Planet Earth Earthquake Notes PDF

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Prof. C.Valenti Planet Earth Earthquake Notes 1

Earthquakes Defined Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained beyond their elastic limit. If a strained rock breaks, it then snaps into a new position and, in the process of rebounding, generates vibrations called seismic waves. The rocks on opposites sides of the fault move with respect to each other, typically distances ranging from millimeters to many meters. Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. The source of an earthquake is called the focus, which is an exact location within the Earth were seismic waves are generated by sudden release of stored elastic energy. The epicenter is the point on the surface of the Earth directly above the focus. Sometimes the media get these two terms confused. Origin of Earthquakes Most natural earthquakes are caused by sudden slippage along a fault zone. Within the Earth rocks are constantly subjected to forces that tend to bend, twist, or fracture them. When rocks bend, twist or fracture they are said to deform or strain (change shape or size). The forces that cause deformation are referred to as stresses. The elastic rebound theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up in the deforming rocks on either side of the fault, when the slippage does occur, the energy released causes an earthquake. This theory was discovered by making measurements at a number of points across a fault. Prior to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake.

Planet Earth Earthquake Notes

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Earthquakes are an energy release in the form of seismic waves caused by the sudden rupture of strained rocks. Strain is deformation of rocks resulting from stress (e.g., tectonic forces). Mechanism of fault movement is explained by the Elastic Rebound Theory which states that strain energy (deformation and friction) builds up on rocks as they are forced in different directions. Rocks on either side of a fault undergo elastic strain as they are stressed by tectonic forces. When the stress exceeds the strength of the rock, it breaks and the rocks abruptly slip past one another along the rupture (fault). When slippage and rupture occur along the fault the stored energy is released as seismic waves that radiate out in all directions and the rocks "rebound" to their original undeformed shape.

Stages of Elastic Rebound Model EQ Cycle Demo: bend a stick until it snaps. Energy is stored in the elastic bending and is released if rupture occurs, causing the fractured ends to vibrate and send out sound waves.  Fault is a break in a rock.  Tectonic forces push on two rock slabs in different directions. Friction between the rocks along the fault causes the rocks to remain put.  Continual forces 'deform the rock' and causes the rocks to build up energy (deformation is called strain).  Eventually the forces pushing on the rocks exceed the strength of friction between the rocks along a fault, the rocks break, are displaced (slippage along the fault occurs) and seismic energy is released. The deformed rock snaps back to its original shape; strain is relieved.  Pressure builds up again and repeats the process. Buildup of strain may occur over hundreds of years and then suddenly released in one good shot.  Sometimes faults creep along gradually releasing energy in much shorter periods of time.  The recurrence interval is the time it takes to accumulate sufficient elastic strain to cause the next EQ. 

Planet Earth Earthquake Notes

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Fracture of Brittle Rocks Faults - Faults occur when brittle rocks fracture and there is an offset along the fracture. When the offset is small, the displacement can be easily measured, but sometimes the displacement is so large that it is difficult to measure. Types of Faults Faults can be divided into several different types depending on the direction of relative displacement. One division of faults is between dip-slip faults, where the displacement is vertical, and strike-slip faults where the displacement is horizontal. Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane along which the relative displacement or offset is vertical. Note that in looking at the displacement on any fault we don't know which side actually moved or if both sides moved, all we can determine is the relative sense of motion. For any inclined fault plane we define the block above the fault as the hanging wall block and the block below the fault as the footwall block. 

Normal Faults - are faults that result from horizontal tensional stresses in brittle rocks and where the hanging-wall block has moved down relative to the footwall block. Horsts & Gabens - Due to the tensional stress responsible for normal faults, they often occur in a series, with adjacent faults dipping in opposite directions. In such a case the down-dropped blocks form grabens and the uplifted blocks form horsts. In areas where tensional stress has affected the crust, the grabens may form rift valleys and the uplifted horst blocks may form linear mountain ranges. The East African Rift Valley is an example of an area where continental extension has created such a rift. The basin and range

Planet Earth Earthquake Notes

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province of the western U.S. (Nevada, Utah, and Idaho) is also an area that has recently undergone crustal extension. In the basin and range, the basins are elongated grabens that now form valleys, and the ranges are uplifted horst blocks. Earthquakes at Diverging Plate Boundaries. Diverging plate boundaries are zones where two plates move away from each other. Earthquakes along divergent plate boundaries are caused by tensional stress in rift valleys along MOR and within continental plates (Mid-Atlantic Ridge, African Rift Valley). Earthquakes that occur along such boundaries show normal fault motion and have low Richter magnitudes due to the tendency of rocks to break easily under tensional stress. Rupture usually occur before great stress can build in the rocks. The earthquakes tend to be shallow focus earthquakes with focal depths less than about 20 km because the brittle lithosphere is relatively thin along these diverging plate boundaries.  Examples - all oceanic ridges, Mid-Atlantic Ridge, East Pacific rise, and continental rift valleys such as the basin and range province of the western U.S. & the East African Rift Valley.

Planet Earth Earthquake Notes

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Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks, where the hanging-wall block has moved up relative the footwall block. A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata.

Earthquakes at Converging Plate Boundaries. Convergent plate boundaries are boundaries where two plates run into each other. Thus, they tend to be zones where compressional stresses are active and thus reverse faults or thrust faults are common. There are two types of converging plate boundaries. (1) subduction boundaries, where oceanic lithosphere is pushed beneath either oceanic or continental lithosphere; and (2) collision boundaries where two plates with continental lithosphere collide. Subduction boundaries -At subduction boundaries cold oceanic lithosphere is pushed back down into the mantle where two plates converge at an oceanic trench. Because the subducted lithosphere is cold it remains brittle as it descends and thus can fracture under the compressional stress. The subduction of the cold oceanic lithosphere produces a continuum of stress along the subduction zone and generates earthquakes that have progressively deeper foci in the direction of subduction beneath the overriding plate. This zone of earthquakes is called the Benioff Zone. The shallow focus earthquakes are near the oceanic trench. Focal depths of earthquakes in the Benioff Zone can reach down to 700 km. Rocks are strong under compression and can store large amounts of strain energy before they rupture. Therefore, these earthquakes are very powerful.

Planet Earth Earthquake Notes

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Examples - Along coasts of South American, Central America, Mexico, Northwestern U.S., Alaska, Japan, Philippines, Caribbean Islands. Subduction Zones* - generate the largest EQ's (M >8.5); trigger other natural disasters. 1960 southern Chile (Mw = 9.5)* largest EQ ever recorded 1964 Alaska (Mw = 9.2)** 2nd largest EQ recorded 1985 Mexico City (Ms = 8.1)

Collision boundaries - At collision boundaries two plates of continental lithosphere collide resulting in fold-thrust mountain belts. Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km.  Examples - Along the Himalayan Belt into China, along the Northern edge of the Mediterranean Sea through Black Sea and Caspian Sea into Iraq and Iran. 1998 Afghanistan (Ms = 6.1) 1990 Western Iran (M = 7.7) 1988 Armenia (M = 7.0)

Planet Earth Earthquake Notes

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Strike Slip Faults - are faults where the relative motion on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties, depending on the sense of displacement. To an observer standing on one side of the fault and looking across the fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip fault. Displacements on the San Andreas fault are estimated at over 600 km.

Transform-Faults are a special class of strike-slip faults. These are plate boundaries along which two plates slide past one another in a horizontal manner. The most common type of transform faults occur where oceanic ridges are offset. Note that the transform fault only occurs between the two segments of the ridge. Outside of this area there is no relative movement because blocks are moving in the same direction. These areas are called fracture zones. The San Andreas fault in California is also a transform fault. Earthquakes at Transform Fault Boundaries. Transform fault boundaries are plate boundaries where lithospheric plates slide past one another in a horizontal fashion. The plates undergo shear stress as they move horizontally past each other in a bind and lurch pattern. Strain builds as the plates are pressured to move past one another, but friction binds them. The longer the fault segments are locked the greater strain accumulates and the energy released. Earthquakes along these boundaries tend to be shallow focus earthquakes with depths usually less than about 100 km. Richter magnitudes can be large.

Planet Earth Earthquake Notes

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World Distribution of Earthquakes The distribution of earthquakes is called seismicity. Seismicity is highest along relatively narrow belts that coincide with plate boundaries. This makes sense, since plate boundaries are zones along which lithospheric plates move relative to one another. When lithospheric plates move relative to one another their movement is slowed by friction. As a result, rocks along the boundary undergo strain (deformation resulting from stress). When the stress on the rocks exceeds their strength, the rocks rupture, forming a fault. Earthquakes occur along faults (fracture along which rocks have been displaced). The sudden rupture of rocks along faults produces earthquake waves, or seismic waves, that shake the ground. Earthquakes and Plate Tectonics The majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic stresses at active plate margins. Earthquakes along plate boundaries can be divided into shallow focus earthquakes that have focal depths less than about 100 km and deep focus earthquakes that have focal depths between 100 and 700 km. Smaller earthquakes occur in association with volcanic eruptions, land fills, and infection of fluids into deep fractured rock and intraplate due to mantle plumes. Intraplate Earthquakes - These are earthquakes that occur in the stable portions of continents that are not near plate boundaries. Many of them occur as a result of re-activation of ancient faults, although the causes of some intraplate earthquakes are not well understood.  Examples - Basin and Range in Southwestern U.S. is the result of tensional stress of the continental rocks, or rifting. These result in horst/graben, mountain formation. Continuing tension leads to recurring vertical movements along normal faults.

Planet Earth Earthquake Notes

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1811-1812 New Madrid sequence (four EQ's Mw ~7.8 and 8.3) 1886 Charleston, SC (Ms = 7.7)

Human Induced Earthquakes: Fracture zones (faults) are activated by increased load of water on land and increased water pressure in rocks below the reservoir (The construction of Hoover Dam on the Colorado river in Arizona and Nevada produced several hundred minor tremors, up to magnitude 4 and 5, Construction of a reservoir in India caused a magnitude 6 earthquake which killed 200 people). Setting off underground nuclear explosions produces aftershocks that relieve stress in the crust. (Nevada nuclear test site produced earthquakes of magnitude 5-6). Deep waste disposal of material injected into wells increases the fluid pressure in fractures and facilitates slippage along the fractures. (Rocky Mountain Arsenal in Colorado).

Planet Earth Earthquake Notes

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Seismic Waves and Ground Shaking When a fault ruptures, rocks break apart suddenly and violently, releasing energy in the form of seismic waves. It is the propagation of these waves through the earth (ground) that is felt during an earthquake.

TYPES OF SEISMIC WAVES Four types of waves are generated simultaneously from the focus during an earthquake, traveling in different ways at different speeds Body Waves - penetrate the earth and travel through it. They emanate from the focus and travel in all directions through the body of the Earth. There are two types of body waves: P -waves and S-waves:

P - waves – primary (push-pull) compression/tension in direction of wave travel Travel through solids, liquids, or gases.

fast ~5-6 km/sec

S-Waves – secondary (shear) perpendicular to the direction of wave travel~3-4 km/sec shear movement within the horizontal. Travel only through solids. P waves can travel through solids, liquids and gasses; S waves only travel through solids (liquids and gasses cannot shear).

Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth.

Planet Earth Earthquake Notes

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Surface waves are often the cause of the most intense ground motion during an earthquake. They move along the surface of the ground at a rate of ~2km/sec. Slowest and most damaging. Love surface - complex horizontal motion. perpendicular/transverse up/down. slow; perpendicular to wave propagation but in the vertical (shear). Raleigh surface - rolling or elliptical motion in the vertical plane, like a waves on the ocean surface, a little slower than Love waves As these different types of waves move at different speeds away from the focus, they become organized into groups of waves travelling at similar velocities. However, near the source of the earthquake, there is not time for this wave segregation to take place, and the shaking may be severe and complex. In addition, waves traveling through rocks are both reflected and refracted across boundaries between different earth materials and along the surface of the earth, amplifying the waves and the resultant shaking and damage. For these reasons damage tends to be greatest at the epicenter. Recording Seismic Waves  Earthquakes are detected from all points on the globe when their seismic waves reach earthquake monitoring stations. The detecting instrument is a seismograph which times and records the incoming waves on a seismogram (Overhead).  The record of an earthquake, a seismogram, as recorded by a seismograph, will be a plot of vibrations versus time. On the seismogram, time is marked at regular intervals, so that we can determine the time of arrival of the first P-wave and the time of arrival of the first S-wave.

Planet Earth Earthquake Notes

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EARTH'S INTERIOR  No body has ever been through the crust let alone the center of the earth.  Knowledge comes from seismic waves. Seismic body not only can travel through a medium, but can also be reflected and refracted. Reflection is the bouncing of a wave off the surface between two media. Refraction is the change in velocity when a wave passes from on medium to another; causes the wave to bend. Reflection and refracton of seismic body waves are the way information was gained about the different compositional layers in the earth. Seismic Wave Rules  Velocity depends on the type of wave, density of material (the higher the density, the greater the speed), elasticity of the material (more elastic, the faster it springs back), and ductility of the material (more ductile, the slower the speed).  Velocity increases with depth because pressure and density increases with depth. Closer packing of molecules.  P waves travel through solids, liquids and gasses.  S waves (shear) doesn't; water does not vibrate at right angles; liquids have no shear strength.  P waves travel faster in any material compared to other waves traveling through the same material.  When waves pass through materials of different densities they bend (refracted); like light traveling through water and air.  P waves travel faster in solids>liquids>gasses. Layers of different composition If the earth had a homogenous composition, rock density would increase steadily with depth as a result of increasing pressure and therefore body wave velocity would also increase steadily. However this is not the case. Body waves are abruptly refracted and reflected at several depths inside the Earth. This means that density does not increase smoothly, and within the earth there must be some boundaries separating materials having distinctly different densities.

Planet Earth Earthquake Notes

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Compositional/density. Crust, Mantle, Core (Inner core is solid, outer core is liquid), core is iron and nickel. Mantle has high iron and magnesium, crust has higher silica and oxygen.

Moho discontinuity: Boundary between the ...