 Earthquakes and the Richter Scale
A cache by Trail Jogger Hidden: 8/10/2007
Size:  (Not chosen) Difficulty:  Terrain: (1 is easiest, 5 is hardest)
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Dr. Charles Richter
Charles F. Richter (1900-1985) was born at Sunnyside Farm on Wehr
Road in Overpeck, Ohio. He developed a means to measure the
magnitude of earthquakes, which was published as the Richter Scale
in 1935.
Earthquakes
Earthquakes are the result of forces deep within Earth's interior
that continuously affect its surface. The energy from these forces
is stored in a variety of ways within the rocks. When this energy
is released suddenly—by shearing movements along faults in
the crust of Earth, for example—an earthquake results. The
area of the fault where the sudden rupture takes place is called
the focus or hypocenter of the earthquake. The point on Earth's
surface directly above the focus is called the epicenter of the
earthquake.
The focal depth of an earthquake is the depth from the Earth's
surface to the region where an earthquake's energy originates (the
focus). Earthquakes with focal depths from the surface to about 40
miles are classified as shallow. Earthquakes with focal depths from
40 to 180 miles are classified as intermediate. The focus of deep
earthquakes may reach depths of more than 435 miles. The focuses of
most earthquakes are concentrated in the crust and upper mantle.
The depth to the center of the Earth's core is about 3,960 miles,
so even the deepest earthquakes originate in relatively shallow
parts of the Earth's interior.
The Richter Scale
The Richter Scale is the measure of the magnitude of seismic waves
from an earthquake and measures how much the ground shakes 60 miles
from the earthquakes epicenter. The scale is logarithmic; that is,
the amplitude of the waves increases by powers of 10 in relation to
the Richter magnitude numbers. The energy released in an earthquake
can be approximated by an equation that includes this magnitude and
the distance from the seismograph to the earthquake's epicenter.
Like ripples formed when a pebble is dropped into water, earthquake
waves travel outward in all directions, gradually losing energy,
with the intensity of earth movement and ground damage generally
decreasing at greater distances from the earthquake focus.
Seismic waves are the vibrations from earthquakes that travel
through Earth; they are recorded on instruments called
seismographs. Seismographs record a zigzag trace that shows the
varying amplitude of ground oscillations beneath the instrument.
Sensitive seismographs, which greatly magnify these ground motions,
can detect strong earthquakes from sources anywhere in the world.
The time, location, and magnitude of an earthquake can be
determined from the data recorded by seismograph stations.
| Micro |
1.0 |
Earthquakes this small happen below ground. You
can't feel them although they can be detected by seismometers. |
| Very Minor |
2.0 |
Locally, they are felt only by a few persons at
rest, especially on upper floors of buildings; delicately suspended
objects may swing. Trees sway. Small ponds ripple. Doors swing
slowly. |
| Minor |
3.0 |
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 hitting
building; standing autos rock noticeably. Felt by most people; some
breakage of dishes, windows, and plaster; unstable objects
overturned; disturbance of trees, poles, and other tall
objects. |
| Light |
4.0 |
Felt by all, many frightened and run outdoors; some
heavy furniture may move; falling plaster and chimneys, damage
slight. |
| Moderate |
5.0 |
Everyone runs outdoors; damage to buildings varies
depending on quality of construction; noticed by people driving
autos. If you are in a car, it may rock. Glasses and dishes may
rattle. Windows may break. |
| Strong |
6.0 |
Can be destructive in areas up to about 100 miles
across in populated areas. Panel walls thrown out of frames; walls,
monuments, chimneys fall; sand and mud ejected; drivers of autos
disturbed. Buildings shifted off foundations, frame structures
thrown out of plumb; ground cracked; underground pipes broken. |
| Major |
7.0 |
Can cause serious damage over larger areas. Most
masonry and frame structures destroyed; ground badly cracked, rails
bent, landslides; sand and mud shift; water splashes over river
banks. It is hard to keep your balance. |
| Great |
8.0 |
Can cause serious damage in areas several hundred
miles across. Few structures remain standing; bridges destroyed;
broad fissures in ground, pipes broken, landslides, rails bent.
Large rocks move. Smaller objects are tossed into the air. Some
objects are swallowed up by the earth. |
| Massive |
9.0 |
Devastating in areas several thousand miles across.
Damage total; waves seen on ground surface, lines of sight and
level distorted, objects thrown up into the air. |
| Meteoric |
10.0 |
Never recorded; equivalent to a 20 km rocky
meteorite impacting earth at 25 km/sec. Complete devastation to the
region. |
Earthquakes in Ohio
Earthquakes occur frequently in Ohio but are generally minor and
unnoticed. The Anna, Ohio (Shelby County in western Ohio)
earthquake on March 9, 1937 was the strongest earthquake to strike
Ohio. The U.S. Geological Survey assigned a magnitude of 5.4 to
this earthquake. The three-story schoolhouse at Anna was cracked
severely. Almost every chimney was broken or twisted, and house
foundations and walls were cracked. A few chimneys fell at Sidney,
about 7 miles south of Anna, and plaster was damaged. Subsurface
changes caused by the two earthquakes included renewed activity of
springs, conversion of ordinary wells to artesian wells, and an
increase in the flow of other water wells; the output of both oil
and gas wells was reduced. This shock was felt in upper stories of
multistory buildings in Chicago and Milwaukee and in Toronto,
Canada. Tremors were also felt in Kentucky, Michigan, Missouri,
Pennsylvania, and West Virginia. Earthquakes of similar magnitude
have since occurred in Lake and Ashtabula Counties in northeastern
Ohio as recently as 2001.
As a general rule, the size or magnitude of an earthquake is
related to the length of rupture of a fault. Seismologists have
speculated that active seismic zones in Ohio could theoretically
generate an earthquake at least an order of magnitude larger than
the largest historic event. Conservatively, they have suggested
that the western Ohio seismic zone could generate an earthquake
with a magnitude of between 6.5 and 7.0 and the northeastern
seismic zone could generate an earthquake with a magnitude of
between 6.0 and 6.5.
The Big One – New Madrid
Many seismologists and emergency planners consider the New Madrid,
Missouri seismic zone to pose the greatest threat to Ohio,
particularly the greater Cincinnati area of southwestern Ohio. One
major earthquake happened in New Madrid on Dec. 16, 1811 and
measured 8.6 on the Richter scale. Five weeks later, on Jan. 23,
1812, a second one occurred that measured 8.4. Finally, a third
earthquake occurred two weeks later on Feb. 7, 1812, and measured
8.7. The three earthquakes were strongly felt throughout most of
Ohio and caused chimneys to fall as far away as Cincinnati.
If a big earthquake occurred in New Madrid, Missouri region, would
you have enough time to be warned before the seismic waves hit your
area?
The mechanical properties of the rocks that seismic waves travel
through quickly organize the waves into two types. Compressional
waves, also known as primary or P waves, travel fastest, at speeds
between 1 and 5 miles per second in the Earth's crust. Shear waves,
also known as secondary or S waves, travel more slowly, usually at
60% to 70% of the speed of P waves. P waves shake the ground in the
direction they are propagating (back and forth), while S waves
shake perpendicularly or transverse to the direction of propagation
(up and down).
The Cincinnati area is about 300 miles from the New Madrid zone;
therefore, P waves from a large New Madrid event would reach
southwestern Ohio in a little less than a minute and a half and the
more damaging S waves would arrive about a minute later. This would
not be enough time to warn people to take cover, but automatic
electronic warning systems could be triggered to sound alarms and
shut down to protect some critical facilities, thanks in large part
to the work of Dr. Richter!
To claim successful completion of this
earthcache, please complete Items 1 and 2 below.
(1) Take a picture of yourself beside the Dr. Charles F.
Richter historical sign located at N39-27.941 W084-29.154 and
upload it with your posting.
(2) Answer the following questions and post your
responses since most home towns will differ.
(a) On your GPS or map, calculate the distance in miles from
your home to New Madrid, Missouri. If the primary wave travels at a
speed of at 5 miles/sec, how long would it take for you to feel the
first tremor after an 8.7 earthquake occurs in New Madrid?
(b) If the damage-causing secondary wave travels at 3
miles/sec, how long would it take for you to start seeing damage
after an 8.7 earthquake occurs in New Madrid?
(c) By subtracting (a) from (b), how much warning would you
have to take cover?
(BONUS) If the Richter number drops by approximately 1 for
every 125 miles the waves travel, using the table above, what
degree of damage would another 8.7 earthquake in New Madrid,
Missouri be expected to cause at your residence?
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