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  • 1.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 Planet Earth Earthquake Notes

2. Prof. C.ValentiPlanet Earth Earthquake Notes 2that the energy stored in bending the rocks was suddenly released duringthe earthquake.Earthquakes are an energy release in the form of seismic waves causedby the sudden rupture of strained rocks. Strain is deformation of rocksresulting from stress (e.g., tectonic forces). Mechanism of faultmovement is explained by the Elastic Rebound Theory which states thatstrain energy (deformation and friction) builds up on rocks as they areforced in different directions. Rocks on either side of a fault undergoelastic strain as they are stressed by tectonic forces. When the stressexceeds the strength of the rock, it breaks and the rocks abruptly slippast one another along the rupture (fault). When slippage and ruptureoccur along the fault the stored energy is released as seismic waves thatradiate out in all directions and the rocks "rebound" to their originalundeformed shape.Stages of Elastic Rebound Model EQ CycleDemo: bend a stick until it snaps. Energy is stored in the elasticbending and is released if rupture occurs, causing the fractured endsto vibrate and send out sound waves. Fault is a break in a rock. Tectonic forces push on two rock slabs in different directions. Frictionbetween the rocks along the fault causes the rocks to remain put. Continual forces deform the rock and causes the rocks to build upenergy (deformation is called strain). Eventually the forces pushing on the rocks exceed the strength offriction between the rocks along a fault, the rocks break, are displaced(slippage along the fault occurs) and seismic energy is released. Thedeformed 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 suddenlyreleased in one good shot. Sometimes faults creep along gradually releasing energy in much shorterperiods of time. The recurrence interval is the time it takes to accumulate sufficientelastic strain to cause the next EQ.Planet Earth Earthquake Notes 3. Prof. C.ValentiPlanet Earth Earthquake Notes 3Fracture of Brittle RocksFaults - Faults occur when brittle rocks fracture and there is an offsetalong the fracture. When the offset is small, the displacement can beeasily measured, but sometimes the displacement is so large that it isdifficult to measure.Types of FaultsFaults can be divided into several different types depending on thedirection of relative displacement. One division of faults is between dip-slip faults, where the displacement is vertical, and strike-slip faultswhere the displacement is horizontal.Dip Slip Faults - Dip slip faults are faults that have an inclined faultplane along which the relative displacement or offset is vertical. Notethat in looking at the displacement on any fault we dont know which sideactually moved or if both sides moved, all we can determine is therelative sense of motion.For any inclined fault plane we define the block above the fault as thehanging wall block and the block below the fault as the footwall block. Normal Faults - are faults that resultfrom horizontal tensional stresses inbrittle rocks and where the hanging-wallblock has moved down relative to thefootwall block.Horsts & Gabens - Due to the tensional stress responsible fornormal faults, they often occur in a series, with adjacent faultsdipping in opposite directions. In such a case the down-droppedblocks form grabens and the uplifted blocks form horsts. In areaswhere tensional stress has affected the crust, the grabens mayform rift valleys and the uplifted horst blocks may form linearmountain ranges.The East African Rift Valley is anexample of an area where continentalPlanet Earth Earthquake Notes 4. Prof. C.ValentiPlanet Earth Earthquake Notes 4extension has created such a rift. The basin and range province ofthe western U.S. (Nevada, Utah, and Idaho) is also an area thathas recently undergone crustal extension. In the basin and range,the basins are elongated grabens that now form valleys, and theranges are uplifted horst blocks.Earthquakes at Diverging Plate Boundaries. Diverging plate boundariesare zones where two plates move away from each other. Earthquakesalong divergent plate boundaries are caused by tensional stress in riftvalleys along MOR and within continental plates (Mid-Atlantic Ridge,African Rift Valley). Earthquakes that occur along such boundaries shownormal fault motion and have low Richter magnitudes due to thetendency of rocks to break easily under tensional stress. Rupture usuallyoccur before great stress can build in the rocks. The earthquakes tendto be shallow focus earthquakes with focal depths less than about 20 kmbecause the brittle lithosphere is relatively thin along these divergingplate boundaries. Examples - all oceanic ridges, Mid-Atlantic Ridge, East Pacific rise, andcontinental rift valleys such as the basin and range province of thewestern U.S. & the East African Rift Valley.Planet Earth Earthquake Notes 5. Prof. C.ValentiPlanet Earth Earthquake Notes 5 Reverse Faults - are faults that resultfrom horizontal compressional stressesin brittle rocks, where the hanging-wallblock has moved up relative thefootwall block.A Thrust Fault is a special case ofa reverse fault where the dip ofthe fault is less than 15o. Thrustfaults can have considerabledisplacement, measuring hundredsof kilometers, and can result inolder strata overlying younger strata.Earthquakes at Converging Plate Boundaries. Convergent plateboundaries are boundaries where two plates run into each other. Thus,they tend to be zones where compressional stresses are active and thusreverse faults or thrust faults are common. There are two types ofconverging plate boundaries. (1) subduction boundaries, where oceaniclithosphere is pushed beneath either oceanic or continental lithosphere;and (2) collision boundaries where two plates with continental lithospherecollide. 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 largePlanet Earth Earthquake Notes 6. Prof. C.ValentiPlanet Earth Earthquake Notes 6amounts of strain energy before they rupture. Therefore, theseearthquakes are very powerful. Examples - Along coasts of South American, Central America, Mexico,Northwestern U.S., Alaska, Japan, Philippines, Caribbean Islands. Subduction Zones* - generate the largest EQs (M >8.5); triggerother natural disasters.1960 southern Chile (Mw = 9.5)* largest EQ ever recorded1964 Alaska (Mw = 9.2)** 2nd largest EQ recorded1985 Mexico City (Ms = 8.1)Collision boundaries - At collision boundaries two plates of continentallithosphere collide resulting in fold-thrust mountain belts. Earthquakesoccur due to the thrust faulting and range in depth from shallow toabout 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 7. Prof. C.ValentiPlanet Earth Earthquake Notes 7Strike Slip Faults - are faults where the relative motion on the fault hastaken place along a horizontal direction. Such faults result from shearstresses acting in the crust. Strike slip faults can be of two varieties,depending on the sense of displacement. To an observer standing on oneside of the fault and looking acr