GC151TZ Earthquakes and the Richter Scale (Earthcache) in Ohio, United States created by Trail Jogger

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GC151TZ ▼

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Hidden : 8/10/2007

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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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