The Richter Scale and the pressure of ocean water.

Let’s see if I can articulate this question clearly. My understanding is that the earthquake that triggered last weeks’ awful tsunami was a 9.0 on the open-ended Richter Scale. ( if that is wrong, let it slide- I’m using the number for example only in this question )

Does this mean that it is a 9.0 of force of one plate moving against the other, or is the 9.0 referring to the raw energy released? That’s not even the real question for me. Here’s my question.

If you say we had a 9.0 in northern California ( god forbid ), it would be occurring at sea level, with nothing above the two plates as they shift but air.

However, the earthquake that triggered the tsunami happened at the floor of the ocean- where the downward pressure is enormous. How many atmospheres down? Is a 9.0 earthquake the same force when it happens with all of that water compressing downwards on the two plates? I am imagining that a 9.0 under the ocean floor is incredibly more powerful than a 9.0 on the surface of the earth in open air. If I take two 8x10 inch sheets of foam core, overlap them a la tectonic plates and shift one suddenly against the other there is precious little reaction.

If I put an 8 ounce weight, it takes more force by my hand to move the upper “plate” against the lower "plate. And of course, if I place a 5 lb. barbell on top of the upper “plate” it may well be impossible to move the upper against the lower, or vice versa. The fact that a huge violent earthquake occurred beneath that weight and pressure says to me ( the layman ) that it took a lot more force to move the plates than it would have at sea level.

How then can we say an earthquake that is so violent but happens under the pressure of an entire ocean’s worth of water ( s.i.c. ) is of equal magnitude to one that happens on the earth’s surface in the air? Is not a 9.0 underwater thousands of times stronger than a 9.0 on the surface?

Cartooniverse

There’s a lot of details to be left out, but most of the magnitude ratings scales for earthquakes are based on the amount of energy released (and there’s many ways of trying to figure that number out). On any given scale a 9.0 is a 9.0 - same amount of energy. It doesn’t matter whether that energy is released underwater or in the middle of the Sahara.

However the “intensity” of an earthquake is a different rating, intensity refers to how violent the shaking is at any given point. I remember one scale that rated local intensity with categories such as “People mildly alarmed”, “People panic”, “windows broken” and so on. Intensity will vary widely depending on where you are, how powerful the quake was, the soil type, the structure that you are in, how much breakable stuff is around you, etc.

Keep in mind that the actual faults are tens of kilometers beneath the surface (whether it’s your backyard or the seafloor). The pressure of that much rock is pretty huge compared to what a few miles of water adds. I’ll grant that it would take more energy to raise a square mile of surface if you start underwater than if you start above sea level (all other things being equal) but it’s not going to be “thousands of times” greater.

Hope that helps!

It does. I suppose as a landlubber I was giving more weight ( heh ) to the ocean water than it deserved, in comparison to the forces at work below the earth’s surface. I did not know that the shifting happened many kilometers below the surface- one is led to believe that the tectonic plates shifting in northern california are of course very deep- but they shift right up to the surface, and don’t just shift kilometers below the surface.

Well again there’s some haziness and I’m sure that a geologist will be along to explain this stuff much better than I can (civil engineer by training, not a geologist).

It may not always be easy to point to the exact location of a fault; faultlines tend to be big (it’s not a hairline crack but a zone which can be hundreds of yards or maybe many miles across) and it’d be easy to say “there’s a crack in the ground, that must be the fault itself”.

Suppose you take a table and cover it with dirt. That represents a tectonic plate with the soil and whatnot on top of it. Now give it a solid smack from below. You’ll undoubtedly see cracks appearing in the dirt but they aren’t necessarily right where the energy was released.

On the other hand, if you are standing on bare rock then you might indeed be standing right on top of the actual slipping point between two plates.

Faults can be deep or they can be right on the surface. On the south edge of Palmdale, CA, state route 14 crosses the San Andreas fault. There is a ridge about 30’ high that goes along the fault line and a cut was made through the ridge for the highway. When the cut was first made the fault line as a thin, very distinct more-or-less vertical line with the rock strata on either side running diagonally downward to the fault forming a series of shallow V’s. since that time there has been quite a bit of creep in that area and the sides of the cut have broken away at the fault line which is no longer distinct.

Here, for what it is worth isa description of how the Richter magnitude is determined. Basically, the amplitude of the trace on a seismograph is measured. The time between the arrival of the S and P waves (two sorts of earth movements in earthquakes) is measured. Using the nomograph, the amplitude of the seismograph is marked on the right hand scale and the S-P difference on the left hand. The two points are then connected by a straight line and the Richter magnitude is read off on the center scale. The S-P difference also allows the distance of the seismograph from the epicenter to be read off.

The nomograph solves the equation that Richter, and others since, developed tying magnitude on the seismograph to the amount of energy in the quake.

And, by the way, no all earthquakes are on plate boundries were one plate overlies the other. Many faults, such as the San Andreas at Palmdale described above, are what might be called a butt joint.

And besides if the fault of the overlying type it might require more strain in the fault to make it slip but when it slips the seismographs measure the actual effect of the movement. The fact that this one was a 9 could lend some credence to speculation that this was one of the overlying fault regions and was held together by pressure so that a lot of strain built up before it slipped. However, wouldn’t all faults that are deep underground have a lot of pressure on them?

From reading the previously cited references, the Richter scale gives an estimate of the energy released as sound waves, not the total energy released by the earthquake, which could be very different.

See The Physics of Earthquakes.

I don’t have anything close to the expertise needed to defend the Richter scale measurement of total energy released. However, the seismograph records actual earth movement. What part of the cited references leads you to the conclusion that only sound waves are measured?

The reference gives the example of the 1994 deep Bolivian earthquake where only 3% of the total energy was released in the form of seismic waves. Energy is also released in the form of shear zone heating, fracture of material, and gravitational potential energy.

The Richter scale is based on an estimate of the peak magnitude of the seismic waves. It is useful as a tool for comparing similar types of earthquakes but its relationship to the total energy and seismic wave energy is indirect.

I’m not a geologist, I’m just interested in the subject of measurement.

I guess your statement “the Richter scale gives an estimate of the energy released as sound waves” was what threw me. Seismic waves are not sound waves but rather are the actual motion of the ground.

In any case, the Richter scale give a measure of ground motion which is the thing that causes damage to us and our works. Heat released at the fracture and energy that goes into smashing things up deep underground isn’t the thing we worry about. The are of theoretical interest to geologists of course, and the fact that it is known that the Bolivian quake you mentioned only put out 3% as seismic waves shows that they must have a way to find that out.

The most fascinating part of the Richter Magnitude link provided above is :

A 12.0 Richter Scale earthquake is that powerful and yet… it’s the amount of solar energy striking the earth daily?

Cool. :cool:

A plate can also drop on the sea floor, can’t it? It’s my ubderstanding that movement, whether up or down, will generate a tsunami. I seem to remember the one that did all that damage in Alaska years ago was caused by a plate dropping.
Peace,
mangeorge