The U.S. Geological Survey (USGS) initially estimated Wednesday's earthquake off the west coast of Northern Sumatra, at 8.9 magnitude. But it was subsequently lowered to 8.7 and then to 8.6. Similarly, the focus of the quake was first thought to be 33 km from the surface, but was later changed to 22.9 km.
The signals from a high-magnitude quake flood the nearby earthquake recording station, leading to an initial estimation of high magnitude.
Click here, for a PDF version of the graphic.
Explaining why tremors were felt in several Indian cities, R.K. Chadha, Chief Scientist at the National Geophysical Research Institute (NGRI), Hyderabad, said: “Tremors are felt at faraway locations due to surface waves produced by an earthquake. Surface waves cause a lateral movement of the particles in the earth's medium. The earth behaves like an elastic medium when seismic waves are travelling.”
Unlike the December 26, 2004 quake caused by a thrust fault, Wednesday's quake was caused by a strike-slip fault. The fault had moved in a north northwest-south southeast direction. In the case of a strike-slip fault, the fractured crust slides past each other laterally.
“The movement along the fault should be in the order of a few metres,” said Dr. Chadha. “Only a detailed modelling using data from 40-50 stations can reveal the actual amount of displacement.”
The reason why the 8.6-magnitude quake did not cause killer tsunami waves was the nature of the faulting. “To generate giant tsunami waves, there should be great vertical displacement of the water column,” Dr. Chadha said. “This happens only in the case of a thrust fault [the December 2004 quake] or a dip-slip fault. Strike-slip fault will not generate tsunami waves.”
According to him, though the quake was a strike-slip fault, there should have been a small amount of oblique movement along the fault. This is the reason why Wednesday's quake caused small tsunami waves.
Even the 7.2-magnitude quake of January 10, 2012, at a depth of 20.5 km from the surface off the west coast of northern Sumatra, was due to strike-slip faulting.
Good Article:-)
It appears that the tree in the picture represents habitated land,as such the 'normal fault' causes it to be on higher ground,thereby tsunamis created,if any,donot cause any devastation,assuming height of tsunami waves created is less than the height of the ground from sea level.
Whereas in case of 'thrust fault' the sea bed is dislocated to higher ground vis-a-vis the habitated land,as such even small waves created due to this dislocation can cause huge devastation on land.
I think many have a confusion in understanding the picture. We must imagine that, of the two portions of the earth in the picture, one is fixed and the other only moves. In which case, a normal fault will cause sinking of the earth and sea level should sink with it. Only when a thrust happens, the earth pushes up and tsunami occurs. I hope I am correct.
Good topic, I did have this question in my mind after the Tsunami watch
was cancelled. The images are little off the mark. I was able to get
more details in eHow and wikipedia sites.
Middle Picture is Thrust Fault(i.e. Reverse Dip slip)
Right side Picture is Normal Fault(i.e. Normal Dip slip)
In Reverse dip slip portion of earth above fault plane moves upwards and that below of it moves downwards.Both can cause a considerable amount of vertical displacement of water column and hence a tsunami
Explanation is simply super.Graphics also easily understand the diff. b/w the thrust & strike- slip fault.
Interesting and informative article published at the right time to educate people on the
difference between types of earthquakes caused due to various faults caused due to earths
tectonic activity. However, the graphics could have been made more informative by adding
few more important details associated with faults - footwall, hanging wall. This would made
graphics 2 & 3 to differentiate properly?
How could a dip slip cause a tsunami? I assume that a dip slip involves a plate slipping down. This would mean that the sea level should go down and not cause a wave. Could some one shed some light on this?
The only thing required to generate tsunami is vertical displacement.
Normal fault can also cause Tsunami but the amplitude will be very low.
water column may rise a few mm to cm. Because the vertical displacement
due to normal fault(deep seated) are generally very small. the mechanism
is very well understood but it is not as simple as these visuals are.
finding out Source parameters of an earthquake requires very complex
mathematical computation.
A simple explanation is that although the magnitude of 2004 and the
present one are similar, most of the energy this time is transmitted
along the rock resulting in tremors being felt far far away, but no
tsunami as energy dissipated in water is less.
Very lucidly written by this author as to why there was no tsunami even after 8.6 magnitude quake in the sea.
The difference between thrust fault and normal fault is that the slip occurs along the natural slope, which is equivalent to a boulder or a rock rolling down a hill slope. However, in the Thrust fault the direction of movement is against the slope, which is to say the boulder or the rock moves up the slope against gravity. That is the reason it is also called a reverse fault. Since a tremendous amount of energy is required to thrust the plate up the slope, Thrust faults do the most damage!
A normal fault is combination of strike-slip and thrust fault. When the vertical displacement is much higher compared to lateral movement then its called a thrust and same for strike-slip if its lateral displacement. If difference between vertical and lateral movement isn't measurable then it a normal or oblique fault.
in thrust fault, one plate move from other in two directions, one is vertical and other is horizontal (collectively we call that as oblique). but this displacement will not give such a major shift of water because that the height of the displacement is less compare to the vertical shift only as in normal fault.
this is i understood from the graphics.. i regret, if i am wrong.
It is easy to understand the phenomenon as the black band, with white dotted line, on top of 3 graphics show the change of level of the 2 blocks, arrows showing the shift direction & the consequential effect is explained.
Good Article about different types of Tsunami with graphical representation. It would be great if more detailed description is given with graphical representation.
I dont find any difference between the middle picture and the right hand picture.
Middle and Right image shows like Dip-slip default.
In my pointof view, If you compare tree which is shown in the images, if the movement towards up like middle image (Thurst fault) then the tsunami will create.
else like Right image (Normal fault) then there is no posibility for Tsunami. This is my view.
@Sonu : I think Normal faults actually make the sea level recede from its shore. If we take the picture 3 above, that means the shore is on the side of the plate which moved up and consequently the sea plate moved down. However, this is not the case in picture 2 where the sea bearing plate has risen sharply unleashing a devastating Tsunami.
The difference is in the Normal fault the plates move apart with a
slight dip. However in the thrust slip, the plate dips to a great
level without moving apart from each other. This is what I can infer
from the images !
So much of information with Graphics, truly reflects the human potential
and so much understanding on Tsunami. At this it is my opinion that
this could be man made. Sounds crazy, but possibilities are high.
It is again on the basis of detailed explanation.
This time the shock waves are until Chennai, Tamilnadu., Guess what!!
Also we have to see the source of the explanation coming out, which
should trigger the process of thoughts of our so called high end people.
Very simple explanation. Graphic is good but dip slip is not shown in the graphic. besides what is the difference between thrust slip and normal fault besides reverse direction of the plate movement?
What about normal faulting, can it produce Tsunami. As normal faulting also cause vertical movement.
This is a reasonable explanation. Informative article. Graphics is great, but the middle portion of the image causes a slight confusion at first sight, reading few more times, I was able to get the gist. But still I am not sure if everyone can understand from the image at first sight.
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