Seismic spike of WTC7 collapse

[k-robjoe]

"John Doe II" wrote on another thread :

"I don't have the right yet to start another topic. Nonetheless I'd like to point out another seimic observation but it concerns WTC 7.
According to the FEMA report at 5:20:33 the building starts to collapse (although not yet visibly)as the two penthouses start moving. The first penthouse disappears only thirty seconds later. Seven further seconds later at 5:21:08 the building starts to collapse in the speed of free fall.
Question is when is a seismic spike registered. I would expect at 5:21:08 the verymoment the whole building collapsed not before when the collapse wasn't visible. But strangely the seismic observation gives 5:20:33 as the time for the spike. (The signal lasts 18 seconds) 37 seconds before the building really started to collapse.
http://www.ldeo.columbia.edu/LCSN/Eq/20010911_wtc.html

I don't understand this. How can a spike be before the actual movement that is supposed to cause the spike??
Can anybody help me out? Feel free to start a new topic for that if you like as this doesn't have any connection to Flight 93."

[On the Road]

1. I've Heard That Argument About the First Two Towers

My problem with it is that it's difficult to believe that a seismic spike was NOT caused by the buildings themselves when they collapsed, but WAS caused by some other unobserved blast.

The key is synchronization. I would have to believe the biggest spike was caused by the collapse of the building.

[John Doe II]

2. I don't want to jump to conclusions

(Thanks for posting k-robjoe!)
I haven't heard about any bizarrity of the seismic recordings concerning the collapse of the Towers (but maybe I'm wrong).
I simply think it's very very strange that the spike is recorded clearly before the actual collapse.
Are the figures by amateurs and could be wrong?
Unlikely it's the LCSN Station Palisades.
Maybe it's just nonetheless a simple mistake?
Sure that could be. Only: 37 seconds is a long time and would be a huge mistake. Moreover it's exactly the time when FEMA remarked the first small movement of the building.
I'm not a scientist. Just what could make a building of that size move a bit (not visible for the normal spectator) create a seismic spike that last 18 seconds and 37 seconds later the whole building collapsed in the speed of free fall???

[On the Road]

3. If There Were an Explosion That Caused the Collapse

There should be two seismic events. If there's only one, the open question would seem to be how the collapse could leave no seismic trace whereas something that was not particularly notable to people at the site caused a significant seismic event. That's why I have trouble with this line of argument. For it to be considered more seriously, there has to be a lot more spelled out.

[stickdog]

4. Do you think that seismic data lie? People lie. Seismic data don't.

The secondary event of the collapse gets lost in the aftershocks of the far less temporally distended (and thus far more seismically measurable) explosive event that initiated the collapse.

It's not some deep mystery. It's basic seismology.

[On the Road]

5. Basic Seismology Says

that when an explosion takes down a building, the blast causes a seismic spike but the building falling down does not?

It seems counterintuitive. Can you provide a source?

[stickdog]

6. There aren't a lot of forensic seismology textbooks on the internet.

But allow me to help with lesson one.

Seismograph readings can be roughly thought of as measuring total energy output vs. TIME. Therefore a smaller release of energy over a very short period of time (like an explosion or sudden fault slippage) often results in a more prominent seismic spike than a larger release of energy occurring over a longer period of time (like a building falling down or the cumulative energy of all the aftershocks of sudden fault slippage).

[John Doe II]

7. Thanks for the explanation, stickdog

I myself don't know what to make of the seismic observation. But I don't have to explain that the crash of the WTC 7 is an important issue.
So, either somebody proves that the given time by the Institute is wrong or that FEMA is wrong or what the 37 seconds gap scientifically means.

[On the Road]

8. I Understand the Concept

but would the impact of a building collapsing be so slow that it wouldn't register whereas a much smaller bomb blast would register? This is what seems counterintuitive to me.

The simplest explanation for a single seismic spike seems to be to be a difference in timing. 37 seconds seems a little long in any case.

[demodewd]

9. above ground level

If the bomb were set off below ground level it may create a seismic spike whereas a collapsing building does not register much seismic activity because it is above ground level. This is my understanding.

[John Doe II]

10. Interesting!

To be more precise:
the data sais that while the crash of the Tower has clearly a stronger magnitude 2.1 and 2.3 vs 0.6 for WTC 7 it's rather strange to note that the signal lasted in the case of the collapse of the Towers only 10 and 8 seconds while for the "collapse" of WTC 7 (which is not even half as high)lasts 18 seconds that means double as long. Quite a significant difference!
http://www.ldeo.columbia.edu/LCSN/Eq/20010911_wtc.html

But just to be sure: Maybe somebody who is really into the seismic analysis like stickdog: Could you please check if the given time (see first entry) are correct. (I take it for granted that the time given by FEMA for the collapse is correct)

[On the Road]

12. Thanks for the Explanation

I don't mean to be obtuse, but most of us here don't know much about seismology, and it's difficult to balance skepticism and openmindedness in an area you don't know much about.

What would be most convincing to me would be seismic images of the WTC7 collapse alongside images of another building collapse or of a bomb blast like the original WTC bombing -- something that would make the unusual features of the WTC7 collapse stand out.

Maybe that's not readily available, but extraordinary claims require extraordinary evidence. You may be able too look at the seismic pattern and from your experience see immediately that there's something funny about, but others can't -- at least without an image and another incident to relate it to.

[John Doe II]

13. I agree

I don't want to make any claim so far. That's what I wanted to point out with "not jumping to conclusions". I just wanted to stress the significance of various oddities concerning the seismic data of WTC 7. And see what other people think etc.
The diagramms are a good idea. I'll look around. I believe I saw them but it's quite a while ago.

[LARED]

25. WTC 7 seismic pattern

According to the FEMA report at 5:20:33 the building starts to collapse

The first penthouse disappears only thirty seconds later.

Seven further seconds later at 5:21:08 the building starts to collapse

I'm not an expert in reading these things but the chart and the time line do not seem worlds apart.

[LARED]

30. A question for stickdog

Seeing as you claim to be the local expert, please provide an interpretation of the above data and explain how it helps or discredits the offical story.

Thanks in advance.

[k-robjoe]

32. May I just ask

Isn´t the most interesting question : Is it or is it not the case that the seismic spike appeared "37 seconds before the building really started to collapse."

Is Lared saying that this is not the case? How did John Doe2 find that this was the case?

Is Lareds graph starting up at the time when FEMA has pinpointed that the WTC7 started to collapse? Or is it starting up at an earlier point? ( Would have to be, if the spike is to be 37 seconds before the collapse?)

Maybe my ignorance can contribute to calm you guys down a bit...?

[stickdog]

37. I never made any such claim. The only claim I made was the ability to

read a seismogram and to possess the basic understanding of seismology of any first year geology student.

My initial guess is the time FEMA reported wasn't 100% accurate. This isn't like Flight 93 in which we're comparing two sets of data, each of which requires exact synchronization. In the case of WTC-7, planes wouldn't be crashing if FEMA's reported collapse timing was a little off the mark.

However, if someone can provide me with a scientifically verified timeline of WTC 7's collapse, I'll ask Terry Wallace about it. He'd certainly want more data than just LDEO before making an assessment, but hopefully he already has it.

[stickdog]

14. I have more confidence in the seismic data than in FEMA's data.

Note that the seismic data absolutely pegged the timing of the planes hitting both towers.

I doubt it would suddenly go out out synch.

[LARED]

11. I disagree. Seismographs do not measure energy

They measure movement verses time. There is a relationship to the amount of energy released, but it is not measuring energy.

This is why the pentagon impact did not register on a seismograph even though the amount of energy released was similar to the WTC impacts.

The WTC are built on bedrock. Rock that more easily transmits the movement induced by the collapse away from the collapse. The Pentagon was build on marshy land that effectively dampens the movement induced by the impact.

Seeing as you feel qualified to provide lessons. What are your credentials?

[stickdog]

15. What are your credentials to question my accurate lesson?

The best measure of earthquakes is the seismic moment. Seismic moment is literally measured in energy (nm, joules, kcals).

http://www.geo.mtu.edu/UPSeis/waves.html

What Are Seismic Waves?

Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.

http://www.seismo.unr.edu/ftp/pub/louie/class/plate/seismology.html

The sudden release of stored elastic potential energy during an earthquake releases both heat energy and seismic waves. The total energy is called the moment M0.



By essentially taking the logarithm of the moment, we can create a magnitude rating. Every earthquake has a unique total energy, and therefore a unique magnitude.

Forensic Seismology lecture:

http://mnw.eas.slu.edu/People/KKoper/Teaching/EASA-130/2003/Lecture_14/

[John Doe II]

16. Where are seismic diagrams?

stickdog, good to have you here!

I had a look around to find the seismic diagram of the WTC 7 collaps. All I found over and over again are the diagrams of the collaps of the Towers not of WTC 7. (But the search confirmed that my data were never in confilct with any other source: so question remains: How is it possible that the spike occured 37 seconds before the actual collaps? I'd like to challenge especiallt those for an answer who are convinced that WTC 7 collapsed due to fire).

Question for help: Did anybody around here came across a seismic diagram? This could be very much of a help to understand what really happened?

[LARED]

22. Perhaps this is just semantics

but let me point out a few things.

You stated

The best measure of earthquakes is the seismic moment. Seismic moment is literally measured in energy (nm, joules, kcal's).

Well, I'm not sure what the best measure of earthquakes are, but based on the information you provided seismic moment is calculated not measured.

To accurately find an earthquake's magnitude one needs to know the location, geological conditions, plus some other things before the amount of energy release can be estimated. I do not process intimate knowledge of how a seismograph works. But the transfer of energy only happens though well known paths. I know a seismograph measures ground movement and time; determining displacement, velocity and acceleration. How much energy is released cannot be determined using this data alone, some manifestation of mass must find its way into the calculation

As for my credentials, I am a BSME with over twenty years experience. A lot of it doing failure analysis in my industry.

Having this experience tells that unless the time data from FEMA and the seismic calculations are reconciled (something that is often very difficult and not worth the effort) all you have is speculation about the timing of the collapse in WTC 7.

So what are your credentials?

[LARED]

24. A correction

FEMA did sort of reconcile the timing.

They used the seismic data to determine the start of the collapse and then used the video tape to determine the time intervals in the collapse sequence.

[stickdog]

27. Once again you are attacking an argument I never made while shaking

me down for my diplomas and employment history.

Why? Have I claimed anything that I haven't backed up? If so, what?

And while I'm happy for you that you're duly proud of your engineering penis, I'm not particularly interested.

By the way, a post of yours on this thread is titled, "Seismographs do not measure energy."

First, the preferred term is "seismograms." Second, they do indeed measure energy. In fact, that is exactly what they measure, as any first year geology student knows. So perhaps, being such an impressive civil engineer, you might want to consider reinforcing the brittle dwelling you inhabit.

[LARED]

29. Are you sure you're qualified to give lessons?

The only reason I ask is you don't seem to have a clear understanding of the common terms.

http://vulcan.wr.usgs.gov/Glossary/Seismicity/description_seismic_monitoring.html


Seismogram:

A graph showing the motion of the ground versus time. -- Excerpt from: Noson, Qamar, and Thorsen, 1988, Washington State Earthquake Hazards: Washington Division of Geology and Earth Resources Information Circular 85

Seismograms are the records (paper copy) produced by seismographs used to calculate the location and magnitude of an EQ. They show how the ground moves with the passage of time. On a seismogram, the HORIZONTAL axis = time (measured in seconds) and the VERTICAL axis= ground displacement (usually measured in millimeters). When there is NO EQ reading there is just a straight line except for small wiggles caused by local disturbance or "noise" and the time markers. -- Excerpt from: USGS Earthquake Hazards Program Website, 2002

Seismograph:

A sensitive instrument that can detect, amplify, and record ground vibrations too small to be perceived by human beings. -- Excerpt from: Noson, Qamar, and Thorsen, 1988, Washington State Earthquake Hazards: Washington Division of Geology and Earth Resources Information Circular 85

Seismographs are instruments used to record the motion of the ground during an EQ--installed in the ground throughout the world and operate as seismographic network. The first one was developed in 1890. The earliest "seismoscope" was invented by the Chinese philosopher Chang Heng in A.D. 132. This did not record earthquakes, however. It only indicated that there was one occurring. A seismograph is securely mounted onto the surface of the earth so that when the earth shakes, the entire unit shakes with it, EXCEPT for the mass on the spring which has inertia, and remains in the same place. As the seismograph shakes under (in the example below) the mass, the recording device on the mass records the realtive motion between itself and the rest of the instrument, thus recording the ground motion. In reality, these mechanisms are no longer manual, but instead work by measuring electronic changes produced by the motion of the ground with respect to the mass. -- Excerpt from: USGS Earthquake Hazards Program Website, 2002

Seismometer:

A seismometer is the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". -- Excerpt from: USGS Earthquake Hazards Program Website, 2002

_________________________________________


Not to be picky, but guys offering lesson on the internet should be able to provide some -- a little information -- as to why they possess the experience and knowledge to have credibility. I told everyone that is paying attetion or cares that all I have is a geology diploma from google and a Mechanical engineering degree.

So what makes you qualified?

[stickdog]

36. Another important lesson.

About seismographs/seismograms, I made an ill-advised attempt to take issue with something you said. I was quite clearly wrong. Your use of the word "seismograph" was entirely correct, and I had no business whatsoever to dispute it. Please accept my sincere apologies.

****

Contrast my reply to you concerning my obvious error with your reply to me after you erroneously stated that "seismographs do not measure energy."

Did I demand your qualifications for making clearly accurate statements or did I instead simply own up to my obviously mistaken criticism? I mean, you're certainly not a trained and credentialed seismologist. So we can hardly trust your understanding of the difference between seismograms and seismographs -- no matter how much evidence you present or how clear and explicit that evidence is.

Or can we? You obviously feel that you are entitled to present information about the field of seismology -- even though, considering your initial contention that seismographs don't measure energy, it is clearly not your area of expertise, credentialed or otherwise. I agree completely, and ask only that you consider extending the same courtesy to others. Accurate information and insightful analysis has inherent merit, regardless of its disseminator. And even when it's been fully certified by every PhD at the Hoover Institute, bullshit is still bullshit. Wouldn't you agree?

Just as an aside, do you consider yourself an expert in chess?

[John Doe II]

38. As nobody answers ...

I'm in/was an expert in chess ....

[LARED]

39. Not to worry

A answer will be coming.

Played chess for fun. Decent player, but nowhere near an expert.

[LARED]

40. Additional lessons

Apology accepted

Contrast my reply to you concerning my obvious error with your reply to me after you erroneously stated that "seismographs do not measure energy."

Would that contrast include you wonderful statements that went like this;

And while I'm happy for you that you're duly proud of your engineering penis, I'm not particularly interested.

Or

….. being such an impressive civil engineer, you might want to consider reinforcing the brittle dwelling you inhabit.

You seem to have forgotten YOU asked for my credentials. I gave them to you, what did you expect? I also clearly stated I am no expert in seismology. Btw, I clearly stated I am a Mechanical engineer, not a civil.

Did I demand your qualifications for making clearly accurate statements or did I instead simply own up to my obviously mistaken criticism?

Sure you owned up to your mistake. That is rarely witnessed in this forum. But why would you demand my qualification for making clearly accurate statement? What does that even mean?

I mean, you're certainly not a trained and credentialed seismologist. So we can hardly trust your understanding of the difference between seismograms and seismographs -- no matter how much evidence you present or how clear and explicit that evidence is.

Very true, but I make no claims as having the ability to provide lessons as you did. I fact I made it clear my seismology training came from google.com. Without knowing your credentials I have no reason to believe anything you say. So why should I or anyone else believe what you’re talking about? Do you have any credentials to back up your opinions – any? Did you read a few good books? Did you get some training, education? Or are you just good at blowing smoke.

The forum is chock full of google cowboys that claim expertise in engineering, forensics, air traffic control, etc, etc, etc so when someone starts offering lesson on seismology it’s a good ides to see if they someone other than their mom thinks they know what they are talking about.

You obviously feel that you are entitled to present information about the field of seismology -- even though, considering your initial contention that seismographs don't measure energy, it is clearly not your area of expertise, credentialed or otherwise.


Yes, anyone is entitled to present information in any field. No one is asking you to stop, only that if you are going to provide interpretations of that information you should tell folks why we should pay attention.

even though, considering your initial contention that seismographs don't measure energy, it is clearly not your area of expertise, credentialed or otherwise.

As I pointed out this is really about semantics, but seismographs do not have the ability to measure energy. I am now aware that the seismic moment is considered an indirect measurement of energy based on empirical data, but the simple fact is without considering mass one cannot really measure the extensive property of the energy of something. In other words as an extensive property it is a measurement of the sums of all values in all part of the system and must include its mass.

The seismic moment as I understand it is estimated based on empirical data.

See here http://www.seismo.unr.edu/ftp/pub/louie/class/100/magnitude.html

Again let me point out this is largely semantics. In reality the scientists that study earthquakes use energy units to estimate magnitude as a convenience not as a true measure of energy. I don’t blame them as it works, is understandable and obviously useful to them. But from a technical viewpoint they are not using a seismograph to measure the energy of an earthquake.

And even when it's been fully certified by every PhD at the Hoover Institute, bullshit is still bullshit. Wouldn't you agree?

I agree. But I fail to understand you reluctance to explain why you are qualified to give lessons.

[stickdog]

41. Basically, what's good for the goose simply ain't for the gander.

The whole point of last TWO answers to you was to illustrate your hypocrisy in demanding for my credentials. Such a demand is common logical flaw indicative of a weak counterargument. As such, anyone could make such a demand from anyone else claiming anything at any time -- no matter how well sourced or obviously true -- and all discussion would be limited to those able to prove they were tenured in the fields they endeavored to discuss.

I clearly illustrated this point above, but it's not surprising that you chose to simply ignore it.

[LARED]

42. What the hell are you talking about?

The whole point of last TWO answers to you was to illustrate your hypocrisy in demanding for my credentials.

My hypocrisy? Please point out where I have been hypocritical? You are providing lessons plans. All I asked was why I should take you seriously.

Such a demand is common logical flaw indicative of a weak counterargument.

Against what argument have I made a counterargument? I've only stated that seismographs don't really measure energy in the technical sense. I think I explained my position. So far all you have said is I'm wrong, because you say so.

As such, anyone could make such a demand from anyone else claiming anything at any time -- no matter how well sourced or obviously true -- and all discussion would be limited to those able to prove they were tenured in the fields they endeavored to discuss.

Can you say utter horse-pucky. No one here is asking for such a thing. All I've asked is why your view holds any weight. Not if your tenured, not even if you graduated from 13th grade. All I've asked for is a reason to believe you over someone else.

I have been up front about my experience and credentials. Why is asking for someone's experience regarding a issue they speak to, a problem for you?

[stickdog]

43. You have been up front that you have no seismology credentials.

Yet that didn't stop you from taking issue with an accurate statement I made about seismology while demanding my credentials. If that's not a double standard, what is?

Saying that seismographs measure movement, not energy is like saying that thermometers measure the expansion of mercury, not temperature.

What I originally wrote:

Seismograph readings can be roughly thought of as measuring total energy output vs. TIME. Therefore a smaller release of energy over a very short period of time (like an explosion or sudden fault slippage) often results in a more prominent seismic spike than a larger release of energy occurring over a longer period of time (like a building falling down or the cumulative energy of all the aftershocks of sudden fault slippage).

was, is and remains 100% accurate.

[LARED]

44. 100% ?

I agree with almost everything in your statement except

..measuring total energy output vs. TIME.

is not correct.

See here near the bottom.

http://www.seismo.unr.edu/ftp/pub/louie/class/100/magnitude.html

Also as an illustration

Saying that seismographs measures energy, is like saying that thermometers measure heat output though measuring the expansion of mercury.

[stickdog]

47. You forgot the part that makes my statement 100% correct.

"can be roughly thought of as measuring total energy output vs. TIME."

From your own link:

Short answer:

Seismologists use a Magnitude scale to express the seismic energy released by each earthquake.

[LARED]

48. As I said a while ago this is largly semanitics

"express" is not the same as "measure".

My only point is a technical one. Without considering mass you cannot truly measure energy.

[Robb]

17. To be specific

...they actually measure velocity over time in three directions. You integrate to get displacement (movement) for those three basic graphs. :)

My understanding is that it wasn't so much the type of rock between the WTC and the Pentagon but rather the transmission of the impact -- Pentagon itself absorbed the relatively small energy of the airplane impact like a collapsing Volvo front-end, whereas the WTC towers bent and released the energy into the ground like reed.

And obviously the collapse of a building that size is much more significant than any airplane crash. As far as the spike appearing before the collapse, I would imagine the energy released would decrease after the intial "clunk" as it began collapsing; with each passing moment there's less and less building slamming into the ground.

I got myself a hell of a crash education in forensic seismology, and two things I'd offer to anyone else trying to figure it out: 1) remember to look at all three components, not just your favorite graphs, and 2) don't think you understand seismic waves until you talk to a prof about it, because the nuances of the interaction between compression and shear waves are pretty complicated.

(edited to sound like English is my first language)

[RH]

45. That is a remarkably uncommitted comment.

Was there no serious attempt to resolve the missing three minute issue since September 2002, by way of a second seismic opinion or whatever?

With forensic seismology I fear an element of bandwagonning. Anybody unusually keen to make a name for themselves I take with a pinch of salt.

[TrogL]

21. so where does your information on seismology come from?

Do you even understand how a seismograph works?

[stickdog]

28. So many critics.

So little merit.

[tngledwebb]

31. What qualifications do you have?

DU rules do not request that posters have special qualifications, but if you are asking this of others, please tell us yours, if any. Do you understand how a seismograph works? If so, please tell how you came upon this special knowledge.

[MercutioATC]

18. Isn't the makeup of the ground under the buildings "basic seismology" too?

Without knowing the distance from the collapses to the sensors, the makeup of the ground between the collapses and the sensors, and the depth of the foundations of the buildings (and what they rested on) I don't see how any conclusions can be reached from basic seismic data...

[John Doe II]

19. MercutioATC would you agree

that in general it's rather strange that the seismic spike is recorded 37 seconds before the actual collapse? And if so, don't you find it rather bizarre that no investigation bothered to answer this oddity? And if you don't think it's strange please feel free to explain this phenomen.

[MercutioATC]

20. If, in fact, the spike WAS the collapse, yes, I'd consider it odd.

What I wouldn't consider odd is if the spike was caused by something else and the collapse of WTC7 wasn't detected because of geological conditions.

[demodewd]

23. What the heck!

Doesn't quite fit your OCT so you have to contrive some half cocked explanation for the apparent anomaly.Eh..Merc? What would that something else be??? Hmmm?? Within that very limited local? What geological conditions would be different for the WTC 7 as opposed to the WTC1 and WTC2?

[MercutioATC]

26. So why didn't the sensors register the WTC7 collapse?

If the building didn't collapse until 37 seconds AFTER the seismic event, obviously the sensors didn't register the collapse. Why, then, is it logical to assume the seismic event that WAS registered had anything to do with WTC7?

What geological conditions would be different? Mainly the depth of the foundation and whether the building was pinned to bedrock. If memory serves, the WTC towers were pinned to the underlying bedrock. I have no idea if WTC7 was or not.

[RH]

46. So the point is....?

Are you now trying to make out that WTC7 did not collapse?

Or is this more to do with the inviolable right of an anomaly to be anomalous?

[John Doe II]

33. For clarification

In order to cal down the heated debate:

LARED states (see 24) that FEMA used the time of the seismic spike in order to calculate the exact time of the crash. (By the way, LARED, could you please be so kind and give a source for this). Of course this would explain very simply the 37 difference (as the crash started invisbily at the top and was only wisible 37 seconds later). But this explanation amazes me. It's seems very unscientific if FEMA would have used the exact time of the seismic spike in order to calculate the time of the "invisble" movement of WTC 7 and not the real collapse that should have created the seismic spike. FEMA's method simply seems wrong.
(Although I ceratinly agree if this happened nonetheless one couldn't read anything out of the seismic data)

[LARED]

34. Link

See the bottom of page 23.

http://www.fema.gov/pdf/library/fema403_ch5.pdf

Plus a comment regarding the use of the term spike. Based on my seismology degree from google.com, I think there may not be a direct relationship between what looks like a seismic spike to the layman, and the actual start of a seismic event.

The different waves that are measured arrive at different times with different signatures and magnitudes. One wave has a spike like appearance and the other does not. So it is not clear to me if the spike is the best way for a novice to relate time to an event.

Perhaps our resident teacher can provide some insight?

Some links I found instructive while I earned my google sheep skin.

http://schools.matter.org.uk/Content/Seismology/seismometer.html

http://www.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.html

[LARED]

35. How to read a seismogram

http://staff.imsa.edu/science/geophysics/geosphere/tectonics/seismogram.html

I thought it was instructive.