Highly Allochthonous http://all-geo.org/highlyallochthonous NEWS & COMMENTARY FROM THE WORLD OF GEOLOGY & EARTH SCIENCE Mon, 07 May 2012 19:14:56 +0000 en hourly 1 http://wordpress.org/?v=3.2.1 Hope Jahren, isotope detective http://all-geo.org/highlyallochthonous/2012/05/hope-jahren-isotope-detective/?utm_source=rss&utm_medium=rss&utm_campaign=hope-jahren-isotope-detective http://all-geo.org/highlyallochthonous/2012/05/hope-jahren-isotope-detective/#comments Mon, 07 May 2012 19:14:56 +0000 Anne Jefferson http://all-geo.org/highlyallochthonous/?p=7439 Continue reading →]]> A post by Anne Jefferson The waning days of the academic year seem like an apt time to recognize the mentors who have had an important influences on my careers. I could wax lyrical about my Ph.D. advisor, but he reads the blog and I’ll save him the embarrassment (for now). And I’ve already written about some of my memories of Reds Wolman, who was a huge influence on my and the whole field of geomorphology. Today, I’d like to introduce another key figure in my education. Even better, you’ll get to meet her in her own words.

Photo of Hope Jahren near Andalsnes, Norway. Photo provided by Jahren and used with permission.

Hope Jahren near Andalsnes, Norway, one of her favorite places.

Hope Jahren is a Professor in the School of Ocean and Earth Science and Technology at the University of Hawaii at Manoa. She uses stable isotopes to understand how organisms are linked to their environment, both in modern times and in the fossil record. Her work has taken her all over the world, from the fossil forests of the High Arctic to the fast food restaurants of the American midwest. Like me, Hope is a native Minnesotan. She earned a PhD in Soil Science at Berkeley, and then held positions at Georgia Tech and Johns Hopkins (where I met her), before moving to Hawaii in 2008. Along the way she has held several Fulbrights, earned both the GSA and AGU young scientist medals and been named one of Popular Science’s “Brilliant 10.” She’s also currently the editor of GSA Bulletin, a recent Leopold fellow, and a fun voice on Twitter. I had the privilege to work with hope on my undergraduate thesis on soil water isotopes and again as an MS student, when she sent me to the fossil forests of Ellesmere Island. Recently, I emailed her to pick her brain on research, teaching, and how she got to be the fantastic scientist I know and love. Here’s our exchange:

Anne: You were the person who introduced me to the amazing world of stable isotopes, when I worked on my senior thesis with you. That thesis was on soil water isotopes in the western US. I went on use stable isotopes of water in my MS and PhD research, and now they are a core analytical technique for me. Thanks for that! So I’m wondering, what got you into isotopes?

Hope: My Ph.D. thesis explored the use of terrestrial plant carbonate — a biomineralized seed — to reconstruct paleoclimate. Some of the fundamental studies on past climate (e.g., reconstruction of the ice ages) had relied upon stable isotope measurements of marine fossil carbonates — we were hoping to do something similar with terrestrial fossils. However, as I got deeper into the project, we realized that plant physiology was exerting a great deal of control over the system … in this way, I got interested in studying how plants work, and how photosynthesis may have worked in the past.

I’ve got a new-fangled cavity ringdown spectrometer (CRDS) for analyzing water isotopes, and it is so much cheaper and easier to use than a traditional mass spectrometer. But I’m also limited to a just hydrogen and oxygen in water, unlike the versatility of a mass spec, so that’s a big downside. Do you care to say what you think the future of stable isotope spectrometry will be? Will the CRDS systems displace the old-school mass spec or am I buying into a passing fad?

I’ve never been much of a hipster instrumentalist, I’m much more interested in talking about data. I think that any method that yields quality measurements is a bonus to the field. Since I started in isotopes, many measurements that we used to do one by one “offline” are now run using automated online accessory instrumentation. This has resulted in many more data points being produced each year, but still fundamentally relies on smart people to generate and test hypotheses.

I also owe you a huge heap of thanks for sending me to Ellesmere Island for my MS research on spectacularly preserved Paleocene-Eocene fossilized forests. You went to the High Arctic for several field seasons, and I know you’ve also done field work in China. Has your science taken you to other exotic locales? Any particular favorites?

I have had the fortune to visit many places in the course of my research, including Japan, Colombia, Brazil and Ireland … as well as many places in the US. My favorite place to work is in Norway, where I spent 2010-2011. There I had the fortune to work with foresters, who are doing fascinating experiments looking at the effects of climate change on the ways in which spruce trees integrate developmental stimuli.

Your “geobiology” class also introduced me to nutrient cycles, soils, and the unforgettable concept of soil forming factors and sequences. I still have my copy of Jenny’s 1944 “Factors of Soil Formation” and I use the concept of chrono- and climo-sequences to understand the evolution of volcanic landscapes. Do you have a favorite soil somewhere in the world?

There’s an ultisol in Coweta county, south of Atlanta that is just stunning. I saw it from the car window in 1996 and have exposed it several times since then. I used to take my courses there several times a year when I was a professor at Georgia Tech.

You said on Twitter that you don’t really consider yourself a teacher, but you definitely had a lasting influence on me, both through your classroom teaching and your research mentorship. I’m sure I’m not the only student you’ve reached. What do you think makes someone a good teacher or mentor?

I don’t think of myself as a teacher in the traditional sense, because I am suspicious of the idea that research (my main interest) can be taught. What I try to do is provide an opportunity, and try to pull a student towards success. However, the student has to decide how much they push themselves in order to make something of that opportunity. I have provided a huge number of opportunities to do research over the years, to undergraduates, to grad students and to post-docs. Occasionally someone really special makes great use of the possibilities offered and succeeds, and I feel lucky to be able to facilitate that success.

When I was an undergraduate, you were early in your career. Now you’re mid-career, are spectacularly successful at research, and have been on the faculty at three universities. Now I’m an early career professor, but I’m starting to move into that mid-career phase too. Yet there’s still so much I feel like I need to learn! Any advice on successfully navigating the real and psychological transition from early career to mid-career faculty?

I think that tenure comes with new obligations, namely to push yourself harder and into riskier areas. I often want to say to newly-tenured faculty, ok, now you’ve got tenure, what are you going to use it *for*? As in, you’ve got 30-odd years of guaranteed job security and very few concrete tasks will be required of you. The concept of tenure doesn’t really exist outside of academia, and it was intended to protect academics from the standard societal pressures toward conformity. So, how will you take advantage of that protection, by putting yourself out there, and what risks will you take? The sabbatical that often follows tenure is the perfect time to reflect, switch gears, and begin taking risks.

I love seeing you pop-up on Twitter, running across your name in various journals, and having a strong female role model in my professional life. Thanks very much for your help with this interview and for being such an influential figure in my career.

]]> http://all-geo.org/highlyallochthonous/2012/05/hope-jahren-isotope-detective/feed/ 3 Scenic Saturday: Upper Mississippi Islands http://all-geo.org/highlyallochthonous/2012/05/scenic-saturday-upper-mississippi-islands/?utm_source=rss&utm_medium=rss&utm_campaign=scenic-saturday-upper-mississippi-islands http://all-geo.org/highlyallochthonous/2012/05/scenic-saturday-upper-mississippi-islands/#comments Sat, 05 May 2012 10:35:12 +0000 Anne Jefferson http://all-geo.org/highlyallochthonous/?p=7450 Continue reading →]]> A post by Anne JeffersonThe last few weeks have seen me overwhelmingly busy with #sciwrite, #gradingjail, #proposalpurgatory, and #deathbydataanalysis, and it doesn’t look like I’ll come up for air for a little while longer. But to give the blog a little freshening, and help me avoid grading early on a Saturday morning, I thought I’d show a picture of my homeland, the Driftless area of southeastern Minnesota and western Wisconsin.

Mobile Islands near Trempealeau, Wisconsin. Photo by A. Jefferson, June 2011

Mobile Islands near Trempealeau, Wisconsin. Photo by A. Jefferson, June 2011

This is a view from the top of Brady’s Bluff in Perrot State Park near Trempealeau, Wisconsin. I am looking south towards Minnesota, across the Upper Mississippi River. The river here is not the classic, loopy meanders of the lower Mississippi. Instead, it is termed “island braided.” The channel is wide and subdivided by multiple, somewhat stable islands. Some of these islands have been around since the first mapping of the river in the 1870s. Other islands appear after each large flood, but then seem to persist. Still others have their whole existence occur within a few decades.

Looking from the left of the picture, the first treed island is mapped as Island 81 by the Mississippi River Commission in the 1890s. Parts of the island have upland oak forest vegetation with huge old trees, and lots of poison ivy. The next low island with shorter trees is one my family calls Gull Island, because it first appeared as the waters receded after the flood of 1993. At first, it was an emergent sandbar that had accumulated around a piece of large wood on which gulls liked to rest. Now it is completely covered by 4 m or higher willows. Behind Gull Island is Lower Mobile Island. This island first appeared in the aftermath of the 1965 flood, and was the subject of study by Winona State University biology professors for several decades. Upstream of Lower Mobile Island is Upper Mobile Island. This island was created when water levels in the river were raised in 1935 by the construction of the Lock and Dam System. It has mostly been eroding ever since. Behind Lower Mobile Island, you might be able to identify another discrete line of trees before your eye reaches the Minnesota shore. That line of trees is all that remains of Lower LaMoille Island, another created by raised water levels in 1936. Upper LaMoille Island disappeared from the surface in the 1990s. Finally, there’s the river bank on the Minnesota side and the classic bluffland topography for which the Driftless Area is famous.

These islands are special to me because their dynamism is really what got me hooked on the field of fluvial geomorphology. I did a science fair project on the geomorphology and history of the islands when I was 16, and I have an MS student who worked on them before being lured to the wilds of Alaska. I’m not sure he’ll ever finish his thesis or that I’ll ever get a paper out of it, but if you want to learn more about the dynamic Upper Mississippi, you should learn from Cal Fremling, the man who wrote the book: “Immortal River: The Upper Mississippi in Ancient and Modern Times.” He’s the biology professor that started studying the Mobile Islands back in 1965.

]]> http://all-geo.org/highlyallochthonous/2012/05/scenic-saturday-upper-mississippi-islands/feed/ 0 Stuff we linked to on Twitter last week http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-43/?utm_source=rss&utm_medium=rss&utm_campaign=stuff-we-linked-to-on-twitter-last-week-43 http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-43/#comments Sun, 25 Mar 2012 16:36:39 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7428 Continue reading →]]> A post by Chris RowanA post by Anne JeffersonWelcome to the weekly links fest from your friendly Highly Allochthonous bloggers. If you’re thinking the format looks a bit different this week, it’s because Chris has been tinkering a bit with the script that generates the links in an attempt to improve readability. Let us know what you think.

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]]> http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-43/feed/ 0 Friday Focal Mechanism: M 7.4, Oaxaca, Mexico http://all-geo.org/highlyallochthonous/2012/03/friday-focal-mechanism-m-7-4-oxaca-mexico/?utm_source=rss&utm_medium=rss&utm_campaign=friday-focal-mechanism-m-7-4-oxaca-mexico http://all-geo.org/highlyallochthonous/2012/03/friday-focal-mechanism-m-7-4-oxaca-mexico/#comments Fri, 23 Mar 2012 23:05:59 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7417 Continue reading →]]> A post by Chris RowanThe largest earthquake to hit the planet this week was in Mexico, which was shaken on Tuesday by a magnitude 7.4 earthquake. The epicentre was in the Oaxaca region about 300 kilometres southwest of Mexico city, and the rupture was quite shallow at about 20 kilometres below the surface. The focal mechanism indicates that the earthquake was due to northeast-southwest thrusting (see my primer on focal mechanisms).

Focal mechanism for Oaxaca earthquake

Location and focal mechanism of the 20th March M7.4 earthquake near Oaxaca, Mexico. Imagery from Google Earth, earthquake locations from the USGS.

The rupture was about 100 km inland from Middle America Trench, where the Cocos plate is being subducted to the northeast beneath Mexico, and is at the right depth to be on the subduction thrust. Interestingly, it also seems to be at about the distance from the trench Aftershocks of the Oaxaca quake

Close up of region around the epicentre of the Oaxaca earthquake, showing the distribution of aftershocks (biggest circle is the main shock). Imagery from Google Earth, earthquake locations from the USGS.

One of the reasons that I was thinking about this is that although this earthquake was quite large, and caused a fair amount of damage close to the epicentre, Mexico as a whole seems to have got off quite lightly. This contrasts with the 1985 magnitude 8.0 earthquake that killed 10,000 people and caused severe damage to buildings in Mexico City (which is built on unconsolidated sediments that can amplify the shaking caused by the passage of seismic waves). The epicentre of that earthquake was up the coast to the northwest, in a similar location relative to the Middle America Trench as this week’s tremor, and a similar distance from Mexico City. So why the large difference in their impacts? The 1985 event was stronger: a magnitude 8 releases about 8 times as much energy as a magnitude 7.4, which is probably a major factor in the reduced casualties and damage. Also, one of the legacies of the 1985 disaster has been more rigorous building codes and efforts to increase earthquake preparedness; it is a more resilient Mexico than it was 25 years ago. But if much of the seismic energy from this earthquake was directed away from Mexico City, as the aftershocks might indicate, then this might have helped to reduce the regional impacts as well.

]]> http://all-geo.org/highlyallochthonous/2012/03/friday-focal-mechanism-m-7-4-oxaca-mexico/feed/ 2 Geological maps: still interesting even when there’s only one rock type http://all-geo.org/highlyallochthonous/2012/03/geological-maps-still-interesting-even-when-theres-only-one-rock-type/?utm_source=rss&utm_medium=rss&utm_campaign=geological-maps-still-interesting-even-when-theres-only-one-rock-type http://all-geo.org/highlyallochthonous/2012/03/geological-maps-still-interesting-even-when-theres-only-one-rock-type/#comments Thu, 22 Mar 2012 03:04:06 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7404 Continue reading →]]> A post by Chris RowanThe USGS, in collaboration with NASA, have just released a geological map of Jupiter’s ultra-volcanically active moon Io, based on images from the Voyager and Galileo probes. It is a thing of beauty.

Geological Map of Io

Geological map of Io. Click to enlarge. Source: USGS

The sheer variety of different geological units that Io’s surface has been divided into is a little eye-opening, especially when you consider that the entire surface of the moon is constantly being resurfaced by the most active volcanoes in the solar system. How can there be so much variety when the whole surface is relatively young volcanic lava?

Volcanism on Io

A volcanic eruption on Io captured by the New Horizons probe.

In fact, the answer is simple: there is so much variety because Io is so geologically active. Hundreds of volanoes are regularly producing individual lava flows, which cover and cut through older flows from the same vent, and other flows from neighbouring vents. Big eruptions dust the surrounding plains with ash. Crater walls collapse and produce debris flows. The surface is faulted and deformed as volcanic edifices are built on top of the crust and magma is extracted from beneath it.

Geology of Io's northern hemisphere

Geological map of Io, zoomed in to part of Io's northern hemisphere. Different lava flows (f_), vents or patera (p_) and crustal blocks uplifted by tectonics (m_) have been distinguished and mapped as separate units. Source: USGS.

So, whilst two bits of rock on Io’s surface might be fairly similar in terms of their composition and mineralogy, they can be very different in terms of their history – how they have got where they are. For geologists, the concept of a ‘mappable unit’ basically boils down to ‘if you can reliably distinguish it, you can map it’, and rocks can just as easily be distinguished by the processes that deposited them, and the relative timing of those processes, as chemistry and mineral content. In the case of Io, for example, lava flows of different ages can be distinguished from each other based on their albedo, colour, and the way they cut across older units, and are themselves cut across in turn. They are thus mappable units, and can be assigned their own colours on a map.

Back on planet Earth, we can see similar principles at work on geological maps of volcanic islands like Hawaii. The whole of the Big Island is effectively made up of variations on a theme of basalt, but assigning it all into one unit would give you a boring – and uninformative – island-shaped blob of a single colour. But different lava flows can be individually mapped, and dated, and traced to different vents, allowing you to produce something a lot more interesting: a map that clearly depicts the rich volcanic history of the Big Island.

Geological map of the Big Island

Geological map of the Big Island, Hawaii. Not simply all basalt, but all sorts of different basalt. Source: USGS.

It’s not just volcanic rocks either. A single sandstone formation can be mapped as one unit, or divided into individual units representing the different pulses of sediment that have filled up a basin. Glacial tills formed in the last million years can be lumped together, or divided up into markers of the waxing and waning of the ice sheets.

In geology, the composition of a rock and the process that formed it are often intertwined; but even when the type of rock you’re looking at stays the same, the processes forming it can change and give you something entirely new, and distinct – and mappable.

]]> http://all-geo.org/highlyallochthonous/2012/03/geological-maps-still-interesting-even-when-theres-only-one-rock-type/feed/ 5 Stuff we linked to on Twitter last week http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-42/?utm_source=rss&utm_medium=rss&utm_campaign=stuff-we-linked-to-on-twitter-last-week-42 http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-42/#comments Sun, 18 Mar 2012 16:57:12 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7398 Continue reading →]]> A post by Chris RowanA post by Anne Jefferson

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]]> http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-42/feed/ 1 Scenic Saturday: from desert to verdant grassland in 10 miles (and 1000 m) http://all-geo.org/highlyallochthonous/2012/03/scenic-saturday-from-desert-to-verdant-grassland-in-10-miles-and-1000-m/?utm_source=rss&utm_medium=rss&utm_campaign=scenic-saturday-from-desert-to-verdant-grassland-in-10-miles-and-1000-m http://all-geo.org/highlyallochthonous/2012/03/scenic-saturday-from-desert-to-verdant-grassland-in-10-miles-and-1000-m/#comments Sat, 17 Mar 2012 15:54:44 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7383 Continue reading →]]> A post by Chris RowanThis view up the south-western slopes of Kohala volcano on the Big Island of Hawaii is a study in climatic contrasts.

South-west flank of Kohala

Looking north-east towards the summit of Kohala volcano, Big Island, Hawaii. Photo: Chris Rowan, 2012.

The ground that I’m standing on is dry, and clearly arid: the sparse and obviously parched vegetation suggests that this landscape sees very little rainfall most of the time. Yet at the very moment I took this photo, the top of Kohala itself is wreathed in storm clouds. When we visited the summit earlier in the day, the weather was fortunately a little more amenable, but the green, green grass and the forests on the summits of the cinder cones show that it is no stranger to precipitation.

Kohala Cinder Cones

Cinder cones on the summit of Kohala, Big Island, Hawaii. Photo: Chris Rowan, 2012.

From altitude, the contrast between the lower and upper slopes is even more striking.

Perspective view of Kohala from the southwest.

Kohala viewed from the southwest in Google Earth. Note the sharp change in colour/vegetation.

This is all caused by Hawaii’s position in the midst of the northern hemisphere’s trade winds, which blow moist air against the north-east slopes of all the Hawaiian islands.
Forced upwards on the other side of Kohala, the air cools, condenses into clouds and rains out all of its moisture as it crosses the summit, created a dry rain shadow on the south-west slope. This orographic precipitation, or “relief rain”, is hardly unique to Hawaii, but the combination of a very tall volcanic mountain, isolated in the middle of an ocean, and fairly constant wind patterns do mean that on the Big Island and other ocean islands experience very large changes in rainfall over exceedingly short distances. From where I was standing, it was less than 10 miles to the other side of Kohala as the crow flies, but along that line the average annual rainfall increases 20-fold (up to 4 metres, compared to 20 cm or less).

Mean annual rainfall for the Big Island.

Mean annual rainfall for the Big Island. Perspective view from the north: Kohala is the promontory in the right foreground. Google Earth overlay from the Rainfall Atlas of Hawaii (http://rainfall.geography.hawaii.edu/)

The people who guided our trip around Kohala are interested in how these large variations in rainfall affect the development of the volcanic landscape. One obvious effect is on the rate of soil development. The lava in this roadcut is probably around 100,000 years old, and whilst weathering in the dry conditions here has produced some top-soil, it’s fairly thin – perhaps a metre or so. Up near the summit, more rainfall has led to a lot more soil.

Soil on the summit of Kohala

Thick soil horizon revealed by a slump on the summit of Kohala, Big Island, Hawaii. Photo: Chris Rowan, 2012.

It’s easy to see why you can get some pretty cool science done in a place with high climatic variability and an easily datable landscape. It’s relatively straightforward to trace a lava flow across many different climatic zones, and since you know that the composition of the initial bedrock and the elapsed time are the same, any changes you observe in processes like soil development will be almost entirely be due to differences in rainfall. It’s rarely so easy to untangle all of these variables. I guess this is why our trip guides endure the extreme hardship of a field season on Hawaii: it gives them the rare opportunity to do fieldwork with good experimental controls. Yes, that must be the reason…

]]> http://all-geo.org/highlyallochthonous/2012/03/scenic-saturday-from-desert-to-verdant-grassland-in-10-miles-and-1000-m/feed/ 6 The humbling legacy of the Tohoku earthquake http://all-geo.org/highlyallochthonous/2012/03/the-humbling-legacy-of-the-tohoku-earthquake/?utm_source=rss&utm_medium=rss&utm_campaign=the-humbling-legacy-of-the-tohoku-earthquake http://all-geo.org/highlyallochthonous/2012/03/the-humbling-legacy-of-the-tohoku-earthquake/#comments Tue, 13 Mar 2012 18:24:43 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7349 Continue reading →]]> A post by Chris RowanA year ago on Sunday, one of the biggest earthquakes ever recorded ruptured the subduction megathrust that dips beneath the east coast of Japan. The rupture displaced the seafloor by tens of metres and generated tsunami waves up to 20 metres high, which hit the coast less than an hour later with devastating effect. My abiding memory of the morning I woke up to news of the earthquake, other than the near-constant pinging of my iPhone with alerts for all the aftershocks, is watching a helicopter video of the tsunami, having overwhelmed any coastal defenses, rushing across the Japanese countryside, sweeping away everything in its path. It was hard to believe wasn’t just something generated in a computer for a new disaster movie.

There were 19,000 casualties, with an additional 3,155 people still listed as missing; more than 370,000 buildings were destroyed or damaged; 300,000 people are still homeless. The estimated total cost to the Japanese economy is more than 300 billion US dollars. All this in a country that is well aware of its violent seismic history, and has taken the risk of future large earthquakes very seriously indeed. This investment was not in vain: compare the casualties and damage caused by the Tohuku event to the magnitude 9.2 Sumatra–Andaman earthquake on Boxing Day 2004, and the resulting tsunami that killed 230,000 people; or the much, much smaller magnitude 7 earthquake that struck Haiti in January 2010 and killed over 300,000. Japan’s strict building codes, sea walls, warning systems and preparedness drills were surely mostly responsible for this order-of-magnitude difference, and yet it still wasn’t enough. That is a humbling reminder of how even our most advanced societies are still at the mercy of the planet we occupy.

But the Tohuku earthquake has handed earth scientists an even more humbling lesson. One reason that the tsunami overwhelmed Japan’s coastal defences so thoroughly is that it met seawalls whose designers had underestimated how big a wave they might have to repel; an underestimate that was based on our faulty understanding of how big faults actually behave. Here’s how we thought it worked: long plate boundary faults such as the subduction megathrust beneath Japan could be divided into discrete segments that behaved quasi-independently. Each segment produced earthquakes of a characteristic size with a characteristic average repeat time, although an earthquake on one segment could affect the timing of rupture on its neighbours due to changes in the local stress.

This understanding was based on our instrumental and historical records of earthquake activity. These records have relatively short durations compared to the timescales of strain build up and release on large faults, so we’ve always known that extrapolating recent behavour over longer time periods might not give us the full picture. But what Tohoku and other recent megaquakes have shown is that it’s not just parameters such as the length of the earthquake cycle on individual segments that can vary over longer timescales, but the nature of the segmentation itself. The really big earthquakes seem to be cases where several segments of a fault all rupture in concert; even more disconcerting is the fact that magnitude 7 and 8 earthquakes every few decades or centuries along a particular stretch of the subduction zone does not rule out a magnitude 9 occuring in the same region every couple of millenia. Rather than being a simple saw-tooth wave of slow strain build-up and rapid release, we’re starting to see that faults sing a much more complex song, with a number of different frequencies mixed in. For most of the time we’ve been recording earthquakes, we’ve been hearing the high notes; we’re only now becoming aware of the base line – the longer earthquake cycles that govern larger, more potentially catastrophic ruptures.

The past decade has been full of rude surprises for geologists: starting with the 2004 magnitude 9.2 Sumatra–Andaman earthquake, the earth has rung with the vibrations of 5 quakes of magnitude 8.5 or greater. As Thorne Lay puts it in this week’s Nature, The Tohuku earthquake is just the most recent in this sequence, all of which “have violated some theories of where and when great earthquakes can occur and what their consequences can be.” We’re starting to realise that there is a lot more tectonic and seismic complexity than we thought in the space between the smooth motion of plates over million-year timescales and the jerky motion of faults over decades, centuries and millennia, and that looking at the long-term behaviour of dangerous faults using tools like paleoseismology (which had uncovered evidence of past events that were similar in scale to last years tsunami, even before it happened) is just as important as measuring contemporary deformation using GPS.

But the most important lesson is this: that no matter how much we think we know, our planet still has an infinite capacity to surprise us.

]]> http://all-geo.org/highlyallochthonous/2012/03/the-humbling-legacy-of-the-tohoku-earthquake/feed/ 4 Stuff we linked to on Twitter last week http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-41/?utm_source=rss&utm_medium=rss&utm_campaign=stuff-we-linked-to-on-twitter-last-week-41 http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-41/#comments Sun, 11 Mar 2012 18:29:17 +0000 Chris Rowan http://all-geo.org/highlyallochthonous/?p=7343 Continue reading →]]> A post by Chris RowanA post by Anne Jefferson

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Anniversary of the Tohuku Earthquake

It is one year today since a magnitude 9.0 earthquake and accompanying tsunami devastated the east coast of Japan, prompting many stories about the event and its aftermath.

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]]> http://all-geo.org/highlyallochthonous/2012/03/stuff-we-linked-to-on-twitter-last-week-41/feed/ 0 Scenic Saturday: The Temple http://all-geo.org/highlyallochthonous/2012/03/scenic-saturday-the-temple/?utm_source=rss&utm_medium=rss&utm_campaign=scenic-saturday-the-temple http://all-geo.org/highlyallochthonous/2012/03/scenic-saturday-the-temple/#comments Sat, 10 Mar 2012 12:20:16 +0000 Anne Jefferson http://all-geo.org/highlyallochthonous/?p=7332 Continue reading →]]> A post by Anne JeffersonRight now I have a graduate student working on a project to understand the effects of stream restoration in altering patterns of groundwater-stream exchange. She’s working in four stream reaches with varying restoration patterns and watershed land uses. In one of her streams, there is a restoration structure she calls “the temple.” I’d walked the lower bit of stream and the upper bit of stream, but somehow I’d managed to miss this feature that is having remarkable effects on the transient storage dynamics of the stream. This week, I rectified my omission and visited the temple.

Stream restoration in BD3

The stream restoration feature known as "the temple". The white pipes are our piezometers for measuring exchange between stream and subsurface water. Photo by A. Jefferson, March 2012.

What the photo above doesn’t fully capture is how large a volume of water is stored behind each of the rock step structures. This picture was taken as water was receding after a pretty high flow event (notice all the debris trapped at the top of one step), but at lower flows there is little or no water going over the top of the steps. Instead, all of the water goes under or around the rock structures and pools of water more than 1 m deep occur between each of the structures. Such big pools at such low flow volumes could have a dramatic effect on things like stream temperature and nutrient dynamics.

This structure is all the more remarkable because it occurs at an interesting geomorphic transition in the stream. Upstream of the temple, the stream is restored using typical features like cross-vanes and riffle/pool features. It is low gradient and not confined in a valley.

A large pool and riffle

Upstream of the temple, restoration structures include a riffle (in the background) and cross vanes (foreground, here submerged by high water). Photo by A. Jefferson, March 2012.

Immediately downstream of the temple, there is a big pool and then a long reach floored with bedrock (obviously not restored). Downstream of the bedrock reach, the stream crosses into the floodplain of a larger watershed and has lots of fine grained alluvium to contend with.

Stream with pools and exposed bedrock in a forest

Downstream of the temple, the bedrock-rich stream reach. Photo by A Jefferson, March 2012.

Thus, it appears that “the temple” restoration feature is placed at an important geomorphic transition in this stream. It’s in the place where the stream briefly enters a more confined valley and it’s the steepest part of the stream. In other Piedmont streams, I’ve seen bedrock cascades in some places like this, but the stream restoration designers wouldn’t have covered over a feature like that. Instead, maybe there was a knickpoint retreating through soil, saprolite or colluvium that could cause a lot of potential instability in the reach above. The “temple” feature then would be a way of preventing further knickpoint retreat by creating a short high-gradient section. I really wish I’d seen this area before the stream was restored. Instead, I marvel at the highly engineered form of the stream as it passes through the temple, and look forward to seeing what my grad student finds out about the effects of this structure on the transient storage and water quality in the stream.

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