The people who do this sort of thing have upgraded the 2004 Indian Ocean Earthquake to the longest ever recorded, and one of the most powerful. It may have been a 9.3 on the Moment magnitude scale.
It seems to have released the equivalent energy of a 100 gigaton atomic weapon. (on the other hand, it released this energy over ten minutes and over a large (800 mile) geographic area, not like a bomb.)
OK, so my question is this, if I were scuba diving over this earthquake at a distance of (say) 1M, would I survive? Does all this energy go into the water as a shock wave? Or what?
What if the quake happened on dry land and I was 1M over it in a balloon?
When earthquakes occur, the potential energy they release gets dispersed chiefly by three means: seismic radiation (that is, the generation of P-waves, S-waves, and L-waves, the kinds of ground motion you might feel), mechanical actions like the formation of cracks, and frictional heating (from the two sides of the fault zone rubbing against each other). The Sumatra-Andaman quake did release a huge amount of energy, but as you noted, the rupture length was 800 km (the longest single rupture we’ve ever witnessed). Once you consider the integrated area involved in the fault displacement, however, which would be 800 km in length by some (as yet unquantified) depth into the Earth along the subduction zone, you see that the energy, however awesome, was really spread out quite a lot. The “number of hydrogen bombs” kind of analogy is terribly misleading in this case.
In your post, what does 1M mean - 1 mile, or 1 meter?
If you found yourself 1 mile above the point on the fault with the greatest amount of displacement, it wouldn’t matter whether you were in water or in air - you wouldn’t be hurt. For all practical purposes, water is incompressible and thus can transmit seismic energy very efficiently (heck, that’s where tsunamis come from), but in deep water directly above any given point on the rupture, the seismic energy is distributed through the entire water column (several thousands of feet in thickness), so at most you could expect to bob a little bit like a cork. By contrast, air is highly compressible, so although the rapid displacement of air by ground motion would certainly produce shock waves (i.e., sound), you wouldn’t be in any physical danger.
If you meant 1M = 1 meter, though, you would be in a bit of a pickle in either water or air, because you would have been slammed by the ground displacement (estimated to have reached a maximum near 20 meters (!) for this quake).
Sorry, rupture length was 1600 kilometers, or 1000 miles… even more impressive.
Another interesting factoid, from the papers just published in the journal Science: The amount of slip (displacement) along the fault, and the rate at which the rupture traveled along the fault zone, were not uniform, because the fault strength varied with distance (strong fault=larger displacement and faster rupture speed). Estimates of the maximum rupture speed are around 2.0 to 2.5 km per second.
So (assuming the being underwater thing doesn’t bother me) how could such a quake hurt me? If I was standing on top of it, the shock would just knock me down, right? I presume I am missing something.
Well, if the ground rises 20 meters and you’re 1 meter above it, it’s gonna smack you. Especially if it is moving at 2 km/sec during that time. (1/100th of a second, but still… smack!).
Okay, standing on top of the fault surface is a bit different from being suspended above it (by diving in water, or hanging out in a hot-air balloon), as you originally suggested. I haven’t been able to find an estimate of the vertical acceleration for the ground movement in this case, but that would certainly determine what happens next. (The 2 to 2.5 km/sec figure given earlier is actually the speed at which the fault is rupturing along its length, not a measure of the speed of upward motion on the fault.) If the vertical acceleration is less than 9.8 m/s^2 (1 g), you’d likely just be knocked down. More than that, you’d be thrown into the air, a bit like going for a ride in a catapult.
Also consider that in earthquakes like this one, ground on one side of the fault (the overthrust side, or hanging wall) moves upward with respect to the ground on the other side of the fault (the underthrust side, or footwall). If you happened to lose your balance and leaned over the edge of the overthrust block, or got tossed in the appropriate direction, you would take a tumble down the newly formed fault scarp. The USGS estimates the total vertical seafloor displacement to be somewhat less than the 15-20 meter total displacement on the fault, but if you had that on land, you could still expect some bumps and bruises at a minimum.
If instead you were suspended above the ground surface, as you suggested in the OP, you would be dealing with your impact with a moving ground surface. I imagine that would be at least as damaging as hitting a brick wall at 35 km/hr, and I imagine it would hurt. All this assumes too that you’re standing out in the open, and not near any structures or large objects that could also be in motion and land on you at any time.
Magnitude (Mw): Loma Prieta, 6.9; Sumatra-Andaman, 9.1
Vertical displacement: Loma Prieta, 1.6 m average; Sumatra-Andaman, 15 m average
Length of rupture: Loma Prieta, 40 km; Sumatra-Andaman, 1600 km
Duration of quake: Loma Prieta, 15 seconds; Sumatra-Andaman, 600 seconds
Now, there were reports that objects as large as cars were tossed about by the Loma Prieta quake, suggesting that at least locally the vertical acceleration was in excess of 1 g. Had the Sumatra-Andaman rupture been on land rather than under the sea, I suspect those reports would have been commonplace. The closest comparable earthquake may be the 1964 Alaska quake (Mw = 9.2*), so reading about that event might give you a better sense of what you could have experienced.
Although the magnitude given for the 1964 Alaska quake is larger than that calculated for the Sumatra-Andaman quake, the statistics available on ground displacement, etc. don’t show effects as profound as last year’s quake. This discrepancy probably relates the differences in instrumentation and data coverage between the two quakes.
Many earthquakes of any size can be detected in multiple places on the Earth, and the moderate to large ones certainly do get registered on seismographs worldwide.
In this case, though, the Washington Post is reporting the fact that every place on Earth actually experienced ground motion, although the magnitude of that ground motion (less than a centimeter) wasn’t perceptible to people in most places like North America and Europe. That sort of impact is still, however, unprecented in recorded history.
I find it remarkable that all that energy could be released and yet it would be (mostly) harmless to me if I had been there. It is hard to wrap my head around that.
Ok I have some practical first hand experience on this subject. I think the original question was about divers getting affected by the tsunami. The short answer is very little.
Tsunamis are very difficult to predict because in deepwater they may only generate a small wave, it is only when they reach the shore that the mass of water builds up to cause a tsunami.
A diver off-shore would feel little more than a bob in the water, same if you have a boat and a tsunami is coming take it out into deep water.
All this assumes too that you’re standing out in the open, and not near any structures or large objects that could also be in motion and land on you at any time.