Transcript Earthquakes
Earthquakes Chapter 6 February 2012 This topic will shake you up Stress Stress- a force (energy) that acts on rock to change its shape or volume Types of Stress Tension- pulls or stretches rock so that it becomes thinner in the middle. This happens at divergent plate boundaries. Compression- squeezes rock until it folds or breaks. This happens at convergent plate boundaries. Stress continued • Shearing- stress that pushes a mass of rock in opposite directions. This occurs at transform boundaries. Faults Most faults occur along plate boundaries. Normal faults- fault is at an angle so one block lies above the fault and the other is below. Reverse fault- Rock pushed together. Blocks move in opposite directions from a normal fault. To correctly identify a fault, you must first figure out which block Normal Faults is the footwall and which is the hanging wall. Then you determine the relative motion between the hanging wall and footwall. Every fault tilted from the vertical has a hanging wall and footwall. To determine which is which, visualize yourself creating a mine in along the fault plane. The block below your feet is the footwall, and the one upon which you would hang your miner's lamp is the hanging wall. It is that simple. Strikeslip faults are vertical and thus do not have hanging walls or footwalls. If the hanging wall drops relative to the footwall, you have a normal fault. Normal faults occur in areas undergoing extension (stretching). If you imagine undoing the motion of a normal fault, you will undo the stretching and thus shorten the horizontal distance between two points on either side of the fault. Faults continued Strike slip faults- rocks slip past each other sideways. Changing the Earth’s Surface Plate movement can cause the crust to fold and bend. Anticline- a fold that bends upward Syncline- a fold that bends downward Changing the E’s surface cont. The collision of two plates can cause compression and folding over a wide area (mountain ranges). - Himalayas in Asia - Alps in Europe Stretching the Earth’s Crust Fault Block Mountains pg 167 Uplifting- a plateau is a large area of flat land pushed up high above sea level. The kaibab Plateau Earthquakes Section 2 Earthquakes are the shaking and trembling that results from the movement of rock beneath Earth’s surface. Stress builds up in rock along a fault until the rock breaks releasing enormous amounts of energy. Most EQ’s begin in the lithosphere. Earthquakes continued Focus- the area beneath the surface where rock that is under stress breaks. Epicenter- the point on the surface directly above the focus. Types of Seismic Waves Seismic waves- carry energy away from the focus, through the Earth’s interior and across the surface. There are three categories of seismic waves: 1. P- waves (primary waves) 2. S- waves (secondary waves) 3. Surface waves Types of Seismic Waves cont. P- waves- primary waves. First to arrive. Compress and expand the ground like an accordion. S- waves- Secondary waves vibrate side to side and up and down. S waves cannot travel through a liquid. These are the waves that shake structures. Surface waves- when S and P waves meet the surface. They move slower than P and S waves. Produce severe ground movements. Measuring EQ’s Mercalli scale- rates EQ’s according to level of damage. 12 steps (Low, moderate, severe, etc..) Measuring EQ’s cont. Richter scale- a number assigned based on EQ size. Measures seismic waves using a seismograph. Moment magnitude scale- measures total energy released. Locating the Epicenter Scientists use the speed of the waves to determine the epicenter. Scientists use the arrival time of the different waves. The further away the earthquake, the greater the arrival time. Monitoring Earthquakes The seismograph- an instrument that uses a rotating drum and a weighted pen to record vibrations as zigzagging lines. Records the P waves first, then the S waves. Measuring Faults Tiltmeters- measures the tilt or raising of the ground. Similar to a carpenters level. Measuring Faults cont. Creep meter- uses a laser to measure horizontal fault movements. (Distance) GPS- measures changes in elevation as well as horizontal movements. Using Seismograph Data page 182 Seismic waves are reflected at faults. This enables scientists to map the faults in an area by studying the wave patterns. So with all this technology can scientists predict earthquakes? Nope! Earthquake Safety Scientists can determine risk by locating where faults are active. How EQ’s cause damage Shaking- can trigger landslides, destroy buildings etc. Liquefaction- turns soft loose soil into mud. Aftershocks- EQ that occurs after a larger EQ hours or days later. May collapse already weakened buildings Tsunamis- when an EQ occurs under water it can displace water causing a large wave Steps to EQ safety The best way to protect yourself is to drop cover and hold. Designing safer Buildings Buildings can be made stronger and more flexible by using: -springs -dampers -shear walls -ties -reinforcement -flex pipes Tuned mass damper 730 tons