h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Kobe was built with specifications to handle earthquakes, and yet suffered significant, widespread damage. That alone should illustrate the power of the quake. One has to wonder what standard is being applied here to qualify this 6.9 magnitude earthquake as "minor" and "relatively small."

For comparison's sake, the 1906 San Francisco quake, one of the most famous quakes in history, formerly estimated at 8.25 on the Richter scale, is being re-examined. And a comparison between the old seismographs and our new ones leads Caltech to believe it measured 6.9.

http://www.johnmartin.com/earthquakes/eqpapers/00000048.htm

Since quakes of the 7 range are indeed common in Japan:

http://en.wikipedia.org/wiki/List_of_earthquakes

(Please note that the Moment Magnitude Scale used in this list is directly comparable to the term *Richter scale*)

And the author twice mentioned that fact:

*earthquakes with a magnitude of 6.9 on the Richter scale are common in Japan*

*a minor earthquake...(footnote)..Minor for Japan, anyway.*

The reference to the quake being *minor* and *relatively small* appears to be in regard to in Japan itself. Indeed the greatest quakes recorded and those with the most damage have not been in Japan and yet it has a continuous history of many more quakes in the quoted range and above than most countries that have suffered the greatest damage.

As to the severity of the damage that is was explained by the author using the time of day, the wooden structures which caught fire and particular geographical location of the city.

<>

The "significant earthquakes" chart lists 9 earthquakes of magnitude 7 or higher in Japan in over 100 years. This must be some new definition of "common" with which I am unfamiliar.

I live in California, and over the same time span, your chart lists 16 total quakes of magnitude 7 or higher. Assuming the Japanese did not have seismic technology until their first major quake reported in 1896, California has had 13 in that time span. So they are more "common" in California than in Japan. Yet here they are not considered common, nor minor.

<>

Were the roadways made of wood? How about the gas pipes? Or the rail lines?

My point exactly, he wasn't talking about California or the rest of the world, he was talking about Japan. Earthquakes are indeed common in Japan and the one that hit Kobe that day wasn't all that unusual in intensity.

*Were the roadways made of wood? How about the gas pipes? Or the rail lines?*

Dumb Bomb?

Obviously not, the buildings were but the roadways, rail lines and gas pipes ran through a corridor that Kobe was in.

Can you read? Why do I bother...

May I point out that the Richter scale (and the attendant Moment Magnitude Scale) are logarithmic in nature. Each whole number is aproximately twice the power of the preceding one. You are thinking linearly.

I'll leave the author and others to set you straight.

Do not make any hasty assumptions. You have no insights into what I am thinking.

Each whole number is TEN TIMES the amplitude, and THIRTY TIMES the energy level.

Just to set *you* straight.

"agnitude is a measure of the amount of energy released during an earthquake. It may be expressed using the Richter scale. To calculate magnitude, the amplitude of waves on a seismogram is measured, correcting for the distance between the recording instrument and the earthquake epicentre. Since magnitude is representative of the earthquake itself, there is thus only one magnitude per earthquake. Taking the Saguenay earthquake of November 25, 1988 as an example, one could not therefore speak of magnitude 6 at Quebec City and magnitude 4 to 5 at Montreal. The effects ( or intensities) at the two places were different, but the magnitude of an earthquake is unique; in this example, it was 6 on the Richter scale. The magnitude scale is logarithmic. This means that, at the same distance, an earthquake of magnitude 6 produces vibrations with amplitudes 10 times greater than those from a magnitude 5 earthquake and 100 times greater than those from a magnitude 4 earthquake. In terms of energy, an earthquake of magnitude 6 releases about 30 times more energy than an earthquake of magnitude 5 and about 1000 times more energy than an earthquake of magnitude 4. It is very unlikely that an earthquake of magnitude less than 5 could cause any damage.

Drat
http://www.seismo.nrcan.gc.ca/questions/faq_e.php#magnitude
"Magnitude is a measure of the amount of energy released during an earthquake. It may be expressed using the Richter scale. To calculate magnitude, the amplitude of waves on a seismogram is measured, correcting for the distance between the recording instrument and the earthquake epicentre. Since magnitude is representative of the earthquake itself, there is thus only one magnitude per earthquake. Taking the Saguenay earthquake of November 25, 1988 as an example, one could not therefore speak of magnitude 6 at Quebec City and magnitude 4 to 5 at Montreal. The effects ( or intensities) at the two places were different, but the magnitude of an earthquake is unique; in this example, it was 6 on the Richter scale. The magnitude scale is logarithmic. This means that, at the same distance, an earthquake of magnitude 6 produces vibrations with amplitudes 10 times greater than those from a magnitude 5 earthquake and 100 times greater than those from a magnitude 4 earthquake. In terms of energy, an earthquake of magnitude 6 releases about 30 times more energy than an earthquake of magnitude 5 and about 1000 times more energy than an earthquake of magnitude 4. It is very unlikely that an earthquake of magnitude less than 5 could cause any damage."

Actually, the NGDC database lists 105 earthquakes in Japan between 1900 and 2003 about 2/3 are over 8.0

Mudhooks: That NGDC database does look like a more reliable source. Looking it over, I do get 106 quakes of magnitude 7 or greater since 1900. Keep in mind, though, that many are aftershocks, and are thus considered to be part of the same event. A magnitude 8 quake can be expected to generate 1 or more aftershocks of 7 or greater.

Filtering the data for multiple quakes for the same event, I still have 68 quakes in the last 105 years, for an average of one every year and a half. That fits a definition of the word "common" with which I am familiar. Thank you.

My original point still stands, however. A 6.9 is still a rather powerful quake, and should never be dismissed as "minor." The USGS classifies a "minor" quake as one that falls in the 3.0-3.9 range, with a 6.9 just shy of "major."
http://neic.usgs.gov/neis/general/magnitude_intensity.html

I pop back to have a look and what do I see, very interesting data.

You are absolutely, completely, and utterly right....about the log multiplier. Thanks for that. So we take a section of a log graph that represents Japan's recent (recorded) historical earthquake intensities. Two thirds of that section of graph is over 8 on the scale (thank you Ms. Dubois). Given that each increase is *TEN TIMES the amplitude, and THIRTY TIMES the energy level* that makes the 6.9 *minor* and *relatively small* for JAPAN, wouldn't you say? And since that was a claim of the author I believe you owe him an apology.

No, I wouldn't say. And you owe me an apology.

By all means, please hold your breath!

No thanks. I have no reason to expect you would suddenly behave honorably.

May I enter the fray?

Just wanted to make some other points on the original post.

The "Great Hanshin-Awaji Earthquake", to give it its English title, effected not only Kobe but many towns and cities in the area, which is why it is not officially called the "Kobe earthquake".

Over 6400 people perished in the quake, not the 3000 quoted here in this article.

I have never once in 15 years of Japanese scholarship come across "Kobe" being spelled "Kobi" except perhaps in misprint.


I was living in Kobe at the time of the earthquake, as I am now, and I can unreservedly say that no Japanese I have met would ever describe the Great Hanshin-Awaji Earthquake as "minor". Split hairs over the terminology and semantics as ye will.

...and forgot to mention- Japanese houses don't have fireplaces, hot embers from which are cited as a cause of fire after the quake.

Hello chouxcream!

Although none of us can speak with complete authority for the meaning(s) the author intended (with of course one exception), several implications seemed evident. I believe that _anyone_ who has experienced an earthquake of any decent magnitude (myself included) wouldn't hesitate to qualify it as greater than average or not minor. I merely pointed out that the author may have meant that for all of Japan it was statistically less than average and therefore minor by comparison. As to your other points I have little or no knowledge except in regard to the fires. In the past I have read that kitchen fires sometimes have caused house fires in Japan after or during earthquakes but that information may be severely dated.

Thank you for your input to the conversation!

Not entirely sure what is being argued about here, and I'm certainly not an expert on earthquakes, but isn't the *depth* of the epicentre just as important as the magnitude and horizontal distance to the epicentre?

So you could have an 8.0 earthquake with a shallow epicentre that is going to cause more damage than a deep one (the asian tsunami was caused by a shallow earthquake I think).

I've been in a 7.1 shallow quake 70 odd km from the epicentre which was bloody scarey and caused almost no damage to human structures (relatively - some chimneys cracked, lots of things fell off shelves, that kind of thing). The official explanation here is that we are not highly populated so we don't get much damage, which doesn't really make that much sense unless highrises and overpass roading are the only thing that get damaged in big quakes.

Maybe the geology of the land is a factor too.

kea, I'm no geologist and have never claimed to be one but can *depth* be a factor when the measurement is made from standard points?

(Please note that last symbol was a question mark, as in the sixth sentence of post four here.)

I don't know where the measurement points were, obviously but wouldn't they be in the same place for all historical measurements? Even if they weren't they are the only statistical data we have so they can be logically assumed to have recorded a less than average strength quake historically in regard to that of all of the rest of Japan (measured).