There is the posting "Insights Into Earthquakes And Tectonic Plates". In that, I explained how the driving force behind earthquakes is the spreading of the seafloor, due to the emergence of magma from below, along a W-shaped line which extends around the world.
The well-known Mid-Atlantic Ridge is a part of this line. It extends from there around the southern part of Africa, across the Indian and Pacific Oceans, to the west coast of North America. There, it forms Mexico's Sea of Cortez (Gulf of California), the valley system of California, and finally the gap between Vancouver Island and mainland British Columbia. The branch of the W extends in the Indian Ocean to form the Red Sea, which is similar in geological nature to the Sea of Cortez.
My reason for the continual emergence of magma from below is explained in "The Story Of Planet Earth", also on this blog. The earth was once at a geological equilibrium. But then an asteroid crashed into our planet. The material from the asteroid "splattered" across the earth, forming the continents. This impact, and the tectonic movement of the continents since, upset the earlier geological equilibrium. The emergence of magma from below, which results in earthquakes, is caused by the seeking of a new equilibrium.
Now, let's go over a brief review of tides.
The moon, and to a lesser extent the sun, creates tides in the earth's oceans by their gravitational pull. Tides are caused not actually by gravity, but by a difference in gravity. The oceans have a certain depth, so that the surface of the ocean is closer to the moon, and is thus more affected by it's gravity, than the water at the bottom of the ocean. This causes a tidal bulge in the ocean under the moon as the earth rotates, producing a high tide and a low tide each day on shore.
The sun has less than half the tidal effect of the moon simply because the sun is about four hundred times as far away as the moon, meaning that the difference in it's gravitational effect between the surface and the bottom of the ocean is much less. This is because of the proportion of the depth of the ocean relative to the distance to the sun, compared with the distance to the moon.
In another posting on this blog, "Earth's Liquid Interior", I showed how we can easily tell that the interior of the earth must be liquid without ever seeing a volcano. If the earth was entirely solid, it's rotation would act as a centrifuge and pull water toward the equator. The earth actually does bulge at the equator because of this, the equatorial circumference is more than the polar circumference. But the average depth of the oceans around the equator is really no greater than it is at higher latitudes. This can only be the case if the earth's interior is liquid, like the oceans.
So, here is the question: If the gravity of the moon, and to a lesser extent the sun, has such an effect on the earth's oceans, why wouldn't it also have an effect on the very deep layer of molten rock below?
We do not experience daily tides in this because it is contained within the solid crust. But it must contribute to moving magma within the earth to where it emerges to the surface, or builds up pressures that eventually culminate in earthquakes along the tectonic plate boundaries. The earth's subterranean layer of molten rock extends so far down that there must be a great difference between the gravity of the moon at it's upper limit, and that at it's lower limit. This means that the tidal effect on it must be many, many times greater than it is on the earth's oceans.
My feeling is that this has got to be a factor in earthquakes.
For more on how the moon's gravity affects the earth, by strengthening it's magnetic field, see "The Moon And Earth's Magnetic Field" on the physics and astronomy blog, www.markmeekphysics.blogspot.com. For the effect of the earth's gravity on the moon, see "The Lunar Balance Hypothesis And Orbital Tilt" also on the physics and astronomy blog.
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