Faults & Earthquakes PDF

2. Reverse Fault-Thisis atype of steep (45-90) dip slip fault in whichthe upper block, abovethefault plane, noves up and over the lower block (This type of Tauing n, common In areas of compression. When the dip angle is shallow (45 or less), a reverse fault is often described a5...

2. Reverse Fault-Thisis atype of steep (45-90) dip slip fault in whichthe upper block, abovethefault plane, noves up and over the lower block (This type of Tauing n, common In areas of compression. When the dip angle is shallow (45 or less), a reverse fault is often described a5a thrustlault hs faulting occurs in response to compresslon (Cornpressional forces) or when the crust is shortened and contracted by forces acting on a common point. It is present along Convergent plate boundaries, near mountain ranges, and in subduction zones. Lxamples; Glarus thrust (Switzerland), Longmenshan Fauit (China), San Rarmón Fault (Chne, Madre Fault Zone (North America), and Lusatian Fault (Germany) 3. Strike-slip fault-This is afault type involving the horizontal rnovement of blocks of rock 0e by-side slidingmovements inwhich rocks, due to the shear stres, are forced to novein aparallel put opposile direction of the strike i you are standing on ong bog4 and look across the fault plane, and t appears that the opposite block is moving to yourleft, then you are observing aleft-lateralstrike Sip fault. The same is trueifyou were to stand on the other block. The other type is the Fight-lateral strike-slip for the same reason, but it appears that the opposite blockis moving to your right. Many Strike-slip faults are present in continental and oceanic crusts, such as offset rívers, parallel valleysS, and abrupt ends to mountain chains. Examples: San Andreas Fault (Californía), Alpine Fault (New Zealand), Kunlun Fault (Tibet), Yarnmourneh Fault (Lebanon), and Philippine Fault (Phíilippínes) Normal fault Reverse fault Stríke-slip fault Figure 10.4 Types of faults Faults Generate Earthquakes Most faults in Earth'scrust do not move for a long time. And when a mass or rock is compressed or stretched over along period, they generate a type of stored energy called elastic potential energy If the rock mass releases the stored energy and does not break after being compressed or stretched, the energy transforms into kinetic energy. It then allows the rock to return to its original shape. But ifit is cornpressed and stretched beyondits limit, it will break. At this point, kinetic energy makes the rock permnanently deforrmed. When ruptures occur along faults, the stored elastic potential energy is released as kinetic energy calledas seismic energy. This energy comes from a point of origin and propagates through thethey Earth's surface are feeling the seismic waves. Most people do not realize that when they experience earthquakes, seismic waves coming from deep within Earth. Earth  An almost similar process goes on during an earthquake. A compressional force can cause stress in Earth's crust, pushing the sides of the faults together. The fault surfaces grind over so they do not promntly slip when pushed sideways; however, as the crust continues to receive stress, the pressure builds up. and it reaches the point that it is enough to cause the rocks to slip suddenly. They release energy in waves that travel through Earth's layers, causing the shaking felt during an earthquake. If you noticed, we have mentioned the term slip several times. This geological feature is the amount of relative ground displacement in an earthquake. In response to stresses or forces, faults in our Earth's crust become displaced and deformed. To better understand such fault movement, rermember that the blocks of rocks move with respect to one another vertically, horizontally, or at an angle. The vertical movements are up or down, while horizontal means sideways. The rock surface along which the fault movement strikes or occurs is called the fault plane. Blocks vertically displaced have several features: the hanging wall and the footwall. If the block is above the fault plane, it is the hanging wall. Fault plane Hanging wall Footwall Figure 10.3 Fault movement If below, it is the footwall. The latter terms are jargon in mining. When miners work underground, they stand on the footwall with their feet and the hanging wall above them. Three Basic Types of Geological Faults There are three kinds of faults: normal, reverse, and strike-slip. Each type results from various forces pushing or pulling on the crust, causing rocksto slide up, down, or past each other. 1. Normal fault-This is a dip-slip fault (inclined split with vertical movement) in which the block above, the fault has moved downward relative to the block below. This faulting occurs in response to extension (tensional forces)or when the crust is stretched and pulled apart by forces in opposite directions It is present along divergent plate boundaries, such as the formation of new oceanic crusts, or long, deep valleys. Examples: AtalantiFault (Greece), Humboldt Fault Zone (North America), Moab Fault (North America), Sierra Nevada Fault (North America), and Wabash Valley Seismic Zone (North America) 332 Science for a Changing World Grade 7  Sci-Checkpoint he focus is the point inside Earth's crust Where an earthquake originates. Seismic waves pronagots vom the focus outward in all directionsup to the epicenter.The epicenter is the point on the surfacat Earth that is directly above the focus We already know what causes an earthquake. Butwhere does an earthquake start? Try a simple activity using a table, sixX boxes, and three stones with your class teàcher. First. get six empty boxes and three stones of different sizes. Pile the empty boxes one on top of the other in groups of three. Then place one pile near one end of the table and the other at the end le stone, strike a box pile under the table. Observe the movement of the boxes. Repeat the steps using the tue other stones one at a time. Figure 10.5 Setup of the earthquake activity In this activity, the table itself is Earth's crust. Each stone represents the focus. The portion of the table hit is the epicenter. The stone sizes represent the magnítudes, while the wobbling boxes are the intensities. We shall discuss the concepts of magnitude and intensity in the next lesson. Today, we willtalk about the cross-section or anatomy of an earthquake. Types of Faults According to Their Level of Activity 1. Fault plane- This is the plarne along which the rupture or breaking has taken place or where one block has moved with respect to the other. 2. Fault line/trace - This is an edge of the fault plane that shows how much movement has taken place. 3. Fault scarp - This is the exposed fault plane above the ground. It serves as a sign that there is afault in the area. 4 Focus/hypocenter- This is the point in thecrust's zone of weakness where the breaking or rupture starts and seismic energy is released. Science for a Changing Worlid Grade 7  Fault scarp Fault trace Epicenter FoCUs Fault plane Figure 10.6 Anatomy of an earthquake 5 Epicenter - This is the point on the surface of Earth that is directly above the focus. It is the that is closest to the populated area. location Being knowledgeable about the their causes of origin and their depthsearthquake's of focus. anatomy allows us to classify earthquakes aside from Types of Earthquakes According to the Depth of Focus 1 Shallow-focus Earthquake -This occurs at depths between 0 and 70 km. Almost 90% of all earthquakes occur close to Earth's surface. The most commonly associated crustal plates are those that move past each other. The seismic waves of this type of earthguake travel shorter distances from the focus to the epicenter,which is why it causes more destruction than any other type of earthquake, though there are other factors to consider in determining the earthquake's potential impacts and damage. Sci-Tídbie In November 2022, a 5.6 magnitude earthquake left at least hundreds of injuries and deaths and thousands of crumbled and collapsed buildings in West Java, Indonesia. If we look at the magnitude, it was much weaker than many other earthquakes that have caused deaths and destruction in Indonesia over the past decades. But why is this one diferent? Seismologists and disaster experts say that the proximity to fault lines, the shallowness of the quake, and inadequate infrastructure that cannot withstand earthquakes all contributed tothe damage. Source: https://reliefweb.int/disaster/eq-2022-000363-idn 2. Intermediate-focus Earthquake-This occurs at depths between 70 km and 300 km. 3. Deep-focus Earthquake -This occurs at depths greater than 300 km. About 3% of globalearthquakes are classified as deep focus. The most commonly associated crustal plates are those in subduction zones where the floating plates on the surface of Earth dive under one another and are "subducted intothe mantle. The seismic waves coming from this type of earthquake lose more of their energy as they travel farther up to the surface. Earth and Space 335