1. 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.
2. 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.
3. H.F. Reid of Johns Hopkins University was the first to explain the actual mechanism by which earthquakes are generated.
4. 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 vibration we call an earthquake.
5. To the contrary, such faults are the most dangerous because the forces are building up rather than being gradually released by continual creep. The result may be a devastating earthquake.
6. 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.
7. 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.
8. Circum-Pacific belt
9. thirty
10. Fire, landslides and ground subsidence, and seismic sea waves (tsunamis) are all capable of adding to the destructive nature of earthquakes.
11. A tsunami is a seismic sea wave formed by the displacement of the ocean floor during an earthquake.
12. The asthenosphere, located between 70–700 kilometers deep, consists of approximately 10% melted rock. This zone lies wholly within the mantle. The lithosphere lies above the asthenosphere and includes the crust and part of the upper mantle (that part above the asthenosphere). The asthenosphere behaves plastically: the lithosphere is rigid.
13. Rock deformation describes how the shape and volume of a rock change in response to stress. Think of a small, reference cube or sphere embedded in an undeformed rock. With the application of stress, the rock deforms (undergoes strain) and any changes in the volume and dimensions of the reference object are recorded by the strain. Depending on the magnitude and type of stress involved, deformation may also produce changes in the location and orientation of a rock.
14. Brittle deformation describes material failure by cracking and rupture.
Faults and joints in rocks are good examples. Brittle deformation is favored by
shallow depths, low rock temperatures, and
massive rigid rocks. Ductile deformation describes material failure by internal
flowage; recrystallization is
usually involved, especially at elevated
temperatures. Ductile deformation is enhanced by elevated temperatures and
confining pressures. Folding at
great depths and elevated temperatures is
accomplished by ductile (plastic) flowage without rupture.
15. Anticlines and synclines are both linear folds, anticlines being upfolds and synclines downfolds. Domes and basins result from upwarping and downwarping, respectively. When exposed by erosion, basins have the youngest rocks exposed at the center and the oldest rocks at the outer edge. The sequence is reversed for domes. Though both anticlines and domes have upwarped strata, anticlines are linear in form while domes are nearly circular, with beds plunging away in all directions from the center.
16. While both result primarily in vertical movement, a normal fault occurs when rock above the fault plane moves down relative to the rock below, and reverse faults occur when the rock above the fault plane moves up relative to the rock below. Normal faults indicate the existence of tensional forces which pull the crust apart. Reverse faults result from compressional forces.
17. Both are dip-slip movements in which one block moves up and the other down along the fault surface. Assume that dip-slip faults with vertical dips (the fault surface is vertical) are normal faults. For dip-slip faults with inclinations or dips other than vertical, the hanging wall-footwall designation is very useful. The hanging wall block is the block that is entirely above the fault surface, and the footwall block is entirely below. In normal fault movement, the hanging wall block slides down along the fault surface with respect to the footwall block. Horizontal distances between points in the blocks are increased (stretched) and the stresses are tensional. In reverse fault movement, the hanging wall block slides upward along the fault surface with respect to the footwall block. Horizontal distances between points in the two blocks are decreased (shortened) and the stresses are compressional.
18. Transform fault
19. Convergent boundaries are most directly associated with mountain building.
20. 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.
21. Continental margins are characterized by their tectonic activity. Passive margins, the east coasts of North and South America for example, exhibit subdued, “quiet,” tectonic movements such as slow uplift and subsidence punctuated by occasional, localized faulting. They have wide continental shelves and their continental slopes merge seaward into abyssal plains. Passive margins form originally by continental rifting and are modified by erosion and deposition as they move away from a mid-ocean ridge. For this reason, they are also known as “trailing” margins.
Active continental margins occupy areas of plate convergence, subduction, and local, transform faulting. Tectonism, intrusion, and volcanism are active and long-lived. The western margins of North and South America are good examples.
22. 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, marine, sedimentary rocks.
23. Terrain is used in a physiographic or geomorphic sense to describe the form and nature of a landscape. For example, the terrain of southeastern Pennsylvania is characterized by gently rolling, forested hills interspersed with cultivated fields and open pastures in the valleys.
Terrane is a much more comprehensive, geologic term describing specific areas or regions with certain, common elements of lithology and geologic history. A terrane can vary from a single, truncated seamount enclosed in an accretionary wedge complex to distinctive rock masses with much larger dimensions. Terranes may have oceanic, island arc, or continental affinities. Some may have rifted apart from the parent continent, then recombined in a subsequent compressive event. Others, such as some along the western margin of North America, formed elsewhere and migrated to their present-day locations.