Chapter 6 Questions - Earthquakes, Geologic Structures, and Mountain Building
How are faults, foci, and epicenters related?
A fault is a large fracture along which there is movement. When movement occurs, the zone within Earth where rock displacement occurs is termed the focus. The point on Earth's surface directly above the focus of an earthquake is called the epicenter.
What is a tsunami? How is one generated?
A tsunami is a seismic sea wave formed by the displacement of the ocean floor during an earthquake.
Describe the principle of a seismograph.
A weight is freely suspended from a support that is attached to bedrock. When earthquake waves reach the seismograph, the inertia of the weight keeps it motionless while the bedrock and the support vibrate.
What is an earthquake? Under what circumstances do earthquakes occur?
An earthquake is the vibration of Earth produced by the rapid release of energy, usually along a fault. A fault is a large fracture along which there is or has been movement. When slippage occurs, an earthquake results.
What is an accretionay wedge? Briefly describe its formation.
As a subducting plate bends and begins its descent into the mantle, most sediments are scraped from basaltic, oceanic crust and piled onto the leading edge of the upper plate, much as snow piles up in front of a moving snowplow. These sediments accumulate (accrete) as a wedge-shaped stack of reverse-fault slices with the tapered edge of the wedge pointing toward the subduction zone. This complexly deformed and faulted sediment pile is the accretionary wedge.
In addition to the destruction created directly by seismic vibrations, list three other types of destruction associated with earthquakes.
Fire, landslides and ground subsidence, and seismic sea waves (tsunamis) are all capable of adding to the destructive nature of earthquakes.
List the major difference between P and S waves.
P waves travel through all materials, whereas S waves are propagated only through solids. Further, in all types of rock, P waves travel faster than S waves.
How does plate tectonics help explain the existence of fossil marine life in rocks atop the Ural Mountains?
The Ural Mountains, a north-south range in west-central Russia, mark the closure site of an ancient marine basin that once existed between the European and Siberian parts of the Eurasian plate. As the two continents converged and joined, the sediments in the former marine basin were lithified, crumpled, and uplifted into a mountain range that includes fossiliferous (containing fossils), marine, and sedimentary rocks.
An earthquake measureing 7 on the Richter scale releases about _____ times more energy than an earthquake with a magnitude of 6.
Thirty-two
Explain what is meant by elastic rebound.
When stress is applied to crustal rocks, they respond by bending, and in doing so they store elastic energy much like a rubber band does when it is stretched. Once the strength of the rock is exceeded, the rock fractures and movement takes place along this fracture or fault. This slippage allows the deformed rocks to snap back to their original shape: a process called elastic rebound. It is the springing back of the rock that produces the vibrations we call an earthquake.
The San Andreas Fault is an excellent example of a _____ fault.
strike-slip fault
