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From: tm on 5 May 2010 14:45

"Bret Cahill" <BretCahill(a)peoplepc.com> wrote in message
news:7e6480f0-f44c-4759-9d5e-bc59d957f58e(a)g5g2000pre.googlegroups.com...


>> The best way is a laser interferometer for small displacements and GPS or
>> gross measurements.
>
>Adding a +/- nanometer error to a +/- meter error just gives you a +/-
>meter error.
>
>GPS wouldn't be useful for the actual data.

Only in your (extremely small) mind.


From: Bret Cahill on 8 May 2010 14:51
> Take a wooden pencil and very slowly bend it. You will begin to hear
> small cracking noises

I can predict that pretty good with strain sensors.

What we need to do is predict this somehow:

http://maps.google.com/maps?f=q&hl=en&q=32.6766,-115.8096(M5.0+-+Southern+California+-+2010+May+08+18:33:10+UTC)&t=h&z=7&iwloc=A

From: John Larkin on 8 May 2010 16:43
On Sat, 8 May 2010 11:51:22 -0700 (PDT), Bret Cahill
<Bret_E_Cahill(a)yahoo.com> wrote:

>> Take a wooden pencil and very slowly bend it. You will begin to hear
>> small cracking noises
>
>I can predict that pretty good with strain sensors.

No, you can't. You won't get a 10 millisecond warning as to when the
pencil will break. Now imagine only being able to place a few surface
sensors on one face of literally millions of cubic miles of complex,
fractured, stressed, moving subsurface stuff. It's like predicting the
weather, but much worse: hopeless.

Move to Mississippi.

John


From: Bret Cahill on 9 May 2010 13:48
> >> Take a wooden pencil and very slowly bend it. You will begin to hear
> >> small cracking noises
>
> >I can predict that pretty good with strain sensors.
>
> No, you can't. You won't get a 10 millisecond warning as to when the
> pencil will break.

Depends on how fast you load the pencil. If you load it over several
months or years I'll get at least some warning that it is being
loaded.

The linearity / non linearity allows for even better predictions.

> Now imagine only being able to place a few surface
> sensors on one face

It's not really on one face. It's several miles above from what may
be a thousands of faces.

No one said it was a easy problem, just that there might be some
additional information to exploit.

Distance metrology needs to get the error down to a few microns over
100 km. Apparently laser interferometry is just for short distances,
even turbulence will mess up the measurement.

The strain sensor might not work for the simple reason the strains
would be so low.

What is the best resolution possible with a bridge?

Another solution might be the World's Largest Seismometer, a proof
mass of hundreds of tons.

> of literally millions of cubic miles of complex,
> fractured, stressed, moving subsurface stuff.

There might be some hope in averages here.

> It's like predicting the
> weather, but much worse: hopeless.

Geologists seem to be able to make really low confidence predictions.
For example the risk of a 7.5+ earthquake over the next month or so is
about 5X what it was on average over the past century.

The question is will everything quiet down for awhile after the next
6.0 or will the next 3.0 set thinks up for an 8.5?

> Move to Mississippi.

High humidity = high entropy

On the other hand low entropy can get you killed.


Bret Cahill



From: John Larkin on 9 May 2010 19:06
On Sun, 9 May 2010 10:48:44 -0700 (PDT), Bret Cahill
<BretCahill(a)peoplepc.com> wrote:

>> >> Take a wooden pencil and very slowly bend it. You will begin to hear
>> >> small cracking noises
>>
>> >I can predict that pretty good with strain sensors.
>>
>> No, you can't. You won't get a 10 millisecond warning as to when the
>> pencil will break.
>
>Depends on how fast you load the pencil. If you load it over several
>months or years I'll get at least some warning that it is being
>loaded.
>
>The linearity / non linearity allows for even better predictions.
>
>> Now imagine only being able to place a few surface
>> sensors on one face
>
>It's not really on one face. It's several miles above from what may
>be a thousands of faces.
>
>No one said it was a easy problem, just that there might be some
>additional information to exploit.
>
>Distance metrology needs to get the error down to a few microns over
>100 km. Apparently laser interferometry is just for short distances,
>even turbulence will mess up the measurement.
>
>The strain sensor might not work for the simple reason the strains
>would be so low.
>
>What is the best resolution possible with a bridge?
>
>Another solution might be the World's Largest Seismometer, a proof
>mass of hundreds of tons.
>
>> of literally millions of cubic miles of complex,
>> fractured, stressed, moving subsurface stuff.
>
>There might be some hope in averages here.

Absolutely. You can probably estimate the average time between
earthquakes. Numbers like 300 +-250 years.

John


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