Earthquakes – A trembling, solid wake-up call in the United States

Large, powerful earthquakes with the potential to kill and injure people and destroy or damage property occur periodically in many areas of the United States and the world.

Earthquakes have probably been occurring for billions of years, playing a major role in the shaping of the earth’s surface. Today, most earthquakes happen in areas bordering the Pacific Ocean, which are also prone to volcanic activity, known as the “ring of fire,” and in the Eurasian and Mediterranean regions.

It is estimated that the lives and property of 100 million or more Americans in 39 states and the territories are at risk from earthquakes. The potential for damage is not confined to those areas on fault lines. Severe earthquakes have been experienced in seven widely separated states, and the areas affected have included large surrounding regions. Federal authorities say earthquakes threaten the U.S. with potential annual losses approaching $10 billion.

A bit of earthquakes history

Large parts of the western U.S. are known to be particularly vulnerable to quakes. The State of California and its famous San Andreas Fault, which is more than 800 miles long and at least 10 miles deep, is considered to be an especially quake-prone area. Thousands of minor earthquakes are recorded along the fault and in other parts of California every year, but geological evidence indicates that large earthquakes occur along southern sections of the fault about every 150 years. A massive earthquake rocked Southern California in 1857.

The great San Francisco earthquake that struck in 1906 taking about 700 lives and causing millions of dollars in damage was estimated at a magnitude of 8.3 on the Richter scale (more about earthquake measurements later) and was apparently about the same magnitude as the 1857 quake along the fault’s southern portion.

But other, smaller though still strong quakes in California since 1906 have caused many millions of dollars more in damage and killed and injured many people.

On March 27, 1964, the Alaska earthquake was of even greater magnitude than the San Francisco earthquake. The Alaska quake released perhaps twice as much energy and was felt over an area of almost 500.000 square miles. Ground motion near the epicenter was so incredibly violent that the tops of some trees were snapped off. One hundred and fourteen people were killed, some as far away as northern California, where a seismic sea wave (tsunami) generated by the Alaskan disturbance struck a small coastal town.

House destroyed by earthquakes

One of the largest earthquakes in the history of eastern North America occurred in Charleston, South Carolina, in 1886, which caused extensive damage and 60 deaths. Other damaging quakes have occurred in areas of the Midwest and the eastern U.S.

Additionally, the northeastern U.S. has a history of small to moderate earthquakes, some of them damaging, but it could also have major earthquakes in the future, according to a report by the U.S. Geological Survey.


Nuclear reactors, dams, schools, high-rise apartment buildings, and housing developments are being planned and built in areas where the danger of major earthquakes is ever-present. This has created a crucial need for more information on the nature, causes, and effects of earthquakes.

Many scientists, including those of the U.S. Geological Survey’s National Center for Earthquake Research in Menlo Park, California, are studying earthquakes hoping to find methods of prediction and to develop practices that will reduce their destructive effects.

An earthquake is the wave-like, sometimes violent movement of the earth’s surface that follows the release of energy in the earth’s crust. The energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or even by man-made explosions. The most destructive quakes, however, are caused by dislocations of the crust.

When subjected to deep-seated forces (whose origins and natures are largely un-known), the crust may first bend, then, when the stress exceeds the strength of the rocks, break and snap to a new position. During the breaking process, vibrations called “seismic waves” are generated, which travel from the source of the earthquake to more distant places. This happens along the surface and through the earth at varying speeds, depending on the medium through which they move.

In certain cases, some of the vibrations can actually be heard, as they are of a high enough frequency, while others are of very low, inaudible frequency. The vibrations caused by earthquakes cause the entire planet to quiver or ring like a bell tuning fork.

A fault can be described as a fracture in the earth’s crust along which two blocks of the crust have slid with respect to each other. The faults are distinguished by the types of movements they make. One crustal block may move horizontally in one direction while the block facing it moves in the opposite direction. Or one block may move up while the opposite block moves downward.

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Movement along California’s San Andreas Fault is mostly horizontal, and the fault is known as a “strike-slip” fault. This is a fault in which the movement is vertical is called a “dip-slip” fault. The movements along many faults can be both horizontal and vertical.

Earth scientists have discovered that earthquakes tend to recur along faults, which reflect zones of weakness in the earth’s crust. When a fault zone experiences an earthquake, there are no assurances that enough stress has been relieved to prevent another quake.

The focus, or region where an earthquake’s energy originates, is usually concentrated in the crust and upper mantle. Since there is a depth of about 4,000 miles to the center of the earth’s core, earthquakes can be considered to originate in relatively shallow parts of the earth’s interior.

The focus of deep earthquakes will reach depths of 700 kilometers (440 miles). Earthquakes along the San Andreas and associated faults have shallow focal depths. Most have a depth of fewer than 10 miles. In the past 150 years, earth movements have occurred along more than half the entire length of the San Andreas Fault. The rupture itself is even now visible at the land surface in many places and it was one of the main inspirations for the disaster movie with the same name.

It is believed that very shallow earth-quakes are probably caused by fracturing of the brittle rock in the crust. Another cause is the internal stresses that overcome the frictional resistance locking opposite sides of a fault. The immediate cause or causes of intermediate and deep earthquakes is not fully understood.

An earthquake’s epicenter is the point on the earth’s surface directly above the focus. As a rule, the location of an earthquake is usually described by the geographic position of its epicenter and by its focal depth. Some of the earthquakes beneath the ocean floor can generate immense seismic sea waves or tsunamis (Japan’s dreaded “huge wave”).

These waves travel across the ocean floor at speeds up to 960 kilometers (600 miles) per hour and may reach 15 meters (50 feet) high or higher by the time they reach the shore. The 1964 Alaska earthquake tsunamis consuming coastal areas produced most of the destruction at Kodiak, Cordova, and Seward. It also caused severe damage along the west coast of North America, particularly at Crescent City. California. Some waves even raced across the ocean to Japan.

In addition, landslides triggered by earthquakes often cause more destruction than the earthquake shocks themselves. During the Alaskan quake, shock-induced landslides destroyed parts of the Turnagain Heights residential development and downtown areas of Anchorage.

Landslide following small earthquakes

Earthquakes Vibrations

Earthquakes produce two general types of vibrations, such as:

  • surface waves that travel along the earth’s surface
  • body waves that travel through the earth.

It is known that surface waves usually have the strongest vibrations, and they cause most of the damage done by earthquakes.

Body waves are of two types:

  • compressional or primary (P) waves,
  • shear or secondary (S) waves.

Both types of body waves pass through the earth’s interior from the focus of an earthquake to distant points on the surface, but only P waves travel through the earth’s molten core. Because P waves travel at great speeds and ordinarily reach the surface first, they are referred to as primary waves.

Since S waves usually reach the surface after the P waves. they are referred to as secondary waves. One of the first signs of an earthquake will often be a sharp thud, signaling the arrival of compression waves. This is usually followed by the shear waves and then the “ground roll” caused by the surface waves.

Measuring Earthquakes

The magnitude of an earthquake is expressed by the Richter Scale. It is a measure of the amplitude of the seismic waves and is related to the amount of energy released. The intensity of a quake is ex-pressed by the Modified Mercalli Scale and is a subjective measure describing how severe a shock was felt at a particular location where the quake hit.

The Richter Scale, was named after Dr. Charles F. Richter of the California Institute of Technology, is the best-known by most people, a scale for measuring the magnitude of earthquakes. The scale is logarithmic, so a recording of 6, for example, indicates a disturbance with ground motion 10 times as large as a recording of 5.

A quake with a magnitude of 2 is the smallest quake normally felt by humans. Earthquakes with a Richter rate of 6 or more are commonly considered major in magnitude.

Great earthquakes such as the 1906 San Francisco and 1964 Alaskan quakes have magnitudes of 8.0 or higher. One such great earthquake occurs somewhere in the world every year, on average.

The Richer Scale has no upper limit, but the largest known shocks have had magnitudes of 8.8 to 8.9.

The Modified Mercalli Scale expresses the power of an earthquake’s effects in a given location. It has values ranging from I to XII. The most frequently used adaptation covers the range of intensity from I—not felt except by a very few under especially positive conditions to XlI—Total damage total where lines of sight and level distorted, and objects being thrown upward in the air.

The Modified Mercalli intensity value assigned to an earthquake site is a more meaningful measure of severity to the lay-man because intensity refers to the effects actually experienced.

Evaluation of earth-quake intensity can be made only after eye-witness reports and results of field investigations are studied and interpreted. The maximum intensity experienced in the Alaska quake of 1964 was X. On the other hand, the San Francisco earthquake that occurred in 1906 reached an extreme intensity of XI.

The earthquakes of large magnitude do not necessarily cause the most intense surface effects. The effect is a given region depends on local surface and subsurface conditions. An area that is underlain by unstable ground such as sand, clay, or silt is likely to experience much more visible effects than an area equally distant from an earthquake’s epicenter but underlain by firm ground such as granite.

The earthquake’s destructiveness depends on many factors. Besides magnitude, these include the focal depth, distance from the epicenter, local geologic conditions, and the design of buildings and other human construction in the area shaken by the quake.

Reading earthquakes

Much of our present understanding of earthquakes is related to the field of plate tectonics. According to what is known about plate tectonics, the earth’s crust is broken into enormous, slow-moving plates. There are seven very large planes, each consisting of both oceanic and continental portions, and a dozen or more small plates.

It is estimated that each plate is about 80 kilometers (50 miles) thick and can be envisioned as having a shallow part that deforms by elastic bending or by brittle breaking, and a deeper part that yields plastically, beneath which is a viscous layer on which the entire plate slides.

These plates tend to be internally rigid and interact mostly at their edges. It was established that plates slide past one another along strike-slip faults. This can occur either on land or at sea. The best known of these faults is the San Andreas Fault of California, now being studied intensively by geoscientists of the U.S. Geological Survey and other institutions.

Large parts of some of the continents depart from ideal rigid-plate behavior and undergo much internal deformation. The western continental U.S. is now broadly stretched and sheared. The San Andreas Fault marks the boundary between the Pacific Plate on the west and the North American Mate on the east.

Although most of the relatively northwest-ward motion of the Pacific Plate past the North American Plate Is taken up along the San Andreas Fault, a part is distributed far inland, and the continental crust is stretched and shattered into the many block ranges and basins and other structures.

Scientists say that tens of millions of years from now, coastal California, including what is now Los Angeles and San Francisco, will be an island due to the northwest motion of the Pacific Plate relative to the North American Plate at the San Andreas Fault.

Read next: How To Survive An Earthquake

Predicting Earthquakes

The current ability to predict the time, place and size of earthquakes is very limited. However, considerable progress has been made in formulating the statistical probability that an earthquake of a certain magnitude will occur within a specific period of time.

Fifty years ago the National Center for Earthquake Research at the U.S. Geological Survey’s Western Region facilities in Menlo Park, California, began a program to study the causes of earthquakes, their influence on humans and property, and techniques for predicting the occurrence of earthquakes.

The greatest emphasis in the National Center’s investigations of earthquake-prone areas is fault zones in California and Nevada, especially the San Andreas Fault. The faults are being mapped and identified by type and movements. Quake shocks along the faults are continuously recorded. Fault zones in other western states and in Hawaii and Alaska are also being studied.

Highway destroyed by earthquakes

Scientists are hopeful that this research will reveal patterns assisting in earthquake prediction. Scientists in Russia, China, and other countries have mounted major studies of earthquakes.

A 15-mile-long stretch of the San Andreas Fault, located roughly at its mid-point in central California near the small town of Parkfield, has become perhaps the best-understood earthquake source region in the world. Scientists are exhaustively monitoring this section of the fault.

Moderate-sized earthquakes have occurred at Parkfield in 1881, 1901, 1922, 1934, and 1966. The only exception to an approximate 21 to 22-year recurrence time is the 1934 shock. The 44 years between 1922 and 1966 is just twice the usual time between the shocks. The 1966 earthquake reestablished the regular timing pattern of the Parkfield earthquakes.

Besides the predictable 22 years between shocks, Park field earthquakes share remarkably similar or characteristic features: magnitude, length, location of rupture, and epicenter. The Parkfield experiment is providing important experience in the design and operation of strain-monitoring instruments that ultimately may be adapted for use in a prototype earthquake prediction network.

The Parkfield earthquake study may provide data fundamental to the successful design of earthquake prediction strategies for southern California and other earthquake-prone areas of the country.

What to do when earthquakes strike

At present times, there is insufficient information available to scientists for the effective and accurate prediction of the location and size of earthquakes.

The best protection from earthquakes appears to be avoiding construction in high-risk areas and the use of earthquake-resistant construction techniques and materials. In areas already inhabited which are at high risk for earthquakes, the emphasis should be on improved construction methods and zoning of damage-prone un-developed land for nonresidential use.

The real movement of the ground in an earthquake is seldom the direct cause of death or serious injury. In general, most casualties are the result of falling objects and debris from buildings shaken and damaged by quakes. Quakes can also trigger landslides and generate seismic sea waves, in some areas, which can cause great damage.

Collapsing buildings or parts of buildings, flying glass from broken windows, overturned bookcases, furniture and appliances, fires from broken chimneys, broken gas lines, fallen power lines, and drastic human actions resulting from Panic commonly caused Injuries and deaths in earthquakes.

There is no way to reduce all earthquake dangers, but there are many actions you can take to help reduce the risks

Before An Earthquake

Support local programs for correcting hazardous unreinforced masonry buildings.

Support school building programs that provide resources for the strengthening of old, weak school buildings or their replacement with structures that earthquake-resistive and are built on ground reasonably safe during a strong earthquake.

Support the community efforts to replace old, weak buildings and to ensure that loose objects on building exteriors (cornices, etc.) are either removed or securely fastened.

In your community, organize and support programs to inform people and prepare them for future earthquakes.

Schools and civic organizations should provide support by holding earthquake drills and training sessions to prepare citizens to react correctly when earthquakes occur.

Support research programs to identify hazardous areas (active faults potential landslides, etc.) and to supply information needed to plan wisely the siting, design, and construction of man-made structures.

Check your home tor earthquake hazards.

Earthquake and damaged car

Bolt down or otherwise provide strong support for water heaters and other gas appliances to pre: vent lurching or toppling. Fires and explosions can result from broken gas lines and appliance connections. Use flexible gas connections wherever possible. You should place all large and heavy objects on the lower shelves. Also, make sure you fasten shelves to walls, and brace or anchor high or top, heavy objects.

In new construction or alternations, follow building codes to minimize earthquake hazards.

Each site for construction should be selected and engineered to reduce the hazard of damage from an earthquake. Ask your realtor about any hazards near your future home.

Move beds away from glass, bookshelves, and other heavy overhanging objects. Remove heavy objects from shelves.

Store caustic, flammable or poisonous materials in unbreakable containers and in protected areas. Hook latches on cupboard doors will keep them closed when bumped from within.

Make sure to hold occasional home earthquake drills. This will provide your family with the knowledge to avoid injury and panic during an earthquake.

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Learn and teach members of your family how to turn off electricity, gas, and water at main switch valves. Keep tools for such turnoffs In a readily available location, such as near the main gas valve. Do not turn off gas unless you actually smell a gas leak. Check with your local utilities of lice for Instructions, or to have the gas turned on.

Have family members receive basic first aid Instruction, as medical facilities and personnel may be overloaded in an earthquake. Keep a flashlight and a battery-powered radio in the home, readily available at all times.

Keep immunizations up to date for all family members.

Conduct calm family discussions about earth-quakes and other possible disasters. Do not tell frightening stories about disasters.

Keep an emergency supply of fresh food, water, and medical supplies. Water should be stored in a low place and in unbreakable containers.

Determine If you are in a possible inundation area In case of a dam failure. If so. be especially alert for news of damage following a quake. and discuss with family members what they should do and where you would meet if the neighborhood is evacuated.

Think about what you should do in case an earthquake strikes when:

  • you are at home;
  • at work;
  • driving your car;
  • in a store;
  • a public hall;
  • a theater or a stadium;
  • visiting friends;
  • involved in your other regular activities.

Proper planning will enable you to act calmly and constructively in an emergency.

During An Earthquake

The first thing you need to do is to remain calm. Think about what it would happen, regardless of any action you take. Try to calm and reassure others.

If indoors, watch out for falling plaster, bricks, light fixtures, and other objects. Watch out for high bookcases, china cabinets, shelves, and other furniture which might slide or topple.

Stay away from windows, mirrors, and chimneys. If in danger, get under a desk, table or bed, in a corner away from windows or in a strong doorway. Encourage others to follow your example. Usually, it’s best not to run outside.

If in a high-rise office or apartment building, get under similar cover. Do not go for the exits since stairways may be broken and jammed with people. Elevators may stop operating in an earthquake.

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

If outside, keep away from building walls, block or brick fences, power poles, and other objects which could fall. Do not run through the streets. If possible, try to move to an open area away from all hazards.

If you find yourself in an automobile, stop in the safest place available, preferably an open area away from overpasses.

After the Earthquake

If you have any type of medical training, check for injuries in your family and neighborhood. Avoid any attempt to move seriously injured persons unless they are in immediate danger of further injury. Check for fires or fire and explosion hazards. It is mandatory to wear shoes in all areas near debris or broken glass.

It is critical to check utility lines and appliances for damage. If discover gas leaks exist, shut off the main gas valve. Do not attempt to turn on the main gas valve. Only the gas company or a qualified plumber or mechanic may do this. If your home’s wiring is damaged, shut off electrical power. Report damage to the appropriate utility companies. Follow their instructions to the letter.

Avoid using matches, lighters, or open flame appliances until you ate sure no gas leaks exist.  Keep in mind to avoid operating electrical switches or appliances if gas leaks are suspected. You will create sparks that will ignite gas from broken lines.

Avoid touching downed power lines or objects touched by the downed wires.

Suggested reading: Generator Safety Rules To Implement Before A Natural Disaster Occurs

Immediately clean up spilled medicines and Other potentially harmful materials.

If water is off, and you do not have an emergency supply, emergency water may be obtained from water heaters and toilet tanks. Read this article on how to find emergency water in your home.

Before flushing the toilets, you should check to see that sewage lines are intact.

Avoid eating or drink anything from open containers near shattered glass. In case you need to, you may strain liquids through a clean handkerchief or cloth if the danger of glass contamination exists.

If the grid is down, check your freezer and plan meals to use up foods that will spoil quickly.

Use outdoor charcoal broilers or camping stoves for emergency cooking.

Possible body inside building

Turn on your radio for damage reports, information, and emergency services available.

Check your chimney, its entire length for cracks and damage, particularly in the attic and at the roofline. Unnoticed damage could lead to a house fire. The initial checkup should be made from a safe distance. Always approach chimneys with caution.

When needed, check the closets and storage shelf areas. The doors of closets and cupboards should be opened carefully as falling objects may cause injuries.

Do not go sightseeing immediately, particularly it beach and waterfront areas where seismic sea waves could strike.

Keep the streets clear for the passage of emergency vehicles and other first responders.

Be prepared for every additional earthquake shock called “aftershocks.” Even though most of these are smaller than the first shock, it should be anticipated that a second earthquake of equal magnitude could occur shortly afterward. Move vulnerable objects to safer positions. Review other preparation steps.

Calmly respond to requests for help from police, fire fighting, civil defense, and relief organizations. However, do not go into damaged areas unless your help has been requested. Cooperate fully with public safety officials.

In certain areas, you will be arrested for getting in the way of disaster relief operations. Conserve water and food for you and your family’s needs.

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