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Old 2007-10-31, 16:47   #1
ewmayer
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Default Magnitude 5.6 Earthquake in Silicon Valley

There was a magnitude 5.6 earthquake last night on the Calaveras fault in the East San Jose foothills, epicenter about 10-15 miles east of my apartment. I was just settling in to watch It's the Great Pumpkin, Charlie Brown [which came on at 8pm], and a few minutes in, it felt as if my upstairs neighbor [a nice but somewhat heavy-set fellow] had started stomping around heavily ... times 50. That initial rumbling is apparently due to the p-wave component of the shock, which travels faster than the more-rolling s-wave, which in my case arrived a few seconds later. I quickly went out onto the back patio - not from fright, since I've experienced decent-sized quakes before [though this was the strongest we've had since I moved to California in 1999 - strongest in these parts since the '89 Loma Prieta quake, in fact] and it was all over in about 10 seconds - but rather to watch the water slosh back and forth in the community pool. The last sensible quake we had here a couple years ago was a 4.5 and caused a sloshing amplitude of just 2-3 inches, last night's was closer to a foot, so I guessed "about five-and-a-half." Pretty close!

Using the well-known rule of thumb [cf. the above Wikipedia page] for earthquake shock arrival times:
Quote:
A quick way to determine the distance from a location to the origin of a seismic wave less than 200 km away is to take the difference in arrival time of the P wave and the S wave in seconds and multiply by 8 kilometers per second
... and estimating roughly 2 seconds delta-T gives 16km, which is again pretty close to the actual distance 'twixt my abode and the reported epicenter. Shaky scientific reasoning, to be sure. ;)
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Old 2007-10-31, 18:52   #2
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Quote:
Originally Posted by ewmayer View Post
There was a magnitude 5.6 earthquake last night on the Calaveras fault in the East San Jose foothills, epicenter about 10-15 miles east of my apartment. I was just settling in to watch It's the Great Pumpkin, Charlie Brown [which came on at 8pm], and a few minutes in, it felt as if my upstairs neighbor [a nice but somewhat heavy-set fellow] had started stomping around heavily ... times 50. That initial rumbling is apparently due to the p-wave component of the shock, which travels faster than the more-rolling s-wave, which in my case arrived a few seconds later. I quickly went out onto the back patio - not from fright, since I've experienced decent-sized quakes before [though this was the strongest we've had since I moved to California in 1999 - strongest in these parts since the '89 Loma Prieta quake, in fact] and it was all over in about 10 seconds - but rather to watch the water slosh back and forth in the community pool. The last sensible quake we had here a couple years ago was a 4.5 and caused a sloshing amplitude of just 2-3 inches, last night's was closer to a foot, so I guessed "about five-and-a-half." Pretty close!

Using the well-known rule of thumb [cf. the above Wikipedia page] for earthquake shock arrival times:

... and estimating roughly 2 seconds delta-T gives 16km, which is again pretty close to the actual distance 'twixt my abode and the reported epicenter. Shaky scientific reasoning, to be sure. ;)
Fun times :D

I was there for the '89 quake, I remember being a bit freaked out by the main quake, but kind of enjoying the aftershocks.
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Old 2007-10-31, 20:28   #3
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Quote:
Originally Posted by ewmayer View Post
...the community pool. The last sensible quake we had here a couple years ago was a 4.5 and caused a sloshing amplitude of just 2-3 inches, last night's was closer to a foot, so I guessed "about five-and-a-half." Pretty close!

Shaky scientific reasoning, to be sure. ;)

Yes. Without knowing the distance of the 4.5 quake or the depth
of either, even knowledge of the relationship between sloshing
amplitude and strength of the quake makes "one more on the
Richter scale" a lucky guess.
I imagine direction relative to fault line comes into play as well.
I think Sri Lanka copped the Boxing day tsunami off Sumatra much
worse than Bangla Desh did was down to the direction being
perpendicular to the fault line.

Last fiddled with by davieddy on 2007-10-31 at 20:40
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Old 2007-10-31, 20:41   #4
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Quote:
Originally Posted by davieddy View Post

Yes. Without knowing the distance of the 4.5 quake or the depth
of either, even knowledge of the relationship between sloshing
amplitude and strength of the quake makes "one more on the
Richter scale" a lucky guess.
Yes, any such extrapolation was only justified if yesterday's quake was at a similar distance as the earlier 4.5 - which turned out to be the case although I couldn't have known it at the time, obviously. OTOH the assumption of yesterday's being stronger based on it feeling stronger at my locale was not totally unjustified, since for a 4.5-or-weaker to feel stronger, we would have had to have been essentially right atop the rupture site, which is statistically improbable. [Though not impossible, since the San Andreas fault runs through the coastal hills just a few miles W of where I live. The Hayward and Calaveras faults run roughly || to the SAF, but are on the opposite side of the valley.]

"It's 'one stronger', isn't it?" -- Nigel Tufnel, famous geologist.

Anyway, admit it: you're just jealous of my pulling-numbers-out-of-my-butt quake postdiction skills. ;)
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Old 2007-10-31, 21:47   #5
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Yeah, I felt the earthquake.
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Old 2007-10-31, 21:54   #6
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Gee :>

And I was kinda freaked out when a 5.8 hit Roermond in 1992 (which is about 150 km from where I live, so pretty much 3.0 was left at where I live).
Of course, that one hit in the middle of the night, and it was my first earthquake. (And the last one I hope)

That one was at a 17km depth, and didn't do too much damage.

Isn't it so that the whole area you live in is bound to have a very major earthquake in the near future?
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Old 2007-10-31, 22:45   #7
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Quote:
Originally Posted by michaf View Post
Isn't it so that the whole area you live in is bound to have a very major earthquake in the near future?
Not sure what the latest doomsday predictions of the USGS are, but historically the San Andreas or one of its branches has a major rupture [magnitude ~7] every century or so. I expect it's likely we'll have another 6+ in the next 20 years.

However, the *really* big quakes along the US west coast, the magnitude > 8 ones, are in an area whose danger many people don't know about: The pacific northwest, in particular the population centers of Seattle and Vancouver. The problem is that these monster quakes are infrequent enough [last one was in 1700, it was recently confirmed that the Japanese have detailed historical records of a massive tsunami caused by that one] that there is no modern-historical record. The whole Seattle and Vancouver area was built up in blissful ignorance of these massive recurring quakes. There have been such quakes in the general area [viewed at large] in living memory [in particular the 1964 alaska quake, magnitude ~9], but none has happened to be near a major population center.
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Old 2007-11-01, 13:20   #8
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The richter scale is ~ log10(amplitude) but the energy rises a factor of 1000 when richter scale rises 2 values, so the Richter scale is ~ logsqrt(1000)(energy) ~ log32(energy).

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

http://science.howstuffworks.com/earthquake5.htm

http://www.ux1.eiu.edu/~cfjps/1300/magnitude.html
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Old 2007-11-01, 13:37   #9
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Quote:
Originally Posted by ATH View Post
The richter scale is ~ log10(amplitude) but the energy rises a factor of 1000 when richter scale rises 2 values, so the Richter scale is ~ logsqrt(1000)(energy) ~ log32(energy).
Not sure if you glimpsed my post (which I deleted swiftly) but it
was on this topic.

In my book, "energy" is proportional to amplitude^2 so when log10(amplitude) rises by 2, energy rises by a factor of 10,000.

Since "decibel" refers to log(energy) why didn't Richter conform
to this previously established convention?

David

Last fiddled with by davieddy on 2007-11-01 at 13:47
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Old 2007-11-01, 14:05   #10
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Quote:
Originally Posted by ewmayer View Post
"It's 'one stronger', isn't it?" -- Nigel Tufnel, famous geologist.
I think this answers my last post.


Quote:
Anyway, admit it: you're just jealous of my pulling-numbers-out-of-my-butt quake postdiction skills. ;)
I'm quite proficient in this respect myself

BTW If Richter meant log (intensity) then an increase of sloshing amplitude
by a factor of SQR(10) would be "one stronger".

Last fiddled with by davieddy on 2007-11-01 at 14:20
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Old 2007-11-01, 16:44   #11
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Quote:
Originally Posted by davieddy View Post
Since "decibel" refers to log(energy) why didn't Richter conform to this previously established convention?
IIRC decibel [as applied to acoustics] multiplies the sound energy [actually the power-per-sq-meter, I believe, since the total energy and power depend on the areal extent of the acoustic front] by a factor of 10, so that scale also has an arbitrary constant factor mixed in. I don't think there is a "standard" convention in this respect - for an example of yet-another-and-completely-different logarithmic multiplier, look at astronomical magnitudes.

In one way, expressing earthquake magnitude as log10(amplitude) actually makes prefect sense, since the shaking you feel is proportional to wave amplitude, not total-energy-release. But again, it all comes down to picking one's arbitrary logarithmic multiplier.
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