What exactly does that mean? Well, having a dead horse, in more modern navy slang, is having a debt that comes out of your pay to cover short term loan that the paymaster gave to you. It’s money you don’t see until it’s paid off. There’s nothing wrong with it, you should always pay your debts, but that is just a term that is applied to it. It comes from older nautical days when sailing was prevalent. If your ship was becalmed for extensive periods, odds are, you would run out of feed any livestock that you were transporting. The last thing you want laying around while you are stuck waiting for the wind… are dead horses. I’ve been around the stench of decaying animal copses (MV Livestock Express in the Red Sea) and it is not a pleasant aroma. (the Livestock Express had about 300 sheep stacked in one corner and couldn’t dump them due to environmental regulations)
Yeah, it’s an obtuse approach to the meat of this, but it is related… sort of.
The Horse latitudes are between about 30 and 35 degrees latitude. It’s pretty close to the boundary that I used in this chart.
In case you missed it, this is one of the follow up charts that I used down in the discussion in the Moonie post. And, since I’m beating a dead horse, I felt the intro was appropriate.
That chart is a plot of the major plate boundaries between 30°N and °N and 30°S. It’s also the region traversed by the apparent sub Lunar and sub Solar points. That means that they both pass directly over head in this region at some point during the year. (plus about 5° of slop just to make it even)
Edward Lane at April 22, 2012 at 14:54 brought up a good point that spurred me back into thinking about the forces at work on the different ends of the plate boundaries. As many of you remember, I mentioned that if there were an effect, that there should be a physical explanation for it… at least an idea of a mechanism. Something that could be examined to see what the merits of the parent idea were. (Solar/Lunar influence).
Simplifying that plot into something that we can sort of measure, you get this:
This is the angular measure of the extents of the plates in longitude as measured from the center of the Earth. The vertical boundaries are misleading, they actually get closer to each other the further from the equator they go. But for getting a general idea of their size, this works.
Now, since the Moon is pretty close, and it’s generally the most touted astronomical body that influences seismic activity, let’s look at what it’s effects might be.
Gravitational acceleration on the surface of the Earth is 9.8 m/s per second. This can also be expressed as N/kg of force. (9.8 N/kg). What is the comparable acceleration effect of the Moon on an object on the Earth? Roughly 0.00003319 m/s² towards the Moon.
How about the variation from one end of a plate to another, allowing for the extra distance from the Moon? Well, for a 30° plate, from directly under the Moon to the end furthest away… 0.00000015 m/s² less.
Here it is in graphic form:
Referring back to the rectangular boxes on the plates… the largest extent was about 114°. That plate has an acceleration difference from the Moon of about 0.00000150 m/s², or ten times greater than the 30° plate.
Either way, that variation in force from the moon is still about 0.0000153% that of the gravitational force from the Earth.
This also explains why the researchers who have found a Lunar effect on some already seismically active areas have such a hard time extracting that signal, it is excruciatingly small.