1 Mr. R & Ms. C – Earth Science 2016https://www.youtube.com/watch?v=LpLDY04kYXE Earthquakes https://www.youtube.com/watch?v=jBdvvXyS-r4 Mr. R & Ms. C – Earth Science 2016
2 Does anyone see a pattern?World of Earthquakes Does anyone see a pattern? The Earth’s crust near tectonic plate edges are forced to bend, compress, and stretch due to the internal forces within the earth, causing earthquakes.
3 Awesome Earthquakes There are about 1,300,000 earthquakes per year (two per day) with magnitudes of 2.9 or lower The 2004 Indian Ocean earthquake lasted nearly 10 minutes—the longest on record Approximately one earthquake with a magnitude of 8.0 or higher occurs per year. The largest earthquake in recent history was a 9.5 and occurred in Chile in It caused giant ocean waves as far as 6,000 miles (10,000 km) away The San Andreas Fault is moving about 2 inches a year, about the same rate fingernails grow. At this rate, San Francisco and Los Angeles will be next to each other in 15 million years The Indian Ocean earthquake in 2004 generated enough energy to power all the homes and businesses in the United States for three days The earliest recorded earthquake is from 1831 B.C. in the Shandong province in China The most earthquake-prone state in the United States is Alaska Aristotle was the first to note that soft ground shakes more than hard ground Japan is one of the most earthquake-prone nations in the world. Thousands of earthquakes occur in Japan every year, but most of them are very weak Japan’s 9.0 earthquake in 2011 not only moved the island closer to the United States, it also shifted the planet’s axis by 6.5 inches An average earthquake lasts around a minute Earthquakes kill approximately 8,000 people each year and have caused an estimated 13 million deaths in the past 4,000 years. The deadliest earthquake known hit Shansi, China on January 23, An estimated 830,000 people died
4 Rock Deformation? Due to plate tectonics, the Earth’s lithosphere and crust are constantly being moved, shifted and re-shaped by HUGE forces. Rock deformation occurs when STRESS (force/area) is applied to the rock. There are different ways STRESS can be applied to a rock: Gravity and the weight of the overlying rocks create a UNIFORM amount of stress, called CONFINING STRESS When STRESS is not EQUALLY applied to the rock it becomes deformed The types of deformation of rocks is shown in the picture STRAIN = a change in: SHAPE SIZE VOLUME
5 Rock Deformation? Once unequal stress is applied the rock will go through different stages before it finally breaks = FRACTURE or FAULT Ductile = has elastic properties (BEND) Brittle = stress exceeds elastic limit (SNAP) Elastic = stress is applied and it can go back to original shape/size/volume (ELASTIC BAND) Every material has an elastic limit, the maximum amount of stress a material can feel and still recover to its original shape.
6 Whose Fault Is It!!! DIP FAULT OBLIQUE FAULT STRIKE-SLIP FAULTMovement or displacement of rock along a plane DIP FAULT OBLIQUE FAULT STRIKE-SLIP FAULT
7 What is an Earthquake? An earthquake is: “A release of energy in the form of seismic waves (vibrations)” Earthquakes occur along fractures in the rock, found on or near plate boundaries. These points called faults. The release of energy is produced by deformation of rock common with a transform plate boundary or subduction zone Elastic rebound theory explains how the energy is released through a fault line in the rock and as the rock breaks the vibration propagate out (earthquake). The energy is released but the rock does not go back to its original position – as shown in the diagram.
8 Find the FAULT?
9 What is an Earthquake? Definitions:Propagation – movement of energy waves. Fault scarp – A raised topographic feature caused by a geologic fault line Epicenter – is the point on the Earth’s surface that is directly above the earthquake’s focus or origin. Focus – is the underground point of origin or hypocenter. Wave fronts – the leading edge of wave propagation. Fault – fractures in the Earth where movement has occurred.
10 Seismic Waves: Body WavesThere are 2 types of body waves (meaning to go through the ‘body’ of the Earth) P-Waves or Primary Waves (Compressional) S-Waves or Secondary Waves (Transverse)
11 Seismic Wave Types Body - Seismic Waves:P waves: Primary (COMPRESSIONAL) P waves arrive first. Primary, pressure waves. Analogous to sound waves. Travel at 12,000 miles per hour (average) Max – 31,000mph Particle motion is along the direction of travel (propagation) of the wave, i.e., longitudinal waves. P waves can travel through solids, liquids or gases. P waves are compression waves - the wave pulse or pulses travels through the rock in a series of compression pulses. On either side of the compression the rock is stretched. The expand and compression of the rock is relatively small, allowing the wave to travel very quickly. A P wave arrives first and are heard and felt as a sharp thud.
12 Differences between “P” and “S” wavesSeismic Wave Types Body - Seismic Waves: S Waves – ‘Shear Waves or Transverse) S waves are characterized by a sideways movement. The rock materials are moved from side to side as the wave passes. S waves are like water waves, the wave pulses travel along by moving the medium from side to side. As the pulse moves along, each section of rope moves to the side then back again in succession. Rocks are more resistant to sideways motion so the S wave travels more slowly. Shakes the ground perpendicular to the direction of movement 3.5km/s = 7,800mph Differences between “P” and “S” waves
13 Types of Seismic Waves Love Waves 2. Rayleigh WavesSURFACE WAVES: Two basic types of surface waves Love Waves 2. Rayleigh Waves 1. L-waves or long waves. Complex motion. Up-and-down and side-to-side. Slowest. Causes damage to structures during an earthquake. 2. Rayleigh waves involve orbital motions, like water waves. A surface particle moves in a circle or ellipse in the direction of propagation.
14 What is an Earthquake? Long Waves – Waves on the Surface of the Earth
15 Measuring an EarthquakeRichter Scale: 1935 design Used in California (not accurate with large Earthquakes <7. Recorded from the amplitude of largest wave recorded Less than 3 = micro-quake – not felt by humans In the 1970’s replaced with a more modern approach and viewpoint of earthquake measurement. Moment magnitude scale – measures the energy released in the rocks – easy comparison for worldwide E.Q’s. Logarithmic
16 Measuring an Earthquake
17 Measuring an EarthquakeMercalli Damage Scale A scale based on ‘how much you feel the earthquake’ – relative damage to people, structures and area Magnitude 1-4 = minor shaking Magnitude 4-6 = cracking, running from buildings, things fall Magnitude 6-7 = ground cracks, landslides, buildings collapse, underground pipes burst
18 Earthquake Hazards Earthquake hazards are:Tsunamis: Tsunamis are long wavelength oceanic waves generated by the sudden displacement of seawater by a shallow earthquake, volcanic eruption or submarine landslide. https://www.youtube.com/watch?v=3xKMFzKOIfQ https://www.youtube.com/watch?v=IuUygn7BZis Fires and Gas explosions: burst and/or broken underground pipes Landslides/rockslides Liquefaction: Liquefaction occurs when waterlogged sediments are agitated by seismic shaking. This separates the grains from each other, reducing their load bearing capacity. Buildings and other structures can sink down into the ground or tilt over, whilst underground pipes and tanks may rise up to the surface. When the vibrations stop the sediments settle down again, squeezing groundwater out of fissures and holes in the ground to cause flooding. The aftermath of liquefaction can leave large areas covered in a deep layer of mud. (Solid becomes a liquid) Lateral ground spreading: Cracking and spreading in the ground Subsidence: Lowering of the ground surface
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20 Earthquake Hazards – Building Buildings Better!While it is not possible to accurately predict earthquakes, measures can be taken to reduce the devastation by constructing earthquake-resistant structures. In some cases, building practices are not up to code, and in the event of an earthquake, the loss of life is catastrophic e.g. Nepal (2015), Haiti (2010), San Francisco (1906), Indonesia (2004). In earthquake-prone areas, buildings are now being constructed with moorings filled with alternating layers of rubber and steel. These are called base isolators. The rubber acts as an “earthquake absorber.” Or placing the building in a new pair of ‘Air Jordan’s’ Buildings with these types of moorings are designed to withstand a magnitude 8.3 earthquake. In attempts to reduce damage to structures, engineers try to: 1. Increase the natural period of the structure through “base isolation.” 2. Install “energy dissipating devices” to dampen the system. Simple reinforcement methods used by engineers include using large bolts to secure buildings to their foundations, as well as providing supporting walls, or shear walls, made of reinforced concrete. This can help to reduce the rocking effect of a building during and after a seismic event. Shear walls at the center of the building (around an elevator shaft) can form a shear core. Employing cross-braces, where walls are built with diagonal steel beams, adds extra support.