What is the difference between a body seismic wave and a surface seismic wave? H
ID: 235597 • Letter: W
Question
What is the difference between a body seismic wave and a surface seismic wave? How do P- and S- waves differ from each other? What are the differences between L- and R -wavcs? How does seismicity differ at tectonic plate boundaries (convergent, divergent, transform)? What type(s) of deformation (brittle, elastic, plastic is/are involved in the release of seismic energy? Describe how the deformation leads to the energy release. What are the factors that would tend to produce a mass wasting event? What are the key differences between rock falls, debris flows, and landslides?Explanation / Answer
1)
Seismic waves are the waves of energy due to the rapid breaking of rock within the earth or an explosion. These energy travels through the earth and is recorded on seismographs. There are numerous kinds of seismic waves, and they move in diverse ways. The two major types of waves are body waves and surface waves. Body waves can move through the earth's inner layers, but the surface waves can move along the surface of the planet like undulations on water. Earthquakes radiate seismic energy as both body and surface waves. On traveling through the inner layers of the earth, body waves enter before the surface waves released by an earthquake. These waves have a high frequency than the surface waves.
2)
Divergent boundaries:- There are places on earth where two plates are spreading apart. Rift valleys and faults arise when the lithosphere is under tensional stress. From this dispersion zone, new magma comes up from the mantle, pushing two plates apart and adding new material at their edges. These zones are generally found in oceans along with mid-ocean ridges. This leads a different type of boundary.
Earthquakes with small Richter magnitudes along the boundaries with normal fault motion have a shallow focus with focal depths of less than 20km.This indicates the weak lithosphere must be thin along the diverging plate boundaries.
Transform boundaries:- These are found where plates move one another. Earthquakes along these boundaries experiences strike-slip motion on the faults, and tend to be shallow focus earthquakes with depths typically less than about 100 km. Richter magnitudes could be large.
Convergent boundaries:- These are the place where two tectonic plates converge (i.e. two plates move toward each other). The compressional stresses are active in these areas and it causes thrust or reverse faults being common. These are of two types:
Subduction boundaries occur where oceanic lithosphere is pushed underneath continental or oceanic lithospheres. When one plate overrides the other, pushing it slowly downward into the mantle, where it melts and form magma. A subducted lithosphere remains cold and brittle as it descends and can rupture under compressional stress. These fractures generate earthquakes with increasing focal depth under the overriding plate. This zone is defined as Benioff Zone. Depths of up to 700km are reached in the Benioff Zone.
Collisional boundaries occurred where two plates of continental lithosphere collide and result a fold-thrust mountain belts. The continental crust is compressed together as the plates push together and is forced upwards. This is called folding. Fold mountains are created by this process. Earthquakes takes place due to the thrust and faulting with focal depth range from shallow to about 200 km.
3)
Generally earthquakes occur along zones where the Earth's crust is experiencing deformation. Deformation is due to the plate tectonic forces and gravitational forces. The type of deformation usually occurs along zones where rocks fracture to create faults.
Interior of the Earth rocks are constantly subjected to forces/stresses and they tend to bend, twist, or fracture. When rocks bend, twist or fracture it is said to deform or strain. The forces which results in deformation are referred to as stresses.
When a rock is experiencing an increasing stress it changes its shape, size or volume. These changes in shape, size or volume is referred to as strain. When stress is applied to rock, the rock passes through 3 successive stages of deformation.
Ductile Deformation takes place where the strain is irreversible andElastic Deformation occurs when the strain is reversible.
Fracture occurs when irreversible strain wherein the material breaks.
Elastic deformation occurs due to relatively small forces and small displacements. The shape of material is restored when force is removed and energy can pass as waves.
When plastic (ductile) deformation occurs the materials react to forces by changing its shape and no storage of energy occurred.
In brittle deformation the material stores energy put in by the force and they build up stress and at some point the material will break. A catastrophic release of energy occurred due to frequent breaking of pre existing weak surfaces.
4)
When the gravitational force acting on a slope increases beyond its resisting force, slope failure or mass wasting takes place. The resisting forces helping to maintain stability of the slope including the slope material's strength, cohesion, the amount of internal friction between grains and any external support of the slope. These factors together define shear strength of the slope.
Many factors can cause mass wasting:- a change in slope gradient, declining of materials by weathering, increase in the amount of water content, changes in the vegetation cover, and overloading.
5)
Mass movements are usually classified on the basis of the major criteria 1) rate of movement 2) type of movement and 3) type of material involved in the movement (rock, soil, or debris).
Rockfalls are a regular type of tremendously rapid mass movement in which rocks of any size fall through the air. Rockfalls takes place along steep canyons, cliffs, and road cuts and build up gathering of loose rocks and rock waste at their base known as talus.
Debris flows are consisted of larger-sized particles and do not contain large amount of water. As a result, these are usually more viscous and normally do not move as rapidly, and not often are confined to preexisting channels. Debris flows may be damaging because they can bring large objects.
Landslide :-Geologists mainly use the term landslide to cover a large variety of mass movements which may cause loss of life, property damage, or a general disturbance of human activities.
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