Anatomy of Earthquake PDF

## Lesson 6.5: Faults Generate Earthquakes **Main Idea:** When an earthquake occurs strong enough for you to feel it, you recognize it as the shaking that affects anyone and anything that stands on the ground. **EQ:** What is the relationship between faults and earthquakes? When a material is co...

## Lesson 6.5: Faults Generate Earthquakes **Main Idea:** When an earthquake occurs strong enough for you to feel it, you recognize it as the shaking that affects anyone and anything that stands on the ground. **EQ:** What is the relationship between faults and earthquakes? When a material is compressed or stretched, a type of potential energy called elastic potential energy is stored in the material. If the stress is released and the material does not break after compression or stretching, the stored energy is released as kinetic energy and the material returns to its original shape. But if the material is compressed or stretched past its limit, it will break. The stored energy is released as kinetic energy, but the material becomes permanently deformed. You know that the plates of the crust are continuously moving away, toward or against each other. These plate movements cause the rocks of the plates to be stretched or compressed, and elastic energy is stored in the rocks as this happens. When the rocks break along a fault, the stored elastic energy is released as a special type of kinetic energy called seismic energy. This energy passes through rocks as seismic waves, and spread from their point of origin toward Earth's surface. Most people do not realize that when they experience earthquakes, they are actually feeling the arrival of seismic waves from deep within Earth. ## Lesson 6.6: Anatomy of an Earthquake **Main Idea:** Earthquakes usually begin several kilometers deep inside Earth at a single point called focus, and then travel toward the surface as seismic waves. **EQ:** How do seismic waves reach Earth's surface? The *focus* is the point in the rock's zone of weakness where the breaking of the rocks first starts and seismic energy is released. This point is also known as *hypocenter*. It is located within the fault plane, which is the breaking surface underground. The fault plane itself is where movement takes place, while the fault line is any edge of the fault plane that shows how much movement has taken place. When the fault plane is exposed above ground, it is called a *fault scarp*, which is usually the best sign that there is a fault in the area. | Description | | |---|---| | --- | *Fault Scarp* | | --- | *Epicenter* | | --- | *Wave Fronts* | | --- | *Focus* | | --- | *Fault Plane* | | --- | *Fault* | ___ Imagine a still pond or lake. When you throw a stone into the water, ripples form on the water surface in circles that are centered on the entry point of the stone. The ripples then move away from the center in circles that grow in size until they disappear or hit the edge of the pond. When seismic energy is released during an earthquake, the focus can be compared to the stone's entry point in the water, and the seismic waves move away from the focus like the ripples until they reach Earth's surface. But unlike ripples on the water's surface, seismic energy moves three-dimensionally as spheres that radiate in all directions away from the focus and toward Earth's surface. When an earthquake happens, news reports usually announce the location of the *epicenter*, which is the point on Earth's surface directly above the focus. The epicenter is usually described by its distance to the closest populated area, which gives an idea of whether the earthquake has caused a significant amount of damage. Do you think a strong earthquake would have the same impact if it happened in an area where there were no communities present? ## Earthquakes and Their Depth Earthquakes may be classified according to the depth of their focus as shown in the table below. About 90 percent of all earthquakes occur less than 100 km from the surface, whereas only about 3 percent may be classified as deep. In general, shallow-focus earthquakes cause more destruction than deep-focus earthquakes because the seismic waves travel a shorter distance from the focus to the surface. Less energy is lost along the way from shallow-focus earthquakes than from deep-focus earthquakes. However, there are many other factors that determine whether an earthquake will do a lot of damage to a certain area, though the most important factor to consider is the presence of people. | Type of Earthquake | Focal Depth from Earth's Surface | |---|---| | Shallow-focus | < 70 km | | Intermediate-focus | 70-300 km | | Deep-focus | >300 km | ## Tsunamis Earthquakes that occur underwater can generate tsunamis. Large undersea earthquakes can cause a section of the ocean floor to rise which, in turn, displaces and causes the water above it to rise. A large volume of water is pushed to the surface, creating a huge wave referred to as tsunami. The waves generated in the deep ocean are small. But as they approach the coast, the waves become bigger, which can cause tremendous damage when they reach land.