A perilous beauty

Ever heard of the "Bonneville Slide?"

It sounds like some obscure country line dance, but the real story is more interesting, and it comes with a connection to a curious Native legend that turns out to refer to a real historical event.

The Klickitat People have lived for centuries on both sides of the Columbia River, up into what is now Skamania and Klickitat Counties, Washington, and down into Multnomah and Clackamas Counties, Oregon.  They tell the tale of Pahto and Wy'east, the two sons of the chief of all the gods, Tyhee Saghalie.  The two young men did not get along, and fought over who would rule over which parcel of land.  Their father shot one arrow south and the other north; Pahto was given the lands around where the northern arrow landed, and Wy'east the territory surrounding where the southern arrow fell to the ground.  Tyhee Saghalie then shook the Earth and created a great bridge across the Columbia River so the two could visit each other.

But soon trouble broke out again.  Pahto and Wy'east both fell in love with the same young woman, the beautiful Loowit, and began to fight, burning villages and destroying forests and crops.  Tyhee Saghalie tried to reason with them, but to no avail.  In the end he grew angry himself and shook the Earth again, destroying the bridge; the cataclysm created a flood that washed away whole forests.  He turned all three into mountains -- Wy'east became Mount Hood, Pahto Mount Adams, and the lovely Loowit Mount Saint Helens.  But even in mountain form they never forgot either their anger or their burning love, and all three still rumble and fume to this day.

What is fascinating is that this odd story actually appears to have some basis in fact.

In around 1450 C.E., an earthquake knocked loose about a cubic kilometer of rock, soil, and debris from Table Mountain and Greenleaf Peak.  The resulting landslide -- the Bonneville Slide  -- roared down the Columbia Gorge, creating a dam and what amounted to a natural bridge something like sixty meters high across one of the biggest rivers in the world.  

Greenleaf Peak today [Image licensed under the Creative Commons Eric Prado, Greenleaf Peak, Washington, CC BY-SA 4.0]

The dam couldn't last, however.  The Columbia River has a huge watershed, and the lake that built up behind the dam eventually overtopped the natural "Bridge of the Gods."  The whole thing collapsed -- probably during a second earthquake -- releasing all that pent-up river water in a giant flood.  It left behind geological evidence, both in the form of a layer of flood-damaged strata west of the slide, and the remains of drowned forests to the east, where trees had died as the dammed lake rose to fill the gorge.

Despite the reminder we got in 1980 -- with the eruption of Mount Saint Helens -- it's easy to forget how geologically active the Pacific Northwest is.  Not only is there the terrifying Cascadia Subduction Zone just offshore (about which I wrote two years ago), the other Cascade volcanoes, from Silverthrone Caldera (British Columbia) in the north to Lassen Peak (California) in the south, are still very much active.  Right in the middle is the massive Mount Rainier, visible from Seattle, Tacoma, and Olympia on clear days, which is one of the most potentially destructive volcanoes in the world.  Not only is it capable of producing lava and pyroclastic flows, it's capped by huge glaciers that would melt during an eruption and generate the catastrophic mudflows called lahars.  The remnants of two historical flows from Rainier -- the Osceola and Electron Lahars -- underlie the towns of Kent, Orting, Enumclaw, Puyallup, Auburn, Buckley, and Sumner, and in some places are twenty to thirty meters deep.

The Earth can be a scary, violent place, but somehow, humans manage to survive even catastrophic natural disasters.  And, in the case of the Bridge of the Gods, to incorporate them into our stories and legends.  Our determination to live in geologically-active areas is due to two things; volcanic soils tend to be highly fertile, and we have short memories.  Fortunately, though, we couple what seems like a foolhardy willingness to take risks with a deep resilience -- allowing us to live in places like the Cascades, which are bountiful, and filled with a perilous beauty.

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The bellringer

Between December 16, 1811 and February 7, 1812, a series of four earthquakes -- each estimated to be above magnitude 7, with the first and last perhaps at magnitude 8 -- hit what you might think is one of the most unlikely places on Earth; southeastern Missouri.

The centers of continents are ordinarily thought to be tectonically stable, as they are generally far from any of the three typical sorts of faults -- divergences, or rifts, where plates are moving apart (e.g. the East African Rift Zone); convergences, or thrust faults, where plates are moving together (e.g. the Cascadia Subduction Zone); and strike-slip faults, where plates are moving in opposite directions parallel to the fault (e.g. the San Andreas Fault).  The Midwest is located in the middle of the North American Craton, an enormous block of what should, according to the conventional wisdom, be old, stable, geologically inactive rock. 

But the 1811-1812 earthquake series happened anyhow.  If they'd occurred today, it would likely have flattened the nearby city of Memphis, Tennessee.

So much for conventional wisdom.

The fault responsible was named the New Madrid Seismic Zone for the county right in the center of it, and its capacity for huge temblors is staggering.  The biggest (and final) earthquake of the four was powerful enough that it was felt thousands of kilometers away, and rang church bells in Charleston, South Carolina.  The shift in terrain changed the course of the Mississippi River, cutting off a meander and creating horseshoe-shaped Reelfoot Lake.

So what created a seismic zone where one shouldn't be?

[Image is in the Public Domain courtesy of the USGS]

The topic comes up because I just finished reading seismologist Susan Elizabeth Hough's excellent book Earthshaking Science: What We Know (and Don't Know) About Earthquakes, which is one of the best laypersons' introductions to plate tectonics and seismicity I've come across.  She devotes a good bit of space to the New Madrid earthquakes, and -- ultimately -- admits that the answer to this particular question is, "We're still not sure."  The problem is, the fault is deeply buried under layers of sediments; current estimates are that the hypocenter (the point directly underneath the epicenter where the fault rupture occurred) is between fifteen and thirty kilometers beneath the surface.  And since the quakes in question happened before seismometers were invented, we're going off inferences from written records, and such traces that were left on the surface (such as "sand blows," where compression forces subsurface sand upward through cracks in the stratum, and it explodes through the surface).

As far as the cause, Hough has a plausible explanation; the New Madrid Seismic Zone is an example of a failed rift, where a mantle plume (or hotspot) tried to crack the continent in half, but didn't succeed.  This stretched the plate and created a weak point -- called the Reelfoot Rift -- where any subsequent stresses were likely to trigger a rupture.  Since that time, the North American Plate has been continuously pushed by convection at the Mid Atlantic Rift, which is compressing the entire plate from east to west; those stresses cause buckling at vulnerable points, and may well have been the origin of the New Madrid earthquakes.

One puzzle, though, is what happened to the hotspot since then.  This is still a matter of speculation.  Some geologists think that friction with the rigid and (relatively) cold underside of the plate damped down the mantle plume and ultimately shut down convection.  Others think that as the North American Plate moved, it simply slid off the hotspot, making the plume appear to move eastward (when in actuality, the plate itself was moving westward).  This may be why another anomalous mid-plate earthquake zone is in coastal South Carolina, and it might also be the cause of the Bermuda Rise.

That point is still being debated.

Another open question is the current risk of the fault failing again.  There's paleoseismic data suggesting major earthquake sequences from the Reelfoot Rift/New Madrid Seismic Zone in around 900 and 1400 C.E., suggesting a timing between events of about four to five hundred years.  But these are estimates themselves, and I probably don't need to tell you that earthquake prediction is still far from precise.  Faults don't fail on a schedule -- which is why it annoys me every time I see someone say that an area is "overdue for an earthquake," as if they were on some kind of calendar.

Still, I can say with at least moderate confidence that it's unlikely to generate another big earthquake soon, which is kind of a relief.

So that's our geological curiosity of the day.  I have a curious family connection to the area; my wandering ancestor Sarah (Handsberry) Overby-Biles-Rulong (she married three times, had nine children, and outlived all three husbands) lived in the town of New Madrid in 1800, after traveling there from her home near Philadelphia as a single woman in the last decade of the eighteenth century.  I've never been able to discover what impelled her to leave her home and, with a group of relative strangers, cross what was then trackless wilderness to a remote outpost, and I've often wondered if she might have been either running away from something, or perhaps might have been a prostitute.  I'm not trying to malign her memory; it bears mention that a good eighty percent of my forebears were rogues, ne'er-do-wells, miscreants, and petty criminals, so it would hardly be a surprise to add prostitution to the mix.  And whatever else you can say about my family members, they were interesting.  I've often wished I could magically get a hold of Sarah's diary.

In any case, Sarah was in Lafayette, Louisiana by 1801, so she missed the New Madrid earthquakes by ten years.  But kind of interesting that she lived for a time in the little village that was about to be the epicenter of one of the biggest earthquakes ever to hit the continental United States, one that rang bells thousands of kilometers away, and which created a geological mystery the scientists are still trying to work out.

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The cliff's edge

The universe is a dangerous place.

Much of what we've created -- the whole superstructure of civilized life, really -- is built to give us a sense of security.  And it works, or well enough.  During much of human history, we were one bad harvest, one natural disaster, one epidemic from starvation, disease, and death.  Our ancestors were constantly aware that they had no real security -- probably one of the main drivers of the development of religion.

The world is a capricious, dangerous place, but maybe the gods will help me if only I pray hard enough.

When the Enlightenment rolled around in the eighteenth century, science seemed to step in to provide a similar function.  Maybe the world could be tamed if we only understood it better.  Once again, it succeeded -- at least partially.  Industrial agriculture and modern medicine certainly saved millions of lives, and have allowed us to live longer, healthier lives than ever before.  Further reassuring us that it was possible to make the universe a secure, harm-free place for such creatures as us.

And we still have that sense, don't we?  When there's a natural disaster, many people respond, "Why did this happen?"  There's an almost indignant reaction of "the world should be safe, dammit."

[Image licensed under the Creative Commons svantassel, Danger Keep Away Sign, CC BY-SA 3.0]

This is why in 2012 a judge in Italy sentenced six geologists to six years in prison and a hefty fines for failing to predict the deadly 2009 L'Aquila earthquake.  There was the sense that if the best experts on the geology of Italy didn't see it coming... well, they should have, shouldn't they?  

That in the present state of our scientific knowledge, it's not possible to predict earthquakes, didn't seem to sway the judge's mind.  "The world is chaotic, dangerous, and incompletely understood" was simply too hard to swallow.  If something happened, and people died, there had to be someone to blame.  (Fortunately, eventually wiser heads prevailed, the charges were thrown out on appeal, and the geologists were released.)

In fact, I started thinking about this because of a study out of the University of California - Riverside that is investigating a technique for predicting earthquake severity based on the direction of propagation of the shock wave front.  This can make a huge difference -- for example, an earthquake on the San Andreas Fault that begins with failure near the Salton Sea and propagates northward will direct more energy toward Los Angeles than one that begins closer in but spreads in the opposite direction.

The scientists are using telltale scratch marks -- scoring left as the rocks slide across each other -- to determine the direction of motion of the quake's shock wave.  "The scratches indicate the direction and origin of a past earthquake, potentially giving us clues about where a future quake might start and where it will go," said Nic Barth, the paper's lead author. " This is key for California, where anticipating the direction of a quake on faults like San Andreas or San Jacinto could mean a more accurate forecast of its impact...  We can now take the techniques and expertise we have developed on the Alpine Fault [in New Zealand] to examine faults in the rest of the world.  Because there is a high probability of a large earthquake occurring in Southern California in the near-term, looking for these curved marks on the San Andreas fault is an obvious goal."

The thing is, this is still short of the ultimate goal of predicting fault failure accurately, and with enough time to warn people to evacuate.  Knowing the timing of earthquakes is something that is still out of reach.

Then there's the study out of the Max Planck Institute for Solar System Research that found that the Sun and other stars like it are prone to violent flare-ups -- on the average, once every century.  These "superflares" can release an octillion joules of energy in only a few hours.

The once-every-hundred-years estimate was based on a survey of over fifty-six thousand Sun-like stars, and the upshot is that so far, we've lucked out.  The last serious solar storm was the Carrington Event of 1859, and that was the weakest of the known Miyake Events, coronal mass ejections so big that they left traces in tree rings.  (One about fourteen thousand years ago was so powerful that if it occurred today, it would completely fry everything from communications satellites to electrical grids to home computers.)

The problem, once again, is that we still can't predict them; like earthquakes, we can know likelihood but not exactitude.  In the case of a coronal mass ejection, we'd probably have a few hours' notice -- enough time to unplug stuff in our houses, but not enough to protect the satellites and grids and networks.  (If that's even possible.  "An octillion joules" is what is known in scientific circles as "a metric shit tonne of energy.")

"The new data are a stark reminder that even the most extreme solar events are part of the Sun's natural repertoire," said study co-author Natalie Krivova.  "During the Carrington event of 1859, one of the most violent solar storms of the past two hundred years, the telegraph network collapsed in large parts of northern Europe and North America.  According to estimates, the associated flare released only a hundredth of the energy of a superflare.  Today, in addition to the infrastructure on the Earth's surface, especially satellites would be at risk."

All of this, by the way, is not meant to scare you.  In my opinion, the point is to emphasize the fragility of life and of our world, and to encourage you to work toward mitigating what we can.  No matter what we do, we'll still be subject to the vagaries of geology, meteorology, and astrophysics, but right now we are needless adding to our risk by ignoring climate change and pollution, and encouraging the ignorant and ill-founded claims of the anti-vaxxers.  (Just yesterday I saw that RFK Jr., who has been nominated as Secretary of the Department of Health and Human Services, is pursuing the de-authorization of the polio vaccine -- an extremely low-risk preventative that has saved millions of lives.)

Life's risky enough without adding to it by listening to reckless short-term profit hogs and dubiously sane conspiracy theorists.

My point here is that the chaotic nature of the universe shouldn't freeze us into despairing immobility; it should galvanize us to protect what we have.  The unpredictable dangers are a fact of life, and for most of our evolutionary history we were unable to do much about any of them.  Now, for the first time, we have figured out how to protect ourselves from many of the risks that our ancestors faced every day.  How foolish do we as a species have to be to add to those risks needlessly, heedlessly, rushing toward the edge of the cliff when we have the capacity simply to stop?

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Make a little noise

Sometimes, you can mislead people not only by what you say, but by what you leave out.

Take, for example, the "Moodus noises," that have been reported for centuries near the village of Moodus, Connecticut, in the town of East Haddam.  The sounds themselves are real enough; in fact, the village's name comes from the Algonquian matchitmoodus, which translates to "place of noises."  Rumblings and deep booms are frequent, especially in the vicinity of nearby Mount Tom, and were apparently part of the inspiration for H. P. Lovecraft's terrifying short story "The Dunwich Horror":

No one, even those who have the facts concerning the recent horror, can say just what is the matter with Dunwich; though old legends speak of unhallowed rites and conclaves of the Indians, amidst which they called forbidden shapes of shadow out of the great rounded hills and made wild orgiastic prayers that were answered by loud crackings and rumblings from the ground below...  Noises in the hills continue to be reported from year to year, and still form a puzzle to geologists and other physiographers.  Other traditions tell of foul odors near the hill-crowning circles of stone pillars, and of rushing airy presences to be heard faintly at certain hours from stated points at the bottom of the great ravines; while still others try to explain the Devil's Hop Yard -- a bleak, blasted hillside where no tree, shrub, or grass-blade will grow.

Which is pretty damn atmospheric, you have to admit.

[Image licensed under the Creative Commons Reuben C. Dodd - DeviantArt - Facebook, The Dunwich Horror - "Wilbur Whateley's Twin" by Reuben C. Dodd, CC BY-SA 3.0]

Interestingly, not only was Lovecraft springboarding off a real phenomenon of subterranean noises; the Devil's Hop Yard is also a real place, but it's not as eerie as Lovecraft would have you believe.  In fact, it's pretty enough that it was set aside as a state park, and as far as its diabolical name, no one's quite sure where it came from.  One theory is that a brewer who lived there was named Dibble, and the locals thought using the name for his hop fields was an amusing pun.

Of course, Lovecraft was writing fiction, and actually, he himself was not at all superstitious.  When fans wrote him letters asking for the directions to Dunwich or Arkham or Innsmouth -- or, worse, said they'd been there and wanted to tell him all about it -- he'd respond with admirable patience, "None of those are real places.  I know that for certain, you see, because I made them up."  But the fact remains that the Moodus noises are quite real, even if he and others spun fictional tales around them.  So what are they?

There are dozens of websites and books and YouTube videos claiming that they're supernatural in origin -- citing Native or early colonial legends but not going any further.  They often quote the passage from Charles Skinner's Myths and Legends of Our Own Land:

It was finally understood that Haddam witches, who practiced black magic, met the Moodus witches, who used white magic, in a cave beneath Mount Tom, and fought them in the light of a giant carbuncle [ruby] that was fastened to the roof...

If the witch-fights were continued too long the king of Machimoddi, who sat on a throne of solid sapphire in the cave whence the noises came, raised his wand: then the light of the carbuncle went out, peals of thunder rolled through the rocky chambers, and the witches rushed into the sky.

Most of the paranormal-leaning sources claim the area is haunted -- either by demons, or nature-spirits, or the ghosts of dead humans (or some combination).  They claim that there's a grand mystery still surrounding the place; you'll frequently see phrases like "no good explanation" and "unexplained phenomenon" and "scientists are baffled" (given the frequency of this one, you'd think scientists do little more than shrug their shoulders in helpless puzzlement all day long).  What these books, articles, and websites conveniently leave out is that in fact, a cogent scientific explanation for the Moodus noises was published by a geologist named Elwyn Perry...

... all the way back in 1941.

Perry proposed -- and the explanation has borne up under scrutiny -- that the Moodus noises are caused by minor seismic activity.  The area around Moodus is prone to earthquake swarms, despite its being far from obvious active fault lines.  In the 1980s there were four separate clusters of small quakes, numbering more than one hundred temblors in all, accompanied by a corresponding upswing of reports of booming and rumbling noises, and another swarm occurred in 2011.  Later studies found that the culprit is the Lake Char Fault, the subterranean suture line of a terrane (a microcontinent that ends up welded to a larger land mass) that stuck to North America during the lockup of Pangaea 250 million years ago.  The boundary was a weak spot when the Atlantic Ocean opened, and the tensional stress of rifting is still being released as the land settles.

So there's a completely natural explanation for the Moodus noises, however reluctant some people are to say so.  In a way, I get it; there's a certain frisson you get from accounts of orgiastic rites and conjuring evil spirits from underground caverns, that "it's a geologic fault zone and what you're hearing are small, shallow earthquakes" simply doesn't provide.

But predictably, I'd much rather know the real answer, and if I want to scare myself, I'll just read "The Dunwich Horror."  As far as the supernatural explanations, I tend to agree with journalist/skeptic Carrie Poppy: "We use these as stopgaps for things we can't explain.  We don't believe them because of evidence, we believe them because of a lack of evidence."

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The sleeping dragon

When most people think of seismically-active regions, Bangladesh is not ordinarily near the top of the list.

Cyclones, sure.  They come roaring up the Bay of Bengal with a horrifying regularity, and most of the country is low enough in elevation that the storms barely even slow down.  The worst was the 1970 Bhola cyclone, which still holds a record as the deadliest storm in recorded history.  The official death toll was five hundred thousand, but is likely higher than that, mostly people who lived in the lowlands near the city of Chittagong.

Unfortunately for the citizens of Bangladesh, though, they're also at high risk for earthquakes -- something that has only been recognized recently.  A 2021 study led by Muhammad Qumrul Hassan of the University of Dhaka found that the region is right on top of the junction of three different tectonic plates, the Eurasian Plate, the Indian Plate, and the small Burma Plate ("small" here means geographic area, not capacity for damage -- the devastating 2004 earthquake and tsunami was caused by a slippage of the Burma Plate relative to the Indian Plate).  But the compression and twisting of the land near the junction has created enough stresses that the entire country is crisscrossed with faults, most notably the Dauki Fault and the Haflong Thrust (which crosses into the Indian states of Meghalaya and Assam to the north).

The whole thing is exceedingly complex, and still poorly understood.  Imagine laying a sheet of pie crust on a table, and you and two friends each stand around it and push, pull, or twist it from the edge.  The sheet will wrinkle, tear, and hump up in places, but exactly where those deformations will end up isn't easily predictable because it depends on where there was weakness in the dough before you started messing with it.  This is the situation with the chunk of the Earth's crust that underlies Bangladesh.  Add to that the fact that the region is poor, and much of it is jungle- or swamp-covered and pretty inaccessible to study, and you have a picture of the extent to which we don't understand the situation.

However -- alarmingly -- a 2016 study found that the entire region has been building up stress for at least four hundred years, meaning when the some piece of fault slips, it's likely to be catastrophic.

The whole topic comes up because of a rather terrifying discovery that was the subject of a paper this week in Nature Communications.  Geoscientists Elizabeth Chamberlain (of Wageningen University). Michael Steckler (of Columbia Univeristy), and colleagues were studying a puzzling historical shift in the channel of the Ganges River, and quite by accident -- it was in an area some locals were digging in to create a pond -- they saw the unmistakable signs of seismites.  These are features in rock layers created by massive earthquakes, in this case a column of sand that had erupted through pre-existing strata during a colossal temblor.  Upon analysis, they found that the river had changed course because of a massive earthquake about 2,500 years ago.

Imagine an event big enough to shift the path of a river that size.

A change in the course of a river is called an avulsion, and it normally takes decades or centuries.  (It's an avulsion of the Mississippi River that the levee system in southern Louisiana is attempting to prevent -- something I wrote about a couple of weeks ago.)  Seismic avulsions are much less common, but when they happen it's sudden and spectacular.  The only other one I've ever heard of is the shift in the Mississippi caused by the 1812 New Madrid earthquake, which dropped the land so much it cut off a meander and created Reelfoot Lake.

The seismic record in Bangladesh indicates that they're dangerously at risk for another earthquake -- and because of the complexity and our lack of comprehension of the fault system underlying the country, the geologists aren't certain where is likeliest to rupture.  There's a sleeping dragon underneath one of the poorest countries in Asia -- and we're only beginning to understand when and how it might suddenly awaken.

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Shaky ground

A little less than six years apart -- on 1 November 1755 and 31 March 1761 -- two major earthquakes struck the country of Portugal, each time generating a tsunami that devastated the capital city of Lisbon.

They were both huge, although given that this was before the invention of the seismometer, we can only guess at how big; estimates are that the 1761 quake was around 8.5 on the Richter Scale, while the 1755 one may have been as high as 9.0.  Each time, the tremors were felt far from the epicenter.  The shaking from the 1755 quake was recorded as far away as Finland.

The effects in Portugal and nearby nations were devastating.  In 1755 the combined death toll in Portugal, Spain, and Morocco -- mostly from the tsunami -- is estimated at fifty thousand.  Over eighty percent of the buildings in Lisbon were damaged or completely destroyed -- and five and a half years later, many of the ones that had survived in 1755 collapsed.

Ruins of the Convento do Carmo, which was destroyed in the Great Lisbon Earthquake of 1755 [Image licensed under the Creative Commons Chris Adams, Convento do Carmo ruins in Lisbon, CC BY-SA 3.0]

What's curious is that Portugal isn't ordinarily thought to be high on the list of seismically-active nations.  It's not on the Ring of Fire, where the majority of the world's earthquakes and volcanoes occur.  The fact is, though, there is a poorly-studied (and poorly-understood) fault zone offshore -- the Azores-Gibraltar Transform Fault -- that is thought to have been responsible for both of the huge eighteenth century quakes, as well as a smaller (but still considerable) earthquake in 1816.

The AGTF, and how it's evolving, was the subject of a paper in the journal Geology last week.  The big picture here has to do with the Wilson Cycle -- named after plate tectonics pioneer John Tuzo Wilson -- which has to do with how the Earth's crust is formed, moved, and eventually destroyed.

At its simplest level, the Wilson Cycle has two main pieces -- divergent zones (or rifts) where oceanic crust is created, pushing plates apart, and convergent zones (or trenches) where oceanic crust is subducted back into the mantle and destroyed.  Right now, one of the main divergent zones is the Mid-Atlantic Rift, which is why the Atlantic Ocean is gradually widening; the Pacific, on the other hand, is largely surrounded by convergent zones, so it's getting smaller.

Of course, the real situation is considerably more complex.  In some places the plates are moving parallel to the faults; these are transform (or strike-slip) faults, like the AGTF and the more famous San Andreas Fault.  And what the new paper found was that the movement along the AGTF doesn't just involve side-by-side movement, but there's a component of compression.

So the Azores-Gibraltar Transform Fault, in essence, is trying to turn into a new subduction zone.

"[These are] some of the oldest pieces of crust on Earth, super strong and rigid -- if it were any younger, the subducting plate would just break off and subduction would come to a halt," said João Duarte, of the University of Lisbon, who lead the research, in an interview with Science Daily.  "Still, it is just barely strong enough to make it, and thus moves very slowly."

The upshot is that subduction appears to be invading the eastern Atlantic, a process that (in tens or hundreds of millions of years) will result in the Atlantic Ocean closing up once more.  The authors write:

[T]he Atlantic already has two subduction zones, the Lesser Antilles and the Scotia arcs.  These subduction zones have been forced from the nearby Pacific subduction zones.  The Gibraltar arc is another place where a subduction zone is invading the Atlantic.  This corresponds to a direct migration of a subduction zone that developed in the closing Mediterranean Basin.  Nevertheless, few authors consider the Gibraltar subduction to be still active because it has significantly slowed down in the past millions of years.  Here, we use new gravity-driven geodynamic models that reproduce the evolution of the Western Mediterranean, show how the Gibraltar arc formed, and test if it is still active.  The results suggest that the arc will propagate farther into the Atlantic after a period of quiescence.  The models also show how a subduction zone starting in a closing ocean (Ligurian Ocean) can migrate into a new opening ocean (Atlantic) through a narrow oceanic corridor.

So the massive Portugal quakes of the eighteenth and nineteenth centuries seem to be part of a larger process, where compression along a (mostly) transform fault is going to result in the formation of a trench.  It's amazing to me how much we've learned in only sixty-odd years -- Wilson and his colleagues only published their seminal papers that established the science of plate tectonics between 1963 and 1968 -- and how much we are still continuing to learn.

And along the way elucidating the processes that generated some of the biggest earthquakes ever recorded.

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The hidden fault

Between December 16, 1811 and February 7, 1812, a series of four earthquakes -- each estimated to be above magnitude 7 -- hit what you might think is one of the most unlikely places on Earth; southeastern Missouri.

The fault (named the New Madrid Seismic Zone for the county right in the center of it) is located in the middle of the North American craton, an enormous block of what should be old, stable, geologically inactive rock.  But even so, the biggest (and final) earthquake of the four was powerful enough that it was felt thousands of kilometers away, and allegedly rang church bells in Charleston, South Carolina.  The shift in terrain changed the course of the Mississippi River, cutting off a meander and creating horseshoe-shaped Reelfoot Lake.

It's well known that most of the world's earthquakes take place along the "Ring of Fire" and other junctions between tectonic plates, but it's not always so.  The New Madrid Fault is thought to be either a failed rift zone -- when a convection current in the mantle tried, but failed, to split the continent, but created a weakness in the middle of the plate -- or else the rebound of the crust from the passage of the Bermuda Hotspot, which is also one possible explanation for the process that created the Ozark Mountains.

The point is, earthquakes don't always occur where you might expect, and sometimes fault lines can stay hidden until suddenly they slip and catch everyone off-guard.  This is the situation much closer to where I live; the Saint Lawrence Rift System, aligned (as you'd expect) with the Saint Lawrence River, is an active seismic zone in northern New York and southern Canada, and like New Madrid, is very far away from any plate margins.  Here, the weakness is very old -- geologists believe the fault actually dates to the early Paleozoic, and may be related to the Charlevoix Asteroid Impact 450 million years ago -- and has been reactivated by something that is causing super slow convergence on opposite sides of the fault (on the order of 0.5 millimeters a year).

What that something might be, no one is certain.

The reason the topic comes up is a paper in the journal Tectonics this week that I found out about because of my friend, the wonderful author Andrew Butters, who is an avid science buff and a frequent contributor of topics for Skeptophilia.  It describes a newly-discovered 72-kilometer-long fault that runs right down the middle of Vancouver Island -- passing just northeast of the city of Victoria.

To be fair, British Columbia isn't exactly seismically inactive; as I described last month, it's in the bullseye (along with the rest of the coastal Pacific Northwest) of the horrifyingly huge Cascadia Subduction Zone.  But even so, the discovery of a hitherto-unknown fault right near a major city is a little alarming, especially since the southeast corner of Vancouver Island is actually pretty far away from Cascadia.  The authors write:

Subduction forearcs are subject to seismic hazard from upper plate faults that are often invisible to instrumental monitoring networks.  Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data.  We demonstrate the utility of a combined approach in a densely populated region of Vancouver Island, Canada, by combining remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, the XEOLXELEK-Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria...  Fault scaling relations suggest a M 6.1–7.6 earthquake with a 13 to 73-km-long surface rupture and 2.3–3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench.  An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarly challenging environments.

So that's a little alarming.  Another thing to file under "You Think You're Safe, But..."  I've frequently given thanks for the fact that I live in a relatively calm part of the world.  Upstate New York gets snowstorms sometimes, but nothing like the howling blizzards of the upper Midwest; and we're very far away from the target areas for hurricanes, mudslides, wildfires, and volcanoes.

But the scary truth is that nowhere is natural-disaster-proof.  As New Madrid, the Saint Lawrence Rift System, and -- now -- Victoria, British Columbia show, we live on an active, turbulent planet that is constantly in motion.  And sometimes that motion makes it a little dangerous for us fragile humans.

The Earth is awe-inspiring and beautiful, but also has little regard for our day-to-day affairs.  You can do what is possible to minimize your risk; forewarned is forearmed, as the old saying goes.  But the reality is that the natural world is full of surprises -- and some of those surprises can be downright dangerous.

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Analysis of a monster

In my post a couple of days ago about the unstable geology of the Greek island of Thera, I stated that this is far from the only place in the world where lots of people live in harm's way from the vagaries of plate tectonics.  I mentioned the Cascadia Subduction Zone, off the coast of the Pacific Northwest, and included a link to the rather desultory post I'd made a while back about what's going on over there.  A loyal reader of Skeptophilia wondered if I might write a more thorough piece about the terrifying situation looming in that beautiful part of the world, so here I am to elaborate, and probably to scare the absolute shit out of anyone living in coastal British Columbia, Washington, Oregon, and northern California.

Even after the general acceptance of plate tectonics by geologists in the 1960s and 1970s, it took a long time for them to see what was happening in the northeastern Pacific.  The presence of a ridge (divergent zone) meant that the seafloor was spreading on both sides; the fact that the small Explorer, Juan de Fuca, and Gorda Plates were being shoved eastward meant that there had to be a trench somewhere between the ridge and continental North America.  But the earliest sounding techniques couldn't find one.  It turned out that it was buried -- submerged under hundreds of meters of muck, silt and sand washed out of the region's numerous rivers.

This, and the fact that there hadn't been a big earthquake in the Northwest since settlement by people of European descent, led a lot of geologists to the conclusion that the trench was "aseismic."  Either the small plates east of the ridge weren't moving, or they were slipping underneath the North American Plate so smoothly that there were no measurable earthquakes.

This wasn't just a little bit wrong.  This was stunningly wrong.  This was wrong with whipped cream and a cherry on top.

The red dots represent earthquakes within the seafloor; the green dots are earthquakes within the continental crust of North America.  [Image is in the Public Domain courtesy of the United States Geological Survey]

The Explorer, Juan de Fuca, and Gorda Ridges are very much active spreading centers, and the fact that there haven't been any recent big earthquakes along the trench -- the Cascadia Subduction Zone, denoted on the map by the line with black triangles -- is not good news.  The entire coastline of the Pacific Northwest is compressing as the three small plates get shoved under North America, just like trying to slide something underneath a throw rug makes it rumple and hump up.  In fact, surveys measuring the positions of the peaks in the Cascade Range and on Vancouver Island have found that the whole terrain is being squished west-to-east, so entire mountains are being pushed toward each other.

Imagine the power required to do that.

Further, the fact that the trench is filled with mud doesn't mean the subduction zone is aseismic; quite the opposite.  It turns out that a large part of the mud deposits there are turbidites -- the result of colossal underwater landslides.

Such as might occur during an enormous earthquake.

More of the mechanism was elucidated in 2003, when researchers found that the whole region was experiencing a phenomenon called episodic tremor and slip, where deeper parts of the conjoined plates -- the bits that are hotter and more plastic -- slip against each other, causing barely a rumble.  This slip/tremor happens like clockwork every fourteen months.  While this may sound like a good thing, it's actually the opposite.  Releasing stress that has built up in the deep parts of the fault merely passes that stress upward to the colder, shallower parts that are still locked together, each ETS episode dialing up the energy like the clicking of another tooth in a ratchet.

So along the subduction zone, the two opposing sides of the plates are stuck together, building up more and more tension -- tension that will one day be released as the faultline unzips, and the whole northwest coast of the continent springs back toward the west.

To say the result will be catastrophic is understatement of the year.

It's happened before.  In fact, geologists taking cores of the aforementioned turbidite sediments off the coast of Washington found evidence that in the past ten thousand years it's happened nineteen times.  The spacing between megathrust earthquakes -- as these are called -- varies between three hundred and nine hundred years, with the average being around five.  And the last one happened a little over 323 years ago.

We actually know down to the hour when it happened -- about 9 PM local time, January 26, 1700.  Indigenous tribes in the area have a long tradition that many years ago, there was a terrible earthquake one midwinter night, during which the seashore dropped and salt water flooded in, killing many people.  Evidence from tree rings in "ghost forests" -- the trunks of hundreds of western red cedars that had all been killed simultaneously by an influx of salt -- showed that some time in the 1690s or early 1700s there had been a massive flood from the ocean as the coastline suddenly dropped by several meters.  The exact date was determined from records across the Pacific, where Japanese scribes describe what they called an "orphan tsunami" (a huge wave that, from their perspective at least, was not preceded by an earthquake) striking coastal Japan.  Knowing the speed with which such waves travel across the ocean, geologists were able to determine exactly when the fault last unzipped from end to end.

The earthquake that resulted is estimated to have been somewhere between 8.7 and 9.2 on the Richter Scale, and to have resulted in land movement averaging around twenty meters.

Not pleasant to consider how that would play out if it happened today.

The worst part, for coastal communities today, is how close the Cascadia Subduction Zone is to shore.  At its closest approaches -- near the west coast of Vancouver Island, and from central Oregon south to Cape Mendocino -- it's estimated that the lag time between the ground shaking and the first of the tsunami waves striking the shore will be around eight minutes.  That's eight minutes between being thrown all over the place by an enormous earthquake, and somehow getting yourself to high ground before you're hit by a giant wall of salt water.

I remember when I first heard in detail about the dangers of the Cascadia Subduction Zone -- in 2015, from Kathryn Schulz's brilliant analysis in The New Yorker called "The Really Big One."  It impressed me so much I actually used the fault as a plot point in my novel In the Midst of Lions, where the story is bracketed by earthquakes (one of them massive).  But when I was a Seattle resident in the 1980s, I had no idea.  I still dearly love the Northwest; not only does it have the ideal climate for a fanatical gardener like myself, it has amazing spots for hiking and camping.  During my time there I spent many happy days on the coast of the Olympic Peninsula -- never realizing that a monster lurked offshore.

So while I miss many things about the Northwest, I know I could never live there again.

It may be that the fault won't rupture for another two hundred years; on the other hand, it could happen tomorrow.  While our ability to analyze plate tectonics is light years beyond what it was even thirty years ago, when the situation in the Northwest first began to come clear, we still don't have any way to determine when the earthquake will happen with any kind of precision.  At the moment, all we know is that it will rupture, sooner or later.

And I don't want to be anywhere near it when it does.

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A dangerous beauty

The Greek island of Thera -- often known by its Italianized name of Santorini -- is the southernmost of the Cyclades, an island chain in the Aegean Sea southeast of mainland Greece.  Like much of the region, it's a stunningly beautiful place.  In fact, one of Thera's names in antiquity was Καλλίστη -- "the most beautiful one."

[Image licensed under the Creative Commons Pedro Szekely from Los Angeles, USA, Santorini, Greece (38051518795), CC BY-SA 2.0]

The steep, rugged, rocky terrain, though, didn't happen by accident.  Thera and the other Cyclades formed because they sit near the margin of the Hellenic Subduction Zone, where the northern edge of the enormous African Plate is being shoved underneath the much smaller Aegean Sea Plate.  The result is the formation of an island arc, where the material in the subducted plate is pushed downward to a depth were it melts, and the blobs of magma rise toward the surface to create a chain of volcanoes.  (Most of the islands in the Caribbean, the Aleutians, and pretty much the entirety of the nations of Japan and Indonesia were formed this way.)

This makes it a dangerous place to live.  It was the site of the Minoan-era city Akrotiri, which became prosperous because of being a central port for the copper trade out of Cyprus (the Latin word for copper, cuprum, actually means "metal from Cyprus").  It was second only to Crete as a center of civilization for the Minoan Empire, and was famed for its art, especially elaborate and beautiful frescoes, pottery, and sculpture.  Many of the houses there had running water carried by bronze pipes, and geothermal heat.

The geothermal heat might have clued its residents in that something was going on underground.  All of the high times came to an end with a colossal eruption of the volcano just offshore in around 1600 B.C.E. 

[Nota bene: this is not what inspired the myth of Atlantis, despite the claims you see all over the place on the interwebz.  Plato made it clear that the legend said Atlantis was "west of the Pillars of Hercules" (the Straits of Gibraltar), somewhere out in the Atlantic (thus the name).  But... allow me to stress this point... Atlantis never existed.  Because it's a myth.]

Anyhow, the eruption of Thera not only destroyed pretty much the entire island, but blew an estimated forty cubic kilometers of dust and ash into the air, triggering atmospheric and climatic effects that were recorded by contemporaneous scholars in Egypt and China and draw comparisons from modern geologists to the Mount Tambora eruption of 1815 that caused "The Year Without A Summer."  The eruption generated a tsunami that devastated coastal cities all over the Mediterranean, including the Minoan city of Knossos on the north shore of Crete.  (The Minoan civilization limped along for another couple of hundred years after this calamity, but was finally finished off by a massive earthquake in 1350 B.C.E. that destroyed Knossos completely.)

Here's the thing, though.

The volcano off the coast of Thera is still active.

A paper last week in Nature Communications looked not at the enormous 1600 B.C.E. eruption, but a much smaller eruption in 1650 C.E.  The leadup to this eruption, however, was about as ominous as you could get.  People noticed the water in the seas off the north coast of Thera boiling and changing color -- and dead fish rising to the surface as well, cooked in situ.  Sulfurous gases wafted over the island.  This was followed by a cinder cone emerging from the sea, which proceeded to fling around molten rocks and ash plumes.

Then... boom.

The new research suggests that what triggered the eruption was a landslide, similar to what kicked off the famous Mount Saint Helens eruption of 1980.  In this case, though, the landslide was underwater, off the northwest flank of the volcano.  This landslide did two things -- it displaced huge amounts of water, generating a twenty-meter-high tsunami, and it took the pressure off the top of the magma chamber, causing it to explode.

The combination killed seventy people and hundreds of domestic animals -- horrible, but nowhere near what the island proved itself capable of 3,600 years ago.  The study found that the magma chamber is refilling at a rate of four million cubic meters per year, meaning with regards to subsequent eruptions -- to invoke the old cliché so often used in connection to active volcanoes and tectonic faults, it's not a matter of "if," it's a matter of "when."

Unsurprisingly, the people in the region seem unaware of the time bomb they're sitting on.  "Local populations, decision-makers, and scientists are currently unprepared for the threats posed by submarine eruptions and slope failures, as has been demonstrated by the recent 2018 sector collapse of Anak Krakatau and the 2022 [Hunga Tonga] eruption," the authors write.  "Therefore, new shore-line crossing monitoring strategies... are required that are capable of being deployed as part of rapid response initiatives during volcanic unrest and which enable real-time observation of slope movement."

It remains to be seen how this could help the almost two thousand people who currently live on the slopes of the island, many of them living in houses sitting on layers of fused ash deposited there during the 1600 B.C.E. eruption.  It's something we've seen here before; people like living in tectonically active regions because (1) the terrain is often dramatic and beautiful, (2) volcanic soils are good for agriculture, and (3) people have short memories.  If the last time things went kablooie was almost three hundred years ago, it's easy for folks to say, "What, me worry?"  (Witness the millions of people living near the terrifying Cascadia Subduction Zone, about which I wrote three years ago.  As well as all the people in the aforementioned countries of Japan and Indonesia.)

Anyhow, that's our rather ominous scientific study of the day.  The Earth is a beautiful and dangerous place, and nowhere does that combination come into sharper focus than the Greek islands.  Makes me glad I live where I do -- despite the cold winters, at least I don't have to worry about the place blowing up.

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