Speaker 1: I want you to picture a devastating earthquake like you
Speaker 1: have undoubtedly seen the footage on the news at some point, right, Oh, yeah, absolutely.
Speaker 1: The air is thick with dust, infrastructure is just crushed,
Speaker 1: and it basically looks like a war zone. But if
Speaker 1: you look closely at the aftermath of these events, you
Speaker 1: almost always notice this one detail that is frankly, deeply unsettling.
Speaker 2: Yeah, it's the contrast exactly.
Speaker 1: You will see one building completely collapsed, just pancaked into rubble,
Speaker 1: and right next door, sometimes literally fifty feet away, another
Speaker 1: building stands there perfectly intact, like it might not even
Speaker 1: have a shattered window.
Speaker 2: Right, And it creates this immediate I guess, cognitive dissonance.
Speaker 2: You see two identical environments, but you get totally different outcomes.
Speaker 1: Yeah, And the immediate assumption is usually about the architecture.
Speaker 2: Right.
Speaker 1: We assume the collapse building was I don't know, poorly
Speaker 1: constructed by someone cutting corners, and the surviving one was
Speaker 1: built like.
Speaker 2: A fortress, which is what anyone would think, right.
Speaker 1: But today's deep dive is going to completely flip that
Speaker 1: assumption because it is not always about how the building
Speaker 1: was built. It is often entirely about what the building
Speaker 1: was built.
Speaker 2: On, which requires a really fundamental shift in how you
Speaker 2: view structural survival. I mean, as a society, we get
Speaker 2: so fixated on the steel and the concrete above the surface,
Speaker 2: shiny stuff, exactly the visible stuff, and we completely ignore
Speaker 2: the invisible brutal physics happening right below it.
Speaker 1: And uncovering those physics is our mission today. We are
Speaker 1: exploring the science of structural survival and we're being guided
Speaker 1: by Steven L. Kramer's foundational work Geotechnical Earthquake Engineering.
Speaker 2: Yeah, that's an incredible resource.
Speaker 1: It essentially is the bible for engineers trying to figure
Speaker 1: out exactly what happens in the dirt beneath your feet
Speaker 1: when the big one hits. And to set up the
Speaker 1: mechanical problem we're dealing with here, let me let me
Speaker 1: offer an analogy.
Speaker 2: Go for it.
Speaker 1: So, if you put a heavy wooden block on a
Speaker 1: solid oak table and you hit that table with a sledgehammer,
Speaker 1: the wooden block shakes a bit, it vibrates. But if
Speaker 1: you take that exact same block of wood, place it
Speaker 1: on a bowl of jelly and hit the table with
Speaker 1: the exact same force, that wood block is going to
Speaker 1: bounce wildly, it'll probably just flip over.
Speaker 2: That is, Yeah, that captures the cord a lemma perfectly,
Speaker 2: because you know, to the untrained eye, the ground just
Speaker 2: looks like the ground, it looks solid, its pavement. But
Speaker 2: to a geotechnical engineer, certain soil profiles act exactly like
Speaker 2: that bowl of jelly.
Speaker 1: So the question then becomes, well, how do you engineer
Speaker 1: for the jelly? Like, how do you mathematically calculate a
Speaker 1: building's survival when the ground underneath it is fundamentally unstable.
Speaker 2: Well, you start by capturing the reality of the wave.
Speaker 2: If you can't just you know, theorize how the jelly
Speaker 2: will behave in a vacuum, you have to measure it
Speaker 2: under immense strut actual data exactly. The entire field of
Speaker 2: geotechnical engineering relies heavily on real world data, and a
Speaker 2: prime example that Kramer uses to really anchor the science
Speaker 2: is the nineteen eighty nine Loma Prieta earthquake in northern California.
Speaker 1: Oh wow, Yeah, that was a massive event. I famously
Speaker 1: interrupted the world series.
Speaker 2: Right it did. Yeah, it was devastating for the area,
Speaker 2: But from an engineering standpoint, it provided this incredible set
Speaker 2: of data. Specifically, scientists looked at two different ground motions
Speaker 2: recorded during that exact same earthquake. Okay, one reading was
Speaker 2: taken from a seismograph sitting on a solid rock outcrop,
Speaker 2: and the other reading was taken from the surface of
Speaker 2: a deep soil deposit, but relatively close by.
Speaker 1: Ah so literally the table and the jelly side by side. Yes.
Speaker 2: And this is actually where the global nature of this
Speaker 2: science comes into play, because when engineers analyze data like
Speaker 2: the Loma Prieta readings, they are pulling from a massive
Speaker 2: collaborative effort. Interestingly, in the engineering literature, Cramer's work included,
Speaker 2: you will often see both metric and imperial units used interchangeably.
Speaker 1: I noticed that it was it could be a bit
Speaker 1: disorienting at first to see meters and feet living next
Speaker 1: to each other in the same equation.
Speaker 2: I know it drives some students crazy, but it reflects
Speaker 2: the reality of the profession. I mean, the current state
Speaker 2: of knowledge for keeping buildings falling down is cobbled together
Speaker 2: from advances across the entire.
Speaker 1: Globe, right, because earthquakes happen everywhere exactly.
Speaker 2: You have seismologists in Japan, structural engineers in California, and
Speaker 2: geotechnical researchers in Europe, and they're all sharing data from
Speaker 2: their respective seismic events. If you try to standardize on
Speaker 2: just one system of measurement, you honestly alienate half the
Speaker 2: global defense effort. Everyone basically needs to be fluent in
Speaker 2: both dialects to use the data effectively.
Speaker 1: That makes a lot of sense actually, So going back
Speaker 1: to that shared data from Luma Prieta, we have the
Speaker 1: reading from the solid rock and the reading from the
Speaker 1: deep soil. What are the engineers actually looking for? I imagine
Speaker 1: they're just comparing how hard the shaking was.
Speaker 2: Well, the intensity of the shaking is part of it,
Speaker 2: for sure, but it is much much more nuanced than that.
Speaker 2: They are comparing three specific quantifiable characteristics.
Speaker 1: Okay, what are they?
Speaker 2: The amplitude of the wave, the frequency content, and the duration.
Speaker 1: Okay, I am going to push back a little here
Speaker 1: on the complexity of this, just you know, looking at
Speaker 1: it from the perspective of someone who just wants a
Speaker 1: building to stay upright. If an earthquake only lasts, say,
Speaker 1: fifteen seconds, why do we need to dive into complex
Speaker 1: variables like frequency contents? Isn't the goal just to make
Speaker 1: sure the concrete can survive that fifteen second window of
Speaker 1: extreme violence, why overcomplicate it?
Speaker 2: That's a great question. But if you only design a
Speaker 2: building to survive the time domain that fifteen second window,
Speaker 2: you are going to completely miss the very mechanism that
Speaker 2: takes the building down. Wait really, yeah, the time domain
Speaker 2: is only half the story. To truly understand how the
Speaker 2: ground is going to respond, engineers have to analyze the
Speaker 2: frequency domain.
Speaker 1: The frequency domain, let's break that down. I am familiar
Speaker 1: with the concept of sound frequencies, obviously, but what does
Speaker 1: that mean when we were talking about dirt and earthquakes?
Speaker 2: So it requires a mathematical process called a Fouria analysis.
Speaker 2: In modern engineering, students and professionals rely on pretty heavy
Speaker 2: computational software like matt Lab to run these.
Speaker 1: Hold on Fouria analysis That sounds like something pulled from
Speaker 1: a graduate level calculus exam. What is the software actually
Speaker 1: doing to the earthquake data.
Speaker 2: Think of a raw earthquake wave as a chaotic wall
Speaker 2: of noise, like it is a messy, highly complex.
Speaker 1: Wave, just a jumble of energy, right.
Speaker 2: A Fourier analysis is a mathematical tool that takes that
Speaker 2: chaotic wave and breaks it down into its simplest components,
Speaker 2: so it turns it into a series of clean, simple
Speaker 2: sine waves, each with its own frequency.
Speaker 1: Oh I see, So it's essentially acting like a sound
Speaker 1: engineer isolating the individual audio tracks from a really noisy
Speaker 1: live concert recording. Yes, it separates the baseline, the vocals,
Speaker 1: and the drums, so you can see exactly how loud
Speaker 1: each individual element is.
Speaker 2: That is a brilliant way to conceptualize it. It shows
Speaker 2: you exactly which frequencies of vibration were present in the
Speaker 2: earthquake and how powerful each specific frequency was. And that
Speaker 2: is vital because buildings, much like tuning forks, have their
Speaker 2: own natural frequencies.
Speaker 1: Meaning a tall sky scraper naturally sways at a different
Speaker 1: frequency than say a two story house exactly.
Speaker 2: And this is where the soil, the jelly becomes the
Speaker 2: real danger. Engineers use what are called transfer functions. These
Speaker 2: are mathematical models that predict how a specific layer of
Speaker 2: soil will alter the seismic wave as it travels from
Speaker 2: the deep bedrock up to the surface.
Speaker 1: But how they actually build that function though? They can't
Speaker 1: just guess what the dirt.
Speaker 2: Is going to do, No, they can't. They have to
Speaker 2: test the physical reality of the site. Geotechnical engineers will
Speaker 2: drill deep boreholes into the earth, pull up core samples
Speaker 2: of the soil, and test them in laboratories to see
Speaker 2: what it's made of, right, to determine their stiffness, their density,
Speaker 2: and their sheer strength. Sometimes they even send artificial shock
Speaker 2: waves into the ground right there on site and measure
Speaker 2: how long it takes for the wave to travel between
Speaker 2: two points. They combine all that physical data to build
Speaker 2: the transfer.
Speaker 1: Function, and what does that function ultimately tell them?
Speaker 2: It tells them if the soil is going to act
Speaker 2: like a rogue audio equalizer.
Speaker 1: Oh wow, Yeah.
Speaker 2: Certain deep soil deposits will take the raw energy of
Speaker 2: an earthquake and actually amplify specific frequencies while damping others.
Speaker 2: So they turn up the base exactly. If the soil
Speaker 2: deposit amplifies the exact base frequency that the building sitting
Speaker 2: on top of it is vulnerable to, you get resonance,
Speaker 2: the swaying of the building amplifies violently, and you get
Speaker 2: catastrophic structural failure.
Speaker 1: That is genuinely terrifying.
Speaker 2: It is, but that is why understanding the frequency domain
Speaker 2: is literally the difference between life and death.
Speaker 1: Structurally, it makes perfect sense. So okay, we've established that
Speaker 1: engineers measure the surface shaking, break it down with Fouria analysis,
Speaker 1: and use transfer functions to predict the soil's behavior. But
Speaker 1: to truly understand why that wave behaves the way it does,
Speaker 1: we have to follow the wave back to its origin.
Speaker 1: We have to look downward.
Speaker 2: We have to move from engineering into pure planetary physics.
Speaker 2: To understand the wave, you have to understand the medium
Speaker 2: it is traveling through.
Speaker 1: You basically have to X ray the Earth, and the
Speaker 1: numbers involved in doing that are just staggering. Just looking
Speaker 1: at the physical makeup of our planet. In the text,
Speaker 1: the Earth weighs about five point four times ten to
Speaker 1: the twenty first power tons or four point nine times
Speaker 1: ten to the twenty fourth kilograms.
Speaker 2: It is a volume and mass that is frankly difficult
Speaker 2: for the human brain to truly process. But that staggering
Speaker 2: weight gives us a crucial engineering metric. Based on its
Speaker 2: size and mass, the Earth has an average specific gravity
Speaker 2: of about five point.
Speaker 1: Five Let's define specific gravity for a second, just to
Speaker 1: make sure we all on the same page.
Speaker 2: Sure, specific gravity is simply a way to measure density
Speaker 2: by comparing a material to water. Water has a specific
Speaker 2: gravity of one. Okay, so if something has a specific
Speaker 2: gravity of five point five, it is five and a
Speaker 2: half times denser or heavier than an equal volume of water.
Speaker 1: Okay, So the planet as a whole averages a five
Speaker 1: point five. But the s official rocks, the granite and
Speaker 1: limestone we see on the crust, the stuffs we actually
Speaker 1: drill into delay our foundations, those only have a specific
Speaker 1: gravity of about two point seventy three.
Speaker 2: Which leaves us with a massive mathematical descripant. And see right.
Speaker 2: If the crust is a three, but the planetary average
Speaker 2: is a five point five, what does that dictate about
Speaker 2: the unseen interior of the Earth.
Speaker 1: It dictates that there must be something incredibly dense, heavy,
Speaker 1: and massive hidden deep down near the core to pull
Speaker 1: that overall average up so high. We are essentially floating
Speaker 1: on a thin, relatively lightweight, fragile shell that surrounds this massive,
Speaker 1: ultra dense center.
Speaker 2: Improving the existence of that dense center is basically where
Speaker 2: seismology was born. Because you know, you obviously cannot dig
Speaker 2: a hole to the core to see what is down there.
Speaker 2: Definitely not the deepest hole ever drilled barely scratch the crust.
Speaker 2: So early scientists use the devastating power of earthquakes as
Speaker 2: a tool. A large earthquake produces an unfathomable amount of
Speaker 2: energy enough to send seismic waves entirely through the globe
Speaker 2: from one side to the other, so.
Speaker 1: It's acting like a planetary scale ultrasound.
Speaker 2: That is precisely how it works. As these seismic waves
Speaker 2: travel through the deep interior, they hit boundaries between different materials,
Speaker 2: for example, the boundary between the solid rocky mantle and
Speaker 2: the ultra dense.
Speaker 1: Outer core, and what happens when they hit.
Speaker 2: When the waves hit those boundaries, the change in density
Speaker 2: causes them to refract, to bend, or to reflect back entirely.
Speaker 1: So by placing seismographs all over the world and just
Speaker 1: waiting for major earthquakes, these early scientists were listening to
Speaker 1: the echoes. By measuring the angles at which these waves
Speaker 1: emerged on the other side of the planet, they mapped
Speaker 1: the Earth's internal layered structure without ever actually laying eyes
Speaker 1: on it.
Speaker 2: It's pretty genius when you think about it.
Speaker 1: It really is which brings us to the waves themselves,
Speaker 1: because understanding the journey of those waves is the key
Speaker 1: to designing a structure that survives them. Now, I have
Speaker 1: to admit I struggle a bit with visualizing the mechanics
Speaker 1: of this.
Speaker 2: A lot of people do.
Speaker 1: I understand surface waves like dropping a pebble in a
Speaker 1: pond and watching the ripples travel along the top of
Speaker 1: the water, But how does a wave travel through thousands
Speaker 1: of miles of solid rock.
Speaker 2: Yeah, it is a totally different mechanism than a water ripple.
Speaker 2: What you are talking about are called body waves, waves
Speaker 2: that travel through the interior body of the Earth. And
Speaker 2: there are two main types of body waves that dictate
Speaker 2: everything in geotechnical engineering, P waves and S waves.
Speaker 1: Okay, let's start with PE waves. These are also called primary,
Speaker 1: compressional or longitudinal waves.
Speaker 2: Right, And the mechanical concept here is compression. PA waves
Speaker 2: are essentially massive sound waves traveling through solid rock.
Speaker 1: So I picture a P wave like taking a metal slinky,
Speaker 1: stretching it out on a long table, and then suddenly
Speaker 1: pushing one end straightforward.
Speaker 2: Yes, perfect.
Speaker 1: You watch that clustered ring of energy travel straight down
Speaker 1: the slinky. It compresses the rings together and they stretch apart,
Speaker 1: which I think is called rare faction. But the energy
Speaker 1: in the rings are all moving in the exact same
Speaker 1: forward direction.
Speaker 2: That slinky analogy is the perfect visualization. The crucial detail
Speaker 2: is that in a P wave, the motion of an
Speaker 2: individual particle of rock is parallel to the direction the
Speaker 2: wave is traveling. It just compresses and expands right along
Speaker 2: its path. Okay, because it relies purely on compression, P
Speaker 2: waves can pass through almost anything. They travel through the
Speaker 2: solid rock of the mantle, and they travel right through
Speaker 2: the liquid outer core of the Earth.
Speaker 1: Okay, parallel motion like a slinky. Now, let's contrast that
Speaker 1: with the S waves. These are known as secondary shear
Speaker 1: or transverse.
Speaker 2: Waves, and these are the waves that cause the catastrophic
Speaker 2: shearing deformations in rock and soil. They are generally much
Speaker 2: more destructive to human made structures. With an S wave,
Speaker 2: the motion of the individual rock particle is perpendicular to
Speaker 2: the direction the wave is traveling.
Speaker 1: So if the P wave is the slinky being pushed forward,
Speaker 1: the S wave is like holding a heavy bull whip
Speaker 1: and snapping it. Yes, the wave of energy is traveling
Speaker 1: forward away from your hand, but the actual leather of
Speaker 1: the whip is violently moving up and down or side
Speaker 1: to side perpendicular to the path.
Speaker 2: Yes, the energy moves forward, but the ground violently shears
Speaker 2: upward or sideways, and this perpendicular shearing motion and leads
Speaker 2: to a fascinating physical limitation.
Speaker 1: What's that?
Speaker 2: Fluids do not have sheer strength.
Speaker 1: Wait, what do you mean by sheer strength?
Speaker 2: Well, think about it this way. You can compress water
Speaker 2: slightly under immense pressure, but you cannot shear a glass
Speaker 2: of water in half with a sideways force. Right, The
Speaker 2: water just flows around it. Because liquids have no sheer strength,
Speaker 2: S waves physically cannot travel through them.
Speaker 1: Oh wow. So when that whipping S wave hits a
Speaker 1: massive body of liquid, it just stops dead.
Speaker 2: It dissipates entirely. And that mechanical limitation is actually how
Speaker 2: those early seismologists proved, beyond a shadow of a doubt,
Speaker 2: that the outer core of the Earth was liquid metal.
Speaker 1: No way.
Speaker 2: Yeah, The S waves from large earthquakes would travel deep
Speaker 2: into the planet and then just vanish. They left a
Speaker 2: massive shadow zone on the other side of the planet
Speaker 2: where no S waves were recorded, proving they had hit
Speaker 2: a massive liquid barrier.
Speaker 1: That is such an elegant piece of deduction, it really is.
Speaker 1: But let's bring this immense planetary physics back to the surface, back.
Speaker 2: To the city's sure, let's grown it.
Speaker 1: The S wave has traveled from the epicenter up through
Speaker 1: the bedrock, and now it hits the soil deposit directly
Speaker 1: beneath a high rise apartment building. This is where we
Speaker 1: hit the real hazards.
Speaker 2: This is where the theory has to become life saving practice.
Speaker 2: The goal of a geotechnical engineer isn't just to memorize
Speaker 2: wave properties. It is to calculate how that violent perpendicular
Speaker 2: S wave sheer force is going to alter the physical
Speaker 2: state of the soil. And the most dangerous alteration is
Speaker 2: a phenomenon called liquefaction.
Speaker 1: Liquefaction, just the word itself sounds ominous. This is when solid,
Speaker 1: seemingly stable ground literally turns to liquid mush under the
Speaker 1: stress of the shaking. Right, But how does solid dirt
Speaker 1: just become liquid?
Speaker 2: It all comes down to water and pressure. In many
Speaker 2: deep soil deposits, the microscopic spaces between the individual grains
Speaker 2: of sand or dirt are filled with water. Okay, when
Speaker 2: that violent S wave hits the soil, it tries to
Speaker 2: compress those poor spaces but because the shake happens so fast,
Speaker 2: the water doesn't have time to drain away, so.
Speaker 1: The water gets trapped and squeezed.
Speaker 2: And as it gets squeezed, the water pressure inside the
Speaker 2: soil skyrockets. The pressure pushes outward with so much force
Speaker 2: that it literally pushes the individual soil particles apart. The
Speaker 2: dirt grains lose contact with each other, and there suddenly
Speaker 2: just well floating in highly pressurized water.
Speaker 1: So the dirt beneath the building loses all of its internal.
Speaker 2: Friction exactly, it completely loses its baring capacity. It ceases
Speaker 2: to behave like a solid and begins to behave like
Speaker 2: a heavy, viscous fluid. That is exactly what happened to
Speaker 2: those tilted apartment buildings we mentioned at the very beginning
Speaker 2: of the deep dive.
Speaker 1: Oh right, The soil liquefied, the foundation lost its support,
Speaker 1: and the massive concrete structures simply sank and tilted into
Speaker 1: the earth, just like a heavy block of wood sinking
Speaker 1: into a bowl of jelly.
Speaker 2: And the buildings right next door, the ones that survived,
Speaker 2: they must have been sitting on a different soil profile,
Speaker 2: like maybe their foundations were anchored directly into the bedrock,
Speaker 2: or they were sitting on dense, dry soil that couldn't liquefy.
Speaker 1: That is usually the case, yeah, which is why understanding
Speaker 1: dynamic soil structure interaction is so paramount For engineers. You
Speaker 1: use the fundamental physics of wave propagation to design a solution.
Speaker 1: For instance, if you are building a massive retaining wall
Speaker 1: to hold back a hillside, you don't just calculate for
Speaker 1: the static weight of the dirt. You calculate for the
Speaker 1: transfer functions. You calculate the specific gravity of the layers
Speaker 1: beneath your sight. You determine what will happen if the
Speaker 1: soil behind that wall liquefies and suddenly pushes against the concrete,
Speaker 1: not with the force of solid john, but with the
Speaker 1: massive hydrostatic pressure of a heavy fluid. You essentially design
Speaker 1: the structure to survive the exact symphony of pe waves
Speaker 1: and s waves that the local geology is going to
Speaker 1: throw at.
Speaker 2: It exactly, and when it is done correctly, it saves
Speaker 2: thousands of lives.
Speaker 1: It is deeply reassuring. Honestly, earthquakes are terrifying, chaotic forces
Speaker 1: of nature, but they aren't magic. They follow strict measurable laws.
Speaker 1: Of physics, waves, propagation, and soil dynamics.
Speaker 2: I absolutely do.
Speaker 1: Whether you are walking across a massive suspension bridge today
Speaker 1: or just sitting in your apartment listening to this, there
Speaker 1: is a very good chance that the complex principles of
Speaker 1: geotechnical earthquake engineering, the careful calculation of amplitude, frequency, content,
Speaker 1: and duration, are actively quietly keeping you safe right now.
Speaker 2: And you know, the more accurately we can map and
Speaker 2: understand the invisible earth beneath us, the more resilient our
Speaker 2: cities become. Now, as a quick review exercise for everyone listening,
Speaker 2: if you were to analyze a newly recorded ground motion,
Speaker 2: what three specific characteristics would you compare to determine how
Speaker 2: a local soil deposit amplified the shaking compared to nearby bedrock.
Speaker 1: A pop quiz, It's just a.
Speaker 2: Little one, and the answer is you'd look at the amplitudes,
Speaker 2: the frequency contents, and the durations.
Speaker 1: Amplitudes, frequency contents and durations. Perfect. Yeah, And before we
Speaker 1: wrap up today, I want to leave you with a
Speaker 1: final forward looking thought based on that mapping, just something
Speaker 1: to mull over as you walk around your city. We
Speaker 1: talked extensively about how early scientists use the massive, destructive
Speaker 1: seismic waves of giant earthquakes to map the deep hidden
Speaker 1: interior of our planet. But the earth beneath our cities
Speaker 1: is always vibrating.
Speaker 2: Oh yeah, it is a constant hum of kinetic energy, traffic,
Speaker 2: heavy construction, subway trains right.
Speaker 1: So here is the thought to explore how might future engineers,
Speaker 1: armed with advanced AI and incredibly sensitive underground sensors utilize
Speaker 1: those daily micro vibrations. Could the constant rumble of a
Speaker 1: subway train act as an artificial seismic wave? Wow? Can
Speaker 1: we use that daily urban noise to run continuous Fourier
Speaker 1: analyses constantly in real time, mapping and monitoring the structural
Speaker 1: integrity of the soil right beneath our feet.
Speaker 2: It is a fascinating frontier moving from just reacting to
Speaker 2: giant earthquakes to using the city's own kinetic heartbeat to
Speaker 2: predict how its foundations will react before the real quake
Speaker 2: ever hits.
Speaker 1: Yeah, it really makes you look at the pavement a
Speaker 1: little differently, doesn't it. Next time you see a building
Speaker 1: perfectly intact while the one next door struggling. Remember, yeah,
Speaker 1: it is not always about the wood block. Sometimes it
Speaker 1: is entirely about the jelly. Keep questioning, keep learning, and
Speaker 1: we will see you on the next deep dive.