WEBVTT - Does Earth's Crust Have Its Own Tides? 0:00:02.040 --> 0:00:07.080 Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff, 0:00:07.120 --> 0:00:09.920 Lauren Vogel bomb here. If you earn your living on 0:00:09.960 --> 0:00:12.959 the ocean, you'd better know how to read a tide table. 0:00:13.520 --> 0:00:16.759 Around the world, most coastal communities witness sea levels rise 0:00:16.840 --> 0:00:20.159 and fall multiple times every day. The effect can be 0:00:20.239 --> 0:00:23.400 quite dramatic. On certain days. There's a fifty three foot 0:00:23.520 --> 0:00:26.600 that's sixteen meter difference between the low and high tides 0:00:26.640 --> 0:00:30.720 in Canada's Immunas Basin Inlet. For example, working fishermen, divers, 0:00:30.760 --> 0:00:34.080 and ship captains must take fluctuations like these into account. 0:00:34.560 --> 0:00:37.360 For this reason, government's released tables that predict the heights 0:00:37.400 --> 0:00:40.800 of future tides four different corners of the oceans. Yet, 0:00:41.120 --> 0:00:44.800 unbeknownst to us, the ground beneath our feet experiences tides 0:00:44.800 --> 0:00:48.199 of its own. The phenomenon goes by many names, including 0:00:48.360 --> 0:00:53.000 land tides, crustal tides, earth tides, and more specifically, solid 0:00:53.120 --> 0:00:56.560 earth tides. No matter what you call the process, it's 0:00:56.600 --> 0:00:59.560 caused by the same forces that generate our better known 0:00:59.560 --> 0:01:03.560 ocean nick tides. Tides are complicated beasts. They are the 0:01:03.600 --> 0:01:06.920 result of several different factors all working together. The most 0:01:06.959 --> 0:01:11.039 significant contributing forces are the gravitational polls that the Sun, Moon, 0:01:11.160 --> 0:01:14.480 and the Earth exert on one another. The Sun actually 0:01:14.560 --> 0:01:17.080 has less influence over our tides than the Moon does, 0:01:17.120 --> 0:01:20.480 despite being twenty two million times larger. That's because the 0:01:20.480 --> 0:01:23.199 Moon is so much closer to Planet Earth. As such, 0:01:23.319 --> 0:01:26.000 on the surface of Earth, the Moon's gravitational force is 0:01:26.040 --> 0:01:30.120 about two point two times stronger than the Sun's. High 0:01:30.120 --> 0:01:32.399 ocean tides, at least in most parts of the world, 0:01:32.600 --> 0:01:35.560 happened twice a day. We experience one when the Moon 0:01:35.680 --> 0:01:39.440 is overhead, and counterintuitive as it may sound, a second 0:01:39.480 --> 0:01:41.560 hide tide takes place when the Moon is on the 0:01:41.600 --> 0:01:44.959 opposite side of Earth. Low ocean tides occurred during the 0:01:45.040 --> 0:01:48.680 periods between these two points. The centrifugal force of our 0:01:48.800 --> 0:01:53.080 rotating planet helps account for this strange bulging effect. While 0:01:53.160 --> 0:01:56.120 this is happening, a similar cycle unfolds within the very 0:01:56.200 --> 0:01:59.559 crust of our planet to a minuscule degree. The ground 0:01:59.640 --> 0:02:02.920 level self rises and falls every day in accordance with 0:02:03.000 --> 0:02:07.000 the Moon's whereabouts. We spoke with Duncan Agnew, a geophysicist 0:02:07.040 --> 0:02:10.640 at Scripps Institution of Oceanography at the University of California, 0:02:10.680 --> 0:02:14.239 San Diego. He said The motion extends through the whole 0:02:14.320 --> 0:02:16.880 of the solid Earth, not just the crust, but is 0:02:16.960 --> 0:02:21.600 largest at the surface. The earth is slightly elastic. Your 0:02:21.680 --> 0:02:24.519 naked eyes all that's required to watch the ocean tide 0:02:24.560 --> 0:02:27.840 come in and go out. However, it's straight up impossible 0:02:27.880 --> 0:02:31.720 to observe solid earth tides without using scientific instruments. At 0:02:31.800 --> 0:02:34.840 high tide, New York City can rise upward by fourteen 0:02:34.880 --> 0:02:38.280 inches that's about thirty six cimes. The Big Apple then 0:02:38.360 --> 0:02:41.560 falls by the same degree at low tide. A pedestrian 0:02:41.639 --> 0:02:44.320 standing in Times Square or the Bronx Zoo wouldn't notice 0:02:44.360 --> 0:02:47.520 any of this, though, because all of the buildings, trees, streets, 0:02:47.520 --> 0:02:51.239 and people in the Five Burrows rise and fall in concert. 0:02:52.000 --> 0:02:55.080 Though that's just one example. Agnew says that the vertical 0:02:55.120 --> 0:02:57.960 motion at the surface varies from place to place. Some 0:02:58.040 --> 0:03:00.880 areas bulge and descend less dramatically than New York does. 0:03:01.120 --> 0:03:05.040 Other locales outperformed the Big City on that score. Okay, 0:03:05.240 --> 0:03:08.040 so far we focused on how the Moon affects both 0:03:08.040 --> 0:03:11.440 solid earth and oceanic tides, but the Sun should not 0:03:11.480 --> 0:03:14.400 be ignored in this discussion. Those who live in coastal 0:03:14.440 --> 0:03:17.400 areas are well aware of how solar activity can affect 0:03:17.400 --> 0:03:20.440 the strength of oceanic tides. When the Sun aligns with 0:03:20.480 --> 0:03:23.560 the Moon, the seas high tides get higher and the 0:03:23.639 --> 0:03:27.080 low tides get lower. The exact opposite happens when those 0:03:27.080 --> 0:03:30.200 two celestial bodies are situated at right angles to one another, 0:03:30.639 --> 0:03:33.320 meaning the planet ends up with low high tides and 0:03:33.520 --> 0:03:37.400 high low tides. That cycle repeats itself every two weeks 0:03:37.480 --> 0:03:41.040 and is therefore known as the Fortnightly cycle. On top 0:03:41.040 --> 0:03:44.440 of giving voters headaches, it also affects solid earth tides. 0:03:46.080 --> 0:03:49.000 Nicholas vander elst of the U S Geological Survey was 0:03:49.040 --> 0:03:52.040 the lead author of a sixteen study that investigated the 0:03:52.040 --> 0:03:56.120 link between the Fortnite cycle, land tides, and seismic activity 0:03:56.200 --> 0:04:00.760 along California's San Andreas Fault. He said via email, when 0:04:00.760 --> 0:04:03.560 the Earth's crust flexes in the direction of the tidal pull, 0:04:03.960 --> 0:04:06.680 this puts a stress on any tectonic faults that cut 0:04:06.720 --> 0:04:09.760 through the rock. If the combination of the title stress 0:04:09.800 --> 0:04:13.080 and the pre existing tectonic stress is just right, this 0:04:13.160 --> 0:04:16.960 can set off an earthquake. For that twenty sixteen research effort, 0:04:17.160 --> 0:04:20.880 vandor Elstz group compared eighty one thousand San Andreas earthquakes. 0:04:21.279 --> 0:04:24.160 They found that the rate of low frequency quakes increases 0:04:24.320 --> 0:04:28.320 right before the fortnightly cycle enters its solar lunar alignment stage. 0:04:28.720 --> 0:04:32.320 But Californians shouldn't lose too much sleep over this news development. 0:04:32.720 --> 0:04:35.400 The earthquakes in question are too weak and occur too 0:04:35.440 --> 0:04:38.440 far below the planet's surface to cause any serious damage 0:04:38.520 --> 0:04:42.480 on the surface. Crystal tides vander eilst notes are generally 0:04:42.800 --> 0:04:46.880 quote far too small to matter for most faults. Nonetheless, 0:04:46.920 --> 0:04:49.760 the geologist has found that it's possible to observe a 0:04:49.880 --> 0:04:54.200 small but measurable influence in some locations, particularly in places 0:04:54.240 --> 0:04:57.359 like mid ocean ridges. There are also special regions of 0:04:57.360 --> 0:04:59.600 the Earth's crust where fault lines appear to be a 0:04:59.680 --> 0:05:02.880 stout endingly weak. These regions tend to be deep at 0:05:02.880 --> 0:05:05.839 the roots of subduction zone faults, like the faults that 0:05:05.920 --> 0:05:10.160 dive beneath Japan and the U. S Pacific Northwest. Down there, 0:05:10.279 --> 0:05:13.080 some twelve to eighteen miles or twenty to thirty kilometers 0:05:13.120 --> 0:05:17.120 beneath the planet's surface. Faults create small scale seismic tremors. 0:05:17.640 --> 0:05:20.200 Van dr Elst said the tides can have a very 0:05:20.240 --> 0:05:23.760 substantial effect on tremors, with tremor rates oscillating by up 0:05:23.800 --> 0:05:27.400 to thirty percent in phase with the tides. However, these 0:05:27.440 --> 0:05:31.280 tiny pseudo earthquakes are totally undetectable by people and do 0:05:31.320 --> 0:05:36.120 not pose any hazard. Still, it's pretty fascinating. Oh and hey, 0:05:36.480 --> 0:05:38.960 the ocean is not the only body of water that 0:05:39.040 --> 0:05:42.440 experiences its own tides. Lakes undergo them as well, but 0:05:42.560 --> 0:05:46.440 on a much smaller scale. For example, the mightiest tides 0:05:46.440 --> 0:05:49.719 on North America's Great Lakes are only five centimeters that's 0:05:49.839 --> 0:05:58.200 zero point four inches in height. Today's episode was written 0:05:58.200 --> 0:06:01.159 by Mark Mancini and produced by tile Acclang. For more 0:06:01.200 --> 0:06:03.760 on this and lots of other earth moving topics, visit 0:06:03.800 --> 0:06:17.320 our home planet, how stuff works dot com.