A whole lot of shakin'

I didn't realize how complicated it is to calculate the magnitude of an earthquake.

Most of us are probably familiar with the Richter Scale, the one most commonly used in the media.  It was developed in 1935 by seismologist Charles Francis Richter to give a standard scale to measure the power of earthquakes.  The scale is logarithmic; each increase in one on the scale represents a ten-fold increase in intensity.  The scale is based upon the displacement amplitude on a seismograph at a distance of one hundred kilometers from the epicenter, starting with a magnitude 0 earthquake causing the needle to move with an amplitude of one micron.  The scale extends up to an unspecified "greater than 9" -- because at that point, pretty much everything in the vicinity, including the seismograph, gets completely pulverized.

When you start looking more closely, though, the problems with the scale start to become obvious.  First of all, if the measurement is being made one hundred kilometers from the epicenter, the terrain in between is a significant factor.  Tremors passing through material with a high amount of shear (such as sand or mud) will lose intensity fast, as compared to ones going through a material that is rigid (such as solid rock).  Second, the origin of the earthquake usually isn't at the epicenter, which is the point on the surface nearest the source; the origin is the hypocenter, directly underneath -- but which can be at any depth from right near the surface down to hundreds of kilometers down.  (The deepest earthquake ever recorded was a minor tremor off the island of Vanuatu in 2004, which had a hypofocus 736 kilometers deep.)  Then there's the fact that earthquakes can be of different durations -- a less powerful earthquake that lasts longer can do as much damage as a more powerful, but shorter, tremor.

Another problem is that earthquakes can result in differences in the oscillation of the waves relative to the direction they're moving.  This is largely due to the fact that there are three basic sorts of faults.  There are thrust faults or convergent faults, where two tectonic plates are moving toward each other; what happens then can be one plate being pushed underneath the other (subduction), which is what causes the quakes (and the volcanoes) in Indonesia and Japan, or the two plates kind of smashing together into a jumble, which is the process that created the Himalayas.  There are extension faults or divergent faults, where the two plates are moving apart; this usually creates smaller but more frequent quakes, and lots of volcanism as magma bubbles up from the underlying mantle.  This is happening in Iceland, and is also the cause of the Great Rift Valley in Africa, which will eventually peel off the Horn of Africa (Somalia and parts of Ethiopia, Kenya, and Tanzania) and open up a new ocean.  Last, there are strike-slip faults or transform faults, where the plates are moving in opposite directions on each side of the fault, such as the famous San Andreas Fault in California.

Map of the (known) tectonic plates [Image is in the Public Domain courtesy of NASA/JPL]

The problems with the Richter Scale have led to the development of several other scales of intensity, such as the Surface-wave Magnitude Scale (which is pretty much just what it sounds like, and doesn't take into account source depth), the Moment Magnitude Scale (which is based on the amount of energy released as measured by the amount and distance of rock moved), the Duration Magnitude Scale (which figures in how long the tremor lasts), and so on.  But these all use different numerical benchmarks, and given that the Richter Scale is more widely known, a lot of people have continued to use that one despite its downsides.

The reason all this comes up is a new study from the University of Southampton that has identified evidence of what appears to be the biggest earthquake known; an almost unimaginable 9.5 on the Richter Scale quake that happened in Chile 3,800 years ago.  Trying to find the epicenter brings up yet another problem with measuring quake intensity, because the evidence is that this particular quake originated from the rupture of a part of the thrust fault between the Nazca Plate and the South American Plate off the coast of the Atacama Desert -- a rupture that was one thousand kilometers long.

The result was a tsunami that deposited marine sediments and fossils of oceanic animals several kilometers inland, and then traveled across the Pacific Ocean and slammed into New Zealand, tossing boulders the size of cars over distances of hundreds of meters.  That region of the Atacama Desert had been inhabited prior to the quake -- astonishing considering how dry and inhospitable the place is -- but it was (understandably) abandoned by the survivors for a long while afterward.

"The local population there were left with nothing," said geologist James Goff, who co-authored the study.  "Our archaeological work found that a huge social upheaval followed as communities moved inland beyond the reach of tsunamis.  It was over a thousand years before people returned to live at the coast again, which is an amazing length of time given that they relied on the sea for food.  It is likely that traditions handed down from generation to generation bolstered this resilient behavior, although we will never know for sure.  This is the oldest example we have found in the Southern Hemisphere where an earthquake and tsunami had such a catastrophic impact on people’s lives.  There is much to learn from this."

The obvious next question is, "Could this happen again?"  The answer is not just that it could, but it will.  Probably not in the same spot, but somewhere along the many tectonic boundaries in the world.  Nor do we know when.  Earthquake prediction is very far from an exact science.  We have instruments like strain gauges to estimate the tension rock is experiencing, but that doesn't tell you what's going on deeper in the ground, nor when the rock will fracture and release that energy as an earthquake.  Predicting volcanic eruptions is much easier; vulcanologists have gotten pretty good at detecting magma movement underground, and recognizing when a volcano is likely to blow.  (This is why the ongoing hoopla about the Yellowstone Supervolcano is all hype; sure, it'll probably erupt again, but some time in the next hundred thousand years or so, and it's showing no signs of an imminent eruption.)

The Earth is a dynamic planet, and the plates on the surface are in constant motion, jostling, coming together, moving apart, a bit like ice sheets on a river when they begin to break up in the spring.  You can't help but be fascinated by the amount of power it's capable of -- a catastrophic release of energy so large that the scales we've developed to measure such things are all but incapable of expressing.

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The rain of glass

A couple of weeks ago I looked at the rather unsettling fact that the seeming benevolence of our home planet is something of an illusion.  As I write this, I'm sitting in a warm house with the calm, clear sunshine sparkling on frost-covered grass, hardly a cloud in the sky, and it's difficult to imagine it ever being any different.  While I don't believe a thoroughly pessimistic outlook helps anything or anyone, it does bear keeping in mind how fragile it all is -- if for no other reason, so that we value what we have.

I started thinking about how quickly and unpredictably a place can go from tranquility to devastation when I ran across a paper that appeared in the journal Geology two weeks ago.  In it, I learned about something I'd never heard about -- a seventy-five-kilometer-wide patch of the Atacama Desert in northern Chile that is covered with shards of black and green glass.

The Atacama Desert is a strange place in and of itself.  Other than the dry valleys of Antarctica, it is far and away the most arid place on Earth; the average rainfall is around fifteen millimeters per year, and there are parts of it that are down in the nearly-unmeasurable range of one to three millimeters.  The few plants and animals that live there have dry-climate adaptations that beggar belief; they get most of the water they need using condensation from fog.  The reason for the peculiar climate is a combination of a more-or-less permanent temperature inversion produced by the South Pacific Anticyclone and the cold, northward-flowing Humboldt Current, combined with a two-sided rain shadow caused by the parallel Andes Mountains and Chilean Coast Range.  It's so dry and barren that it was used by NASA as one of the places to test the Mars Lander's ability to detect the presence of microscopic life.

The aridity is what allowed for the discovery that was the subject of the November 2 paper.  Geologists Peter Schultz (Brown University), R. Scott Harris (Fernbank Science Center), Sebastián Perroud (Universidad Santo Tomás), and Nicolas Blanco and Andrew Tomlinson (Servicio Nacional de Geología y Minería de Chile) analyzed the peculiar shards that cover the patch on the northern end of the desert, and found out that they were all formed in one event -- the mid-air explosion of a comet about twelve thousand years ago.

The authors write:

Twisted and folded silicate glasses (up to 50 cm across) concentrated in certain areas across the Atacama Desert near Pica (northern Chile) indicate nearly simultaneous (seconds to minutes) intense airbursts close to Earth’s surface near the end of the Pleistocene.  The evidence includes mineral decompositions that require ultrahigh temperatures, dynamic modes of emplacement for the glasses, and entrained meteoritic dust.  Thousands of identical meteoritic grains trapped in these glasses show compositions and assemblages that resemble those found exclusively in comets and CI group primitive chondrites.  Combined with the broad distribution of the glasses, the Pica glasses provide the first clear evidence for a cometary body (or bodies) exploding at a low altitude.  This occurred soon after the arrival of proto-Archaic hunter-gatherers and around the time of rapid climate change in the Southern Hemisphere.

The dry climate is why we even know about this event.  Cometary collisions almost never leave a crater; given that comets are mostly made of various kinds of ice, the heat of friction from the atmosphere causes them to evaporate and finally explode, creating an airburst but no solid-object impact.  The airburst can be devastating enough, of course.  The 1908 Tunguska Event, the largest such occurrence in recorded history, flattened eighty thousand trees in over two thousand square kilometers of Siberian forest, and registered on seismographs all the way around the world in Washington, D.C.  If Tunguska had happened over a major city, there wouldn't have been a person left alive or a building left standing in the blast zone.

Like Tunguska, at the time and place of the Atacama airburst, there weren't many people in the danger zone.  There was, however, a lot of sand, and the heat from the collision melted it into glass -- indicating temperatures in excess of 1,700 C.  In a climate with ordinary amounts of rainfall, the glass would have been degraded and eroded, but here, it rained out of the sky and then has just kind of sat there for the intervening twelve thousand years.

"It was clear the glass had been thrown around and rolled," study lead author Peter Schultz said, in an interview with Science News.  "It was basically kneaded like bread dough."

The glass shards (the dark bits) in the northern Atacama Desert [photograph by Peter Schultz]

It would have been quite a spectacular thing to witness (from a safe distance), and you have to wonder how the survivors explained it.  "It would have seemed like the entire horizon was on fire," Schultz said. "If you weren’t religious before, you would be after."

So that's our disquieting scientific research for the day.  The reassuring news is that we've gotten pretty skilled at mapping the asteroids, meteors, and comets out there in the Solar System, and none of them seem to be headed our way, at least not for a good long while.  Which is a bit of a relief.  As often as I complain about how dull it is to live in a part of the world where the biggest excitement of the day is when the farmer across the road lets his cows out into the field, this isn't the kind of change of pace I'm really looking for.

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If Monday's post, about the apparent unpredictability of the eruption of the Earth's volcanoes, freaked you out, you should read Robin George Andrews's wonderful new book Super Volcanoes: What They Reveal About the Earth and the Worlds Beyond.

Andrews, a science journalist and trained volcanologist, went all over the world interviewing researchers on the cutting edge of the science of volcanoes -- including those that occur not only here on Earth, but on the Moon, Mars, Venus, and elsewhere.  The book is fascinating enough just from the human aspect of the personalities involved in doing primary research, but looks at a topic it's hard to imagine anyone not being curious about; the restless nature of geology that has generated such catastrophic events as the Yellowstone Supereruptions.

Andrews does a great job not only demystifying what's going on inside volcanoes and faults, but informing us how little we know (especially in the sections on the Moon and Mars, which have extinct volcanoes scientists have yet to completely explain).  Along the way we get the message, "Will all you people just calm down a little?", particularly aimed at the purveyors of hype who have for years made wild claims about the likelihood of an eruption at Yellowstone occurring soon (turns out it's very low) and the chances of a supereruption somewhere causing massive climate change and wiping out humanity (not coincidentally, also very low).

Volcanoes, Andrews says, are awesome, powerful, and fascinating, but if you have a modicum of good sense, nothing to fret about.  And his book is a brilliant look at the natural process that created a great deal of the geology of the Earth and our neighbor planets -- plate tectonics.  If you are interested in geology or just like a wonderful and engrossing book, you should put Super Volcanoes on your to-read list.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]