Earthquake Glossary

Earthquake Glossary

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Earthquake Glossary Aftershocks When one earthquake happens, others usually take place at nearby locations. The earthquake with the largest magnitude is called the mainshock; anything before it is called a foreshock and anything after it is called an aftershock. Bigger earthquakes have more and larger aftershocks. As the magnitude of the mainshock increases, the magnitude of the largest aftershock increases as well. In general, an earthquake large enough to cause damage will produce several felt aftershocks within the first hour. The rate of aftershocks dies off quickly with time. Bolt One of the most common causes of damage to houses in an earthquake is when they slide off their foundation. Bolts are used to secure the house to its foundation which can prevent this major cause of damage. Crust The outermost layer of the Earth is called the crust. The difference between the crust and mantle is chemical, with different, denser rocks in the mantle. Epicenter The epicenter of an earthquake is the point on a fault where an earthquake starts. Earthquakes happen because of sudden slip on a fault. The stress in the Earth's crust pushes on the fault holding it together and causing frictional resistance between the rocks on each side. Eventually enough slip is built up and the rocks slip suddenly, releasing energy in the form of sound waves and shear waves that travel through the rock to cause the shaking that we feel as the earthquakes. Your distance from the fault plane is the most important factor in determining how much shaking you feel from a particular size earthquake. Fault Earthquakes always occur on faults. Faults are places in the earth where the rocks are broken and the rocks on one side have moved in some direction relative to the other. There are three types of faults --thrust, normal and strike-slip. A thrust occurs on a plane that is at an angle to the surface such that one side slides up over the other. Normal faults are also on angled fault planes, but one side moves down and away from the other. Strike-slip events occur on vertical faults and one side slides by the other. Strike-slip and thrust are the most common types in southern California. Every point on the fault plane releases energy so bigger faults produce bigger earthquakes. We can thus estimate how big an

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Earthquake Glossary

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earthquake to expect by mapping the length of the fault. The longest fault in southern California is the San Andreas fault that is long enough to produce a magnitude 8 earthquake. Intensity Magnitude represents the energy released in the earthquake and is not what you feel in the event. Intensity is judged by the size of the area affected by the earthquake and the damage that it does on that area. Magnitude Magnitude is the most commonly reported measure of an earthquake's size. They defined it in terms of the amplitude of ground velocity recorded on a particular seismograph, scaled by the distance from the instrument to the earthquake. P-waves Earthquakes produce three general types of waves to radiate energy. Two are body waves, which means that they travel through the body of the Earth and the other is surface waves, which means that they travel along the surface of the Earth. The two body waves are called P waves and S waves . P-waves are compressional waves while S waves are shear waves. Shear waves cannot travel through a fluid so Pwaves are the only ones that travel through the Earth's core . P waves travel faster, but S waves are usually 2-3 times larger than the P wave. The time between the P and S wave can tell you how far away the earthquake is. Richter Scale The Richter scale is used to describe the magnitude scale, or the power of the earthquake. Seismograph We record earthquakes through the use of seismographs. Seismographs are instruments that create an electrical signal when the ground moves. By comparing the signal from many seismographs at different locations we can determine if an earthquake occurred and by timing the waves that travel from the earthquake, determine where and when it occurred. Shaking The shaking one feels during an earthquake is not the direct movement of the fault but rather the movement carried by the waves generated because of that slip. This shaking, and not direct fault motion, is the primary cause of damage in an earthquake. Both thrust and strike-slip earthquakes produce both vertical and horizontal shaking. Strike-slip Strike-slip faults are fault planes where the plane of the fault is vertical and the rock blocks on either side move horizontally during an earthquake. The San Andreas is an example of a strike-slip fault in southern California.

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Earthquake Glossary

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Tsunamis When thrust-faulting earthquakes happen under the ocean, the earthquake can push large blocks of ocean floor up. When the ocean floor moves up, the water that was in that spot has to go somewhere else. That somewhere else is into a large wave called a tsunami. Uplift Uplift describes how the ground is lifted up during earthquakes. Many of southern California's mountains have formed because of repeated earthquakes. How tall the mountain depends both on how fast the faults are pushing it up and how long that fault has been active. How steep a mountain is tells you how fast its fault is moving. Waves A wave travels through a material when a force pushes on that material and the material resists being pushed. Earthquakes produce two types of body waves, called P- and S-waves. P waves are sound waves. The particle motion in all sound waves is compressional which means that the particles move in the same direction as the wave as described in the example above. S waves are shear waves. The particle motion in shear waves is perpendicular to the direction of the wave. Shear waves are only seen in solids and cannot transmit through a liquid or a gas.

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