Earthquake Lesson Notes PDF

Unit 4 Lesson 1 An earthquake is a phenomenon caused by the sudden and rapid movement of large volumes of rock along fractures on the surface of the earth called faults The outermost shell of the earth, which includes the crust and the uppermost mantle, consists of tectonic plates. These are massi...

Unit 4 Lesson 1 An earthquake is a phenomenon caused by the sudden and rapid movement of large volumes of rock along fractures on the surface of the earth called faults The outermost shell of the earth, which includes the crust and the uppermost mantle, consists of tectonic plates. These are massive and irregularly shaped slabs of rock. Due to their roughness, the edges of these plates get stuck while the plates continue moving. As these waves travel through rocks beneath the earth, they radiate energy in all directions like ripples on water. This produces the shaking felt on the surface. There are two types of seismic waves: body waves and surface waves. Body waves are seismic waves that travel through the interior of the earth. Primary (P) waves are the first waves that reach the surface of the earth and make the ground shake. They move the ground back and forth along the direction they are traveling. The shaking from P waves is light or sometimes not felt. Secondary (S) waves are body waves that vibrate perpendicular to their propagation direction, producing an up and down motion. They move slower but shake the ground more strongly than P waves. Surface waves are waves that are trapped near the surface. Love waves are surface waves that have a horizontal motion perpendicular to the direction they are traveling. Rayleigh waves shake the ground in a rotational manner with no transverse motion. The severity of an earthquake is measured based on the energy it releases, or its magnitude, and its effects on people and man-made structures, or what we call intensity. Frequency is the number of times a movement such as ground shaking is repeated within a certain amount of time. High frequency earthquakes affect small buildings more than the high rise ones. Ground shaking is the primary cause of earthquake damage to man-made structures. It causes buildings and other infrastructures to collapse which may result in injuries or casualties. In cases where it breaks water dams, flash floods may occur. Fire is a secondary hazard if the shaking sufficiently damages electric and gas lines. Ground shaking also triggers other earthquake hazards such as landslides and liquefaction. A building code is a set of rules that set standards on the construction of buildings. If properly implemented, it can ensure that buildings are constructed properly. Unit 4 Lesson 2 Ground rupture is the visible breaking and displacement of the Earth’s surface along the trace of a fault. It may be a vertical movement, a lateral movement, or a combination of both, depending on the type of fault generating it Strike-slip faults are vertical or near-vertical faults that displace rock horizontally. If the block opposite you when looking across a fault moves to the left, it is a sinistral (or left-lateral) fault. If the block moves to the right, it is a dextral (or right-lateral) fault. Normal faults are characterized by the downward movement of the hanging wall with respect to the footwall. Reverse or thrust faults are faults which move the hanging wall up. These faults are dip-slip faults, which causes a vertical displacement of the ground. Unit 4 Lesson 3 Liquefaction occurs when the ground loses stiffness and behaves like liquid in response to earthquakes. When shaking happens, grains of the underlying material vibrate and undergo compaction, wherein the sediments are compressed and fluids in the pore spaces are squeezed out. As a result of the decrease in the spaces between grains, the pore water pressure increases. When the pore water pressure is equal to the weight of the overlying material, liquefaction occurs. dd Liquefaction at depth can cause geysers of sand and water to shoot from the ground, a phenomenon called sand boils. Unit 4 Lesson 4 Mass wasting, more commonly known as a landslide, is the downslope movement of rocks or sediments under the influence of gravity. It is the process that follows weathering or the removal of rock or debris from their original source. Landslides can be triggered by factors such as heavy or prolonged rain, oversteepened slopes, removal of vegetation, and earthquakes. Creep is the gradual, almost imperceptible downward displacement of sloping rock or soil caused by buildup of significant strain. The following factors affect the occurence of an earthquake-induced landslide: the strength of the earthquake, the distance from the fault, topography, climate, and the characteristics of the rock or soil, and the man-made changes or structures on the area. Unit 4 Lesson 5 A tsunami is a series of waves generated by large-scale displacements of water which are usually triggered by earthquakes or major submarine landslides. It is a Japanese word which translates to ‘harbor wave’ (tsu meaning ‘harbor’ and nami meaning ‘wave’). When an earthquake occurs beneath the sea, the water above the affected area moves upward and produces tsunamis. These waves initially have a small height but grow as they move through shallower depths, a process called wave shoaling, and approach the shore. When an earthquake occurs beneath the sea, the water above the affected area moves upward and produces tsunamis. These waves initially have a small height but grow as they move through shallower depths, a process called wave shoaling, and approach the shore. Tsunamis usually originate from earthquakes that occur in subduction zones. Subduction zones are sites at convergent boundaries, where two or more tectonic plates collide, wherein one plate descends (subducts) beneath another. The downward movement of the subducting plate is driven by temperature. Colder lithosphere is denser and will sink. A tsunami can be local or regional. Local tsunamis are from a nearby source. They are confined to coasts within 100 km or the distance they travel within less than an hour. Regional tsunamis affect a wide geographical area, typically within 1,000 km or 1-3 hours of the wave travel time. Tsunamis usually originate from underwater earthquakes due to movements in subduction zones. Tsunamis can be predicted but the damage and destruction it will cause cannot be precisely anticipated.