1.B.4 Comparing Earthquakes

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來自 University of Illinois at Urbana-Champaign 的課程

Planet Earth...and You!

23 個評分

University of Illinois at Urbana-Champaign

Planet Earth...and You!

23 個評分

Earthquakes, volcanoes, mountain building, ice ages, landslides, floods, life evolution, plate motions—all of these phenomena have interacted over the vast expanses of deep time to sculpt the dynamic planet that we live on today. Planet Earth presents an overview of several aspects of our home, from a geological perspective. We begin with earthquakes—what they are, what causes them, what effects they have, and what we can do about them. We will emphasize that plate tectonics—the grand unifying theory of geology—explains how the map of our planet's surface has changed radically over geologic time, and why present-day geologic activity—including a variety of devastating natural disasters such as earthquakes—occur where they do. We consider volcanoes, types of eruptions, and typical rocks found there. Finally, we will delve into the processes that produce the energy and mineral resources that modern society depends on, to help understand the context of the environment and sustainability challenges that we will face in the future.

從本節課中

Week 1: Earthquakes!

In the lectures for this week, we will consider what an earthquake is, how it occurs, how we can record and measure its size, and what we can do to mitigate damage. Our first weekly assignment consists of reporting your own experience with earthquakes (if any). The Week 1 Lab gives you a chance to work with seismograms, locate an earthquake epicenter, and determine magnitude. Finally, the Week 1 Discussion addresses the balance between risks from quakes and costs of preventive measures.

1.B.1 The Intensity and Magnitude of Earthquakes6:26

1.B.2 Seismometers4:30

1.B.3 Interpreting Seismograms and Earthquake Magnitude4:36

1.B.4 Comparing Earthquakes5:51

與講師見面

Dr. Stephen Marshak
Professor and Director of the School of Earth, Society, and Environment
Department of Geology
Dr. Eileen Herrstrom
Lecturer
Geology

[MUSIC]

Let's think a little bit about the size or

the meaning of the numbers on a magnitude scale.

Often, the numbers of earthquakes that are recorded are usually three,

four, five or larger.

That's because earthquakes with magnitudes that are smaller than about three or

four are almost inconsequential.

You may feel them if you're near the epicenter as a small vibration, but

they really don't have any significant consequence.

When you start getting earthquakes four, five,

and larger, they have consequences, and may be newsworthy.

Now, one of the things about the magnitude scale is that it's logarithmic.

That means that each number represents and

increase of a factor of 10 over the number before it.

Now remember, we were saying that a magnitude is a measure of the amplitude or

is based on a measure of the amplitude of the waves recorded by a seismograph.

So that means that if we have an earthquake of magnitude 5, the vibration

is about a factor of 10 larger than that of a magnitude 4.

So here's the record for a day as recorded by a seismometer in Japan.

And you can see that for much of the day, hardly anything was happening and

the lines are fairly smooth.

They're not perfectly straight, because remember, they're also recording so

called noise vibrations due to the environment around the seismometer.

But every now and then you see a little blip where

there's been a bigger vibration and that's an earthquake.

And you an see where the smaller blips occur, that's a smaller earthquake and

where the larger blips occur, that's a larger earthquake.

So let's now return to the question of how to compare the size of earthquakes.

This time, using the magnitude scale.

Seismologists distinguish among earthquakes based on the magnitude.

Another way to look at the size of an earthquake is to consider the amount of

energy released by an earthquake.

Now when we were talking about the size of the vibration,

we said that each increase of an integer in the magnitude scale

represented an increase in a factor of 10 in the size of the vibration.

But, it turns out at an increase of one integer

in a magnitude scale represents an increase

in a factor of 32 in the amount of energy released by an earthquake.

That means that an earthquake of magnitude 6 releases about a million

times more energy than does an earthquake of magnitude 2.

There's a huge energy difference between different size earthquakes.

In fact, one great earthquake,

one earthquake greater than magnitude 8 can release more energy in a matter

of minutes than all the other earthquakes that happen on Earth in a given year.

Because we can think of earthquakes as being sources of energy,

we can compare the size of an earthquake to other sources of energy.

So for example, we can create the scale here

where we represent on a vertical scale the magnitude and

then plot on that different events and compare them to more familiar events.

So for example, a large lightning bolt is about

the equivalent of magnitude 3 earthquake [SOUND].

Tornado is the equivalent of about a magnitude 4.6 earthquake.

The Hiroshima atomic bomb is the equivalent of about a 6.2 earthquake.

The largest nuclear test ever recorded on Earth was the equivalent of a magnitude 8.

And the Krakatoa explosion or the explosion of Krakatoa in 1883 that

created sound that was heard around the world was equivalent of about an 8.5.

The largest earthquake that has ever been recorded on Earth is

about a magnitude 9.5.

It was a earthquake that happened in 1960 in Chile.

Earthquakes larger than about 9.5 probably can't happen

because there really isn't a fault big enough that could have enough slip

over a small interval of time that can generate that size earthquake.

So 9.5 is about as big as earthquakes get on the planet.

The last thing we'll mention here, and

we are not going to into detail during the lecture part here,

is how you can measure the location of an epicenter of an earthquake.

That's done by measuring the earthquake at three different seismograph stations.

And what we can determine is the distance between that seismograph station and

the epicenter.

And if we do that for three different earthquakes and

draw a circle around the seismograph.

Where those circles intersect represents the location of the earthquake.

So just to summarize,

we've seen today that not all earthquakes are the same size.

And that we can describe the differences between different size earthquakes

using one of two scales.

The Mercalli Intensity scale,

which is a subjective measurement of the amount of damage cause by an earthquake.

And the magnitude scale, which is a objective measurement of the size of

the vibration generated by the earthquake.

[MUSIC]

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