The Trembling Earth

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Earthquake-resistant engineering

The Best Earthquake Apps

Have you felt an earthquake while sitting at home or in the office, and wanted to be the first to know how big it was and where it was centered? Do you have relatives who live in earthquake country, and would like to know immediately when a big earthquake strikes and swamps the phone lines? If you live in Mexico or Japan, apps already exist that can warn you when your local seismic network detects strong shaking, affording you precious seconds to scramble to shelter. For the rest of us, the best we can do for now is figure out the when, where, and how big as or after the shaking subsides. The quickest way to find out the deets on an earthquake is through your mobile phone.

The world of mobile software abounds with apps designed to bring real-time earthquake information to their users around the globe. There are apps that ______________ With so many options, and so many $0.99 bills to pay, you’ll obviously want to make an educated decision. I’ve dedicated this post to a personal summary, review, and ranking of all the quake-related apps I could get my hands on, shelling out those hefty sums on my own so that you don’t have to. Others have done this before me, and I encourage you to evaluate each of our opinions before you take the plunge… or you could just go by mine, since obviously it’s the one you’ve stumbled upon and the point is to reduce the amount of decision-making you have to do, right? Right. So without further ado, here’s my ranking:

East Coast (U.S.) earthquakes – What gives?

On Tuesday afternoon, August 23, 2011, people up and down the United States’ East Coast–and as far inland as Chicago–felt an unmistakable tremor. At first, of course, the sensation was not so unmistakable: “earthquake” was not the first explanation that came to mind, especially for the explosion-conscious residents of New York and D.C. mere days before the 10th anniversary of the Sept 11 terrorist attacks, nor for Americans (and Canadians!) on the periphery of the quake’s perceptible shaking, where people were more likely to think they were simply nodding off to sleep or getting dizzy after a heavy lunch. For the tens of millions of people within hundreds of miles of the epicenter, however, this respectably sized M5.8 earthquake was quickly recognized for what it was, surprising as it may have been for many.

Check out this post I compiled, which is chock full of videos from the quake.

The quake immediately became big news nationally, and while East Coasters evacuated buildings, reeling in surprise and alarm, West Coasters took advantage of what they saw as ample opportunity to chide their uptight counterparts for not being laid back enough to deal with a measly 5. Some of it was rather snide. Let me remind West Coasters at this point that when a M5.5 hit Los Angeles in the summer of 2008, buckling subterranean water lines, breaking ceiling sprinkler pipes at LAX, and collapsing brick walls and chimneys throughout the region, the news coverage lasted for days. The startling “little ones” get ya every time. So don’t pretend you’re above it; it was a 5.8!

In the days and weeks following the earthquake (I was in China, blocked from my blog!) several specific questions dominated the discourse:

1) Why did an earthquake happen on the East Coast, away from the active plate boundary out west?

2) Why was this earthquake, apparently considered so puny by Californians, felt over such a great distance?

and, as with most earthquakes 3) Will this happen again? i.e., Should we be worried?

This generalized geologic map of the epicentral region was drafted by Chuck Bailey at the College of William and Mary, showing ancient tectonic structures in the bedrock that may be responsible for this year's tremor.

Some of these questions have been thoroughly answered by articles and blog posts, but I think they’re worth revisiting for clarification.

1)Why did this happen on the East Coast?

This first question is generically answered by the USGS as it is with every quake east of the rockies. Professor Callan Bentley, on the other hand, quite thoroughly explains the geologic and tectonic context of this earthquake on his blog Mountain Beltway. His post is well worth reading if you’re curious about how this quake happened or how others on the east coast might. The gist is that ancient faults that formed as the ancestral Appalachians were pushed up have been gently posthumously reactivated. They’re far from becoming prevalent active structures, but any imperfection in the crust is liable to slip a little given the right stresses.

2) Why was this earthquake felt so far? What does “Old and Cold” have to do with anything?

Perhaps the most conspicuous feature of this anomalously noteworthy eastern quake was the radius over which is was felt. People thousands of miles apart were jostled from below all at once, by a meager 5.8 quake that wouldn’t grab attention for more than a couple hundred miles in California. Popular observation on this front has indeed identified a real phenomenon: earthquakes are felt for longer ranges on the East Coast, or to put it more technically, seismic waves attenuate more gradually there.

For comparison, the Did-You-Feel-It map of the 5.8 Virginia quake is displayed at the SAME SCALE as that of a 6.0 in California in 2004. Note that the radius of perceptible shaking in the West Coast's 6.0 is basically the same as the radius of "moderate" shaking and "light" damage caused by a smaller quake on the East Coast.

You have probably heard this phenomenon explained by describing the crust beneath the East Coast as “old and cold” (and by implication the western crust as young and hot, make of that what you will), but in all likelihood that was the extent of the explanation you got. The crust is older and colder so seismic waves travel farther. To me, this is only a partial answer. Age and temperature of the Earth’s crust are certainly correlated to how far seismic waves travel, but why so? What is the actual mechanism by which the age and temperature/density of the crust affect the passage of seismic waves? Did anybody else’s curiosity bells ring when they got this incomplete explanation?

What does “old and cold” mean anyway? We consider the continental crust of the western U.S. to be “young” because it is actively (i.e., currently) deforming. As the Pacific and North American tectonic plates grind past (or into) each other, they drag and crumple one another. This generates cracks (faults), wrinkles (folds), and frictional heat, the manifestations of which are earthquakes, surface deformation, and generally warmer crust. On the other side of the continent, the crust hasn’t been beat up like that for hundreds of millions of years, so the faults and folds that formed even before dinosaurs roamed the continent have had plenty of time to seal shut with minerals and pressure. The heat generated during the formation of the ancestral Appalachian mountains has long since diffused away. Hence, it’s been a long time since anything substantially moved over there, and it’s been cooling the whole time: old and cold.

Okay, but what does that have to do with seismic waves and how far earthquakes are felt? Based on the above, we know that the eastern crust is dense, solid, and intact. The western crust is riddled with faults and is warmer (meaning it’s less dense and more pliable). If you have something full of cracks, a force on one side of it will be much harder to transmit to the other side than a force on something solid. Imagine aluminum window blinds versus, say, an aluminum baking sheet.

Ah, the old blinds-versus-cookie sheet comparison. Appropriately enough, aluminum is one of the most abundant elements in the Earth's crust!

If you jostle the bottom of the blinds, the wave you create will make it some way towards the top before it dies out because energy is lost in the interface between each blind. If you apply the same action to a cookie sheet, the whole thing moves at once. Basically, this illustrates the difference between the fractured west coast and the coherent east coast. As one side of an active fault moves, the fault can slip so that the motion of one side is not transmitted across to the next, just like the blinds separated by strings. Any motion approaching faults is stunted because the interface doesn’t represent a solid connection.

The same thing goes on at a microscopic scale within rocks. As a wave of deformation passes through a rock body, small imperfections in mineral grains can migrate, close, or slip, absorbing the incoming deformation. Imagine pushing on one end of a large sponge versus pushing on one end of a brick. The other side of the brick will move, whereas the sponge will absorb your force by deforming internally.

Don't read too much into it.

The imperfections that allow this internal deformation are far more pervasive in the deforming, fractured west coast rock, so there are more of them to absorb the energy of seismic waves. Furthermore the migration, closure, and slip of these features is enabled by higher temperatures. High temperatures are the manifestation of energy stored within a material by vigorously vibrating atoms. The more vigorously atoms vibrate, the easier it is to knock them out of their position and into a new one. Thus, higher temperatures enable greater internal movement of atoms (deformation), which in turn can absorb the energy imparted by seismic waves the way a sponge absorbs your push on it by moving internally.

This analogy is incomplete, since the continental crust of the western U.S. isn’t simply full of gaps and holes that can be compressed like a sponge. Internal shearing is prevalent, however, in a way that the weak sponge helps illustrate. Perhaps imagine the crust more like a hockey puck versus a stack of coasters. Push the top towards one side or shake the top back and forth, and whereas the whole puck will move coherently, the stack of coasters will lean over by sliding along each interface, thereby absorbing your push.

Third time's the charm.

So, as seismic waves ripple outward from an earthquake, their movement is dampened by every imperfection and every crack they cross, until finally they’re so small as to be imperceptible. This happens very quickly in actively deforming, broken up regions, but has a very limited effect in regions that lack serious discontinuities in the rock.

3) Should we be worried?

One of the most important lessons highlighted by this earthquake is that anyone anywhere may be at risk of earthquakes. This is indeed a valuable thing to keep in mind, considering even approximately similar “repeats” of eastern North America’s historic earthquakes would be unbelievably devastating. Most people are vaguely familiar with these historic events (some huge earthquake in Missouri in 1811), but I encourage you to review all of these fascinating earthquakes through the USGS’s info pages about them. I’ll leave the full story of the New Madrid quakes for another post–they’re a remarkable series of quakes, and there’s been much contention regarding their magnitudes and their potential for repetition. For now, I’ll leave you to ponder the consequences of either of these–or anything like them–happening in this generation.

New Madrid, MO earthquakes of 1811-1812: http://earthquake.usgs.gov/earthquakes/states/events/1811-1812.php

Charleston, SC earthquake of 1886: http://earthquake.usgs.gov/earthquakes/states/events/1886_09_01.php

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.

Tsunami misconceptions

Exactly one year ago today we (some of us) sat glued to our TVs as the tsunami generated by the gargantuan 8.8 Chilean earthquake barreled into bays around the Pacific Ocean. This is what we saw:

 

Many people found that anti-climactic, especially in comparison to this famous video of the 2004 Indian Ocean tsunami pouring into Patong Beach, or this dramatic one of the ’04 tsunami crashing ashore at another resort hotel on Phuket, or this disturbing video of the same surge tearing through a hotel restaurant. I, however, found the Hawaii video marvelously awesome, and rather less distressing to boot. I prefer videos of 2010’s rather “anti-climactic” and largely forgotten tsunami because their lack of flashy, splashy drama allows a more nuanced view that reveals the impressive power underlying these waves.

Tsunamis tend to be conceptualized incorrectly by newcasters, artists, and even eye-witnesses. They differ substantially from regular ocean waves, and are thus not well characterized simply as giant versions of their every-day cousins. I myself had difficulty imagining their nature until the eye-opening collection of tourists’ videos emerged documenting the tremendous 2004 Indian Ocean tsunami. Since then YouTube has enabled us to witness nearly every major tsunami that has crashed ashore somewhere in the world, and the videos lend substantial aid to our imaginations in understanding what a tsunami really is.

A tsunami occurs when a large portion of the ocean floor moves–imagine the water in a bathtub when you shift your body from one side to the other. Although submarine landslides may generate tsunamis, more commonly the giant displacements caused by undersea earthquakes heave immense volumes of water upward to spawn these waves. When a fault slips beneath the ocean it lifts a huge column of water by a few to a few tens of meters. This water then has to settle its newly gained gravitational potential and equilibrate back to global sea level. To move back downward, some of it must be displaced outward to account for the new protrusion of land caused by the earthquake below. The energy of the displaced water is transmitted rapidly through the ocean, shoving other water out of the way laterally and rolling along as a large area of elevated sea level.

Whereas regular, wind-driven ocean waves are merely surface disturbances, excited by moving air pushing gently over large areas of the ocean and extending no deeper than a few meters, tsunamis involve motion of the entire water column, top to bottom. As all that moving water approaches a shallowing shore (and especially one where the coast line funnels it to a point), it piles up, growing taller and slower until the leading edge of it begins to crest like an ordinary wave. The difference is the volume of water involved. Because tsunamis are generated by the rapid uplift of entire swaths of the ocean floor, their areal extent is huge. Behind even a modest-height crest are literally kilometers of water at that same elevation, ready to pour up onto shore. In effect we perceive tsunamis as large areas of elevated sea level, like a rapid shift in the tide, not like a steep short wave that merely reaches great heights. Once a tsunami hits, it continues pouring inland, as you can see in all of these videos, until the trough of the multi-kilometer-wavelength wave arrives and the water can finally drain back out to sea.

In the milder videos from the much smaller Chilean tsunami, this effect is very apparent, especially in time lapse. The relative calm with which the tsunami arrives belies the massive force driving it as this humongous ripple inexorably laps up over shorelines around the globe.

As the Chilean tsunami arrived at the northern California coast it merely appeared as a rapid decrease in the period of the tides, with an ebb and a rise accentuated by the narrow geometry of the harbor:

 

The rapid rise and dramatic drawback is even apparent along the wide, flat beach at Santa Monica Pier as the tsunami sweeps past Los Angeles:

 

 

Tsunamis are not predominantly impressive nor dangerous because of their height, but because of their width, or wavelength.

There you have it. I encourage you all to peruse NOAA’s National Center for Tsunami Research website (also in sidebar at right) for some great graphics and model animations of recent and historic tsunamis, like this map of modeled wave amplitude in the Feb 27-28 Chile tsunami last year.

 

Measured and modeled tsunami wave heights from the 2010 M8.8 Chile earthquake, from NOAA / PMEL / Center for Tsunami Research.

Earthquake Basics

Before diving into the wealth of fascinating earthquake phenomena I plan to share with you, I’ll give an extremely brief basic overview of what earthquakes are and why they happen, so that we can all be on the same page.

Let’s start with the big picture: The Earth is a big rocky (and rather metallic) body, like a number of others in our solar system. Within this big orb of solid rock (and both liquid and solid metal), temperature and pressure generally increase with depth toward the center, partially because the original heat from Earth’s formation is insulated within its own massive rocky body.

Solid materials behave differently as their temperatures increase, becoming more pliable with increasing heat. The outside of our rocky orb is cold, crisp, and brittle–this is the Earth’s cracking crust, which we all are the most familiar with because it’s what we live on. Deeper within the Earth, the solid rock is hot enough that over huge distances and long spans of time it can be deformed like silly putty or very thick syrup. As heat from the center of the Earth radiates outward, the solid rocky “mantle” convects like a lava lamp. Heated material is less dense than cold material, so it rises, and vice versa. Over hundreds of millions of years the solid interior of the Earth churns, dragging the cold brittle crust with it. Imagine re-boiling a pot of stew that has formed a cooling skin. (Although the specifics of the manner in which the crust and mantle interact are the subject of ongoing research and controversy, that relationship is beyond the scope of this blog. For our purposes lateral motion of the crust is inherently related to convective motion of the mantle beneath it.)

The mantle’s churning and dragging breaks the brittle crust up into small pieces that grind together, spread apart, or override each other. These “small pieces” are the infamous tectonic plates, atop one [or two] of which you live. While at their centers the tectonic plates are all moving in different directions from each other, their edges are pressed together by the immense weight of rock and so they stick together in the same way that a dresser “sticks to” a carpeted floor, making harder to drag than to lift it up and transport free of carpeted friction. Eventually, of course, the centers of the plates have moved far enough relative to one another that their edges need to catch up. That’s when earthquakes happen.

The cold brittle crust is elastic, meaning that–just like your underwear waistband–it will deform a finite amount in response to an applied force, but once the force is let go it snaps back into place. So, as the rough edges of tectonic plates are held back by the adjacent plate(s), they can only withstand so much stretching before they have to snap back into shape, which they generally do catastrophically. This “elastic recovery” occurs along faults, giant cracks that penetrate the crust, colloquially known as “earthquake faults” or “fault lines”, a few of which are pictured in this blog’s header.

Plate edges are generally a crumpled, fractured mess, from millennia of being dragged along the ragged edges of their neighbors. Many of these fractures are faults, capable of slipping to produce a “fault rupture”, which radiates giant elastic waves as one side of the crack slides along the other. Imagine snapping a twig or a pencil; it bends before it breaks, but once it cracks you hear a loud snap and feel it sharply jolt your fingers. Those are compressional and elastic waves shooting through the air and the pencil: a tiny little analog to an earthquake, just like static shocks are tiny little analogs to lightning.

Every earthquake represents the elastic waves emanated from a small (or large) patch of a fault rupturing and slipping to recover from built-up strain, which is driven by motion of the brittle tectonic plates over a warm, slowly churning solid mantle. In each rupture, the brittle crust we live on deforms just a bit, but a finite amount. Every quake results in finite deformation, which accumulates over time to shape the surface of the planet. We’ll get to that in a later post. Large earthquakes can move the land surface by tens of meters within just a few seconds, forever altering the landscape. Small earthquakes barely affect the surface, moving a tiny patch of a fault by sometimes microscopic amounts. We’ll get to earthquake sizes and magnitudes some other time as well.

So that was your primer on earthquakes. Comments, questions, or clarifications are welcome.

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