WEBVTT 1 00:00:00.080 --> 00:00:05.000 I want you to picture a devastating earthquake like you 2 00:00:05.000 --> 00:00:08.800 have undoubtedly seen the footage on the news at some point, right, Oh, yeah, absolutely. 3 00:00:08.919 --> 00:00:12.000 The air is thick with dust, infrastructure is just crushed, 4 00:00:12.279 --> 00:00:15.000 and it basically looks like a war zone. But if 5 00:00:15.039 --> 00:00:18.519 you look closely at the aftermath of these events, you 6 00:00:18.679 --> 00:00:23.600 almost always notice this one detail that is frankly, deeply unsettling. 7 00:00:23.839 --> 00:00:26.199 Yeah, it's the contrast exactly. 8 00:00:26.559 --> 00:00:30.079 You will see one building completely collapsed, just pancaked into rubble, 9 00:00:30.239 --> 00:00:34.000 and right next door, sometimes literally fifty feet away, another 10 00:00:34.039 --> 00:00:36.719 building stands there perfectly intact, like it might not even 11 00:00:36.719 --> 00:00:37.960 have a shattered window. 12 00:00:38.159 --> 00:00:42.159 Right, And it creates this immediate I guess, cognitive dissonance. 13 00:00:42.960 --> 00:00:46.119 You see two identical environments, but you get totally different outcomes. 14 00:00:46.200 --> 00:00:48.799 Yeah, And the immediate assumption is usually about the architecture. 15 00:00:48.840 --> 00:00:49.240 Right. 16 00:00:49.280 --> 00:00:51.960 We assume the collapse building was I don't know, poorly 17 00:00:52.000 --> 00:00:54.640 constructed by someone cutting corners, and the surviving one was 18 00:00:54.679 --> 00:00:55.039 built like. 19 00:00:55.039 --> 00:00:57.560 A fortress, which is what anyone would think, right. 20 00:00:58.119 --> 00:01:00.719 But today's deep dive is going to completely flip that 21 00:01:00.759 --> 00:01:04.400 assumption because it is not always about how the building 22 00:01:04.480 --> 00:01:07.760 was built. It is often entirely about what the building 23 00:01:07.840 --> 00:01:08.400 was built. 24 00:01:08.200 --> 00:01:11.760 On, which requires a really fundamental shift in how you 25 00:01:11.920 --> 00:01:15.000 view structural survival. I mean, as a society, we get 26 00:01:15.000 --> 00:01:18.200 so fixated on the steel and the concrete above the surface, 27 00:01:18.359 --> 00:01:21.959 shiny stuff, exactly the visible stuff, and we completely ignore 28 00:01:22.079 --> 00:01:24.840 the invisible brutal physics happening right below it. 29 00:01:24.920 --> 00:01:28.000 And uncovering those physics is our mission today. We are 30 00:01:28.040 --> 00:01:31.640 exploring the science of structural survival and we're being guided 31 00:01:31.680 --> 00:01:35.799 by Steven L. Kramer's foundational work Geotechnical Earthquake Engineering. 32 00:01:35.879 --> 00:01:37.239 Yeah, that's an incredible resource. 33 00:01:37.319 --> 00:01:39.879 It essentially is the bible for engineers trying to figure 34 00:01:39.879 --> 00:01:42.239 out exactly what happens in the dirt beneath your feet 35 00:01:42.280 --> 00:01:45.359 when the big one hits. And to set up the 36 00:01:45.400 --> 00:01:48.359 mechanical problem we're dealing with here, let me let me 37 00:01:48.400 --> 00:01:49.319 offer an analogy. 38 00:01:49.400 --> 00:01:49.920 Go for it. 39 00:01:50.200 --> 00:01:52.120 So, if you put a heavy wooden block on a 40 00:01:52.159 --> 00:01:55.280 solid oak table and you hit that table with a sledgehammer, 41 00:01:55.640 --> 00:01:58.680 the wooden block shakes a bit, it vibrates. But if 42 00:01:58.719 --> 00:02:01.760 you take that exact same block of wood, place it 43 00:02:01.799 --> 00:02:04.120 on a bowl of jelly and hit the table with 44 00:02:04.159 --> 00:02:07.319 the exact same force, that wood block is going to 45 00:02:07.359 --> 00:02:09.400 bounce wildly, it'll probably just flip over. 46 00:02:09.759 --> 00:02:12.240 That is, Yeah, that captures the cord a lemma perfectly, 47 00:02:12.319 --> 00:02:14.879 because you know, to the untrained eye, the ground just 48 00:02:14.960 --> 00:02:18.039 looks like the ground, it looks solid, its pavement. But 49 00:02:18.240 --> 00:02:22.960 to a geotechnical engineer, certain soil profiles act exactly like 50 00:02:23.080 --> 00:02:24.080 that bowl of jelly. 51 00:02:24.400 --> 00:02:26.719 So the question then becomes, well, how do you engineer 52 00:02:26.800 --> 00:02:30.080 for the jelly? Like, how do you mathematically calculate a 53 00:02:30.120 --> 00:02:34.360 building's survival when the ground underneath it is fundamentally unstable. 54 00:02:34.400 --> 00:02:36.479 Well, you start by capturing the reality of the wave. 55 00:02:36.479 --> 00:02:38.879 If you can't just you know, theorize how the jelly 56 00:02:38.919 --> 00:02:41.120 will behave in a vacuum, you have to measure it 57 00:02:41.400 --> 00:02:45.479 under immense strut actual data exactly. The entire field of 58 00:02:45.520 --> 00:02:49.919 geotechnical engineering relies heavily on real world data, and a 59 00:02:49.960 --> 00:02:52.960 prime example that Kramer uses to really anchor the science 60 00:02:53.120 --> 00:02:57.080 is the nineteen eighty nine Loma Prieta earthquake in northern California. 61 00:02:57.159 --> 00:02:59.599 Oh wow, Yeah, that was a massive event. I famously 62 00:02:59.599 --> 00:03:00.960 interrupted the world series. 63 00:03:00.759 --> 00:03:03.479 Right it did. Yeah, it was devastating for the area, 64 00:03:03.840 --> 00:03:07.120 But from an engineering standpoint, it provided this incredible set 65 00:03:07.159 --> 00:03:10.960 of data. Specifically, scientists looked at two different ground motions 66 00:03:11.000 --> 00:03:14.639 recorded during that exact same earthquake. Okay, one reading was 67 00:03:14.639 --> 00:03:17.960 taken from a seismograph sitting on a solid rock outcrop, 68 00:03:18.240 --> 00:03:20.319 and the other reading was taken from the surface of 69 00:03:20.360 --> 00:03:23.080 a deep soil deposit, but relatively close by. 70 00:03:23.360 --> 00:03:27.400 Ah so literally the table and the jelly side by side. Yes. 71 00:03:28.039 --> 00:03:32.159 And this is actually where the global nature of this 72 00:03:32.280 --> 00:03:36.680 science comes into play, because when engineers analyze data like 73 00:03:36.759 --> 00:03:39.719 the Loma Prieta readings, they are pulling from a massive 74 00:03:39.759 --> 00:03:44.719 collaborative effort. Interestingly, in the engineering literature, Cramer's work included, 75 00:03:45.120 --> 00:03:48.719 you will often see both metric and imperial units used interchangeably. 76 00:03:48.919 --> 00:03:50.960 I noticed that it was it could be a bit 77 00:03:51.000 --> 00:03:53.280 disorienting at first to see meters and feet living next 78 00:03:53.319 --> 00:03:54.400 to each other in the same equation. 79 00:03:54.520 --> 00:03:56.560 I know it drives some students crazy, but it reflects 80 00:03:56.639 --> 00:03:58.759 the reality of the profession. I mean, the current state 81 00:03:58.800 --> 00:04:01.439 of knowledge for keeping buildings falling down is cobbled together 82 00:04:01.479 --> 00:04:03.080 from advances across the entire. 83 00:04:02.919 --> 00:04:05.840 Globe, right, because earthquakes happen everywhere exactly. 84 00:04:06.280 --> 00:04:09.919 You have seismologists in Japan, structural engineers in California, and 85 00:04:10.000 --> 00:04:14.159 geotechnical researchers in Europe, and they're all sharing data from 86 00:04:14.199 --> 00:04:17.920 their respective seismic events. If you try to standardize on 87 00:04:18.000 --> 00:04:21.800 just one system of measurement, you honestly alienate half the 88 00:04:21.839 --> 00:04:25.279 global defense effort. Everyone basically needs to be fluent in 89 00:04:25.360 --> 00:04:27.879 both dialects to use the data effectively. 90 00:04:28.199 --> 00:04:31.279 That makes a lot of sense actually, So going back 91 00:04:31.319 --> 00:04:33.720 to that shared data from Luma Prieta, we have the 92 00:04:33.800 --> 00:04:36.040 reading from the solid rock and the reading from the 93 00:04:36.079 --> 00:04:40.160 deep soil. What are the engineers actually looking for? I imagine 94 00:04:40.040 --> 00:04:41.920 they're just comparing how hard the shaking was. 95 00:04:42.040 --> 00:04:44.079 Well, the intensity of the shaking is part of it, 96 00:04:44.160 --> 00:04:47.040 for sure, but it is much much more nuanced than that. 97 00:04:47.600 --> 00:04:51.839 They are comparing three specific quantifiable characteristics. 98 00:04:51.879 --> 00:04:52.639 Okay, what are they? 99 00:04:52.759 --> 00:04:56.199 The amplitude of the wave, the frequency content, and the duration. 100 00:04:56.439 --> 00:04:58.160 Okay, I am going to push back a little here 101 00:04:58.199 --> 00:05:00.879 on the complexity of this, just you know, looking at 102 00:05:00.920 --> 00:05:02.720 it from the perspective of someone who just wants a 103 00:05:02.720 --> 00:05:05.800 building to stay upright. If an earthquake only lasts, say, 104 00:05:06.160 --> 00:05:09.360 fifteen seconds, why do we need to dive into complex 105 00:05:09.439 --> 00:05:12.800 variables like frequency contents? Isn't the goal just to make 106 00:05:12.839 --> 00:05:15.759 sure the concrete can survive that fifteen second window of 107 00:05:15.839 --> 00:05:18.600 extreme violence, why overcomplicate it? 108 00:05:19.000 --> 00:05:21.360 That's a great question. But if you only design a 109 00:05:21.360 --> 00:05:24.480 building to survive the time domain that fifteen second window, 110 00:05:25.160 --> 00:05:27.879 you are going to completely miss the very mechanism that 111 00:05:27.920 --> 00:05:31.720 takes the building down. Wait really, yeah, the time domain 112 00:05:31.800 --> 00:05:34.920 is only half the story. To truly understand how the 113 00:05:34.920 --> 00:05:38.319 ground is going to respond, engineers have to analyze the 114 00:05:38.439 --> 00:05:39.399 frequency domain. 115 00:05:39.439 --> 00:05:42.519 The frequency domain, let's break that down. I am familiar 116 00:05:42.519 --> 00:05:45.160 with the concept of sound frequencies, obviously, but what does 117 00:05:45.199 --> 00:05:47.720 that mean when we were talking about dirt and earthquakes? 118 00:05:47.920 --> 00:05:51.480 So it requires a mathematical process called a Fouria analysis. 119 00:05:52.120 --> 00:05:55.680 In modern engineering, students and professionals rely on pretty heavy 120 00:05:55.720 --> 00:05:58.759 computational software like matt Lab to run these. 121 00:05:58.639 --> 00:06:01.480 Hold on Fouria analysis That sounds like something pulled from 122 00:06:01.480 --> 00:06:04.680 a graduate level calculus exam. What is the software actually 123 00:06:04.680 --> 00:06:05.959 doing to the earthquake data. 124 00:06:06.079 --> 00:06:08.759 Think of a raw earthquake wave as a chaotic wall 125 00:06:08.800 --> 00:06:12.160 of noise, like it is a messy, highly complex. 126 00:06:11.680 --> 00:06:13.519 Wave, just a jumble of energy, right. 127 00:06:13.720 --> 00:06:16.519 A Fourier analysis is a mathematical tool that takes that 128 00:06:16.639 --> 00:06:19.879 chaotic wave and breaks it down into its simplest components, 129 00:06:19.879 --> 00:06:22.519 so it turns it into a series of clean, simple 130 00:06:22.600 --> 00:06:24.800 sine waves, each with its own frequency. 131 00:06:24.959 --> 00:06:27.160 Oh I see, So it's essentially acting like a sound 132 00:06:27.160 --> 00:06:31.680 engineer isolating the individual audio tracks from a really noisy 133 00:06:31.759 --> 00:06:35.560 live concert recording. Yes, it separates the baseline, the vocals, 134 00:06:35.560 --> 00:06:37.879 and the drums, so you can see exactly how loud 135 00:06:38.000 --> 00:06:39.399 each individual element is. 136 00:06:39.600 --> 00:06:42.439 That is a brilliant way to conceptualize it. It shows 137 00:06:42.439 --> 00:06:46.319 you exactly which frequencies of vibration were present in the 138 00:06:46.360 --> 00:06:49.639 earthquake and how powerful each specific frequency was. And that 139 00:06:49.800 --> 00:06:53.079 is vital because buildings, much like tuning forks, have their 140 00:06:53.120 --> 00:06:54.920 own natural frequencies. 141 00:06:54.360 --> 00:06:57.279 Meaning a tall sky scraper naturally sways at a different 142 00:06:57.319 --> 00:07:00.639 frequency than say a two story house exactly. 143 00:07:00.680 --> 00:07:02.519 And this is where the soil, the jelly becomes the 144 00:07:02.560 --> 00:07:06.399 real danger. Engineers use what are called transfer functions. These 145 00:07:06.399 --> 00:07:10.079 are mathematical models that predict how a specific layer of 146 00:07:10.199 --> 00:07:13.720 soil will alter the seismic wave as it travels from 147 00:07:13.720 --> 00:07:16.160 the deep bedrock up to the surface. 148 00:07:16.399 --> 00:07:18.839 But how they actually build that function though? They can't 149 00:07:18.879 --> 00:07:20.519 just guess what the dirt. 150 00:07:20.279 --> 00:07:22.360 Is going to do, No, they can't. They have to 151 00:07:22.439 --> 00:07:26.319 test the physical reality of the site. Geotechnical engineers will 152 00:07:26.439 --> 00:07:30.120 drill deep boreholes into the earth, pull up core samples 153 00:07:30.120 --> 00:07:32.600 of the soil, and test them in laboratories to see 154 00:07:32.639 --> 00:07:36.079 what it's made of, right, to determine their stiffness, their density, 155 00:07:36.240 --> 00:07:40.040 and their sheer strength. Sometimes they even send artificial shock 156 00:07:40.079 --> 00:07:43.000 waves into the ground right there on site and measure 157 00:07:43.040 --> 00:07:45.399 how long it takes for the wave to travel between 158 00:07:45.439 --> 00:07:49.199 two points. They combine all that physical data to build 159 00:07:49.199 --> 00:07:50.120 the transfer. 160 00:07:49.759 --> 00:07:52.279 Function, and what does that function ultimately tell them? 161 00:07:52.319 --> 00:07:54.240 It tells them if the soil is going to act 162 00:07:54.279 --> 00:07:56.120 like a rogue audio equalizer. 163 00:07:56.319 --> 00:07:57.399 Oh wow, Yeah. 164 00:07:57.519 --> 00:08:00.120 Certain deep soil deposits will take the raw energy of 165 00:08:00.120 --> 00:08:03.959 an earthquake and actually amplify specific frequencies while damping others. 166 00:08:04.040 --> 00:08:07.279 So they turn up the base exactly. If the soil 167 00:08:07.319 --> 00:08:11.000 deposit amplifies the exact base frequency that the building sitting 168 00:08:11.000 --> 00:08:13.800 on top of it is vulnerable to, you get resonance, 169 00:08:14.360 --> 00:08:17.720 the swaying of the building amplifies violently, and you get 170 00:08:17.759 --> 00:08:19.560 catastrophic structural failure. 171 00:08:19.759 --> 00:08:22.040 That is genuinely terrifying. 172 00:08:22.160 --> 00:08:25.160 It is, but that is why understanding the frequency domain 173 00:08:25.360 --> 00:08:27.439 is literally the difference between life and death. 174 00:08:27.639 --> 00:08:31.240 Structurally, it makes perfect sense. So okay, we've established that 175 00:08:31.399 --> 00:08:34.919 engineers measure the surface shaking, break it down with Fouria analysis, 176 00:08:35.200 --> 00:08:38.320 and use transfer functions to predict the soil's behavior. But 177 00:08:38.440 --> 00:08:41.600 to truly understand why that wave behaves the way it does, 178 00:08:42.000 --> 00:08:44.039 we have to follow the wave back to its origin. 179 00:08:44.120 --> 00:08:45.360 We have to look downward. 180 00:08:45.559 --> 00:08:48.519 We have to move from engineering into pure planetary physics. 181 00:08:48.759 --> 00:08:51.559 To understand the wave, you have to understand the medium 182 00:08:51.600 --> 00:08:52.440 it is traveling through. 183 00:08:52.480 --> 00:08:54.639 You basically have to X ray the Earth, and the 184 00:08:54.720 --> 00:08:57.519 numbers involved in doing that are just staggering. Just looking 185 00:08:57.559 --> 00:08:59.720 at the physical makeup of our planet. In the text, 186 00:09:00.200 --> 00:09:02.799 the Earth weighs about five point four times ten to 187 00:09:02.799 --> 00:09:06.919 the twenty first power tons or four point nine times 188 00:09:06.919 --> 00:09:08.360 ten to the twenty fourth kilograms. 189 00:09:08.440 --> 00:09:11.320 It is a volume and mass that is frankly difficult 190 00:09:11.320 --> 00:09:15.080 for the human brain to truly process. But that staggering 191 00:09:15.120 --> 00:09:19.159 weight gives us a crucial engineering metric. Based on its 192 00:09:19.200 --> 00:09:22.399 size and mass, the Earth has an average specific gravity 193 00:09:22.480 --> 00:09:24.120 of about five point. 194 00:09:23.879 --> 00:09:26.679 Five Let's define specific gravity for a second, just to 195 00:09:26.679 --> 00:09:27.879 make sure we all on the same page. 196 00:09:27.960 --> 00:09:31.080 Sure, specific gravity is simply a way to measure density 197 00:09:31.120 --> 00:09:34.600 by comparing a material to water. Water has a specific 198 00:09:34.600 --> 00:09:37.120 gravity of one. Okay, so if something has a specific 199 00:09:37.120 --> 00:09:39.639 gravity of five point five, it is five and a 200 00:09:39.679 --> 00:09:43.360 half times denser or heavier than an equal volume of water. 201 00:09:43.480 --> 00:09:45.720 Okay, So the planet as a whole averages a five 202 00:09:45.759 --> 00:09:49.000 point five. But the s official rocks, the granite and 203 00:09:49.080 --> 00:09:51.840 limestone we see on the crust, the stuffs we actually 204 00:09:51.919 --> 00:09:55.200 drill into delay our foundations, those only have a specific 205 00:09:55.200 --> 00:09:57.200 gravity of about two point seventy three. 206 00:09:57.399 --> 00:10:00.519 Which leaves us with a massive mathematical descripant. And see right. 207 00:10:00.720 --> 00:10:03.360 If the crust is a three, but the planetary average 208 00:10:03.399 --> 00:10:06.000 is a five point five, what does that dictate about 209 00:10:06.000 --> 00:10:07.639 the unseen interior of the Earth. 210 00:10:07.720 --> 00:10:11.320 It dictates that there must be something incredibly dense, heavy, 211 00:10:11.720 --> 00:10:14.679 and massive hidden deep down near the core to pull 212 00:10:14.720 --> 00:10:17.919 that overall average up so high. We are essentially floating 213 00:10:17.960 --> 00:10:23.000 on a thin, relatively lightweight, fragile shell that surrounds this massive, 214 00:10:23.200 --> 00:10:24.120 ultra dense center. 215 00:10:24.360 --> 00:10:27.440 Improving the existence of that dense center is basically where 216 00:10:27.519 --> 00:10:31.799 seismology was born. Because you know, you obviously cannot dig 217 00:10:31.840 --> 00:10:33.639 a hole to the core to see what is down there. 218 00:10:33.759 --> 00:10:37.480 Definitely not the deepest hole ever drilled barely scratch the crust. 219 00:10:38.080 --> 00:10:42.080 So early scientists use the devastating power of earthquakes as 220 00:10:42.120 --> 00:10:46.120 a tool. A large earthquake produces an unfathomable amount of 221 00:10:46.240 --> 00:10:49.720 energy enough to send seismic waves entirely through the globe 222 00:10:49.759 --> 00:10:51.320 from one side to the other, so. 223 00:10:51.279 --> 00:10:53.759 It's acting like a planetary scale ultrasound. 224 00:10:54.080 --> 00:10:57.240 That is precisely how it works. As these seismic waves 225 00:10:57.240 --> 00:11:01.120 travel through the deep interior, they hit boundaries between different materials, 226 00:11:01.360 --> 00:11:04.480 for example, the boundary between the solid rocky mantle and 227 00:11:04.519 --> 00:11:05.559 the ultra dense. 228 00:11:05.279 --> 00:11:07.399 Outer core, and what happens when they hit. 229 00:11:07.799 --> 00:11:10.360 When the waves hit those boundaries, the change in density 230 00:11:10.480 --> 00:11:13.720 causes them to refract, to bend, or to reflect back entirely. 231 00:11:13.840 --> 00:11:16.000 So by placing seismographs all over the world and just 232 00:11:16.039 --> 00:11:19.480 waiting for major earthquakes, these early scientists were listening to 233 00:11:19.519 --> 00:11:22.799 the echoes. By measuring the angles at which these waves 234 00:11:22.799 --> 00:11:25.639 emerged on the other side of the planet, they mapped 235 00:11:25.720 --> 00:11:29.759 the Earth's internal layered structure without ever actually laying eyes 236 00:11:29.799 --> 00:11:30.159 on it. 237 00:11:30.159 --> 00:11:32.039 It's pretty genius when you think about it. 238 00:11:32.039 --> 00:11:35.360 It really is which brings us to the waves themselves, 239 00:11:35.960 --> 00:11:39.320 because understanding the journey of those waves is the key 240 00:11:39.399 --> 00:11:42.360 to designing a structure that survives them. Now, I have 241 00:11:42.399 --> 00:11:45.399 to admit I struggle a bit with visualizing the mechanics 242 00:11:45.399 --> 00:11:45.639 of this. 243 00:11:45.919 --> 00:11:46.799 A lot of people do. 244 00:11:47.039 --> 00:11:50.000 I understand surface waves like dropping a pebble in a 245 00:11:50.039 --> 00:11:52.559 pond and watching the ripples travel along the top of 246 00:11:52.600 --> 00:11:56.200 the water, But how does a wave travel through thousands 247 00:11:56.200 --> 00:11:57.759 of miles of solid rock. 248 00:11:58.279 --> 00:12:01.399 Yeah, it is a totally different mechanism than a water ripple. 249 00:12:01.679 --> 00:12:04.279 What you are talking about are called body waves, waves 250 00:12:04.320 --> 00:12:07.000 that travel through the interior body of the Earth. And 251 00:12:07.039 --> 00:12:09.360 there are two main types of body waves that dictate 252 00:12:09.519 --> 00:12:13.240 everything in geotechnical engineering, P waves and S waves. 253 00:12:13.279 --> 00:12:15.919 Okay, let's start with PE waves. These are also called primary, 254 00:12:16.000 --> 00:12:18.279 compressional or longitudinal waves. 255 00:12:18.200 --> 00:12:21.960 Right, And the mechanical concept here is compression. PA waves 256 00:12:22.000 --> 00:12:25.320 are essentially massive sound waves traveling through solid rock. 257 00:12:25.600 --> 00:12:29.799 So I picture a P wave like taking a metal slinky, 258 00:12:30.399 --> 00:12:32.559 stretching it out on a long table, and then suddenly 259 00:12:32.559 --> 00:12:33.759 pushing one end straightforward. 260 00:12:33.879 --> 00:12:34.559 Yes, perfect. 261 00:12:34.639 --> 00:12:37.720 You watch that clustered ring of energy travel straight down 262 00:12:37.759 --> 00:12:41.720 the slinky. It compresses the rings together and they stretch apart, 263 00:12:41.720 --> 00:12:45.000 which I think is called rare faction. But the energy 264 00:12:45.000 --> 00:12:47.759 in the rings are all moving in the exact same 265 00:12:47.879 --> 00:12:48.600 forward direction. 266 00:12:49.120 --> 00:12:54.240 That slinky analogy is the perfect visualization. The crucial detail 267 00:12:54.399 --> 00:12:56.679 is that in a P wave, the motion of an 268 00:12:56.720 --> 00:13:00.120 individual particle of rock is parallel to the direction the 269 00:13:00.159 --> 00:13:03.279 wave is traveling. It just compresses and expands right along 270 00:13:03.279 --> 00:13:06.600 its path. Okay, because it relies purely on compression, P 271 00:13:06.759 --> 00:13:09.840 waves can pass through almost anything. They travel through the 272 00:13:09.879 --> 00:13:12.639 solid rock of the mantle, and they travel right through 273 00:13:12.840 --> 00:13:14.759 the liquid outer core of the Earth. 274 00:13:14.759 --> 00:13:18.120 Okay, parallel motion like a slinky. Now, let's contrast that 275 00:13:18.159 --> 00:13:20.519 with the S waves. These are known as secondary shear 276 00:13:20.600 --> 00:13:21.759 or transverse. 277 00:13:21.279 --> 00:13:24.159 Waves, and these are the waves that cause the catastrophic 278 00:13:24.200 --> 00:13:28.159 shearing deformations in rock and soil. They are generally much 279 00:13:28.159 --> 00:13:31.919 more destructive to human made structures. With an S wave, 280 00:13:32.200 --> 00:13:35.360 the motion of the individual rock particle is perpendicular to 281 00:13:35.399 --> 00:13:36.840 the direction the wave is traveling. 282 00:13:37.039 --> 00:13:39.679 So if the P wave is the slinky being pushed forward, 283 00:13:40.039 --> 00:13:42.200 the S wave is like holding a heavy bull whip 284 00:13:42.200 --> 00:13:45.039 and snapping it. Yes, the wave of energy is traveling 285 00:13:45.080 --> 00:13:48.759 forward away from your hand, but the actual leather of 286 00:13:48.799 --> 00:13:51.559 the whip is violently moving up and down or side 287 00:13:51.559 --> 00:13:53.240 to side perpendicular to the path. 288 00:13:53.399 --> 00:13:56.840 Yes, the energy moves forward, but the ground violently shears 289 00:13:56.960 --> 00:14:00.360 upward or sideways, and this perpendicular shearing motion and leads 290 00:14:00.399 --> 00:14:02.200 to a fascinating physical limitation. 291 00:14:02.480 --> 00:14:02.919 What's that? 292 00:14:03.200 --> 00:14:04.960 Fluids do not have sheer strength. 293 00:14:05.279 --> 00:14:06.840 Wait, what do you mean by sheer strength? 294 00:14:06.960 --> 00:14:09.320 Well, think about it this way. You can compress water 295 00:14:09.399 --> 00:14:13.080 slightly under immense pressure, but you cannot shear a glass 296 00:14:13.120 --> 00:14:15.759 of water in half with a sideways force. Right, The 297 00:14:15.799 --> 00:14:19.320 water just flows around it. Because liquids have no sheer strength, 298 00:14:19.679 --> 00:14:21.879 S waves physically cannot travel through them. 299 00:14:22.000 --> 00:14:24.120 Oh wow. So when that whipping S wave hits a 300 00:14:24.279 --> 00:14:26.879 massive body of liquid, it just stops dead. 301 00:14:27.159 --> 00:14:31.200 It dissipates entirely. And that mechanical limitation is actually how 302 00:14:31.240 --> 00:14:34.600 those early seismologists proved, beyond a shadow of a doubt, 303 00:14:34.799 --> 00:14:37.320 that the outer core of the Earth was liquid metal. 304 00:14:37.679 --> 00:14:38.080 No way. 305 00:14:38.320 --> 00:14:41.960 Yeah, The S waves from large earthquakes would travel deep 306 00:14:42.000 --> 00:14:44.759 into the planet and then just vanish. They left a 307 00:14:44.799 --> 00:14:46.799 massive shadow zone on the other side of the planet 308 00:14:46.840 --> 00:14:49.639 where no S waves were recorded, proving they had hit 309 00:14:49.720 --> 00:14:51.320 a massive liquid barrier. 310 00:14:51.519 --> 00:14:54.279 That is such an elegant piece of deduction, it really is. 311 00:14:54.759 --> 00:14:59.559 But let's bring this immense planetary physics back to the surface, back. 312 00:14:59.399 --> 00:15:00.919 To the city's sure, let's grown it. 313 00:15:01.200 --> 00:15:03.799 The S wave has traveled from the epicenter up through 314 00:15:03.840 --> 00:15:07.399 the bedrock, and now it hits the soil deposit directly 315 00:15:07.480 --> 00:15:10.639 beneath a high rise apartment building. This is where we 316 00:15:10.720 --> 00:15:11.600 hit the real hazards. 317 00:15:11.720 --> 00:15:14.679 This is where the theory has to become life saving practice. 318 00:15:14.799 --> 00:15:17.759 The goal of a geotechnical engineer isn't just to memorize 319 00:15:17.799 --> 00:15:21.840 wave properties. It is to calculate how that violent perpendicular 320 00:15:22.039 --> 00:15:25.000 S wave sheer force is going to alter the physical 321 00:15:25.080 --> 00:15:28.080 state of the soil. And the most dangerous alteration is 322 00:15:28.080 --> 00:15:29.799 a phenomenon called liquefaction. 323 00:15:30.279 --> 00:15:34.840 Liquefaction, just the word itself sounds ominous. This is when solid, 324 00:15:35.000 --> 00:15:39.240 seemingly stable ground literally turns to liquid mush under the 325 00:15:39.279 --> 00:15:42.399 stress of the shaking. Right, But how does solid dirt 326 00:15:42.480 --> 00:15:43.399 just become liquid? 327 00:15:43.720 --> 00:15:46.600 It all comes down to water and pressure. In many 328 00:15:46.679 --> 00:15:50.799 deep soil deposits, the microscopic spaces between the individual grains 329 00:15:50.840 --> 00:15:53.840 of sand or dirt are filled with water. Okay, when 330 00:15:53.879 --> 00:15:56.759 that violent S wave hits the soil, it tries to 331 00:15:56.840 --> 00:16:01.279 compress those poor spaces but because the shake happens so fast, 332 00:16:01.360 --> 00:16:03.360 the water doesn't have time to drain away, so. 333 00:16:03.360 --> 00:16:05.519 The water gets trapped and squeezed. 334 00:16:05.080 --> 00:16:07.759 And as it gets squeezed, the water pressure inside the 335 00:16:07.799 --> 00:16:12.399 soil skyrockets. The pressure pushes outward with so much force 336 00:16:12.480 --> 00:16:16.440 that it literally pushes the individual soil particles apart. The 337 00:16:16.519 --> 00:16:19.360 dirt grains lose contact with each other, and there suddenly 338 00:16:19.440 --> 00:16:21.639 just well floating in highly pressurized water. 339 00:16:21.840 --> 00:16:24.679 So the dirt beneath the building loses all of its internal. 340 00:16:24.320 --> 00:16:28.120 Friction exactly, it completely loses its baring capacity. It ceases 341 00:16:28.159 --> 00:16:30.279 to behave like a solid and begins to behave like 342 00:16:30.279 --> 00:16:33.480 a heavy, viscous fluid. That is exactly what happened to 343 00:16:33.480 --> 00:16:36.080 those tilted apartment buildings we mentioned at the very beginning 344 00:16:36.080 --> 00:16:36.799 of the deep dive. 345 00:16:37.240 --> 00:16:40.799 Oh right, The soil liquefied, the foundation lost its support, 346 00:16:41.200 --> 00:16:45.399 and the massive concrete structures simply sank and tilted into 347 00:16:45.440 --> 00:16:48.480 the earth, just like a heavy block of wood sinking 348 00:16:48.480 --> 00:16:49.440 into a bowl of jelly. 349 00:16:49.559 --> 00:16:51.840 And the buildings right next door, the ones that survived, 350 00:16:51.879 --> 00:16:54.159 they must have been sitting on a different soil profile, 351 00:16:54.519 --> 00:16:57.639 like maybe their foundations were anchored directly into the bedrock, 352 00:16:57.960 --> 00:17:01.279 or they were sitting on dense, dry soil that couldn't liquefy. 353 00:17:01.759 --> 00:17:05.079 That is usually the case, yeah, which is why understanding 354 00:17:05.200 --> 00:17:09.640 dynamic soil structure interaction is so paramount For engineers. You 355 00:17:09.720 --> 00:17:13.359 use the fundamental physics of wave propagation to design a solution. 356 00:17:14.039 --> 00:17:16.839 For instance, if you are building a massive retaining wall 357 00:17:16.880 --> 00:17:19.799 to hold back a hillside, you don't just calculate for 358 00:17:19.839 --> 00:17:22.359 the static weight of the dirt. You calculate for the 359 00:17:22.400 --> 00:17:26.079 transfer functions. You calculate the specific gravity of the layers 360 00:17:26.119 --> 00:17:28.759 beneath your sight. You determine what will happen if the 361 00:17:28.759 --> 00:17:32.559 soil behind that wall liquefies and suddenly pushes against the concrete, 362 00:17:32.640 --> 00:17:34.640 not with the force of solid john, but with the 363 00:17:34.680 --> 00:17:38.319 massive hydrostatic pressure of a heavy fluid. You essentially design 364 00:17:38.400 --> 00:17:42.680 the structure to survive the exact symphony of pe waves 365 00:17:42.720 --> 00:17:44.759 and s waves that the local geology is going to 366 00:17:44.799 --> 00:17:45.279 throw at. 367 00:17:45.119 --> 00:17:47.720 It exactly, and when it is done correctly, it saves 368 00:17:47.960 --> 00:17:48.880 thousands of lives. 369 00:17:49.319 --> 00:17:54.000 It is deeply reassuring. Honestly, earthquakes are terrifying, chaotic forces 370 00:17:54.000 --> 00:17:58.960 of nature, but they aren't magic. They follow strict measurable laws. 371 00:17:58.960 --> 00:18:02.039 Of physics, waves, propagation, and soil dynamics. 372 00:18:02.079 --> 00:18:02.960 I absolutely do. 373 00:18:03.119 --> 00:18:06.160 Whether you are walking across a massive suspension bridge today 374 00:18:06.440 --> 00:18:08.799 or just sitting in your apartment listening to this, there 375 00:18:08.839 --> 00:18:11.359 is a very good chance that the complex principles of 376 00:18:11.359 --> 00:18:16.400 geotechnical earthquake engineering, the careful calculation of amplitude, frequency, content, 377 00:18:16.480 --> 00:18:20.480 and duration, are actively quietly keeping you safe right now. 378 00:18:20.680 --> 00:18:23.000 And you know, the more accurately we can map and 379 00:18:23.079 --> 00:18:26.079 understand the invisible earth beneath us, the more resilient our 380 00:18:26.119 --> 00:18:29.279 cities become. Now, as a quick review exercise for everyone listening, 381 00:18:29.920 --> 00:18:32.240 if you were to analyze a newly recorded ground motion, 382 00:18:32.640 --> 00:18:36.240 what three specific characteristics would you compare to determine how 383 00:18:36.279 --> 00:18:40.000 a local soil deposit amplified the shaking compared to nearby bedrock. 384 00:18:40.279 --> 00:18:41.640 A pop quiz, It's just a. 385 00:18:41.559 --> 00:18:44.759 Little one, and the answer is you'd look at the amplitudes, 386 00:18:44.839 --> 00:18:46.640 the frequency contents, and the durations. 387 00:18:46.720 --> 00:18:50.559 Amplitudes, frequency contents and durations. Perfect. Yeah, And before we 388 00:18:50.559 --> 00:18:52.079 wrap up today, I want to leave you with a 389 00:18:52.119 --> 00:18:55.559 final forward looking thought based on that mapping, just something 390 00:18:55.599 --> 00:18:57.799 to mull over as you walk around your city. We 391 00:18:57.880 --> 00:19:01.960 talked extensively about how early scientists use the massive, destructive 392 00:19:02.000 --> 00:19:06.599 seismic waves of giant earthquakes to map the deep hidden 393 00:19:06.680 --> 00:19:09.839 interior of our planet. But the earth beneath our cities 394 00:19:09.920 --> 00:19:10.960 is always vibrating. 395 00:19:11.079 --> 00:19:15.440 Oh yeah, it is a constant hum of kinetic energy, traffic, 396 00:19:15.680 --> 00:19:17.839 heavy construction, subway trains right. 397 00:19:17.920 --> 00:19:21.720 So here is the thought to explore how might future engineers, 398 00:19:22.079 --> 00:19:26.200 armed with advanced AI and incredibly sensitive underground sensors utilize 399 00:19:26.200 --> 00:19:29.319 those daily micro vibrations. Could the constant rumble of a 400 00:19:29.359 --> 00:19:34.000 subway train act as an artificial seismic wave? Wow? Can 401 00:19:34.079 --> 00:19:36.559