Why We Have Earthquakes

Earthquakes generally occur at junction points where the "plates" of the Earth’s outer shell come into contact with each other. The plates are constantly moving and the movement is neither smooth nor constant. The motion of the plates deforms rocks along the plate boundary (fault) until the rocks can no longer withstand the strain. Then, a sudden slip along the fault releases energy, causing earthquake shaking. The relative motion of the plates may be almost entirely horizontal, as is the case with the San Andreas Fault where the Pacific plate and the North American plate join. Major vertical movement can also occur, as with the Cascadia Subduction Zone at the intersection of the Gorda plate, the Juan de Fuca plate and the North American plate. Finally, earthquakes may occur either onshore or offshore.

Location And Risk Variance

Earthquake damage is typically concentrated in locations that can be identified in advance. These areas include:

1. Areas nearest to fault segments that are likely to move.
2. Areas of soft soils where shaking is increased.
3. Areas where the ground may settle or slide.

Failure of the ground during an earthquake can happen in many ways. Faults may break the ground as they slip during an earthquake. Steep or unstable hill slopes may slide, especially if an earthquake hits during the rainy season. Cracks may open along ridge crests close to the earthquake epicenter. Soft ground near lakes, bays or lagoons may amplify the ground acceleration and settle during shaking. Finally, the ground may fail if a sandy layer at shallow depth is saturated with water and flows like a liquid during earthquake shaking.

Where earthquakes occurred in the past, they will happen again.

With respect to offshore earthquakes, the magnitudes have typically fallen between 5.0 and 7.5 on the Richter Scale. A future earthquake of this type will likely affect the coastal communities between Cape Mendocino and Humboldt Bay along the North Coast, with some damage possible as far north as the southern Oregon coast. These earthquakes may damage roads, bridges, chimneys and foundations, move heavy furniture and knock many items off shelves.

Onshore earthquakes have recurred on average every 20 years and, when centered near populated areas, have caused major damage. These earthquakes historically have had magnitudes between 5.0 and 7.1. Geologic evidence, however, suggests this type of earthquake may have magnitudes of 7.5 or higher. Such an earthquake could cause surface rupture and would produce stronger ground shaking than any historic earthquake. The likelihood of such an earthquake in the near future is unknown, but is clearly less than the likelihood of offshore earthquakes along the North Coast.

Studies of the amount of strain released in the 1906 San Francisco earthquake and geologic studies of even older earthquakes suggest an average interval between earthquakes on the northern segment of the San Andreas Fault of several hundred years. Geologists, however, have forecast a 50/50 chance of Southern California being struck by a great earthquake (magnitude 8.0 or greater) along the southern segment of the San Andreas Fault within the next 15 years. When it happens, the impact on Southern California will be enormous.

In this event, police, fire, utilities and medical services will be overwhelmed. Cities within Southern California Counties that would normally help each other in a large emergency may be too busy with their own problems to provide mutual aid. It may take days or weeks in many areas to restore electricity, gas, water or telephones. Devastated roads will make travel anywhere difficult or impossible. This is why disaster planners have warned everyone to be prepared to be "on our own" for at least the first 72 hours after the earthquake.

Earthquake Damage Potential

The actual movement of the ground in an earthquake is seldom the cause of death or injury. Most casualties result from falling object sand debris because the shocks can shake, damage or demolish buildings, bridges and other man-made structures.

Earthquakes can also trigger landslides and generate huge ocean waves called "tsunamis." Both of these events can cause great damage and loss of life.

Earthquake-related injuries are commonly caused by:

1. Partial building collapses, such as toppling chimneys, falling bricks from wall facings, collapsing walls, falling ceiling plaster, light fixtures, and pictures.
2. Flying or falling glass from broken windows, with greater risk around modern high-rise buildings.
3. Overturned bookcases, fixtures, and other furniture and appliances.
4. Fires from broken chimneys, broken gas and electric lines, or spilled flammable liquids. The danger may be aggravated by lack of water to fight fires due to broken water mains.
5. Fallen power lines.
6. Human panic reactions, such as crowds stampeding for stairways or exit

Before The Earthquake

Falling objects and collapsing structures causes injuries and deaths during earthquakes. Show children safe areas to duck and cover. Practice counting how many seconds your "test earthquake" lasts. This will help you keep calm when a real earthquake strikes.

If an earthquake hits during the day, family members may be separated for hours or even days. Your local chapter of the American Red Cross can help you develop a plan at home, at work, and in your neighborhood. A family plan should include:

1. A safe place where your family can reunite after the earthquake. As transportation may be disrupted, select alternate meeting places near work or school.
2. Designation of an out-of-the-area telephone contact. Completing local telephone calls may be difficult. It will probably be easier to telephone someone out of the earthquake area. Select a relative or friend to act as a clearinghouse for information about your family. All family members should call this contact to report their condition and location. Make sure members carry this number with them at all times. Other friends and family should know this number too.
3. Discuss the plan with all family members. Discuss with children what will happen to them if an earthquake occurs while they are at school.
4. Know the safest places in your home. They should be away from heavy furniture or appliances, wood stoves, fireplaces, and large panes of glass, pictures, or mirrors.
5. Know where your gas, electric, and water main shutoffs are and how to turn each off. Now is the time to buy a special wrench that fits your gas turnoff valve and to fasten it next to the valve. Remember, turn off the utilities only if you suspect the lines are damaged, if you smell gas, or if you are instructed to do so. If you do turn the gas off, you must contact your utility company to turn it back on.
6. Know how to fight fires, rescue people trapped under debris, provide first aid, find help for dire emergencies, and assist others, especially the fragile elderly or disabled.
7. Know the policy of your local school concerning release of children after an earthquake. Arrange with neighbors to watch out for your family and property in case you are not at home.

Make plans with your family, your neighbors, and your co-workers. Every business should have an emergency response plan. Contact your American Red Cross chapter about developing a Neighborhood Emergency Service Team (N.E.S.T.) within your neighborhood, schools, and work place.

Additionally, falling objects and toppling furniture can be dangerous and expensive to replace or repair. Therefore, you should remember to:

1. Move heavy items, such as pictures, mirrors or tall dressers away from your bed.
2. Secure tall furniture and bookcases with lag bolts to wall studs. Add lips to shelves to prevent costly items from sliding off their supports.
3. Put latches on cabinet doors, especially at home in your kitchen and at work or school in laboratories.
4. Fasten heavy or precious items to shelves or tables. Secure file cabinets, computers, televisions, and machinery that may overturn during an earthquake.
5. Store potentially hazardous materials such as cleaners, fertilizers, chemicals, and petroleum products in appropriate containers and in sturdy cabinets fastened to the wall or floor.
6. In your office, be sure heavy objects are fastened to the building structure and not just a moveable wall. Ask a carpenter or electrician to determine whether light fixtures and modular ceiling systems are securely fastened.
7. Be sure your water heater is fastened to the wall studs and that all gas heaters and appliances are connected to the gas pipe through flexible tubing. If you use propane gas, be sure the storage tank is secured against overturning and sliding.
8. Secure your wood stove to wall or floor studs. Make sure you have a fire extinguisher close at hand.
9. Check with your school officials to be sure that they have taken the same precautions.

Since services and aid may be unavailable or severely restricted for days or weeks, you should have basic emergency supplies available and protected at home, at work, and in your car including:

1. Flashlights, batteries, an A-B-C- rated fire extinguisher, a battery-operated radio, a first aid kit and handbook, a supply of fresh water and food, warm clothes, sturdy shoes, gloves, and a fresh supply of any medications that you and your family members may need.
2. Make sure emergency supplies are located in a safe and readily available place. For most Californians, the trunk of the automobile is an excellent location.
3. Make sure everyone in your family knows where these supplies are and how to use them.
4. Take a course in first aid from your local chapter of the American Red Cross.
5. Include pets in your planning. Plan for their food and water supplies for at least three days.
6. Make arrangements with a neighbor to care for your pet in the event you are unable to get home.

Finally, you can complete your pre-earthquake preparations by looking at the first aid and survival guide in the introductory pages of your telephone book. Go to your local library and ask the librarian for reference materials. Ask your city or county Office of Emergency Services or your local American Red Cross chapter for pamphlets on preparedness and survival. Invite a speaker to talk to your club or organization about earthquake preparedness.

During The Earthquake

The most important thing you can do during an earthquake is to remain calm. By doing this, you will be in a better position to assess your situation and instill confidence in those around you. Think through the consequences of any actions you take. Try to calm and reassure others.

If you are indoors, stay there. If you happen to be in the kitchen, turn off the stove at the first sign of shaking. Watch for falling plaster, bricks, light fixtures and other objects. Watch out for high bookcases, china cabinets, shelves, and other furniture or appliances, which might fall or topple.

Stay away from windows, mirrors and chimneys. If in danger, get under a table, desk, or bed; in a corner away from windows; or in a strong doorway. Encourage others to follow your example. Do not run outdoors. Falling debris or electrical wires may hit you.

If you are in a high-rise office building, move away from windows and outside walls. Get under a desk or table. Do not dash for exits, since stairways may be broken or jammed with people. Power for elevators may also fail.

If you are in a crowded store, do not rush for a doorway since hundreds of people may have the same idea. If you must leave the building, choose your exit as carefully as possible.

If you are outside, avoid high buildings, walls, power poles, and other objects that could fall during the earthquake. If possible, move to an open area away from all hazards.

If you are in an automobile, pull your car to the side of the road and stop in the safest place possible. Avoid bridges, overpasses, and power lines. Remain inside your car until the shaking is over. Turn on your radio for news and information. Proceed with extreme caution because of the danger of debris in the road or damage to the road itself.

After The Earthquake

When the shaking has stopped:

1. Check for injuries to your family and in your neighborhood. Do not attempt to move seriously injured persons unless they are in danger of further injury.
2. Check for fires or fire hazards.
3. Wear shoes in all areas near debris or broken glass.
4. Do not touch downed power lines or objects touched by the downed wires.
5. Immediately clean up spilled medicines, drugs, and other potentially harmful materials.
6. Check for damaged utilities. Inspect for leaking gas lines by smell only; do not use candles, matches, or other open flames.
7. If you smell gas, open all windows and doors so that gas can escape. Shut off the main valve at your gas meter, leave the house immediately, and notify authorities of the leak. Do not re-enter the house until repairs have been made and the dwelling has been declared safe.
8. If water pipes break shut off the main valve, which brings water into the house.
9. If the house is properly wired, internal trouble with the electricity is very unlikely. If there is a short circuit, turn off the electricity at the meter box.
10. If water is off, emergency water may be obtained from melted ice cubes, from canned vegetables, and from toilet tanks (if no bluing or sanitizing chemicals have been added). Water is also available from swimming pools and spas, and from water heaters (but only if strained through a clean handkerchief or cloth if the danger of glass contamination exists).
11. Check to see that sewer lines are intact before permitting continued flushing of toilets.
12. Do not eat or drink anything from open containers near shattered glass. Liquids can be strained through a clean handkerchief or cloth if the danger of glass contamination exists.
13. If power is off, check your freezer and plan meals to use up food which will spoil quickly.
14. Use outdoor charcoal or propane broilers for emergency cooking. Do not bring these items indoors. The accumulation of fumes from their use can be deadly.
15. Do not use your telephone except for genuine emergency calls.
16. Check your chimney over its entire length for cracks and damage, particularly in the attic and at the roofline. Unnoticed damage could lead toa fire or collapse in aftershocks. The initial check should be made from a distance. Approach your chimneys with caution.
17. Check closets and storage shelf areas. Open closet and cupboard areas carefully and watch for objects falling from shelves.
18. Do not spread rumors. They often do great harm following disasters.
19. Tune-in to local radio stations for information and danger reports.
20. Do not go sightseeing. Do not use your vehicle unless there is a genuine emergency. Keep the streets clear for emergency vehicles.
21. Be prepared for additional aftershocks. Although most of these are smaller than the main shock, some may be strong enough to cause additional damage.
22. Respond to requests for help from police, fire fighting, civil defense and relief organizations, but do not go into damaged areas unless your help has been requested. Cooperate fully with public-safety officials. In some areas, you may be arrested for getting in the way of disaster operations.
23. Finally, the American Red Cross will handle information concerning the welfare of separated family members. Do not call or go to the police or fire department for this information.

By following the safety rules, you and your family should be in a better position to cope with the immediate effects of the disaster, and to get things "back to normal" as soon as possible.

Children And Earthquakes

Earthquakes may be traumatic events for all of us, but they are especially frightening for children, who may have to leave their homes and all that is familiar to them. A child does not usually understand such events and feels anxious and confused. After an earthquake, a child’s fears are those of reoccurrance, injury, death, or of being alone, separated from the rest of the family. Aftershocks can increase these fears.

Parents sometimes ignore the emotional needs of a child once assured of their physical safety. A child’s persistent fears may generate disruptive behavior, surprising and frustrating a parent who is trying to continue with the daily family routine. Parents can help in the following ways:

1. Keep the family together. This provides immediate reassurance to a child, and continuing fears of being abandoned and unprotected are alleviated.
2. Reassure children by words as well as actions. Use phrases like "we are all together and nothing has happened to us," or "you do not have to worry, we will look after you."
3. Encourage the child to talk. It can be helpful to include other family members, neighbors and their children in a talk about reactions to the disaster.
4. Include the child in all family activities. There will be important things to do after an earthquake, such as checking on the damage or cleaning up broken glass and fallen furniture. Whenever possible, a child should also be included in these activities.

At bedtime, a child may have difficulty falling asleep. The child may wake up during the night or have nightmares for weeks or months after the earthquake. These situations may be dealt with by allowing the child to move into a room with another child or sleep in the parents’ room, or by the parents simply spending extra time in the child’s room giving reassurance.

If these remedies don’t seem to be effective with your child, contact your local Mental Health Department for additional suggestions.

How To Purify Water

If water is polluted with dirt or sediment, strain it into a container through paper towels, coffee filters, or several layers of clean cloth to remove any sediment or floating matter. Disinfect the strained water with a 5.25% sodium hypochlorite solution (liquid household chlorine bleach) or with tincture of iodine.

Do not use the granular form of household bleach. It is poisonous!

If liquid chlorine bleach is older than one year, the amount used should be doubled, as it loses strength over time. Purchase an eyedropper to add bleach or iodine to the water. Use the eyedropper for this purpose only.

Mix well by stirring or shaking the water in a container. Let stand for 30 minutes before using. A slight chlorine odor should be detectable in the water. If not, repeat the dosage and let stand for an additional 15 minutes before using.

If the water can be boiled, boil at a rolling boil for 5 minutes (10 minutes is safer). This should remove any harmful bacterial contamination. If you are in the mountains, add one additional minute of boiling time for each 1,000 feet of altitude. Boiling for longer than 5 minutes is safer, but do this only if you have the gas or fuel to spare.

Water purification tablets are available in drug stores and sporting goods stores and are recommended for your first aid kit. Follow the directions on the package to purify water. Water purification tablets have a shelf life of 2 years and lose their effectiveness if they get damp before use.

Purify only enough water at one time to last for 48 hours. This will minimize the chances of re-contamination.

Earthquake Insurance

The insurance company that insures your home for fire is required to offer you earthquake insurance. The most common type of earthquake insurance is normally added as an endorsement on standard homeowners’ insurance policies. Typically there is a deductible of 5 to 10 percent or more of the value of the house. This means that for a home currently insured at $200,000, the homeowner would have to pay $10,000 to $30,000 on damages before the insurance company would pay anything.

Separate deductibles may apply to contents and structure. An important coverage is temporary living expenses. This pays for motel and meals if you have to move out of your home. There is usually no deductible on this coverage. The yearly cost of residential earthquake insurance is normal about $1.50 to $3.00 Per $1,000 of coverage on the structure.

Northern California Recent History And Risks

The North Coast has accounted for about one-quarter of all the earthquake energy released in California during historic times. The size, location, and frequency of past earthquakes give an expectation of what to expect in the future.

The instrumental record of earthquakes on the North Coast extends back only into the early Twentieth Century. However, it is possible to learn about older earthquakes from written accounts in newspapers, church logs, and diaries. From these accounts, seismologists can sometimes reconstruct the pattern of ground shaking and estimate the location and magnitude of early earthquakes. Although locations are imprecise, the record of damage to North Coast communities indicates fourteen earthquakes with strength similar to the April, 1992 earthquakes.

The majority of North Coast earthquakes have been centered offshore in the southeastern portion of the Gorda plate. These earthquakes recur frequently, causing some damage to North Coast communities about every two years. Communities in the coastal region from Cape Mendocino to Eureka have been struck far more frequently than the rest of Humboldt or Del Norte Counties. Northern Humboldt County, Del Norte County and coastal communities of southern Oregon have only rarely been affected by earthquakes in historic times. However, the historic record only gives a short-term view of earthquakes. Areas not affected historically may still be at risk.

Three powerful earthquakes struck the Cape Mendocino area on April 25th and 26th, 1972 (magnitudes 7.1, 6.6, and 6.7). The earthquakes caused 356 reported injuries, destroyed 202 buildings, and damaged 906 additional structures. Fires triggered by the second earthquake destroyed most of the Scotia shopping district. Property losses were estimated at over $60 million, and President Bush declared the region a Major Disaster Area. The earthquakes triggered numerous landslides and rock falls, and caused widespread liquefaction on the Mattole and Eel River Valleys. Many roads and bridges in the Cape Mendocino region suffered damage. The earthquakes were felt from southern Oregon, to Reno, Nevada, and as far south as Salinas.

The first earthquake, on Saturday, April 25th, is particularly important because:

1. It was the largest onshore earthquake in the North Coast region during this century.
2. It produced some of the strongest ground motions ever recorded. Am instrument located within three miles of the earthquake recorded accelerations of the ground nearly twice the acceleration of gravity.
3. It was the first large earthquake probably located along the Cascadia subduction zone boundary and demonstrates that the subduction zone is capable of producing damaging earthquakes.
4. It uplifted the coastline near Cape Mendocino by as much as 4 feet, killing intertidal communities of mussels, barnacles, sea urchins and algae.
5. The motion of the sea floor produced a tsunami that reached coastal communities with tens of minutes of the earthquake and reached a maximum height of 1 1/2 feet at Crescent City.

Southern California Recent History And Risks

Six major earthquakes have rocked the Los Angeles area since 1930. While there have been thousands and thousands of aftershocks of differing magnitudes, the major events worthy of discussion are the Long Beach Earthquake, the San Fernando (Sylmar) Earthquake, the Landers Earthquake, the Whittier Narrows Earthquake, the Sierra Madre Earthquake, and the Northridge Earthquake.

On March 10, 1933 at 5:54 pm, PST, a magnitude 6.4 earthquake occurred on the Newport-Inglewood fault zone. There was no surface rupture associated with this earthquake. It resulted in 120 deaths and over $50 million in property damage. Most of the damaged buildings were of non-reinforced masonry. Many school buildings were destroyed. Fortunately, however, the children were not present in the classrooms. This earthquake led to the passage of the Field Act, which gave the State Division of Architecture authority and responsibility for approving design and supervising construction of public schools. Building codes were also improved following this event.

On February 9, 1971 at 6:01 am, PST, a magnitude 6.6 earthquake occurred on the San Fernando fault, a thrust fault which broke the surface in the Sylmar-San Fernando area. This event is also known as the Sylmar Earthquake. The total surface rupture was roughly 12 miles (19 km) long. The maximum slip was up to 6 feet (2 meters). The quake caused over $500 million in property damage and 65 deaths. Most of the deaths occurred when the Veteran’s Administration hospital collapsed. Many other hospitals also suffered severe damage. Newly constructed freeway overpasses collapsed. In response to this earthquake, building codes were strengthened and the Alquist Priolo Special Studies Zone Act was passed in 1972. The purpose of this act is to prohibit the location of most structures for human occupancy across the traces of active faults and to mitigate, thereby, the hazard of fault rupture.

On October 1, 1987 at 7:42 am, PDT, a magnitude 5.9 earthquake occurred on a previously unknown, concealed thrust fault, about 20 kilometers east of downtown Los Angeles. It resulted in eight fatalities and $358 million in property damage. Severe damage was confined mainly to communities east of Los Angeles and near the epicenter. No severe structural damage to high-rise structures in downtown Los Angeles was reported. The most severe damage occurred in Whittier, Alhambra, and Pasadena. These areas had high concentrations of non-reinforced masonry buildings. Residences which sustained damage usually were constructed of masonry, were not fully anchored to foundations, or were houses built over garages with large door openings. Many chimneys collapsed and in some cases, fell through roofs. Wood frame residences sustained relatively little damage.

On June 28, 1991 at 7:43 am, PDT, a magnitude 5.8 earthquake occurred on the Clamshell-Sawpit Canyon fault, an offshoot of the Sierra Madre fault zone in the San Gabriel Mountains. Because of its depth and moderate size, it caused no surface rupture, though it triggered rockslides that blocked mountain roads. Roughly $40 million in property damage occurred in the San Gabriel Valley. Two deaths resulted from this earthquake: one person was killed in Arcadia, and one person in Glendale died from a heart attack. In all, at least 100 others were injured, though the injuries were mostly minor. None of the aftershocks of this event were strong enough to produce additional damage.

On June 28, 1992 at 4:57 am, PDT, a magnitude 7.3 earthquake occurred six miles north of Yucca Valley, California. The faults that ruptured include the Johnson Valley, Landers, Homestead Valley, Emerson, and Camp Rock. Several other faults experienced minor rupture along the 53 miles (85 km) that were affected, as well as damage during large aftershocks or triggered slips. Property damage and personal injuries were minimal due to the isolated area of the event. However, the quake was widely felt, and peoples’ nerves were on edge for several days anticipating and experiencing aftershocks.

Finally, on January 17, 1994 at 4:31 am, PST, a magnitude 6.7 earthquake occurred on the Northridge Thrust, also known as the Pico Thrust. This was the first earthquake to strike directly under an urban area of the United States since the 1933 Long Beach Earthquake. The earthquake occurred on a blind thrust fault, and produced the strongest ground motions ever instrumentally recorded in an urban setting in North America. Damage was widespread, sections of major freeways collapsed, parking structures and office buildings collapsed, and numerous apartment buildings suffered irreparable damage. Damage to wood-frame apartment houses was very widespread in the San Fernando Valley and Santa Monica areas, especially to structures with "soft" first floor or lower-level parking garages. The high accelerations, both vertical and horizontal, lifted structures off of their foundations and shifted walls laterally.

Despite the losses, gains made through earthquake hazard mitigation efforts of the last two decades were obvious. Retrofits of masonry building helped reduce loss of life, hospitals suffered less structural damage than previously, and emergency response was exemplary. The Northridge Earthquake proved that preparing for earthquakes will greatly reduce risk. The earthquake brought home several important lessons. Thrust faults concealed below Los Angeles present a threat to the region approaching that posed by the San Andreas fault. When earthquakes occur directly beneath a city, it will be subjected to ground motions with peak accelerations approaching the force of gravity, exceeding the levels of shaking anticipated by current building codes. Systems of concealed faults under the Los Angeles area are more complex than previously thought. The significant fracturing of welds in steel frame buildings was unexpected because of the ductility of steel. Therefore, understanding the cause and correcting the problem will be essential to continue building in earthquake prone regions.

Worldwide And United States Earthquake Statistics

The most destructive known earthquakes on record in the World, resulting in 50,000 deaths or more, listed in order of greatest number of deaths are:

Date Location Deaths Mag.

January 23, 1556 Shansi, China 830,000 n/a
October 11, 1737 Calcutta, India 300,000 n/a
July 27, 1976 Tangshan, China 255,000 8.0
August 9, 1138 Aleppo, Syria 230,000 n/a
May 22, 1927 Xining, China 200,000 8.3
December 22, 856* Damghan, Iran 200,000 n/a
December 16, 1920 Gansu, China 200,000 8.6
March 23, 893* Ardabil, Iran 150,000 n/a
September 1, 1923 Kwanto, Japan 143,000 8.3
December 28, 1908 Messina, Italy 100,000 7.5
September, 1290 Chihli, China 100,000 n/a
November, 1667 Shemakha, Caucasia 80,000 n/a
November 18, 1727 Tabriz, Iran 77,000 n/a
November 1, 1755 Lisbon, Portugal 70,000 8.7
December 25, 1932 Gansu, China 70,000 7.6
May 31, 1970 Peru 66,000 7.8
1278 Silicia, Asia Minor 60,000 n/a
January 11, 1696 Sicily, Italy 60,000 n/a
May 30, 1935 Quetta, Pakistan 60,000 7.5
February 4, 1783 Calabria, Italy 50,000 n/a
June 20, 1990 Iran 50,000 7.7

The ten largest magnitude earthquakes in the United States are:

Date Location Mag.
March 28, 1964 Prince William Sound, Alaska 9.2
March 9, 1957 Andreanof Islands, Alaska 8.8
February 4, 1965 Rat Islands, Alaska 8.7
November 10, 1938 East of Shumagin Islands, Alaska 8.3
July 10, 1958 Lituya Bay, Alaska 8.3
September 10, 1899 Yukutat Bay, Alaska 8.2
September 4, 1899 Cape Yakataga, Alaska 8.2
May 7, 1986 Andreanof Islands, Alaska 8.0
February 7, 1812 New Madrid, Missouri 7.9
January 9, 1857 Fort Tejon, California 7.9
April 3, 1868 Ka’u District, Island of Hawaii 7.9
October 9, 1900 Kodiak Island, Alaska 7.9
November 30, 1987 Gulf of Alaska 7.9

The ten largest magnitude earthquakes in the contiguous United States are:

Date Location Mag.
February 7, 1812 New Madrid, Missouri 7.9
January 9, 1857 Fort Tejon, California 7.9
March 26, 1872 Owens Valley, California 7.8
February 24, 1892 Imperial Valley, California 7.8
December 16, 1811 New Madrid, Missouri 7.7
April 18, 1906 San Francisco, California 7.7
October 3, 1915 Pleasant Valley, Nevada 7.7
January 23, 1812 New Madrid, Missouri 7.6
July 21, 1952 Kern County, California 7.5
November 4, 1927 Lompoc, California 7.3
December 16, 1954 Dixie Valley, Nevada 7.3
August 18, 1959 Hebgen Lake, Montana 7.3
October 28, 1983 Borah Peak, Idaho 7.3

The frequencies of worldwide earthquakes on an average annual basis by magnitude, based on observations since 1900, are:

Description Magnitude Average Annual Occurences
Great 8.0 and higher 1
Major 7.0 to 7.9 18
Strong 6.0 to 6.9 120
Moderate 5.0 to 5.9 800
Light 4.0 to 4.9 6,200
Minor 3.0 to 3.9 49,000
Very Minor 3.0 or smaller 3,285,000

The number of earthquakes located by the US Geological Survey National Earthquake Information Center for the past five years follow:

Magnitude 1993 1994 1995 1996 1997
8.0 to 9.9 1 2 3 1 0
7.0 to 7.9 15 13 22 20 16
6.0 to 6.9 141 161 185 150 117
5.0 to 5.9 1449 1542 1327 1126 1016
4.0 to 4.9 5034 4544 8140 8558 7462
3.0 to 3.9 4263 5000 5002 4521 4069
2.0 to 2.9 5390 5369 3838 2065 2160
1.0 to 1.9 1177 779 645 267 349
0.1 to 0.9 9 17 19 1 4
No recording 3997 1944 1826 2155 3471
Total 21476 19371 21007 18864 18664

As more seismographs are installed in the world, more earthquakes have been located.

With respect to magnitude versus ground motion and energy, the following illustrates the ground motion change (displacement) and energy change resulting from selected magnitude changes:

Magnitude Change Ground Motion Change Energy Change

1.0 10.0 times 32.0 times
0.5 3.2 times 5.5 times
0.3 2.0 times 3.0 times
0.1 1.3 times 1.4 times

In other words, a magnitude 7.2 earthquake produces 10 times more ground motion than a magnitude 6.2 earthquake, but it releases about 32 times more energy. The energy release best indicates the destructive power of an earthquake.

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