Earthquakes
Earthquakes
In a single year, on average, more than 900,000 earthquakes are recorded and 150,000 of them are strong enough to be felt. Each year about 18 earthquakes are major, with a Richter magnitude of 7.0 to 7.9, and on average one earthquake has a magnitude of 8 to 8.9.
Magnitude 9 earthquakes are rare. The United States Geological Survey lists five since 1900. All but the Great Indian Ocean Earthquake of 2004 occurred somewhere around the Pacific Ocean basin.
Earthquake Zones
Nearly 95% of all earthquakes take place along one of the three types of plate boundaries.About 80% of all earthquakes strike around the Pacific Ocean basin because it is lined with convergent and transform boundaries .This area is known as the Ring of Fire .About 15% take place in the Mediterranean-Asiatic Belt, where convergence is causing the Indian Plate to run into the Eurasian Plate. The remaining 5% are scattered around other plate boundaries or are intraplate earthquakes.
Seismic Waves
Seismic waves are the energy from earthquakes. Seismic waves move outward in all directions away from their source. Each type of seismic wave travels at different speeds in different materials. All seismic waves travel through rock, but not all travel through liquid or gas. Geologists study seismic waves to learn about earthquakes and the Earth’s interior.
Types of Seismic Waves
There are two major types of seismic waves. Body waves travel through the Earth’s interior.Surface waves travel along the ground surface. In an earthquake, body waves are responsible for the sharp jolts. Surface waves are responsible for the rolling motions that do most of the damage in an earthquake.
Body Waves
Primary waves (P-waves) and secondary waves (S-waves) are the two types of body waves. Body waves move at different speeds through different materials.
P-waves are faster. They travel at about 6 to 7 kilometers (about 4 miles) per second. Primary waves are so named because they are the first waves to reach a seismometer. P-waves squeeze and release rocks as they travel. The material returns to its original size and shape after the P-wave goes by. For this reason, P-waves are not the most damaging earthquake waves. P-waves travel through solids, liquids, and gases.
S-waves are slower than P-waves. They are the second waves to reach a seismometer. S-waves move up and down. They change the rock’s shape as they travel. S-waves are about half as fast as P-waves, at about 3.5 km (2 miles) per second. S-waves can only move through solids. This is because liquids and gases don’t resist changing shape.
Animation http://www.classzone.com/books/earth_science/terc/content/visualizations/es1002/es1002page01.cfm
The third type, surface waves, travel along the ground outward from an earthquake’s epicenter. Surface waves are the slowest of all seismic waves. They travel at 2.5 km (1.5 miles) per secondSurface waves cause objects to fall and rise. They also cause objects to sway back and forth. These motions cause damage to rigid structures during an earthquake.
Activity 1: watch the video and answer the questions on your handout. Video http://www.youtube.com/watch?v=73LNyVYD8Ig
1. What was the magnitude of the Chilean earthquake?
2. What two plates are converging near Chile that caused the earthquake?
3. What is the Ring of Fire? What occurs along this Ring of Fire?
4. What was the strongest earthquake ever recorded? When did it occur?
5. Why are scientists urging Memphis to adopt building codes similar to Chile's?
measuring Earthquakes: http://siovizcenter.ucsd.edu/library/TLTC/TLTCmag.htm
Focus of an earthquake
The focus is also called the hypocenter of an earthquake. The vibrating waves travel away from the focus of the earthquake in all directions. The waves can be so powerful they will reach all parts of the Earth and cause it to vibrate like a turning fork.
Epicenter of an earthquake
Directly above the focus on the Earth's surface is the earthquake epicenter. Earthquake waves start at he focus and travel outward in all directions. Earthquake waves do not originate at the epicenter.
Seismograms come in handy for locating earthquakes too, and being able to see the P wave and the S wave is important. You learned how P & S waves each shake the ground in different ways as they travel through it. P waves are also faster than S waves, and this fact is what allows us to tell where an earthquake was. To understand how this works, let’s compare P and S waves to lightning and thunder. Light travels faster than sound, so during a thunderstorm you will first see the lightning and then you will hear the thunder. If you are close to the lightning, the thunder will boom right after the lightning, but if you are far away from the lightning, you can count several seconds before you hear the thunder. The further you are from the storm, the longer it will take between the lightning and the thunder.
P waves are like the lightning, and S waves are like the thunder. The P waves travel faster and shake the ground where you are first. Then the S waves follow and shake the ground also. If you are close to the earthquake, the P and S wave will come one right after the other, but if you are far away, there will be more time between the two. By looking at the amount of time between the P and S wave on a seismogram recorded on a seismograph, scientists can tell how far away the earthquake was from that location. However, they can’t tell in what direction from the seismograph the earthquake was, only how far away it was. If they draw a circle on a map around the station where the radius of the circle is the determined distance to the earthquake, they know the earthquake lies somewhere on the circle. But where?
Scientists then use a method called triangulation to determine exactly where the earthquake was (figure 6). It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter!
No, and it is unlikely they will ever be able to predict them. Scientists have tried many different ways of predicting earthquakes, but none have been successful. On any particular fault, scientists know there will be another earthquake sometime in the future, but they have no way of telling when it will happen.
Most destructive http://earthquake.usgs.gov/earthquakes/world/historical_mag_big.php
Are you confused? Go to the following site for more information about Earthquakes or open the power point in the "add files" section<
https://ees.as.uky.edu/sites/default/files/elearning/module06swf.swf
Games:
http://www.dropcoverholdon.org/beatthequake/game/
http://www.iknowthat.com/mhscience/Earthquakes/earthquake_movie.html
http://www.insidedisaster.com/experience/Main.html#
extra credit lab http://www.glencoe.com/sites/common_assets/science/virtual_labs/ES09/ES09.htm
Extra Files:
FindingEpicenter.docx (421k)Kathy Dougherty, Jan 29, 2017, 2:00 PM
Notes from the Earthquake website.docx (14k)