The Trembling Earth

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Tag Archives: Tohoku

Amazing liquefaction in Tokyo

A new video from Japan [embedded below] shows liquefaction occurring at a scale and scope that I haven’t seen before in video footage. The video is from Urayasu  town, Chiba Prefecture–an industrial suburb of Tokyo that appears to be sited on made land adjacent to Tokyo Bay. No wonder it sloshes so heavily: made land (fill) is particularly susceptible to liquefaction. We just can’t pack things down the way nature can over millennia.

The video starts in the aftermath of the March 11, 2011 M9.0 Tohoku earthquake, where sediment-filled ground water is bubbling up through gaps in the pavement, or any other fractures that represent escape routes. Within minutes, the M7.9 aftershock hits, and you can see light poles, trees, and buildings shaking violently. All the while, the engineered infrastructure sloshes and bobs, essentially floating on a thick package of fluid-saturated mush. The differential swaying you observe is effectively the dramatic reduction of seismic wave velocity in the loose, fluid-supported substrate. The sound you hear is largely the metal guard rails creaking as they’re stretched and bent. The oscillations last for a very long time.

Liquefaction commonly accompanies large earthquakes in areas where a shallow groundwater table supports suspension of soft soils when it’s shaken. I’ve posted videos of the phenomenon before. It occurs during any earthquake that strikes an area with the right mix of water-saturated sediments. It was widespread throughout Christchurch in each of their big jolts in 2011, and occurred pervasively in the shoreline areas of Japan during their monstrous 2011 quake. Fractures opened during shaking provide conduits for this newly mobile soil slurry to escape surface-ward under the weight of dry material above, producing sand volcanoes, like the one pictured in the righthand image of this blog’s banner.

After watching these videos, it’s clear why this phenomenon results in such destruction, particularly to pipelines and underground utilities. Water main failures may add to the fluid pressure mobilizing all the soil.

When you hear about the danger of living on “fill” and the destruction of liquefaction, these illustrative clips should come to mind.

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First-hand documentaries from the Tohoku quake and tsunami

Several eye-witnesses of the March 11 M9.0 in Japan have posted chilling first-hand footage of their experiences. The following two videos document the entire disaster, from the shaking until deep into the tsunami.

First, here’s the harrowing video taken by a professional storm chaser who happened to be in Otsuchi, Japan doing volunteer work for Save Japan Dolphins when the earthquake roared along the coast. He struggles to keep his balance at the tail end of the earthquake, then they hop in their car and spend eight precious minutes fleeing to high ground with the rest of the alarmed population. These guys are clearly professionals; they have incredible composure. As they navigate their way through town you hear radio correspondence with a second car behind them full of other dolphin activists, the Sea Shepherds. While the tsunami roils its way into the bay from the open ocean, the water level creeps up and floods the seaside industrial buildings. As usual with footage of the tsunami, it keeps on coming, getting higher and higher against all belief.

This next video is unprecedented: a high-def dashboard camera in a bus records the entire event, most of it from in/on the tsunami! It was recently recovered, and we get to witness its journey. It would be marvelous if someone who knows Japanese could do a little translation of this report. Watch this; the following text is a spoiler.

It’s incredible to watch as the bus helplessly bobs with mounds of urban flotsam, but the literally immersive perspective reveals fascinating gradients in the velocity and height of the tsunami’s flow as it rages through city streets. The bus seems for a while fortunately stuck in an eddy behind a huge concrete building… until it gets entrained in a rapid flow and slammed up against something, at which point debris pierces the windshield, water rushes in, and the camera dies.

There’s at least one more street-level recording of the tsunami pouring into town. Once again what begins high and dry ends up so far below water it defies imagination.

Swaying high-rises and resonance frequencies

In a post a few weeks ago I linked to a humbling video of high-rise buildings in Tokyo swaying after the 9.0 quake. Well, those buildings were full of tens of thousands of people, plenty of whom had cameras. That video was only the beginning; below are many more that capture the dramatic oscillation of the towering steel edifices.

The swaying is especially clear in the following video, likely filmed during the 7.9 aftershock that happened shortly after the 9.0. The tsunami is already on the news, and smoke is billowing from the distance. It must have been alarming to start shaking again while watching the startling consequences of the first quake unfold.

The triplet of skyscrapers below is linked together by walkways high in the air. Clearly the walkways were engineered with earthquakes in mind, as they collapse and bend while the buildings swing differently.

From inside one of the swaying towers you can hear the surrounding building creak:

While high-rises do rattle and shake from earthquakes, the “gentle” swaying is a result of their resonant response to the low frequency waves unleashed by large earthquakes. Imagine an especially tall building being pushed to the side, from the bottom. The huge building has a lot of inertia, and it takes time for the force at the bottom to be transmitted up through the beams to the top. When the forces reach the top (a matter of less than a second, probably) the whole building will be in motion, moving to the side. If you suddenly stop the bottom, the top maintains its momentum and overshoots this position until the structure’s stiffness halts it and the elastic property that allowed it to flex forces it to recover that deformation and swing back the other way. This single impulse (pushing the building to the side a finite distance) results in an oscillation of the un-anchored end of the building (the top) as the force imparted at its base is gradually dampened or absorbed by flexing, heating, and creaking of the beams.

Now, if instead of stopping the bottom of the building you reverse it and push it back the other way, this would amplify the distance the top has to travel once it swings back, adding momentum to the return oscillation and enhancing the swaying. The same thing goes on when you push someone on a swing: you give them pushes just as they swing “forth” so that the energy you input into the system is added to the energy they already have being pulled forth by gravity. If you pushed them as they were coming at you, all of your energy would be expended resisting the force of their swing “back,” and the system (you and the swinger) would lose all of its energy. To make a tall building really sway, the seismic waves must drag its base back and forth at a frequency that matches that of the building’s natural oscillation–its resonant frequency. That is to say, if you “plucked” a building and let it wobble it would do so at a frequency that is determined by its material properties, geometry, and weight, among other things. If you then continue to shake it at that same frequency, you’ll accentuate the motion, just like the swing set, causing “resonance.”

Taller buildings have lower natural resonant frequencies than short buildings, meaning that if a broad spectrum of seismic wave frequencies is released, buildings with different resonant frequencies will sway differently. Small 2-story houses have extremely high resonant frequencies, and are thus more susceptible to the very sharp seismic waves experienced most strongly near the epicenter of a quake. Sky-scrapers may start swaying at huge distances from a quake, where high-frequency waves have died off and all that are left are the low-frequency seismic waves people can barely feel (we have even higher resonant frequencies than houses, if you want to think of it that way: it’s sort of why a bus braking hard makes us fall over whereas a train stopping for hundreds of yards leaves us upright).

In fact, there’s an impressive video from the May 2008 M8.3 Wenchuan earthquake in central China… filmed over 1,000 miles away in Taipei! At the top of Taipei101, currently the second tallest building in the world, is a “tuned mass damper”, essentially a giant dense metal pendulum designed to counteract the swaying of the building due to wind or earthquakes. On the afternoon of the Wenchuan quake, tourists looking at the orb witnessed it in action as it counterbalanced the passing seismic waves from the distant quake.

The video illustrates just how slow this swaying is. Without the pendulum for reference it is unlikely anyone would have noticed.

On the other hand, the magnitude of each oscillation in the Tokyo high-rises is almost certainly enough to have made plenty of their occupants nauseous. Fortunately the slow swaying keeps their contents from being thrown around too violently, making them among the safer places to be in a quake.

Japanese videos of the Tohoku earthquake and tsunami

So many videos of the March 11 M9.0 Tohoku earthquake and ensuing tsunami exist that collecting and disseminating them is a daunting task. In my three earlier posts on the subject I referred you to a variety of perspectives of both events (the quake and the tsunami). Since then plenty more clips have emerged, and continue to. I’ll make an attempt to guide you to a few of the more spectacular or interesting ones; the rest can be found through potentially endless browsing among the chain of “related videos” on YouTube.

The earthquake

The following surveillance video is one of the few that I’ve found recorded from a well-mounted stationary camera, removing the shakiness of handheld cameras that makes it difficult to discern real ground motion. As the camera remains strongly braced against the ceiling/wall, we can much more clearly see the motion of the contents of this building as it sways in the quake. The footage is from a 5th floor office in Koto Ward, Tokyo. Note the duration of shaking–over two minutes!–and pay attention to changes in the direction of shaking. The two most remarkable aspects of this footage are the variability in direction and intensity of shaking. The shaking we observe doesn’t reflect true ground motion, but rather the building’s response to that force from below. The building twists and wobbles as it’s shaken from beneath, perhaps adding to the fluctuating intensity of the shaking. The large source area of the earthquake (a huge patch of the subduction interface that ruptured) also contributes to the fluctuations in shaking as waves from early in the earthquake bounce around beneath the city and interfere with incoming waves from the later parts of the rupture. Shaking intensifies as it proceeds, likely representing both the interference of old and new waves and the gradual approach of the earthquake wave source southward as the rupture propagated down the coast, closer to this building.

The next clip was filmed outside and only coincidentally captured the quake, unlike most of the others in which people had enough time to run and grab their cameras.

Here a couple of young American tourists witnesses the quake in a Tokyo park. The rustling trees and alarmed birds provide a glimpse of quakes sans humans, while the warning siren adds an eerie gravity to the situation.

The tsunami

All of the videos of the tsunami I have seen are powerful, terrifying, and staggering to watch. Here are a few Japanese videos of it pouring into coastal harbors on the northeast coast, filmed from shore-front office buildings.

The most astounding part of these videos is how quickly the surge in sea level overtakes the seawalls and surges inland. It happens so quickly and easily it seems almost calm, belying the utter violence of torrents of water pouring into the city streets.

The tsunami easily overtakes the seawall in Shiogama, and the warning siren adds a chillingly stoic urgency to the unfolding disaster. “This is really happening” is the message I get.

Another video from just down the pier in Shiogama, where passenger ferries wait at a floating boarding platform. This video is filled with markers to indicate how high the water rises–watch especially the loading ramps and the bottom of the 2nd or 3rd floor balcony the videographer is standing on.

[Updated 4/18/11 10:35am]: Here’s a 3rd video of the tsunami overtopping a seawall, this time in Kesennuma port. The utter devastation wrought by the tsunami surge is plain to see here: it destroys absolutely everything in its path–everything you think it might wipe out from the beginning of the video… it does. This is how you end up with those scenes of utterly razed coastal towns. I can’t imagine this cameraman wasn’t utterly fearing for his life.

Below is yet another unbelievable video of the tsunami swelling into Kesennuma. This one is chock full of massive ships being swept through town, and like the other ones it shows the sea continue to flood inland to an astonishing depth.

This next video is a long one (~10 minutes), but well worth watching to see the full beginning of the tsunami unfold. There are fascinating eddies, currents, and surges to watch interact; you see how easily boats can get tossed around; and believe me, it doesn’t get truly intense until the end, even after you thought the worst was past. It’s like the first quake video above: don’t be fooled by the duration or waning intensity of the disaster at hand. In both cases it just gets worse and worse.

Finally two much shorter clips of the wave crashing ashore. The first I haven’t been able to find the raw clip of, but the news excerpt will suffice. The perspective is from the ground in what appears to be a similar area to–if not the very same–the region filmed so famously from the helicopter as the flaming black goo surged ashore. The clip of interest is short, and it’s merely of the wave cresting and crashing, but it is a formidable sight to see with its towering height:

Here watch the incredible volume of water surging forth as it’s launched over a breakwater and then speeds rapidly ashore:

The last video I want to share today serves as an incredible illustration of the age-old wisdom that a deep drawback of the sea forebodes a devastating surge back inward. Don’t go collect fish when you see the ocean get sucked out! Throughout this video there are various items for scale that give a staggering perspective on just how low the sea has dropped in advance of the inward surge of the tsunami:

I have hardly referred you to a fraction of the videos out there, so if you’re in awe of these there are plenty more to be found by clicking through the related videos on YouTube. As you watch these videos that bring you closer to the scene and evoke some vicarious experience of the terror, do consider the plight of those millions displaced; there are links to major aid organizations in the right-hand panel.

Unwarranted alarmism, and a hiatus for field work

There have been plenty of doomsday claims circulating the internet in the wake of the massive 9.0 “Tohoku” earthquake–lots about supermoons, some about dead fish, some merely about some alleged “pattern” of recent quakes circling the Pacific, a pattern generally conveniently selected to only include earthquakes that have hit the news. I won’t bother to give any of these claims too much credence by linking to them here, but they’re easy enough to come by.

Fortunately there are some scientifically grounded explanations as well, like this editorial by USGS seismologist Susan Hough.

The bottom line with claims about patterns of earthquakes and predictions for the near future is that for them to be of any value, they have to predict quakes better than random chance. In other words, it’s easy to “predict” that there will be a significant earthquake on the U.S. west coast–we have earthquakes here commonly! A prediction needs to be precise in space, time, and magnitude to be of any value beyond the forecasts we have developed already to understand earthquake hazard.

It is essential to be critical when you hear predictions that concern matters of such importance as whether you will be hit soon by a devastating earthquake. Two helpful guidelines to keep in mind are

1) If you live in a quake-prone area, you’ve had a pretty high risk of such earthquakes all along–is the prediction precise enough to make a difference in your preparation?

2) As Susan Hough points out, very few predictions precede large earthquakes, but plenty of large earthquakes precede predictions. Renewed awareness of quakes inspired by a devastating event tends to foster the anxiety that faux-predictions thrive on.

In that second vein, it’s important to remember how many earthquakes are going on around the world all the time. Most of the largest ones occur far out to sea, deep beneath the ocean’s surface. There’s nothing special at all about Christchurch’s 6.3 except that it hit directly below a city. There are scores of others like it every month. Don’t be fooled by “connections” like that; in fact it’s valuable to check the USGS’s “significant earthquakes” page for reminders of just how many large and/or devastating events have occurred within recent memory.

Other than that little spiel, there’s plenty I’d like to pass on regarding the information and news pouring forth from Japan, but I have limited time as I prepare for two weeks of field work in Mexico. Some other bloggers are doing a great job compiling information, and their posts are well worth mining for informative tidbits about the quake. Notably, the Shaking Earth has a collection of resources, and Chris Rowan at Highly Allochthonous posted an earthquake round-up with links galore.

There are some neat–if somewhat disconcerting–liquefaction videos coming out of Japan, but I’ll devote a later post to discussion of these.

I’m off to the borderland of northern Baja, Mexico for two weeks, to re-survey the escarpment left by surface rupture of the 7.2 earthquake there last Easter day (pictured in the banner of this blog). I’ll come back with pretty pictures and info about that fascinating rupture.

Myself and fellow UCD grad student Peter Gold surveying the fresh April 4, 2010 El Mayor-Cucapah earthquake rupture with a terrestrial LiDAR scanner last spring.

Animation of Japan quake traversing the U.S.

The data-consolidating institutional consortium IRIS–the Incorporated Research Institutions for Seismology–has a spectacular resource to visualize actual seismic waves propagating around the Earth, that everyone should check out.

Here is an animation they put together (they do this for every significant quake) displaying ground motion at recording stations set up around the U.S. You can see the dramatic passage of the seismic waves from the 8.9 quake, and you get glimpses of the waves that have circled the Earth from the other side and are passing back through heading back towards the epicenter. This is a visualization of actual recorded data–note the scale bar on the seismogram at the bottom: nearly a centimeter of ground motion all across the U.S.!

The data are from the “USArray,” a travelling deployment of seismometers meant not to detect earthquakes but to probe the interior of the Earth using seismic waves, like a giant ultrasound. Of course the array thoroughly detects earthquakes, but its density is overkill for the location and characterization of individual seismic events–that’s why it doesn’t matter that it’s migrating across the country.

Obviously right now the array predominantly occupies the great plains and is just entering the midwest. In this animation the amplitude of vertical motion is colored on a red-blue scale and the direction of motion at any given station is indicated in real time (well, sped up) as a little line directed outward from the station. It’s a pretty good way to visualize 3D ground motion if I do say so myself. The seismogram visible on the bottom is a representative one chosen near the center of the array (highlighted in yellow on the map). Those of you in the midwest–look how much was going on beneath your feet yesterday!

I highly encourage you to explore this site and the vast collection of Ground Motion Visualizations (GMVs) they have. Pick your favorite earthquake and watch it ripple through the U.S.!

Enjoy!

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