8.0 Earthquake reported off the Solomon Islands

[LongBlade]

An 8.0-magnitude earthquake struck off the Solomon Islands in the Southwest Pacific early Wednesday, the U.S. Geological Survey reported, sparking a tsunami warning.

The center of the quake was located some 360 miles east-southeast of Honiara, the capital of the Solomon Islands, the USGS said. It was reported to be 3.6 miles deep.

Source: http://www.cnn.com/2013/02/05/world/asia/solomon-islands-quake/index.html

[TheCommandTent]

Hope this doesn't trigger any huge tsunamis.

[LongBlade]

It's not clear. 3 feet were reported in the Solomons, but you know how those things travel.

[Toonces]

When you consider that the big waves on the North Shore here in Hawaii have periods of about 18 seconds, and a tsunami has a period of 18 minutes, brother that is a lot of energy!

[LongBlade]

When you consider that the big waves on the North Shore here in Hawaii have periods of about 18 seconds, and a tsunami has a period of 18 minutes, brother that is a lot of energy!

Yes it is. You guys all clear there?

[Mr. Bigglesworth]

Doesnt that depend on what caused the tsunami? From somewhere I filed away in my brain that a tsunami from an asteroid would travel at about 500 and something miles per hr. The tsunami from the ancient super-volcano in Indonesia dropped the side of a land mass into the ocean. That one probably travelled fairly fast as well. The video of the Japanese coast guard vessel going over the tsunami was < 1 minute by my rough memory of watching the video. I remember the boat climbing while they made these gutteral noises.

What is special about 18 minutes? Please explain.

[Longdan]

Toonces knows waves and I do not.  Period should be wavelength I think.  Normal waves Toonces spoke of
are 18 seconds apart.  A Tsunami would be 60 times that.  That is my guess.  In the Indian Ocean Tsunami
the water drew far away from the normal shoreline seaward as the tsunami approached and then the big
wave showed up after some minutes.

[Mr. Bigglesworth]

Yeah, that indian ocean one was deep IIRC.

I'm sure he knows his waves, I want to know too.

[Toonces]

This is hard to explain in words, and I always struggled with the math anyway, but think of it like this:

Big waves travel faster than small waves, and the longer they travel the further they pull apart.

In general, the period of a wave (the time from one peak to another peak of a wave, or the time it takes for one full wavelength to pass a point) is a measure of it's energy.  A 10 foot wave at 5 seconds (locally wind driven storm waves) will be 10 feet when it hits a beach.  That same wave at 20 seconds (a typical north Pacific swell coming from the Aleutians and hitting Hawaii) might be 30 feet or more when it hits a beach.  The period translates to energy.

A 20 second period means that the peak of one wave hits, then the trough hits 10 seconds later, then 10 seconds after that another peak hits.  If you imagine sea level, then a 10' wave is 5' above sea level at one point and 5' below sea level at one point, with 20 seconds between the 5' above sea level peaks.  That's how much water you have travelling with that wave.

Ok, now imagine a wave with a peak that is 3' above sea level (a 1 meter tsunami), but with a period of 18 minutes.  That's 9 minutes of water travelling 1 meter +/- above sea level and 9 minutes below sea level...this is why you hear about the ocean sucking away for minutes exposing the reef, etc.  The wave is troughing before all of that water comes rushing in.  That is just a TON of water (=energy).  This is why you see these tsunami waves run up a beach for miles, etc.  The volume is just unbelievable.  Also, these waves are travelling very, very fast, on the order of hundreds of miles per hour.  Again, longer period=faster wave.

The magnitude of the earthquake is one thing that helps you determine how likely a tsunami is.  Other factors are the type of earth movement, a slip is the worst because it instantly displaces a ton of water, and the amount of actual earth/water that was displaced makes a big difference.  The whole science of this is evolving.

I hope that explains this some.

[Mr. Bigglesworth]

It does. I did do 1st yr calculus based physics so I have a rudimentary understanding of waves.

It seems to me the energy is proportional to the displaced mass of water x the speed. It also seems to me the wavelength is based on the depth of the area. The wave gets shorter and taller when it runs up a beach. Even entering a shallow bay it piles up. The front slows from friction on the bottom, the back piles over the top making the classic surf wave.

Everything still seems to be dependent on what caused the wave. Amplitude, wavelength, velocity. Some underground event displaces the water. A big slip (many sq miles) in deep water will be slowed by the weight of the water it has to shift (accelerate). Therefore deep origin tsunamis might be slower than an earthquake with a shallow origin. A shallow earthquake will be faster and hit harder.

Yes? No? Should I stick to my day job?

[Toonces]

I'm not an expert on tsunamis but what you're saying sounds about right.  An "earthquake" can be close to the surface or miles beneath the surface.  How much it "slips" and displaces water is what is key.  This is why you might see a 7.0 earthquake 30km deep that prompts no warnings, but one that is 3km deep or something like that is a very big deal.

But basically a large, vertical displacement of the crust is going to create a bigger wave; the larger the area and the deeper the fall, the larger the magnitude and also the potential for a tsunami.

The wavelength is related to the distance from the origin of the wave; a wave generated farther away has a longer period because the waves spread over time, but I can only explain this with math and I'd need to look it up anyway.  Tsunamis are unique because it's an instantaneous wave rather than a storm generated wave train that separates over time.

WRT to last sentence of your post, that's not how I understand it.  Water is essentially incompressible so the depth as it relates to water volume is not relevant.  The amount of water displaced is what counts.  In that sense, it really matters less how deep the epicenter of the quake is as much as how much water the earth's movement displaces manifested on the surface of the crust.  A horizontal movement may not generate any tsunami, but a vertical one will. 

[Mr. Bigglesworth]

Incompressible is not the issue. I am saying that a given force of slip that has to lift a much larger mass of water/earth will be slower. More mass, less acceleration.