Just to add a little interest to Typhoon Morakot. There were a few typhoon induced earthquakes detected during Morakot of high magnitude.
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[quote=“Fox”]Just to add a little interest to Typhoon Morakot. There were a few typhoon induced earthquakes detected during Morakot of high magnitude[/quote]Are you serious? How can a typhoon cause an earthquake?
…butterfly beating its wings and all that…
Yep, scientifically proven. Cool, isn’t it?
It’s from changes in atmospheric pressure - you change the pressure on one side of the fault (on the land surface), but not on the other (in the ocean), and it’ll be more likely to move.
And they’re weird earthquakes - very slow.
I think it’s pretty cool.
Atmospheric pressure is one cause.
The massive weight of billions of tonnes of water and debris shifting is another, especially tidal surge but also in this case rivers.
Besides molten rock currents in the mantle moving the earth’s plates the most common cause of earthquakes is the sun and the moon and their influence on the tidal pulse. As you can imagine the orogeny of Taiwan is quite young in geological time so there is a lot of instability in the crust. I guess a lot of that water flowed down the rift valley which is the suture zone between Eurasia and the Philippines. It’s earthquake prone. It has hundreds of quakes a day.
The typhoon induced earthquakes are what they term unfelt quakes and are like teleseismic quakes in their wave period but they can be strong.
[quote=“Fox”]Atmospheric pressure is one cause.
The massive weight of billions of tonnes of water and debris shifting is another, especially tidal surge but also in this case rivers.
Besides molten rock currents in the mantle moving the earth’s plates the most common cause of earthquakes is the sun and the moon and their influence on the tidal pulse. As you can imagine the orogeny of Taiwan is quite young in geological time so there is a lot of instability in the crust. I guess a lot of that water flowed down the rift valley which is the suture zone between Eurasia and the Philippines. It’s earthquake prone. It has hundreds of quakes a day.
The typhoon induced earthquakes are what they term unfelt quakes and are like teleseismic quakes in their wave period but they can be strong.[/quote]
Well, things like changes in pressure, fluids, etc can trigger earthquakes, but they don’t cause earthquakes. Earthquakes are caused by stress built up by the moving plates. The stress that builds up is resisted by friction along the faults, and when these things get very closely balanced it can take just a very small change in the amount of stress to overcome friction and allow an earthquake, which is where these triggers can come in. Taiwan gets tons of earthquakes because it’s right on top of two plates slamming into each other. All these other factors might change exactly when an earthquake happens, but they are not the reason that Taiwan gets earthquakes.
[quote]Well, things like changes in pressure, fluids, etc can trigger earthquakes, but they don’t cause earthquakes. Earthquakes are caused by stress built up by the moving plates. The stress that builds up is resisted by friction along the faults, and when these things get very closely balanced it can take just a very small change in the amount of stress to overcome friction and allow an earthquake, which is where these triggers can come in. Taiwan gets tons of earthquakes because it’s right on top of two plates slamming into each other. All these other factors might change exactly when an earthquake happens, but they are not the reason that Taiwan gets earthquakes.
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I know forumosa is full of silly semantic bullshit but that takes the cake.
Noone is trying to steal your thunder.
But just to clarify there would be no stress build up if there were no motion and whilst friction is part of the cause of earthquakes it is the combination of gravity and friction that causes earthquakes. It is not one way or the other.
Well sorry, but this is what I do for a living, so I when I see a misconception it’s hard for me to pass it by. But if you guys aren’t interested in knowing how things really work then I’ll just shut up.
By this, then, you are saying that without any motion there WOULD be stress buildup, and that gravity in fact has NOTHING to do with what causes earthquakes? That it IS one way and not the other?
now you’re going to put me in teacher mode…
Depends on which kind of motion you’re talking about. Motion of the plates causes stress (oriented more or less horizontally) to build up. Motion along a fault releases stress, so if the fault doesn’t move then stress builds up. Gravity also provides stress (oriented vertically). What matters is the difference between the magnitudes of different orientations of stress (ie. horizontal - vertical, or NS-EW, etc)). When this difference is large enough to overcome friction along a fault you get an earthquake. So yes, a small change in gravity can push you over the edge, trigger an earthquake, and release some of the built up stress. BUT, if you did not trigger that particular earthquake, the plates will still continue to move, the fault will continue to build up stress, and you’d get an earthquake eventually anyway.
Gravity and friction contribute to earthquakes in the sense that they make it hard for the fault to move, allowing more stress to build up and therefore bigger earthquakes. But the thing that’s causing the fault to want to move in the first place is the plate motions.
So fox was quite right, then. Thanks.
To be honest, it’s not clear what Fox means here. If he’s saying that there’s no stress build up without plate motions and that without gravity the earth would pretty much shut down and you’d have no plate tectonics, then yes, he’s right. If not, then he’s a little off. Not necessarily completely off, but to me it’s worth clarifying.
You can have an earthquake with no change in gravity triggering it. You cannot have an earthquake with no friction along the fault. You will not have an earthquake without plate motions building up stress (unless you want to get into explosions and mining and stuff, which is whole other issue). It’s a little silly to talk about earthquakes in the complete absence of gravity, but you could go up into space, stick a rock in a vise, squeeze it, and it will break, which is basically all an earthquake is.
The role of gravity also depends on the type of fault. Since it’s Taiwan, we of course think of things colliding, but where things are pulling apart gravity plays an opposite role, and where things are sliding past each other it plays pretty much no role at all.
I’m not trying to start a fight here. I find this stuff interesting and I like to help people learn about it.
Precisely. That’s what I thought when I read his post – apart from the “earth shutting down,” which you added yourself. It seemed pretty clear to me. Therefore, I asked you to clarify, and you confirmed that yes, he was right. No fight.
ok, fair e[quote=“sandman”][quote]To be honest, it’s not clear what Fox means here. If he’s saying that there’s no stress build up without plate motions and that without gravity the earth would pretty much shut down and you’d have no plate tectonics, then yes, he’s right.[/quote]
Precisely. That’s what I thought when I read his post – apart from the “earth shutting down,” which you added yourself. It seemed pretty clear to me. Therefore, I asked you to clarify, and you confirmed that yes, he was right. No fight.[/quote]
Ah, sorry then, my first interpretation was that he’d meant something different. So we’re all right 
Wrong. All motion is the result of gravity (acceleration). Earthquakes are caused by gravity and friction. It is basic physics. I don’t know what you do for a living, but I’m the editor of Terrestrial, Atmospheric and Oceanic Sciences (SCI) journal, the Geoscience Union of Taiwan, National Taiwan University’s Dept Earth Science, National Taiwan University’s Dept. of Oceanography, Academia Sinica’s Dept Earth Science, and Academia Sinica’s Dept. Physics, National Taiwan Museum’s Journal of Natural Science. In short, there would hardly be a paper published in earth science out of Taiwan in the past 10 or 11 years that I haven’t read.
So I know something about it, but I’m not an expert. I’d be the first to admit that. I’m self educated out of necessity.
I know when you say gravity might not be the direct cause of an earthquake you are referring to variations in earth’s gravitational field and how these might impact earthquake behavior. However, setting that aside, it is still gravity that causes the motion of the earth’s plates via molten rock in the mantle. It wouldn’t be here and it wouldn’t be melting and there wouldn’t be currents in the mantle or even a crust to move without gravity and there wouldn’t be friction either.
Interestingly enough though in areas where the gravity field is high there are fewer large earthquakes than in areas where it is lower. One possible reason for that is that in higher gravity regions the crust is possibly more settled having undergone more frequent smaller earthquakes. In areas where the field is lower, earthquakes might occur less frequently allowing for stress to build in the crust more gradually thereby allowing for the triggering of a large earthquake by some other event. That event will be gravity related. There is no getting around that, but it might have some other overriding specific reason such as a change in atmospheric pressure. All changes in atmospheric pressure are gravity related.
Of course if you get down to it, everything that happens on or in the Earth is the result of gravity because otherwise Earth wouldn’t exist, but since we started by talking about atmospheric pressure, landslides, and a lot of water, I hope that you can see why I interpreted what you said on the more local scale, not on the driving plate tectonics kind of scale. But you can still have an earthquake that is not triggered by changes in the local gravitational field.
As for what I do, lets just say that I have a copy of TAO on my desk right now.
Hah, I was wondering how long it would take for this to get split off…
Not me, Maoman must have been bored… but i was thinking of it.
Personally, i think that earthquakes are all a myth, and it’s all just a grand synchronized bout of hallucinatory leaping about.
Heh, my first earthquake I really thought I was hallucinating since I hadn’t slept in about 36 hours. It wasn’t until I got up, opened my door, and discovered that the rattling wasn’t someone knocking, that I realized that my dizzy feeling was not all in my head.