Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. The largest, main earthquake is called the mainshock.
Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock! A simplified cartoon of the crust brown , mantle orange , and core liquid in light gray, solid in dark gray of the earth.
Public domain. The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece — it is made up of many pieces like a puzzle covering the surface of the earth.
Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates , and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving.
Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake. The tectonic plates divide the Earth's crust into distinct "plates" that are always slowly moving.
Earthquakes are concentrated along these plate boundaries. This is why S waves are also known as transverse waves. S waves cannot travel through liquids or gases. For one, scientists can use P waves and S waves to identify where an earthquake began.
To do that, they need to have data gathered by seismic instruments at three or more different locations. Triangulation is only possible when there are accurate measurements of the times at which P waves and S waves show up at each seismometer.
Some techniques use only the P waves. Others also consider the time difference between the arrival of the first P waves and S waves. The farther the distance between the seismometer and the source of the quake, the more exaggerated that time difference will be. So with a seismometer as a center, scientists draw a circle of the proper size on a map.
But using only one seismometer, there is no way to tell in which direction the source was. It could be anywhere along the outer edge of that circle. By plotting the circles for at least three instruments on the same map, however, there will be a single point where those circles overlap. The point where a quake originates is called its hypocenter.
Those same seismic waves also can be generated by underground explosions. These might arise from a small blast inside an underground coal mine, for example. Or, they might signal the test detonation of a nuclear weapon such as several that recently took place in North Korea. And P waves, in particular, can strongly point to whether the seismic waves come from a natural quake or an unnatural blast. So they die away pretty quickly. After several repetitions, ask students to describe the rope motion.
Ask them what kind of earthquake wave motion this resembles. The answer is shear waves. Remind them that in shear waves particles of material move back and forth perpendicular to the direction in which the wave itself moves. S-waves S stands for secondary are shear earthquake waves that pass through the interior of the Earth. S-waves don't change the volume of the material through which they propagate, they shear it. Note: The motion of the rope due to shear waves is much easier to observe than the compression waves, but the shear waves travel more slowly than compression waves.
In an earthquake, scientists can observe the arrival of compression waves before the arrival of shear waves using seismographs. You may choose to show a close-up of a record seismogram for a single earthquake event, and ask your students to point out different seismic waves.
In addition, shear waves cause much more damage to structures since it is easier to shake surface rocks than it is to compress them. Encourage your students to critically evaluate their slinky and rope setup. Ask them if they see any limitations associated with their setup. Ask them to compare and contrast their simple setup with actual vibrations caused by seismic waves traveling through the Earth or along its surface. For instance, seismic waves carry energy from the source of the shaking outward in all directions not in one direction only as the setup shows.
Optional Both primary and secondary waves are body waves pass through the interior of the Earth. Surface waves travel along the Earth's surface. Two examples of surface waves are Rayleigh waves and Love waves. Explain to your students that Rayleigh waves cause the ground to ripple up and down like water waves in the ocean before they break at the surf line whereas the Love waves cause the ground to ripple back and forth like the movement of a snake.
Ask the students to recall how scientists use seismic wave observations to investigate the interior structure of the Earth. This is similar to checking the ripeness of a melon by tapping on it. To understand how scientists see into the Earth using vibrations, one needs to understand how waves or vibrations interact with the rocks that make up the Earth. Introduce to your students the two simplest types of wave interactions with rocks: reflection and refraction.
Ask students to define reflection. They should be able to give simple examples like echoes or reflection in a mirror. Explain to your students that echoes are reflected sound waves, and that students' reflections in a mirror are composed of reflected light waves. Tell students that a seismic reflection happens when a wave impinges on a change in rock type. Part of the energy carried by the wave is transmitted through the material refracted wave and part is reflected back into the medium that contained the wave.
Refraction can be demonstrated by dropping a coin in a bottle filled with water. The coin changes direction when it hits the water's surface and won't sink to the bottom vertically. Can animals sense an impending tsunami? What should we do during a tsunami? Why do trees seem to resist more to tsunamis than houses?
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