6th Grade Science - Chapter 11

Explanation
When stress accumulates in Earth's crust, the rocks eventually reach their elastic limit. The elastic limit is the maximum amount of stress that a material, in this case, rocks, can withstand without permanently deforming or breaking. Once the rocks surpass their elastic limit, they can no longer return to their original shape, resulting in an earthquake. This phenomenon occurs due to the release of stored energy in the rocks, causing them to fracture and slip along fault lines.

Explanation
When rocks break because of stress, the energy released is in the form of an earthquake. Earthquakes occur when there is a sudden release of energy in the Earth's crust, causing the ground to shake. This energy is generated when accumulated stress within the rocks exceeds their strength, resulting in the breaking and movement of the rock layers along a fault line. This movement releases seismic waves that propagate through the Earth, causing the shaking and vibrations that we experience as an earthquake.

Explanation
Normal faults are created by tension. Tensional forces cause the hanging wall to move downward relative to the footwall, resulting in the formation of a normal fault. This type of faulting commonly occurs in areas where the Earth's crust is being stretched or pulled apart, such as along divergent plate boundaries or in rift zones. As the tensional forces pull the rocks apart, they create a space for the hanging wall to drop down, forming a fault scarp.

Explanation
When Earth's plates move apart, it creates a divergent boundary. At these boundaries, tension forces act on the rocks. As the plates separate, the rocks are pulled apart, causing them to experience tension. This tension can lead to the formation of faults, fractures, and the creation of new crust through volcanic activity. Therefore, the correct answer is "move apart."

Explanation
Compression forces are present where Earth's plates come together. When tectonic plates collide, they can create compressional forces that push against each other. This can result in the folding and uplifting of rock layers, forming mountains and mountain ranges. Compression forces can also cause earthquakes as the built-up stress is released.

Explanation
A reverse fault is often located where plates come together. In a reverse fault, the hanging wall moves up and over the footwall, resulting in compression and shortening of the crust. This type of fault is commonly found in areas of convergent plate boundaries, where two plates collide and one is forced beneath the other. The compressional forces in these regions cause rocks to break and move along the fault plane, leading to the formation of a reverse fault.

Explanation
The San Andreas Fault in California is an example of a strike-slip fault. In a strike-slip fault, the rocks on either side of the fault move horizontally past each other. This type of fault is characterized by a lack of vertical displacement, meaning that there is no significant up or down movement along the fault line. The San Andreas Fault is a famous example of a strike-slip fault because it marks the boundary between the Pacific Plate and the North American Plate, and is responsible for numerous earthquakes in the region.

Explanation
In a reverse fault, the rocks above the fault surface are pushed upwards and over the rocks below the fault surface. This occurs due to compressional forces that cause the rocks to move in opposite directions, resulting in the overlying rocks being forced up and over the underlying rocks. This type of fault is commonly associated with convergent plate boundaries, where tectonic plates collide and create compression forces that lead to the formation of reverse faults.

Explanation
Secondary waves, also known as shear waves, move through Earth by causing particles in rocks to vibrate at right angles to the direction of the waves. These waves are slower than primary waves and can only travel through solid materials. They are responsible for causing the side-to-side shaking motion during an earthquake.

Explanation
The radius of the circle seismologists draw on a map is equal to the distance from a station to an earthquake's epicenter. The epicenter is the point on the Earth's surface directly above the focus, which is the actual location where the earthquake originates. By measuring the time it takes for seismic waves to reach different stations, seismologists can determine the distance from each station to the epicenter. This information is then used to draw circles around each station, with the radius equal to the distance to the epicenter, allowing them to triangulate the exact location of the earthquake.

Explanation
To locate an earthquake's epicenter, scientists use information from at least three seismograph stations. The epicenter refers to the point on the Earth's surface directly above the focus, which is the actual location where the earthquake originates underground. By analyzing the time it takes for the seismic waves to reach different seismograph stations and comparing the data, scientists can triangulate the epicenter and determine its precise location. This information is crucial for understanding the earthquake's impact and for issuing appropriate warnings and response measures.

Explanation
The farther apart primary, secondary, and surface waves arrive, the farther away the epicenter is. This means that when the waves take longer to reach a certain location, it indicates that the epicenter, which is the point on the Earth's surface directly above the focus of an earthquake, is located at a greater distance from that location. Therefore, the correct answer is "farther away."

Explanation
Primary waves, also known as P-waves, are a type of seismic wave that can move through the Earth. These waves are characterized by their ability to travel through solids, liquids, and gases. As they propagate, primary waves cause particles in rocks to move back and forth in the same direction as the waves. This motion is similar to the way sound waves travel through air, creating compressions and rarefactions. Therefore, primary waves are responsible for the initial shaking experienced during an earthquake and are the fastest seismic waves to arrive at a given location.

Explanation
Most of the destruction during an earthquake is caused by surface waves. Surface waves are seismic waves that travel along the Earth's surface and cause the most damage because they have a larger amplitude and longer period compared to other types of seismic waves. These waves can produce strong shaking and ground displacement, leading to the collapse of buildings, landslides, and other forms of destruction. Primary waves, on the other hand, are the fastest seismic waves but generally cause less damage compared to surface waves.

Explanation
Surface waves are the slowest seismic waves. These waves travel along the Earth's surface and cause the most damage during earthquakes. They have a complex motion that includes both vertical and horizontal components. Surface waves are slower than primary (P) and secondary (S) waves because they travel through the outer layers of the Earth, where the material is less rigid. This causes them to have a lower velocity compared to the other types of seismic waves.

Explanation
Primary waves, also known as P-waves, are the first waves to arrive at a seismograph station during an earthquake. These waves are the fastest and can travel through both solids and liquids, causing particles to move back and forth in the same direction as the wave. Therefore, the correct answer is "first".

Explanation
Seismic waves are vibrations that travel through the Earth's interior during an earthquake. These waves change speed and path as they encounter different materials, allowing scientists to study the structure of the Earth's interior. By analyzing how seismic waves behave and measuring their travel times, scientists can infer the composition and properties of the various layers within the Earth, such as the crust, mantle, and core. Therefore, seismic waves are crucial in determining the structure of Earth's interior.

Explanation
The boundary between the upper mantle and the crust is called the Moho discontinuity. This term is derived from the name of the Croatian scientist Andrija Mohorovičić, who first discovered this boundary in 1909. The Moho discontinuity marks a significant change in seismic wave velocities, indicating the transition from the solid rock of the crust to the denser rock of the upper mantle. It is an important feature in understanding the Earth's structure and plate tectonics.

Explanation
The shadow zone refers to the area on Earth's surface where no seismic waves are detected after they are released by an earthquake. This occurs due to the bending and reflection of seismic waves as they pass through different layers of the Earth's interior. The shadow zone is a result of the refraction of seismic waves by the liquid outer core and the inability of these waves to travel through the solid inner core. As a result, seismic waves are not detected in this specific region, providing valuable information about the Earth's internal structure.

Explanation
The lithosphere is the correct answer because it is the outermost layer of the Earth. It is a rigid and solid layer that includes the crust and the uppermost part of the mantle. The lithosphere is divided into several tectonic plates that float and move on the semi-fluid asthenosphere beneath it. This layer is responsible for the formation of continents, ocean basins, and the Earth's topography.

Explanation
To make your home more seismic-safe, it is recommended to put heavy items on lower shelves. This is because during an earthquake, the lower shelves are less likely to tip over or fall, reducing the risk of injury or damage. Placing heavy items on upper shelves can lead to a higher center of gravity, making them more prone to falling and causing potential harm. By placing heavy items on lower shelves, the stability of the objects is increased, making the home safer during seismic activity.

Explanation
Primary waves, also known as P-waves, are a type of seismic wave that can travel through solids, liquids, and gases. However, they undergo a change in speed and direction when they pass through different mediums. When primary waves encounter the liquid outer core of the Earth, which is made up of molten iron and nickel, they experience a significant decrease in speed. This change in speed causes the waves to refract or bend, ultimately leading to their attenuation or stopping within the liquid outer core. Therefore, primary waves don't pass through the liquid outer core and are stopped when they hit it.

Explanation
The correct answer is "magnitude". The Richter scale is used to measure the magnitude of an earthquake, which is a measure of the energy released by the earthquake. The question is asking for a term that completes the sentence and indicates the factor by which the energy released by a 6.5 magnitude earthquake is greater than a 5.5 magnitude earthquake. Therefore, "magnitude" is the appropriate term to use in this context.

Explanation
Shaking from an earthquake can cause wet soil to become more liquid. The intense vibrations and movement during an earthquake can cause the water within the soil to separate and flow more freely, reducing its overall stability and making it behave more like a liquid. This phenomenon, known as liquefaction, can lead to the loss of soil strength and increased risk of landslides and structural damage during seismic events.