When two blocks of the Earth slip past each other suddenly, that’s what we call an earthquake! From a physics point of view, earthquakes are a release of the elastic potential energy that builds up. Most energy is released as heat, not as shaking, during an earthquake. 90% of all earthquakes happen along the Ring of Fire, which is the active zone that surrounds the Pacific Ocean.
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The Earth has four main layers: the hard skin on the surface is the crust which extends only 30 miles; the hot magma section is a soupy mass of rock that extends approximately 1800 miles; and the core which is made out of two parts: the hot liquid metal outer core surrounds the solid nickel -iron inner core.
The plates on the crust float on magma, which is a lot like the consistency tar. The crust has seven main tectonic plates that slide around and can either slide apart from each other (called a normal fault that is usually found on the sea floor), collide into each other (called a thrust fault), or move in opposite directions (strike-slip fault) at different speeds. Where the plates are sliding apart on the sea floor, the temperature of the water can rise over 1200oF. Scientists measure the speeds of the plates moving from 1 to 10 inches per year.
There are three different waves that travel through the planet when an earthquake happens. The first waves are the compression-type “P-waves” (primary waves), which travel through the entire planet, including liquid water and solid rock. Most people don’t notice the P-waves, but they do notice the secondary “S-waves”, which follow 60-90 seconds after the P-waves. Following the S-waves are the surface waves, which are like when you wave a bed sheet up and down.
Earthquakes are detected and measured by many different types of instruments, like strain gauges, creep meters, tilt sensors, seismometers, x-ray imagery, and more. These detectors aren’t perfect, though. Earthquake detectors not only discover earthquakes but also glacier movement, nuclear explosions, meteor impacts, and volcano eruptions!
After you watch the video, click the link to do your seismology calculations to figure out both the epicenter and the magnitude of four different earthquakes:
- Japan Earthquake 1995 (map answer)
- Los Prieta California Earthquake 1989 (map answer)
- Mexico Earthquake 1978 (map answer)
- Northridge California Earthquake 1994 (map answer)
If you’d like to measure the Earth’s Magnetic Pulse, you can do that here.
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We have almost no direct knowledge of anything beneath the crust — all of our data is inferred from the seismic waves of earthquakes bouncing off the various layers, and from various bits of the Earth’s interior that bubble up to the surface, such as volcanic magma.
How do we know the earth’s crust is only 30 miles deep, if we have only drilled to 6 or 7 miles deep?
Some kinds of lava do actually have some natural radioactivity. Granite – which is formed when magma cools – can contain radioactive elements such as potassium, uranium, and thorium in higher concentrations that the rest of Earth’s crust. This natural radiation isn’t dangerous.
why isn’t lava radioactive if it touches a radioactive iron core?-Gabe williamson
There are four links after the last paragraphs of explanation under the video – do you see them?
where is the seismograph work sheet for this video?
What are the other kinds of rock that give off a voltage when you smack them with a sledge hammer?
-Olivia