This is Jo. I’m calling from the north end of town and we need 
some help down here. The water in 
our well has changed its level and we are trying to put out fires with buckets.”

Caller 6: “Hello Jake, Lynda from Southside City Junior High School. Students 
felt it and did the drop, cover, 
and hold drill. We only had slight damage to the building, with some fallen 
plaster.”

Caller 7: “Hi, this is Hank and I’m calling on my car phone. I just heard the 
news and thought I would report that 
right after I turned south into town off the Interstate, I felt a big bump—but 
there were not any potholes in the 
road. It was disturbing.”

Caller 8: “Hello, this is Mary, and I’m calling from the basement of the First 
Bank in the center of Wattsville. 
The building has partially collapsed and people are trapped down here. Please 
send help.”

Caller 9: “Hi this is Jim. We were at the mall when the quake struck. Everyone 
panicked and ran outside. Luckily 
no one was hurt, and damage was slight.”

Caller 10: “Hi, this is Ron. I work at the MacBest Castle. Many of the tourists 
were frightened and ran outdoors, 
and our chimneys were damaged.”

Caller 11: “Hello Jake. My name is Sue and I’m calling from the Big Bear Lumber 
Camp. The walls of our house 
were swaying and creaking during the earthquake.”

Caller 12: “Hi. This is Len. We were picnicking at the Great Bend Park. Shortly 
after the quake struck we saw 
trees, telephone poles, and the flagpole swaying back and forth.”

Caller 13: “Hi. This is Jim out at the Roundup Truck Stop. When the quake hit, 
our chimney broke and fell 
over.”

Caller 14: “Hello, Jake. This is Marty up at Big Bear Ski Resort. The quake 
rattled our dishes and windows, and 
the walls made creaking sounds.”

Caller 15: “Hi Jake. This is Keith in River City. Our chimney is damaged and 
some plaster has fallen.”

Caller 16: “Hi. This is Diana calling from the Lucky Strike Mine. Over here, we 
thought that a big truck had 
struck the building.”

Caller 17: “Hi Jake. This is Monte from Wattsville University. Everyone in our 
class felt the quake, and some of 
the older, more poorly built buildings have suffered considerable damage.”

Caller 18: “Hello. My name is Marilyn, and I’m calling from Hot Springs Ranch. 
The strangest thing happened! 
Our big pendulum clock stopped dead when the quake struck.”

Caller 19: “Hi Jake, this is Karen. When the quake hit, I was in White Water 
visiting a friend. Nearly everyone 
felt it and we had a lot of windows broken. They were frightened, and I was 
too!”

Caller 20: “Hello Jake. This is Frank. I was at Blue Lake Resort when all the 
cars in the parking lot started 
rocking back and forth.”

Caller 21: “Jake, this is Gene at White Water Manufacturing. All of the heavy 
furniture in the showroom was 
moved by the quake, and some of the plaster fell off the walls.”

3.3d Five Seismograms
Simplified

3.3d Five Seismograms (key)

3.3e Distance, Magnitude, Amplitude
 
3.3f Several Seismographs
 
Chinese seismoscope, 132 A.D. The direction 
in which the first ball fell was thought to 
indicate the direction of the epicenter.
 
Inside view

Modern seismographs

3.3g Sample Seismograms

3.3g Sample Seismograms (key)

3.3h Map of Station Locations
 
3.3h Map of Station Locations (key)

3.3i Time/Distance Reference Table