Talk:Megathrust earthquake
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[edit] What scale are the magnitudes for?
This article doesn't state what scale the magnitudes are for. I suppose it's the moment magnitude scale? Brianjd 06:10, 2004 Dec 29 (UTC)
- Did moment magnitude scale exist in 1700? Or was that 9.0 figure for the 1700 quake guessed (guess, since it can't be accurate) much later on? Cascadia Earthquake says 9.0 on the Richter scale, but Richter magnitude scale says it can't be used for magnitudes > 8.0. -- Paddu 21:03, 29 Dec 2004 (UTC)
The magnitude of the 1700 Cascadia earthquake is estimated from Japanese tsunami data and ranges from 8.7 to 9.2. Articles on this earthquake often state Richter scale. However, I would take it to mean moment magnitude scale. I just did some research and I saw Richter scale mentioned with these numbers here and there. I guess it's just being sloppy. I came across this article http://www.msu.edu/~fujita/earthquake/bigquake.html which gives the moment magnitudes as 1700 Cascadia 9, 1960 Chilean 9.5/9.6 and 1964 Alaska 9.2.
For comparision, I looked up the non-megathrust 1906 San Francisco earthquake. The reference above gives the magnitude as 8.0, the duration was 45-60 seconds and the length of fracture along the San Andreas fault was 430km (270mi). This shows that it requires a megathrust earthquake with its 1000km fracture to generate the energy release of a magnitude 9 plus earthquake. In fact all earthquakes since 1900 of magnitude 9 or greater have been megathrust earthquakes. Another example of a large non-megathrust earthquake is the 2002 Denali (Alaskan) earthquake, magnitude 7.9, duration 1.5-2 minutes, depth 4.2km (3mi) and length of fracture 330km.
[edit] Feb '06
Megathrust earthquakes are not only ocean/continental collisions, but also continental/continental collisions, unable to cause subduction, they are the reason many mountains are formed, such as the Himalayas (Above by User:Jordskjelv)
- The Kashmir quake was not a megathrust - the definition given doesn't fit. Earthquakes are not the reason many mountains are formed, they are a product of the mountain building orogenic event. I remved the addition to the article - didn't fit and was full of errors. Vsmith 15:51, 18 February 2006 (UTC)
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- So, I believe I have read that convergent faultlines are megathrust faultlines, so how can the Pakistani earthquake who occurred in a "subduction" zone according to USGS(Which doesn't subduct), _not_ have been a megathrust earthquake? And that word orogenic, it was hard to understand, but as that article also referred to tectonic plates, and tectonic plates movement are called earthquakes, how can earthquakes _not_ have caused the himalaya to grow? I ask you to look at this website http://www.seismo.ethz.ch/gshap/ict/india.html Where it states "It is now well known that the continued northward collision of the Indian plate with respect to the Eurasian landmass causes the intense seismicity, and has produced the most gigantic topographic features of the world, the Himalaya and the Tibetan plateau." So the way I see it, you don't have all the facts here. As it clearly states that this collition, through intense seismicity, has created the Himalaya and Tibetan plateau. Also, this website http://tectonics.caltech.edu/publications/pdf/bollinger_JGR2004b.pdf states that Himalaya contains an intracontinental megathrust. So I want to know your sources, or how my sources are wrong. Given the sources I have provided, my addition to the article was correct. The sources strongly confirms that the Himalaya are built by intence seismicity, and that there is an intracontinental megathrust thereJordskjelv 16:45, 18 February 2006 (UTC)
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- OK. First, thanks for the link to the JGR pdf, it was an interesting read. But, the authors of that paper do not use the phrase megathrust earthquake. They refer to the seismically active decollement or megathrust fault system that underlies the Himalaya due to the Tibetan plateau overriding the Indian plate and it is indeed a mega thrust fault zone.
- Given that, the megathrust earthquake concept is a rather poorly defined term. All usage I find connects the term to subduction zone thrusting such as the Sumatran earthquake, along the subduction from Japan through the Kuriles, and the Cascadia zone off British Columbia and NW US. I don't find the phrase used in scientific references in connection with continental collisions. The term may be used there, but I haven't seen it - point to a published scientific usage of megathrust earthquake first. Wikipedia is not the place to redefine terms - only to report the way terms are used. The distinction may seem small - but it is real, current usage is for subduction zone earthquakes - and much of the hype for the term comes from its association with tsunami events which requires a coastal or subduction zone environment.
- Next - the Himalaya are the result of the collision between the Indian and Asian plates. The earthquakes of the region are also caused by that ongoing collision. The seismicity and earthquakes are a result, along with the grwoth of the mountains, not a cause (read the quote you provided above).
- Vsmith 18:24, 18 February 2006 (UTC)
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- Ok, I get kinda confused here. Consider myself to know alot about earthquakes, but not to have all the answers. But you say the earthquakes are a result of the ongoing collition, but I thought the earthquakes were the way the collition happen. The collition takes place through earthquakes? But also, if you use the USGS and looks for the earthquake in Pakistan, the fault is called a subduction zone, although subduction does not occur intracontinetally, but collition. actually a quite interesting discussion....
- Actually the word Orogenic, I have never come across it, even though I have read alot of the world's leading seismologists. Not in the Earthquake glossary at USGS either. But now that you mentioned it, I found alittle, and will study that too. But this is a word not much used..... Maybe to make things easier for the readers to simply call it subduction all the way? Jordskjelv 19:09, 18 February 2006 (UTC)
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- Seismologists don't say orogeny a lot because they are concerned with measuring and predicting the individual movements (quakes) which combne to create the orogeny itself. Orogeny isn't neccessarily subduction; the Himalayas aren't subducting, they are colliding. But it's still an orogeny, a mountain building event.
- As for the whole megathrust earthquake term...it is yet another concocted word related to seismology, such as silent earthquake. But whatever...wikipedia is full of concocted stuff.Rolinator 15:01, 2 April 2006 (UTC)
- Seismologists don't say orogeny a lot because they are concerned with measuring and predicting the individual movements (quakes) which combne to create the orogeny itself. Orogeny isn't neccessarily subduction; the Himalayas aren't subducting, they are colliding. But it's still an orogeny, a mountain building event.
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