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how to educate the public?
what are the ways best used to educate the public?
why is educating the public important?
when is the best time to educate the public?

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what techniques can we use to improve construction techniques?
how to improve construction techniques?
why is better construction important to reduce the risk of damage from earthquakes?

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Arrowhead General Insurance Agency, Inc.

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Most of the nation assumes California is the primary state affected by earthquakes. But that's not necessarily true. While the West Coast (California, Oregon and Washington), along with Hawaii and Alaska experience the most quakes, every state can potentially experience temblors. Producers, provide your business owners with these tips to minimize earthquake damage and injury.

This year's Great ShakeOut is this Thursday. This worldwide event includes earthquake drills that are an opportunity to practice how to be safer during earthquakes: " Drop, Cover and Hold On." ShakeOut encourages you, your business and your clients to update emergency plans and supplies, and to secure your space in order to prevent damage and injuries.

Mother Nature's other curve balls, like hurricanes, winter storms, and often wildfires, at least provide an element of warning before they arrive. Not so with temblors. That element of surprise makes earthquake preparedness even more essential. Investing in strong buildings plus stabilizing non-structural elements within your clients' structures can reduce the risk of earthquake damage. These same preparation steps can also help provide protection from a tornado, hurricane or other natural disaster. Urge your clients to consider these steps provided by Zurich to minimize earthquake damage and injury.

Pre-quake preparations to minimize earthquake damage and injury

Precisely because scientists have never been able to accurately predict a major earthquake, being prepared is crucial. After all, "Earthquakes don't kill people. Buildings do."

Structural elements

Encourage clients to retain a qualified structural engineer to evaluate their building, especially if it is older than the seismic design provisions in local building codes.

Consider retrofits for older buildings so that they meet or exceed current building codes.

Unreinforced masonry is especially vulnerable in an earthquake and should be retrofitted.

Non-structural elements

Brace heavy machinery, appliances, containers, tanks and inventory that an earthquake could cause to shift, fall or rupture.

Lock the rollers of heavy furniture and storage shelving.

Attach computers and towers to desks.

Anchor file cabinets, mirrors or pictures to wall studs.

Secure ceiling lights and false ceilings to joists.

Install a main gas shut-off device and flexible connections on gas appliances. Train staff on how and when to shut off gas, electricity and water. Keep wrenches or other necessary tools close by.

Consider applying safety film to windows and glass doors, especially where breakage could cause the most injuries or damage.

Training and preparation

Designate an emergency response team responsible for fire safety equipment checks, evacuation procedures, first-aid training, drills and both internal / external communication in the event of an earthquake.

Regularly check the integrity of fire extinguishers, pumps and water tanks. Fire is the most common hazard after an earthquake.

Assemble disaster preparedness kits and stow in various parts of the building. These should include whistles, dust masks, a portable charger for cell phones, a battery-powered or hand crank radio and a NOAA Weather Radio with tone alert.

Educate staff on how to protect themselves in a quake. Explain that smoke alarms and sprinkler systems may go off in buildings during an earthquake, even if there is no fire. Depending on where they are when the quake strikes, teach them to:

Inside a building: "Duck, cover and hold." Move away from windows and doors. Drop to the floor, take cover under a sturdy table or desk against an inside wall. Grasp one of its legs while covering your head with your other arm.

Outside a building: Move into a clearing and away from power lines, trees and buildings. Drop to the ground and wait for the shaking to stop. If you are near unstable slopes or cliffs, be alert for falling rocks or landslides.

Driving: Pull to the shoulder or off the road, away from traffic, road signs and power lines. Don't stop on or under bridges. Stay in the car with seat belt fastened until the shaking stops. If you resume driving, go slowly and look for possible road damage.

Post-earthquake recovery steps to minimize earthquake damage and injury

Aid your clients in business continuity planning, outlining procedures to organize post-quake recovery. Their plan should address business interruption risks and the possibility that fire protection systems may be damaged by the temblor, rendering them useless or only partially functioning. Their plan should also encompass:

Guidelines for taking an inventory of damage.

Priorities for repairs.

Contact information of contractors critical for repairs and resumption of operations.

Warnings against hot work until fire protection is fully functional.

Suppliers' contact information to order fresh supplies after the temblor.

Communication procedures with customers, informing them of any delays or reductions in their business capabilities.

Share with your clients the Zurich infographic below, with more earthquake facts and tips.

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How to minimize earthquake damage and injury - Arrowhead

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Most of the nation assumes California is the primary state affected by earthquakes. But that's not necessarily true. While the West Coast (California, Oregon and Washington), along with Hawaii and Alaska experience the most quakes, every state can potentially experience temblors. Producers, provide your business owners with these tips to minimize earthquake damage and injury.

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how to upgrade building safety?
what are the characteristics of an earthquake safe building?
what type of furniture to use for reducing earthquake damages?

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Throughout history, we've built impressive structures and cities only for them to encounter the forces of nature. Earthquakes are one of the Earth's most destructive forces - the seismic waves throughout the ground can destroy buildings, take lives, and costs tremendous amounts of money for loss and repair.

According to the National Earthquake Information Center, there is an average of 20,000 earthquakes each year -16 of them being major disasters. On September 20, 2017, a magnitude 7.1 rocked Mexico's capital city and killed approximately 230 people. As with the case with other earthquakes, the damage was not caused by the quake itself but by the collapse of buildings with people inside them, making earthquake-proof buildings a must.

Over the past few decades, engineers have introduced new designs and building materials to better equip buildings to withstand earthquakes. Read on to learn how earthquake-proof buildings are designed today.

How Earthquakes Impact Buildings

Before we look at the features, it's important to understand how earthquakes impact man-made structures. When an earthquake occurs, it sends shockwaves throughout the ground in short rapid intervals in all different directions. While buildings are generally equipped to handle vertical forces from their weight and gravity, they cannot handle side-to-side forces emitted by quakes.

This horizontal load vibrates walls, floors, columns, beams and the connectors that hold them together. The difference in movement between the bottom and top of buildings exerts extreme stress, causing the supporting frame to rupture and the entire structure to collapse.

How to Make A Building Earthquake-Proof

To design an earthquake-proof building, engineers need to reinforce the structure and counteract an earthquake's forces. Since earthquakes release energy that pushes on a building from one direction, the strategy is to have the building push the opposite way. Here are some of the methods used to help buildings withstand earthquakes.

1. Create a Flexible Foundation

One way to resist ground forces is to "lift" the building's foundation above the earth. Base isolation involves constructing a building on top of flexible pads made of steel, rubber, and lead. When the base moves during the earthquake, the isolators vibrate while the structure itself remains steady. This effectively helps to absorb seismic waves and prevent them from traveling through a building.

2. Counter Forces with Damping

You might be aware that cars have shock absorbers. However, you might not know that engineers also use them for making earthquake-resistant buildings. Similar to their use in cars, shock absorbers reduce the magnitude of shockwaves and help buildings slow down. This is accomplished in two ways: vibrational control devices and pendulum dampers.

Vibrational Control Devices

The first method involves placing dampers at each level of a building between a column and beam. Each damper consists of piston heads inside a cylinder filled with silicone oil. When an earthquake occurs, the building transfers the vibration energy into the pistons, pushes against the oil. The energy is transformed into heat, dissipating the force of the vibrations.

Pendulum Power

Another damping method is pendulum power, used primarily in skyscrapers. Engineers suspend a large ball with steel cables with a system of hydraulics at the top of the building. When the building begins the sway, the ball acts as a pendulum and moves in the opposite direction to stabilize the direction. Like damping, these features are tuned to match and counteract the building's frequency in the event of an earthquake.

3. Shield Buildings from Vibrations

Instead of just counteracting forces, researchers are experimenting with ways buildings can deflect and reroute the energy from earthquakes altogether. Dubbed the "seismic invisibility cloak", this innovation involves creating a cloak of 100 concentric plastic and concrete rings in and burying it at least three feet beneath the foundation of the building.

As seismic waves enter the rings, they are forced to move through to the outer rings for easier travel. As a result, they are essentially channeled away from the building and dissipated into the plates in the ground.

4. Reinforce the Building's Structure

To withstand collapse, buildings need to redistribute the forces that travel through them during a seismic event. Shear walls, cross braces, diaphragms, and moment-resisting frames are central to reinforcing a building.

Shear walls are a useful building technology that helps to transfer earthquake forces. Made of panels, these walls help a building keep its shape during movement. Shear walls are often supported by diagonal cross braces. These steel beams have the ability to support compression and tension, which helps to counteract the pressure and push forces back to the foundation.

Diaphragms are a central part of a building's structure. Consisting of the floors of the building, the roof, and the decks placed over them, diaphragms help remove tension from the floor and push force to the vertical structures of the building.

Moment-resisting frames provide more flexibility in a building's design. This structure is placed among the joints of the building and allows for the columns and beams to bend while the joints remain rigid. Thus, the building is able to resist the larger forces of an earthquake while allowing designers more freedom to arrange building elements.

Earthquake-Resistant Materials

While shock absorbers, pendulums, and "invisibility cloaks" may help dispel the energy to an extent, the materials used in a building are equally responsible for its stability.

Steel and Wood

For a building material to resist stress and vibration, it must have high ductility - the ability to undergo large deformations and tension. Modern buildings are often constructed with structural steel - a component of steel that comes in a variety of shapes that allow buildings to bend without breaking. Wood is also a surprising ductile material due to its high strength relative to its lightweight structure.

Innovative Materials

Scientists and engineers are developing new building materials with even greater shape retention. Innovations like shape memory alloys have the ability to both endure heavy strain and revert to their original shape, while fiber-reinforced plastic wrap - made by a variety of polymers - can be wrapped around columns and provide up to 38% greater strength and ductility.

Engineers are also turning to natural elements. The sticky yet rigid fibers of mussels and the strength-to-size ratio of spider silk have promising capabilities in creating structures. Bamboo and 3D printed materials can also function as lightweight, interlocking structures with limitless forms that can potentially provide even greater resistance for buildings.

Over the years, engineers and scientists have devised techniques to create some effective earthquake-proof buildings. As advanced the technology and materials are today, it is not yet possible for building to completely withstand a powerful earthquake unscathed. Still, if a building is able to allow its occupants to escape without collapsing and saves lives and communities, we can consider that a great success.

Related Posts

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How Earthquake-Proof Buildings Are Designed

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Throughout history, we've built impressive structures and cities only for them to encounter the forces of nature. Earthquakes are one of the Earth's most destructive forces - the seismic waves throughout the ground can destroy buildings, take lives, and costs tremendous amounts of money for loss and repair.

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what is fire protection system ?
what is the purpose of fire protection system ?

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https://www.firetrace.com/fire-protection-blog/what-is-a-fire-protection-system

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Fire safety is integral for business owners. A fire can spark in an instant and spread within seconds. Businesses investing in fire protection systems realize significant benefits when the systems perform as expected. These systems reduce damage to the building, expensive equipment, documents, and inventory. Fire suppression systems, smoke detectors, and sprinkler systems are all types of fire protection systems that help with fire detection and protecting building occupants and equipment.

Understanding How a Fire Protection System Functions

It is important to understand the functionality of fire protection systems. Systems work in different ways, but all have a common goal to detect a fire and protect the building, its occupants, and valuables. A common fire protection system is a smoke detector and a sprinkler. If a fire sparks, smoke sets off the detector causing the sprinkler system to activate. The water protects against the spread of fire. While this is an effective method, when dealing with certain critical equipment or special hazards, automatic fire suppression systems using clean agents are a better choice. These systems detect and suppress fire while leaving no residue.

Benefits of Fire Protection Systems

One of the main benefits of a fire protection system is that in the long run, it saves money. Think of a business and what investments are made in manufacturing equipment, information technology hardware, and infrastructure. If a fire impacts operations for a significant period of time, it could cost a company millions of dollars. A prime example is a machine shop that manufactures parts. If the shop were to experience a fire and shut down for multiple days or weeks, the business would suffer immensely. Equipment may need to be replaced, and valuable contracts could be lost. Businesses must take into consideration all factors when they look at the price of a fire protection system.

Another benefit of some types of fire protection systems is the automatic dispatching of emergency services. These systems will work to immediately suppress the fire, while also notifying the authorities to send emergency professionals to your location.

Determining the Appropriate Fire Protection System

How do you go about determining the best system for your business? You must consider the facility you are protecting, compliance with building codes, and potential insurance requirements. Because many fire protection systems are permanent, it may be important to think about your facility's future needs, not just its current needs. For example: if you are protecting a server room and expect to add more servers as time goes on, can you add onto the system?

When selecting and installing a system, it is essential to hire a company with a comprehensive understanding of your needs who can customize your fire protection options. If your building is not up to code or machines and equipment are not adequately protected, you may face exposure and downtime.

Active vs. Passive Fire Protection

Fire protection can be active or passive. When a building is under construction, the contractor and architect can incorporate passive fire protection, such as fire doors and fire escapes. Passive fire protection may also refer to the use of non-flammable materials during construction. By contrast, active fire protection means using a system that reacts in case of a fire. Examples of active fire protection include sprinkler systems and special hazard fire suppression systems.

Active vs. Non-Electric Detection

Deciding whether to use active or non-electric fire detection is also pivotal. Non-electric fire detection does not require electricity. If you lose power, you have the security that your fire protection system will work in case of a fire.

You will also want to think about the system offering and any services that come with it. Will the system give you around the clock detection? Does it notify authorities once triggered? What are the recommendations for ongoing inspection or testing of the equipment?

Fire protection systems play an important role in keeping building occupants, assets, and the building safe. If you are looking to protect critical equipment or machines, a fire suppression system may be your best option.

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Fire safety is integral for business owners. A fire can spark in an instant and spread within seconds. Businesses investing in fire protection systems realize significant benefits when the systems perform as expected. These systems reduce damage to the building, expensive equipment, documents, and inventory.

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what is emergency response team?
how does emergency response team work ?
who is involved in an emergency response team ?

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Move tall and heavy furniture away from the areas where people sleep, sit, or spend lots of time.

Fasten the heavy furniture to at least two studs of your wall

Shift heavy items on shelves closer to the ground.

Add protective plastic window film.

Store glassware inside closed cabinets.

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Living in earthquake country brings a special sense of uncertainty to life. It's time for all of us living in earthquake country to get "earthquake-proof" by securing our homes. Take a few simple steps, and you'll sleep safer at night.

We can't predict the smaller earthquakes, let alone the massive temblors. The next life-changing quake might happen at any time-even while you're reading this article! Preparing for earthquakes means minimizing your risks, not eliminating them.

Earthquake-Proof Your Living Spaces

We now know that the greatest risk in major earthquakes comes from falling and flying objects. The first step for everyone in earthquake country should be to make the interior of your home as earthquake-safe as possible.

How to Earthquake Proof the Interior of Your House

Move tall and heavy furniture away from the areas where people sleep, sit, or spend lots of time.

Fasten the heavy furniture to at least two studs of your wall using flexible earthquake straps.

Shift heavy items on shelves closer to the ground.

Move seats and tables away from large glass windows, or minimize the risk of shattering by adding protective plastic window film.

Store glassware inside closed cabinets.

For objects on open shelves, add a "lip edge" to the shelf or secure objects themselves with museum putty or velcro.

Earthquake-Proof Your Utilities

Ensure that your water heater is strapped to the wall with two metal braces and has flexible corrugated water connectors. Learn the location of your gas-shutoff valve, make sure you know how to turn it off, and keep the correctly sized wrench for it nearby. If you are often away from home, consider an automatic valve that will shut off the gas in the event of an earthquake.

The Earthquake Country Alliance provides an extensive list of instructions for securing most household items and fixtures. The Federal Alliance for Safe Homes also offers this handy video that covers all the basics of securing your interior space.

Assess Your Home's Structure

Now that you've locked down your interior space, consider the bones of your home. Even though most injuries come from flying objects and falls, houses still fall apart in earthquakes. If yours was built more than two decades ago and you're not sure about the foundation, take a look.

Go down to the crawlspace or basement below your house, and inspect the perimeter walls. Seismic strengthening of houses mostly concerns foundation bolting and "cripple wall" bracing. Homes built directly on concrete slabs will not have crawl spaces, however these types on homes will likely have anchor bolts or straps already. Many homes in California and the Pacific Northwest do not have basements. If your home does, a seismic retrofit of the basement can give your home a stronger foundation for the whole house.

You can hire a licensed engineer to perform an inspection of your home and identify the weaknesses in the structure that require a retrofit. The homeowner should also be present for the inspection.

The Role of Moisture

It's also important to understand how moisture will affect your foundation. The foundation of a home is affected by local soil and water conditions. California has soil with high clay content which expands and contracts when moisture is present. This can cause the foundation to move up and down and ultimately damage structure of a home. To help avoid moisture, keep your roof gutters clean, and make sure rainwater is always running into drains and not under the house.

Foundation Bolting

Most importantly, ensure that the "sill plate" of the house-the bottom of the walls-is corrected bolted to the foundation. Look for steel anchor plates that are installed at least every 4 to 6 feet along the sill. These anchors keep the house from rolling off the foundation during the massive shaking of a major earthquake.

If they don't exist, are too few, or are installed incorrectly, you'll likely want to act fast and hire a contractor or tackle the job yourself (if you are skilled and bold).

Bracing 'Cripple Walls'

Cripple walls arise generally when a house has a small area like a crawlspace between the foundation and the first story of the house. These short, wood-framed walls basically act like shock absorbers during earthquakes. They can become a major factor in earthquake damage if they are unreinforced.

To strengthen cripple walls, homeowners usually install vented plywood sheathing or diagonal sheathing along the length of the walls. Properly done work will fasten the foundation, sill, cripple wall, and floor joists into one unbreakable mass.

Earthquake-Proof Planning

Along with making a family emergency plan, storing essential supplies in an earthquake kit, and meeting your neighbors, assessing the safety of your home ranks high on the earthquake preparation checklist.

The overall task might seem daunting, but if you take it one step at a time (perhaps start by securing one piece of furniture per day) you'll advance to "earthquake-proof" soon enough. And you'll be feeling much safer when the Big One finally hits.
Reviewed by Victoria Chames. Original publish date January 10, 2019.

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Living in earthquake country brings a special sense of uncertainty to life. It's time for all of us living in earthquake country to get "earthquake-proof" by securing our homes. Take a few simple steps, and you'll sleep safer at night. We can't predict the smaller earthquakes, let alone the massive temblors.

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Considerable work has been done in seismology to explain the characteristics of the recorded ground motions in earthquakes. Such knowledge is needed to predict ground motions in future earthquakes so that earthquake-resistant structures can be designed. Although earthquakes cause death and destruction through such secondary effects as landslides, tsunamis, fires, and fault rupture, the greatest losses-both of lives and of property-result from the collapse of man-made structures during the violent shaking of the ground. Accordingly, the most effective way to mitigate the damage of earthquakes from an engineering standpoint is to design and construct structures capable of withstanding strong ground motions.

Interpreting recorded ground motions

Most elastic waves recorded close to an extended fault source are complicated and difficult to interpret uniquely. Understanding such near-source motion can be viewed as a three-part problem. The first part stems from the generation of elastic waves by the slipping fault as the moving rupture sweeps out an area of slip along the fault plane within a given time. The pattern of waves produced is dependent on several parameters, such as fault dimension and rupture velocity. Elastic waves of various types radiate from the vicinity of the moving rupture in all directions. The geometry and frictional properties of the fault critically affect the pattern of radiation from it.

The second part of the problem concerns the passage of the waves through the intervening rocks to the site and the effect of geologic conditions. The third part involves the conditions at the recording site itself, such as topography and highly attenuating soils. All these questions must be considered when estimating likely earthquake effects at a site of any proposed structure.

Experience has shown that the ground strong-motion recordings have a variable pattern in detail but predictable regular shapes in general (except in the case of strong multiple earthquakes). An example of actual shaking of the ground (acceleration, velocity, and displacement) recorded during an earthquake is given in the . In a strong horizontal shaking of the ground near the fault source, there is an initial segment of motion made up mainly of P waves, which frequently manifest themselves strongly in the vertical motion. This is followed by the onset of S waves, often associated with a longer-period pulse of ground velocity and displacement related to the near-site fault slip or fling. This pulse is often enhanced in the direction of the fault rupture and normal to it. After the S onset there is shaking that consists of a mixture of S and P waves, but the S motions become dominant as the duration increases. Later, in the horizontal component, surface waves dominate, mixed with some S body waves. Depending on the distance of the site from the fault and the structure of the intervening rocks and soils, surface waves are spread out into long trains.

Constructing seismic hazard maps

In many regions, seismic expectancy maps or hazard maps are now available for planning purposes. The anticipated intensity of ground shaking is represented by a number called the peak acceleration or the peak velocity.

To avoid weaknesses found in earlier earthquake hazard maps, the following general principles are usually adopted today:

1. The map should take into account not only the size but also the frequency of earthquakes.

2. The broad regionalization pattern should use historical seismicity as a database, including the following factors: major tectonic trends, acceleration attenuation curves, and intensity reports.

3. Regionalization should be defined by means of contour lines with design parameters referred to ordered numbers on neighbouring contour lines (this procedure minimizes sensitivity concerning the exact location of boundary lines between separate zones).

4. The map should be simple and not attempt to microzone the region.

5. The mapped contoured surface should not contain discontinuities, so that the level of hazard progresses gradually and in order across any profile drawn on the map.

Developing resistant structures

Learn about the Mexico City earthquake of 1985 and the structural engineering of Torre Mayor building

Contunico © ZDF Enterprises GmbH, Mainz See all videos for this article

View seismic researchers in Japan studying seismic shocks and developing methods to limit the potential damage by future earthquakes

Contunico © ZDF Enterprises GmbH, Mainz See all videos for this article

Developing engineered structural designs that are able to resist the forces generated by seismic waves can be achieved either by following building codes based on hazard maps or by appropriate methods of analysis. Many countries reserve theoretical structural analyses for the larger, more costly, or critical buildings to be constructed in the most seismically active regions, while simply requiring that ordinary structures conform to local building codes. Economic realities usually determine the goal, not of preventing all damage in all earthquakes but of minimizing damage in moderate, more common earthquakes and ensuring no major collapse at the strongest intensities. An essential part of what goes into engineering decisions on design and into the development and revision of earthquake-resistant design codes is therefore seismological, involving measurement of strong seismic waves, field studies of intensity and damage, and the probability of earthquake occurrence.

Earthquake risk can also be reduced by rapid post-earthquake response. Strong-motion accelerographs have been connected in some urban areas, such as Los Angeles, Tokyo, and Mexico City, to interactive computers. The recorded waves are correlated with seismic intensity scales and rapidly displayed graphically on regional maps via the World Wide Web.

Seismological tomography

Seismological data on the Earth's deep structure come from several sources. These include P and S waves in earthquakes and nuclear explosions, the dispersion of surface waves from distant earthquakes, and vibrations of the whole Earth from large earthquakes.

One of the major aims of seismology is to infer the minimum set of properties of the Earth's interior that will explain recorded seismic wave trains in detail. Notwithstanding the tremendous progress made in the exploration of the Earth's deep structure during the first half of the 20th century, realization of this goal was severely limited until the 1960s because of the laborious effort required to evaluate theoretical models and to process the large amounts of earthquake data recorded. The application of high-speed computers with their enormous storage and rapid retrieval capabilities opened the way for major advances in both theoretical work and data handling.

Since the mid-1970s, researchers have studied realistic models of the Earth's structure that include continental and oceanic boundaries, mountains, and river valleys rather than simple structures such as those involving variation only with depth. In addition, various technical developments have benefited observational seismology. For example, the implications of seismic exploratory techniques developed by the petroleum industry (such as seismic reflection) have been recognized and the procedures adopted. Equally significant has been the application of three-dimensional imaging methods to the exploration of the Earth's deep structure. This has been made possible by the development of very fast microprocessors and computers with peripheral display equipment.

The major method for determining the structure of the Earth's deep interior is the detailed analysis of seismograms of seismic waves. (Such earthquake readings also provide estimates of wave velocities, density, and elastic and inelastic parameters in the Earth.) The primary procedure is to measure the travel times of various wave types, such as P and S, from their source to the recording seismograph. First, however, identification of each wave type with its ray path through the Earth must be made.

Seismic rays for many paths of P and S waves leaving the earthquake focus F are shown in the figure. Rays corresponding to waves that have been reflected at the Earth's outer surface (or possibly at one of the interior discontinuity surfaces) are denoted as P P, P S, S P, P S S, and so on. For example, P S corresponds to a wave that is of P type before surface reflection and of S type afterward. In addition, there are rays such as p P P, s P P, and s P S, the symbols p and s corresponding to an initial ascent to the outer surface as P or S waves, respectively, from a deep focus.

An especially important class of rays is associated with a discontinuity surface separating the central core of the Earth from the mantle at a depth of about 2,900 km (1,800 miles) below the outer surface. The symbol c is used to indicate an upward reflection at this discontinuity. Thus, if a P wave travels down from a focus to the discontinuity surface in question, the upward reflection into an S wave is recorded at an observing station as the ray P c S and similarly with P c P, S c S, and S c P. The symbol K is used to denote the part (of P type) of the path of a wave that passes through the liquid central core. Thus, the ray S K S corresponds to a wave that starts as an S wave, is refracted into the central core as a P wave, and is refracted back into the mantle, wherein it finally emerges as an S wave. Such rays as S K K S correspond to waves that have suffered an internal reflection at the boundary of the central core.

The discovery of the existence of an inner core in 1936 by the Danish seismologist Inge Lehmann made it necessary to introduce additional basic symbols. For paths of waves inside the central core, the symbols i and I are used analogously to c and K for the whole Earth; therefore, i indicates reflection upward at the boundary between the outer and inner portions of the central core, and I corresponds to the part (of P type) of the path of a wave that lies inside the inner portion. Thus, for instance, discrimination needs to be made between the rays P K P, P K i K P, and P K I K P. The first of these corresponds to a wave that has entered the outer part of the central core but has not reached the inner core, the second to one that has been reflected upward at the inner core boundary, and the third to one that has penetrated into the inner portion.

By combining the symbols p, s, P, S, c, K, i, and I in various ways, notation is developed for all the main rays associated with body earthquake waves. The symbol J has been introduced to correspond to S waves in the inner core, should evidence ever be found for such waves.

Finally, the use of times of travel along rays to infer hidden structure is analogous to the use of X-rays in medical tomography. The method involves reconstructing an image of internal anomalies from measurements made at the outer surface. Nowadays, hundreds of thousands of travel times of P and S waves are available in earthquake catalogs for the tomographic imaging of the Earth's interior and the mapping of internal structure.

Structure of the Earth's interior

Studies with earthquake recordings have given a picture inside the Earth of a solid but layered and flow-patterned mantle about 2,900 km (1,800 miles) thick, which in places lies within 10 km (6 miles) of the surface under the oceans.

The thin surface rock layer surrounding the mantle is the crust, whose lower boundary is called the Mohorovičić discontinuity. In normal continental regions the crust is about 30 to 40 km thick; there is usually a superficial low-velocity sedimentary layer underlain by a zone in which seismic velocity increases with depth. Beneath this zone there is a layer in which P-wave velocities in some places fall from 6 to 5.6 km per second. The middle part of the crust is characterized by a heterogeneous zone with P velocities of nearly 6 to 6.3 km per second. The lowest layer of the crust (about 10 km thick) has significantly higher P velocities, ranging up to nearly 7 km per second.

In the deep ocean there is a sedimentary layer that is about 1 km thick. Underneath is the lower layer of the oceanic crust, which is about 4 km thick. This layer is inferred to consist of basalt that formed where extrusions of basaltic magma at oceanic ridges have been added to the upper part of lithospheric plates as they spread away from the ridge crests. This crustal layer cools as it moves away from the ridge crest, and its seismic velocities increase correspondingly.

Below the mantle lies a shell that is 2,255 km thick, which seismic waves show to have the properties of a liquid. At the very centre of the planet is a separate solid core with a radius of 1,216 km. Recent work with observed seismic waves has revealed three-dimensional structural details inside the Earth, especially in the crust and lithosphere, under the subduction zones, at the base of the mantle, and in the inner core. These regional variations are important in explaining the dynamic history of the planet.

Long-period oscillations of the globe

Sometimes earthquakes are large enough to cause the whole Earth to ring like a bell. The deepest tone of vibration of the planet is one with a period (the length of time between the arrival of successive crests in a wave train) of 54 minutes. Knowledge of these vibrations has come from a remarkable extension in the range of periods of ground movements that can be recorded by modern digital long-period seismographs that span the entire allowable spectrum of earthquake wave periods: from ordinary P waves with periods of tenths of seconds to vibrations with periods on the order of 12 and 24 hours such as those that occur in Earth tidal movements.

The measurements of vibrations of the whole Earth provide important information on the properties of the interior of the planet. It should be emphasized that these free vibrations are set up by the energy release of the earthquake source but continue for many hours and sometimes even days. For an elastic sphere such as the Earth, two types of vibrations are known to be possible. In one type, called S modes, or spheroidal vibrations, the motions of the elements of the sphere have components along the radius as well as along the tangent. In the second type, which are designated as T modes, or torsional vibrations, there is shear but no radial displacements. The nomenclature is n S l and n T l, where the letters n and l are related to the surfaces in the vibration at which there is zero motion. Four examples are illustrated in the figure. The subscript n gives a count of the number of internal zero-motion (nodal) surfaces, and l indicates the number of surface nodal lines.

Several hundred types of S and T vibrations have been identified and the associated periods measured. The amplitudes of the ground motion in the vibrations have been determined for particular earthquakes, and, more important, the attenuation of each component vibration has been measured. The dimensionless measure of this decay constant is called the quality factor Q. The greater the value of Q, the less the wave or vibration damping. Typically, for o S ₁₀ and o T ₁₀, the Q values are about 250.

The rate of decay of the vibrations of the whole Earth with the passage of time can be seen in the figure, where they appear superimposed for 20 hours of the 12-hour tidal deformations of the Earth. At the bottom of the figure these vibrations have been split up into a series of peaks, each with a definite frequency, similar to that of the spectrum of light. Such a spectrum indicates the relative amplitude of each harmonic present in the free oscillations. If the physical properties of the Earth's interior were known, all these individual peaks could be calculated directly. Instead, the internal structure must be estimated from the observed peaks.

Recent research has shown that observations of long-period oscillations of the Earth discriminate fairly finely between different Earth models. In applying the observations to improve the resolution and precision of such representations of the planet's internal structure, a considerable number of Earth models are set up, and all the periods of their free oscillations are computed and checked against the observations. Models can then be successively eliminated until only a small range remains. In practice, the work starts with existing models; efforts are made to amend them by sequential steps until full compatibility with the observations is achieved, within the uncertainties of the observations. Even so, the resulting computed Earth structure is not a unique solution to the problem.

Bruce A. Bolt

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earthquake - Methods of reducing earthquake hazards

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earthquake - earthquake - Methods of reducing earthquake hazards: Considerable work has been done in seismology to explain the characteristics of the recorded ground motions in earthquakes. Such knowledge is needed to predict ground motions in future earthquakes so that earthquake-resistant structures can be designed.

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- emergency response teams are groups of people who prepare for and respond to emergency incidents such as natural disasters, security threats, public health crises, or other potential business disruptions.
- the goal of an ERT is to restore or maintain operations and minimize losses during an emergency incident.
- roles: emergency commander, emergency coordinator, first aid squad