Does Earth's Crust Have Its Own Tides?

You've probably seen ocean tides making the seas rise and fall, but solid land has tides, too. Learn more in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff,
Lauren Vogel bomb here. If you earn your living on
the ocean, you'd better know how to read a tide table.
Around the world, most coastal communities witness sea levels rise
and fall multiple times every day. The effect can be
quite dramatic. On certain days. There's a fifty three foot

(00:23):
that's sixteen meter difference between the low and high tides
in Canada's Immunas Basin Inlet. For example, working fishermen, divers,
and ship captains must take fluctuations like these into account.
For this reason, government's released tables that predict the heights
of future tides four different corners of the oceans. Yet,
unbeknownst to us, the ground beneath our feet experiences tides

(00:44):
of its own. The phenomenon goes by many names, including
land tides, crustal tides, earth tides, and more specifically, solid
earth tides. No matter what you call the process, it's
caused by the same forces that generate our better known
ocean nick tides. Tides are complicated beasts. They are the
result of several different factors all working together. The most

(01:06):
significant contributing forces are the gravitational polls that the Sun, Moon,
and the Earth exert on one another. The Sun actually
has less influence over our tides than the Moon does,
despite being twenty two million times larger. That's because the
Moon is so much closer to Planet Earth. As such,
on the surface of Earth, the Moon's gravitational force is
about two point two times stronger than the Sun's. High

(01:30):
ocean tides, at least in most parts of the world,
happened twice a day. We experience one when the Moon
is overhead, and counterintuitive as it may sound, a second
hide tide takes place when the Moon is on the
opposite side of Earth. Low ocean tides occurred during the
periods between these two points. The centrifugal force of our
rotating planet helps account for this strange bulging effect. While

(01:53):
this is happening, a similar cycle unfolds within the very
crust of our planet to a minuscule degree. The ground
level self rises and falls every day in accordance with
the Moon's whereabouts. We spoke with Duncan Agnew, a geophysicist
at Scripps Institution of Oceanography at the University of California,
San Diego. He said The motion extends through the whole

(02:14):
of the solid Earth, not just the crust, but is
largest at the surface. The earth is slightly elastic. Your
naked eyes all that's required to watch the ocean tide
come in and go out. However, it's straight up impossible
to observe solid earth tides without using scientific instruments. At
high tide, New York City can rise upward by fourteen

(02:34):
inches that's about thirty six cimes. The Big Apple then
falls by the same degree at low tide. A pedestrian
standing in Times Square or the Bronx Zoo wouldn't notice
any of this, though, because all of the buildings, trees, streets,
and people in the Five Burrows rise and fall in concert.
Though that's just one example. Agnew says that the vertical

(02:55):
motion at the surface varies from place to place. Some
areas bulge and descend less dramatically than New York does.
Other locales outperformed the Big City on that score. Okay,
so far we focused on how the Moon affects both
solid earth and oceanic tides, but the Sun should not
be ignored in this discussion. Those who live in coastal
areas are well aware of how solar activity can affect

(03:17):
the strength of oceanic tides. When the Sun aligns with
the Moon, the seas high tides get higher and the
low tides get lower. The exact opposite happens when those
two celestial bodies are situated at right angles to one another,
meaning the planet ends up with low high tides and
high low tides. That cycle repeats itself every two weeks

(03:37):
and is therefore known as the Fortnightly cycle. On top
of giving voters headaches, it also affects solid earth tides.
Nicholas vander elst of the U S Geological Survey was
the lead author of a sixteen study that investigated the
link between the Fortnite cycle, land tides, and seismic activity
along California's San Andreas Fault. He said via email, when

(04:00):
the Earth's crust flexes in the direction of the tidal pull,
this puts a stress on any tectonic faults that cut
through the rock. If the combination of the title stress
and the pre existing tectonic stress is just right, this
can set off an earthquake. For that twenty sixteen research effort,
vandor Elstz group compared eighty one thousand San Andreas earthquakes.

(04:21):
They found that the rate of low frequency quakes increases
right before the fortnightly cycle enters its solar lunar alignment stage.
But Californians shouldn't lose too much sleep over this news development.
The earthquakes in question are too weak and occur too
far below the planet's surface to cause any serious damage
on the surface. Crystal tides vander eilst notes are generally

(04:42):
quote far too small to matter for most faults. Nonetheless,
the geologist has found that it's possible to observe a
small but measurable influence in some locations, particularly in places
like mid ocean ridges. There are also special regions of
the Earth's crust where fault lines appear to be a
stout endingly weak. These regions tend to be deep at

(05:02):
the roots of subduction zone faults, like the faults that
dive beneath Japan and the U. S Pacific Northwest. Down there,
some twelve to eighteen miles or twenty to thirty kilometers
beneath the planet's surface. Faults create small scale seismic tremors.
Van dr Elst said the tides can have a very
substantial effect on tremors, with tremor rates oscillating by up

(05:23):
to thirty percent in phase with the tides. However, these
tiny pseudo earthquakes are totally undetectable by people and do
not pose any hazard. Still, it's pretty fascinating. Oh and hey,
the ocean is not the only body of water that
experiences its own tides. Lakes undergo them as well, but
on a much smaller scale. For example, the mightiest tides

(05:46):
on North America's Great Lakes are only five centimeters that's
zero point four inches in height. Today's episode was written
by Mark Mancini and produced by tile Acclang. For more
on this and lots of other earth moving topics, visit
our home planet, how stuff works dot com.