Earthquake Information |
About EarthquakesAn 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. 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. 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. WHAT ARE MY RISKS?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 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. 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. EARTHQUAKE PREPAREDNESSYour 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. 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:
Emergency SuppliesThe 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
Medical and General Hygiene
Tools and Hardware
Actions to TakeThe following are actions that should be taken in the event of an earthquake. During an Earthquake
After an Earthquake
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. 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. 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. |