1 00:00:02,040 --> 00:00:07,080 Speaker 1: Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff, 2 00:00:07,120 --> 00:00:09,920 Speaker 1: Lauren Vogel bomb here. If you earn your living on 3 00:00:09,960 --> 00:00:12,959 Speaker 1: the ocean, you'd better know how to read a tide table. 4 00:00:13,520 --> 00:00:16,759 Speaker 1: Around the world, most coastal communities witness sea levels rise 5 00:00:16,840 --> 00:00:20,159 Speaker 1: and fall multiple times every day. The effect can be 6 00:00:20,239 --> 00:00:23,400 Speaker 1: quite dramatic. On certain days. There's a fifty three foot 7 00:00:23,520 --> 00:00:26,600 Speaker 1: that's sixteen meter difference between the low and high tides 8 00:00:26,640 --> 00:00:30,720 Speaker 1: in Canada's Immunas Basin Inlet. For example, working fishermen, divers, 9 00:00:30,760 --> 00:00:34,080 Speaker 1: and ship captains must take fluctuations like these into account. 10 00:00:34,560 --> 00:00:37,360 Speaker 1: For this reason, government's released tables that predict the heights 11 00:00:37,400 --> 00:00:40,800 Speaker 1: of future tides four different corners of the oceans. Yet, 12 00:00:41,120 --> 00:00:44,800 Speaker 1: unbeknownst to us, the ground beneath our feet experiences tides 13 00:00:44,800 --> 00:00:48,199 Speaker 1: of its own. The phenomenon goes by many names, including 14 00:00:48,360 --> 00:00:53,000 Speaker 1: land tides, crustal tides, earth tides, and more specifically, solid 15 00:00:53,120 --> 00:00:56,560 Speaker 1: earth tides. No matter what you call the process, it's 16 00:00:56,600 --> 00:00:59,560 Speaker 1: caused by the same forces that generate our better known 17 00:00:59,560 --> 00:01:03,560 Speaker 1: ocean nick tides. Tides are complicated beasts. They are the 18 00:01:03,600 --> 00:01:06,920 Speaker 1: result of several different factors all working together. The most 19 00:01:06,959 --> 00:01:11,039 Speaker 1: significant contributing forces are the gravitational polls that the Sun, Moon, 20 00:01:11,160 --> 00:01:14,480 Speaker 1: and the Earth exert on one another. The Sun actually 21 00:01:14,560 --> 00:01:17,080 Speaker 1: has less influence over our tides than the Moon does, 22 00:01:17,120 --> 00:01:20,480 Speaker 1: despite being twenty two million times larger. That's because the 23 00:01:20,480 --> 00:01:23,199 Speaker 1: Moon is so much closer to Planet Earth. As such, 24 00:01:23,319 --> 00:01:26,000 Speaker 1: on the surface of Earth, the Moon's gravitational force is 25 00:01:26,040 --> 00:01:30,120 Speaker 1: about two point two times stronger than the Sun's. High 26 00:01:30,120 --> 00:01:32,399 Speaker 1: ocean tides, at least in most parts of the world, 27 00:01:32,600 --> 00:01:35,560 Speaker 1: happened twice a day. We experience one when the Moon 28 00:01:35,680 --> 00:01:39,440 Speaker 1: is overhead, and counterintuitive as it may sound, a second 29 00:01:39,480 --> 00:01:41,560 Speaker 1: hide tide takes place when the Moon is on the 30 00:01:41,600 --> 00:01:44,959 Speaker 1: opposite side of Earth. Low ocean tides occurred during the 31 00:01:45,040 --> 00:01:48,680 Speaker 1: periods between these two points. The centrifugal force of our 32 00:01:48,800 --> 00:01:53,080 Speaker 1: rotating planet helps account for this strange bulging effect. While 33 00:01:53,160 --> 00:01:56,120 Speaker 1: this is happening, a similar cycle unfolds within the very 34 00:01:56,200 --> 00:01:59,559 Speaker 1: crust of our planet to a minuscule degree. The ground 35 00:01:59,640 --> 00:02:02,920 Speaker 1: level self rises and falls every day in accordance with 36 00:02:03,000 --> 00:02:07,000 Speaker 1: the Moon's whereabouts. We spoke with Duncan Agnew, a geophysicist 37 00:02:07,040 --> 00:02:10,640 Speaker 1: at Scripps Institution of Oceanography at the University of California, 38 00:02:10,680 --> 00:02:14,239 Speaker 1: San Diego. He said The motion extends through the whole 39 00:02:14,320 --> 00:02:16,880 Speaker 1: of the solid Earth, not just the crust, but is 40 00:02:16,960 --> 00:02:21,600 Speaker 1: largest at the surface. The earth is slightly elastic. Your 41 00:02:21,680 --> 00:02:24,519 Speaker 1: naked eyes all that's required to watch the ocean tide 42 00:02:24,560 --> 00:02:27,840 Speaker 1: come in and go out. However, it's straight up impossible 43 00:02:27,880 --> 00:02:31,720 Speaker 1: to observe solid earth tides without using scientific instruments. At 44 00:02:31,800 --> 00:02:34,840 Speaker 1: high tide, New York City can rise upward by fourteen 45 00:02:34,880 --> 00:02:38,280 Speaker 1: inches that's about thirty six cimes. The Big Apple then 46 00:02:38,360 --> 00:02:41,560 Speaker 1: falls by the same degree at low tide. A pedestrian 47 00:02:41,639 --> 00:02:44,320 Speaker 1: standing in Times Square or the Bronx Zoo wouldn't notice 48 00:02:44,360 --> 00:02:47,520 Speaker 1: any of this, though, because all of the buildings, trees, streets, 49 00:02:47,520 --> 00:02:51,239 Speaker 1: and people in the Five Burrows rise and fall in concert. 50 00:02:52,000 --> 00:02:55,080 Speaker 1: Though that's just one example. Agnew says that the vertical 51 00:02:55,120 --> 00:02:57,960 Speaker 1: motion at the surface varies from place to place. Some 52 00:02:58,040 --> 00:03:00,880 Speaker 1: areas bulge and descend less dramatically than New York does. 53 00:03:01,120 --> 00:03:05,040 Speaker 1: Other locales outperformed the Big City on that score. Okay, 54 00:03:05,240 --> 00:03:08,040 Speaker 1: so far we focused on how the Moon affects both 55 00:03:08,040 --> 00:03:11,440 Speaker 1: solid earth and oceanic tides, but the Sun should not 56 00:03:11,480 --> 00:03:14,400 Speaker 1: be ignored in this discussion. Those who live in coastal 57 00:03:14,440 --> 00:03:17,400 Speaker 1: areas are well aware of how solar activity can affect 58 00:03:17,400 --> 00:03:20,440 Speaker 1: the strength of oceanic tides. When the Sun aligns with 59 00:03:20,480 --> 00:03:23,560 Speaker 1: the Moon, the seas high tides get higher and the 60 00:03:23,639 --> 00:03:27,080 Speaker 1: low tides get lower. The exact opposite happens when those 61 00:03:27,080 --> 00:03:30,200 Speaker 1: two celestial bodies are situated at right angles to one another, 62 00:03:30,639 --> 00:03:33,320 Speaker 1: meaning the planet ends up with low high tides and 63 00:03:33,520 --> 00:03:37,400 Speaker 1: high low tides. That cycle repeats itself every two weeks 64 00:03:37,480 --> 00:03:41,040 Speaker 1: and is therefore known as the Fortnightly cycle. On top 65 00:03:41,040 --> 00:03:44,440 Speaker 1: of giving voters headaches, it also affects solid earth tides. 66 00:03:46,080 --> 00:03:49,000 Speaker 1: Nicholas vander elst of the U S Geological Survey was 67 00:03:49,040 --> 00:03:52,040 Speaker 1: the lead author of a sixteen study that investigated the 68 00:03:52,040 --> 00:03:56,120 Speaker 1: link between the Fortnite cycle, land tides, and seismic activity 69 00:03:56,200 --> 00:04:00,760 Speaker 1: along California's San Andreas Fault. He said via email, when 70 00:04:00,760 --> 00:04:03,560 Speaker 1: the Earth's crust flexes in the direction of the tidal pull, 71 00:04:03,960 --> 00:04:06,680 Speaker 1: this puts a stress on any tectonic faults that cut 72 00:04:06,720 --> 00:04:09,760 Speaker 1: through the rock. If the combination of the title stress 73 00:04:09,800 --> 00:04:13,080 Speaker 1: and the pre existing tectonic stress is just right, this 74 00:04:13,160 --> 00:04:16,960 Speaker 1: can set off an earthquake. For that twenty sixteen research effort, 75 00:04:17,160 --> 00:04:20,880 Speaker 1: vandor Elstz group compared eighty one thousand San Andreas earthquakes. 76 00:04:21,279 --> 00:04:24,160 Speaker 1: They found that the rate of low frequency quakes increases 77 00:04:24,320 --> 00:04:28,320 Speaker 1: right before the fortnightly cycle enters its solar lunar alignment stage. 78 00:04:28,720 --> 00:04:32,320 Speaker 1: But Californians shouldn't lose too much sleep over this news development. 79 00:04:32,720 --> 00:04:35,400 Speaker 1: The earthquakes in question are too weak and occur too 80 00:04:35,440 --> 00:04:38,440 Speaker 1: far below the planet's surface to cause any serious damage 81 00:04:38,520 --> 00:04:42,480 Speaker 1: on the surface. Crystal tides vander eilst notes are generally 82 00:04:42,800 --> 00:04:46,880 Speaker 1: quote far too small to matter for most faults. Nonetheless, 83 00:04:46,920 --> 00:04:49,760 Speaker 1: the geologist has found that it's possible to observe a 84 00:04:49,880 --> 00:04:54,200 Speaker 1: small but measurable influence in some locations, particularly in places 85 00:04:54,240 --> 00:04:57,359 Speaker 1: like mid ocean ridges. There are also special regions of 86 00:04:57,360 --> 00:04:59,600 Speaker 1: the Earth's crust where fault lines appear to be a 87 00:04:59,680 --> 00:05:02,880 Speaker 1: stout endingly weak. These regions tend to be deep at 88 00:05:02,880 --> 00:05:05,839 Speaker 1: the roots of subduction zone faults, like the faults that 89 00:05:05,920 --> 00:05:10,160 Speaker 1: dive beneath Japan and the U. S Pacific Northwest. Down there, 90 00:05:10,279 --> 00:05:13,080 Speaker 1: some twelve to eighteen miles or twenty to thirty kilometers 91 00:05:13,120 --> 00:05:17,120 Speaker 1: beneath the planet's surface. Faults create small scale seismic tremors. 92 00:05:17,640 --> 00:05:20,200 Speaker 1: Van dr Elst said the tides can have a very 93 00:05:20,240 --> 00:05:23,760 Speaker 1: substantial effect on tremors, with tremor rates oscillating by up 94 00:05:23,800 --> 00:05:27,400 Speaker 1: to thirty percent in phase with the tides. However, these 95 00:05:27,440 --> 00:05:31,280 Speaker 1: tiny pseudo earthquakes are totally undetectable by people and do 96 00:05:31,320 --> 00:05:36,120 Speaker 1: not pose any hazard. Still, it's pretty fascinating. Oh and hey, 97 00:05:36,480 --> 00:05:38,960 Speaker 1: the ocean is not the only body of water that 98 00:05:39,040 --> 00:05:42,440 Speaker 1: experiences its own tides. Lakes undergo them as well, but 99 00:05:42,560 --> 00:05:46,440 Speaker 1: on a much smaller scale. For example, the mightiest tides 100 00:05:46,440 --> 00:05:49,719 Speaker 1: on North America's Great Lakes are only five centimeters that's 101 00:05:49,839 --> 00:05:58,200 Speaker 1: zero point four inches in height. Today's episode was written 102 00:05:58,200 --> 00:06:01,159 Speaker 1: by Mark Mancini and produced by tile Acclang. For more 103 00:06:01,200 --> 00:06:03,760 Speaker 1: on this and lots of other earth moving topics, visit 104 00:06:03,800 --> 00:06:17,320 Speaker 1: our home planet, how stuff works dot com.