An earthquake is the release of energy from the earth's tectonic plates. The zone where two tectonic plates come together is called a fault. Prior to an earthquake, tectonic forces result in a gradual buildup of strain energy stored on either side of the fault. When the local stresses along the fault become too large, the fault slips suddenly or ruptures and releases the stored strain energy. This rupture on the fault plane is called the focus and the projection of this point on the ground surface is called the epicenter.

When a rupture occurs along a fault, the strain energy stored on either side of the fault is released in the form of seismic waves and heat. These seismic waves propagate away from the ruptured fault zone and through the geologic layers of rock and soil. The process of seismic wave propagation causes the ground to shake.

The most common type of earthquake is a shallow event where two tectonic plates slide past one another. Deeper earthquakes usually occur when one plate moves toward and under another plate. Other earthquakes can also occur as a result of volcanic activity, collapses of the ground, and man-made explosions.

Three types of waves are created when energy is released in an earthquake. The P wave, or primary wave, is the fastest and can move through both liquid and solid rock. P waves, like sound waves, are compressional waves, which mean that they compress and expand matter as they move through it. S waves, or secondary waves, are the waves directly following the P waves. S waves travel at right angles to the direction of motion and can only travel through solid matter. S waves are more significant than P waves because they are usually larger and produce both vertical and horizontal motion in the ground surface. Both P and S waves are called body waves because they move within the Earth's interior. The third type of wave is the surface wave, which is the slowest of the three waves. These waves move close to or on the outside surface of the ground.

Geologists use seismographs to record surface and body waves. When motion is recorded, a seismogram is created, which tells how big the waves were and how long they lasted. Using several seismograph stations records, the epicenter and focus can be located through triangulation.

Earthquakes can be quantified in several ways. The first way is to describe the earthquake's intensity. Intensity is the measure of damage to the ground surface and the effects on humans. The most common scale is the Modified Mercalli Scale, which uses a twelve-point Roman numeral scale to describe damage. While intensity helps determine the extent of the damage, it is not an objective measure of the actual earthquake size, because the intensity can vary greatly in the same area due to different geologic conditions.

The second method of measurement is the magnitude of the earthquake. Magnitude depends on wave amplitude and distance measured from seismograms. The most common scale is the Richter scale, which measures the magnitude on a logarithmic scale.

The third type of measurement is called the seismic moment. Using seismic waves and field measurements that describe the fault area, the seismic moment, a parameter related to the angular leverage of the forces producing slip on a fault, can be measured. This moment is related to a corresponding magnitude known as the moment magnitude. This type of measurement gives a consistent and uniform measure of the size of an earthquake of any magnitude anywhere in the world and is considered very accurate because it takes into account fault geometry.

Several geological factors affect the intensity of ground shaking at a site. The magnitude of the earthquake, distance to the epicenter or focus, and soil conditions at a site can greatly affect the amount of damage a region will experience. In general, ground shaking at sites less than 5 kilometers from the fault rupture would be twice as strong as that felt at 10 to 15 km away. Ground shaking near faults can also generate pulses that impose large displacement demands on structures. Significant deposits of soft soil at a site can also amplify seismic waves and increase displacement demands on buildings. Soft, saturated soils also have the potential for liquefaction, in which soils lose shear strength and cannot support the structures founded on them.

Ground shaking in earthquakes causes vibratory motions at the base of structures, and the structure actively responds to these motions. Damage occurs when the displacements imposed on the structure cause the building to deform beyond its elastic state. The severity of the damage depends of the type of inelastic deformations that occur in a building. In general, structures that can deform in a ductile manner, similar to the bending of a tree, are sturdy and have the ability to protect lives. On the other hand, structures that deform in a brittle manner, similar to the snapping of a twig, have the potential for sudden failure and collapse and can cause human casualties.

The chances of a moderate to large earthquake depend on the seismicity of the region. The West Coast is the most active region in the United States for earthquakes since it is where two of the earth's tectonic plates, the Pacific Plate and the North American Plate, meet. However, this does not mean the other parts of the country do not experience earthquakes. The largest earthquake recorded in North America took place on the New Madrid Fault in Missouri in 1811. The Charleston, South Carolina region has also experienced significant earthquakes in the past hundred years.

The time between earthquakes is also important in determining the magnitude of an earthquake. The longer the forces build up along a fault, the more energy will be released when the fault ruptures, creating a larger earthquake. Recent building codes have required buildings to be life-safe from forces generated by an earthquake with an approximate return period of 500 years.

The greatest risk from an earthquake is that to life safety. In past earthquakes, many buildings have collapsed, killing thousands of people. Modern building code requirements are set with the intent of protecting life safety. The building may be damaged beyond repair, but the building has not collapsed, allowing safe evacuation, and the overall risk of life-threatening injury is low.

The problem today is that many buildings were designed and constructed before modern seismic codes were enacted. Therefore, there is still significant risk to life safety in the event of a major earthquake. Some types of buildings are more susceptible to collapse than others. Older, unreinforced masonry buildings are one of the most vulnerable types.

Traditional seismic codes have focused on ensuring a life safe performance and offered some expectation that the damage would be repairable. However, in recent moderate and large earthquakes, while modern buildings have performed as designed, structures have been irreparably damaged or too costly to repair. Economic losses due to property damage have been extensive in recent years and have led to the development of new performance-based methodologies with the intent on controlling property damage and losses.

Another economic factor in assessing damage following earthquakes is the risk of business interruption. Most of the revenue generated by companies is related to the products and services they provide to the public, rather than the physical assets of the company. Any significant interruption to the production of these goods and services can have an adverse effect on companies, including putting them out of business. Technology companies in the Silicon Valley are just such a sector where business interruption is a critical issue. The recent performance-based guidelines developed by the structural engineering community focus on the design of new structures and strengthening of existing buildings with the intent of minimizing business interruption.

The healthcare services industry is another sector where continued operation following an earthquake is critical. Recent legislation enacted by the State of California has required that hospital acute care facilities must upgrade their buildings to be operational following a major earthquake by the year 2030.

Your family should always know how to contact one another following an earthquake to let everyone know your personal status. The most efficient way to accomplish this is to have everyone call a common relative or friend out of state and leave a message. This person should act as a contact and relay any messages from any other family member who calls. This should be done as soon as possible following an earthquake, as phone lines in and out of the region will most likely become jammed with traffic.

For parents with children, you should know your school's policy on where the children are to go following an earthquake. If it is unlikely for you to pick up the children at school, please arrange for a neighbor, relative or family friend to pick them up and go to a prearranged location.

Fires following earthquakes are one of the greatest threats to life and property. Several things that can be done to help reduce the potential for fire following the earthquake:

• Strap the water heater securely to the wall to prevent toppling and a leaking gas line,
• Install smoke detectors throughout the house,
• Install fire extinguishers in high-risk areas (kitchen, etc.) and learn how to use them properly,
• Learn the location of utility shut-off points (gas, water, etc.) and learn how to shut them down,
• Store gasoline, other flammable liquids, pesticides, household chemicals, and other hazardous materials
• in unbreakable containers and move them away from possible sources of ignition.

Emergency Supplies

The average home has on hand some of the supplies and food required for up to three days, which may be the length of time before utilities are restored. The list below will help you determine any items not on hand, or items that should be stored together in case of emergency evacuation. Stocking your car or workplace with some of these supplies may also be desirable, since families ma be physically separated in an emergency.

Water and Food

• Water. Each person requires one to two gallons a day for drinking. Also learn how to purify water, either through tablets or heat.
• Food. Keep pantry shelves well-stocked, preferably with canned foods and dried foods that can be eaten without cooking or refrigeration. Be sure to plan for any special diets. Don't forget a manual can opener.

Medical and General Hygiene

• An ample stock of hygiene products, such as toothpaste, toothbrush, and toilet paper.
• A spare pair of eyeglasses (you may not be able to continue wearing contacts) and an extra supply of any necessary medication.
• Any supplies needed for special care of babies, seniors and the disabled.
• A first aid kit and book. Taking a first aid course or CPR course will also be beneficial.

Tools and Hardware

• A working flashlight in a convenient location.
• A portable radio to listen to emergency information.
• Spare batteries (store them in the refrigerator to extend their life).
• A wrench for shutting off gas and water services.
• A fire extinguisher in high-risk areas.

Actions to Take

The following are actions that should be taken in the event of an earthquake.

During an Earthquake

• If you are inside, stay in, get under a desk or table or brace yourself in a doorway, and stay away from windows, bookcases, chimneys and mirrors.
• If you are outside, stay outside, move to an open area, and beware of overhead wires, chimneys and falling objects.
• If you are in a car, pull over and stop, but not near power lines or under an overpass. Stay in your car until the shaking stops. If a line from a power pole falls onto your car, consider it a live electrical wire even if you see no sparks or arcing.

After an Earthquake

• Check for fires, leaking gas and injuries.
• Look for people who may be hurt or trapped.
• Do not use telephones except to require emergency help.
• Turn on a portable radio for instructions and news.
• Clean up spilled flammable liquids, chemicals and medicines.
• Check for leaking sewage under house or near street before using toilet.
• Organize emergency supplies and important legal documents in case of evacuation.
• Draw a moderate amount of cold water in the bathtub in case water service is disrupted later.
• Keep streets clear for emergency vehicles.

There are two types of problems associated with seismic damage to dwellings. The first type is structural damage. This is damage caused by an earthquake that directly affects the capability of the house to stand up. The second type of damage is classified as nonstructural damage. This type of damage does not affect the integrity of the structure, but may prevent the use of structure after an earthquake.

What are Common Structural Deficiencies?

The most common structural deficiencies in houses are related to the foundation. Since the foundation supports the rest of the house, any damage to the foundation will most like affect the integrity of the structure. Many wood-frames homes, typically older housing, may not be adequately anchored or bolted to the foundation. Without proper anchorage, a house can slide off the foundation during the earthquake, severely damaging or destroying the house.

Another common deficiency in the foundation is an unbraced "cripple wall." This is the short wall that connects the foundation to the floor of the house and encloses the home's crawl space. If these walls are not braced with plywood, they have the potential to lean or collapse, sending the house crashing down to the foundation.

What are Common Nonstructural Deficiencies?

Even if a house is structurally sound, damage can occur due to items in the house. This type of damage can prevent the use of the house and have a significant financial impact on repair of the house following an earthquake.

The most common items that can cause damage are unbraced water heaters (which may explode and cause a fire), unreinforced masonry chimneys, tall shelves and tall file cabinets. In addition to damage, toppling of these elements can cause severe injury.

How Do I Fix These Problems?

Many of the problems described above can be remedied through simple and inexpensive means. Nonstructural deficiencies can usually be mitigated using simple clips and straps bought at a local hardware store. Structural deficiencies may require the hiring of a general contractor or engineer.