Stuff we linked to on Twitter last week

Categories: links

Stuff we linked to on Twitter last week

It’s time to dust off the blog, but let’s ease into it by renewing our committment to sharing all of the interesting things we see on Twitter every 7 days. Here’s a global roundup.

Categories: links

The geodetic fingerprints of shallow thrusting in Nepal

NASA’s Earth Observatory put out this great image last week, which shows the ground displacement in Nepal resulting from last month’s devastating magnitude 7.8 earthquake, which has claimed at least 8,500 lives. The vertical displacements have been calculated by comparing radar altimeter data from satellite passes before and after the earthquake happened.

Displacements in land surface due to the M7.8 Gorkha earthquake, measured by satellite radar altimetry. Source: NASA

Displacements in land surface due to the M7.8 Gorkha earthquake, measured by satellite radar altimetry. Source: NASA

One of the striking things about this image is how, even if this was the only information you had about the earthquake – no geological cross-sections, no focal mechanism – you could still tell that you are looking at the results of a large rupture of a thrust fault that shallowly dips to the north. The broad zone of uplift in front, with an equally broad zone of subsidence behind, is a clear signature of this kind of structure. To understand why, you first have to think about what is happening in the build up to this earthquake. Plate tectonic forces are constantly driving India northwards, thrusting it under the Eurasian plate. The contact is a gently northward dipping fault zone that is usually locked by friction down to depths of 15–20 km or so; this means that the motion of the underlying Indian plate will also drag crust on the overriding Eurasian plate down and to the north with it, causing subsidence above the locked portion of the fault. This effect dies off further north as the fault gets deeper and weaker – higher temperatures mean ductile rather than brittle deformation dominates, so the fault is no longer ‘strong’. In this region, land squashed between India-driven motion to the south, and strong Eurasian crust to the north, is squeezed pushed upwards.

Deformation of the land surface above a frictionally locked, shallowly dipping thrust fault. Subsidence above the shallow part of the fault is balanced out by uplift down-dip of the locked zone.

Deformation of the land surface above a frictionally locked, shallowly dipping thrust fault. Subsidence above the shallow part of the fault is balanced out by uplift down-dip of the locked zone.

However, this is (mostly) elastic strain – remove the forces causing this change of shape, and it will bounce back to its original shape. This is exactly what happens when the accumulated strain across the fault becomes large enough to overcome its friction, and it ruptures in an earthquake. The region above the previously locked thrust pops back up as it catches up with decades or centuries of plate motion; and the uplifted crust to the north drops back down as the tectonic vice it was caught in relaxes.

Deformation of the land surface associated with a large earthquake on a shallow thrust fault. Release of accumulated strain causes uplift above the shallow part of the fault and subsidence further down-dip.

Deformation of the land surface associated with a large earthquake on a shallow thrust fault. Release of accumulated strain causes uplift above the shallow part of the fault and subsidence further down-dip.

To make things even clearer, you can quite nicely project my schematic cross-section above onto the broad patterns of uplift and subsidence seen in the radar data.

How the pattern of land displacement measured by radar altimetry matches up well to the pattern expected from an earthquake on a shallowly dipping thrust fault.

How the pattern of land displacement measured by radar altimetry matches up well to the pattern expected from an earthquake on a shallowly dipping thrust fault.

What makes this even more impressive is how quickly we have this picture – a few decades ago, you’d need months – or years – of levelling surveys to work out how the land surface had changed elevation after a major earthquake. Now we’re only limited by how soon an orbiting radar satellite is going to pass overhead.

Categories: earthquakes, geohazards, structures, tectonics

Of time and rivers flowing

This afternoon, I needed to get out and enjoy the early spring air and sunshine. I popped on some headphones and opened up my current favorite podcast, Backstory. Delighted, I discovered that the most recent episode focuses on America’s relationship with freshwater resources. It starts with a primer on the quirks of America’s water law, relays the fascinating misfortune of colonial Jamestown’s water problems (and how brewing women saved the day), and the lasting legacy of racial segregation in public pools. It’s definitely worth a listen, even if there are a few too many water puns mid-show.

HAGeoDog determinedly steered me towards the trail along the Cuyahoga River, which winds through downtown Kent. The river and its clear influence on our town’s history provided the perfect visual setting from the stories I was hearing in the podcast. While I’ve taken this walk dozens of times in the nearly three years I’ve lived in Kent, I always find new things to appreciate along the way. Here are a few of today’s musings.

When I moved to glaciated northeastern Ohio, the last thing I was expecting was bedrock slot canyons, but here they are. There is a short reach of gorge within Kent, and a much more impressive gorge downstream in Cuyahoga Falls and Akron (but that is a story for another time). The stretch in Kent is known as Brady’s Leap, for a multi-layered story of an Army captain leaping the river in a single bound. The river is wider now then it was when it was leapt, having been blasted during construction of the Pennsylvania and Ohio Canal, and it is surely unleapable, but it is still unexpected and beautiful.

Bedrock gorge at Brady's Leap, Cuyahoga River (Photo by A. Jefferson, March 2015).

Bedrock gorge at Brady’s Leap, Cuyahoga River (Photo by A. Jefferson, March 2015).

A little way downstream, I can stand on former reservoir sediments in what is now a park, an look upstream at the Main Street Bridge. As I do, I can contemplate both what this place looked like until the dam came out in 2004, with much higher and stiller water than we see today, and the importance of river crossings in tying together the Kent community. No one else was going to leap to get across the river, so bridges were key, and their number has fluctuated over the two centuries of white settlement in this spot. Today, our town as 4 of them, and when one is closed as the Main Street bridge was all summer long, the river becomes a pretty good barrier to moving about.

Main Street bridge, Kent. (Photo by A. Jefferson, March 2015)

Main Street bridge, Kent. (Photo by A. Jefferson, March 2015)

But rather than barriers, rivers have long been transportation corridors, and the Cuyahoga is no exception.

Rivers have long been transportation corridors. Historic Kent depot behind the modern freight train running next to the river, with its remnant lock system. (Photo by A. Jefferson, March 2015).

Rivers have long been transportation corridors. Historic Kent depot behind the modern freight train running next to the river, with its remnant lock system. (Photo by A. Jefferson, March 2015).

The lock, which dated from the short-lived Pennsylvania and Ohio Canal era (~1840-1857), blew out in the Great Flood of 1913 (see here for a photo of that event). But the lock is what saved the dam in 2004, because taking out the lock structure enabled the river to be free flowing through town, achieving the restoration objectives while preserving the heritage of the dam itself.

Kent dam. The city will turn on a recirculating pump and make water appear to flow over it when the weather is warmer. (Photo by A. Jefferson, March 2015)

Kent dam. The city will turn on a recirculating pump and make water appear to flow over it when the weather is warmer. (Photo by A. Jefferson, March 2015)

Looking downstream from the dam, the river appears to split into two channels with an island in between. When I first moved to town, I was puzzled by this apparent island braided reach, but I have since learned that this is yet another place where the legacy of the canal continues to shape the river. The left channel is the old canal itself, while the right thread is a pretty little mixed bedrock-alluvial reach.

Looking downstream from the Kent dam (Photo by A. Jefferson, March 2015).

Looking downstream from the Kent dam (Photo by A. Jefferson, March 2015).

It’s clear that while humans may have created the island 160+ years ago, the river is working to reshape it of it’s own accord. Several channels have been cut through to join the left and right threads of the river, and at the bottom of the island, there’s a beautiful gravel and boulder bar that changes every year.

Boulder bar in the Cuyahoga River, downtown Kent. (Photo by A. Jefferson, March 2015)

Boulder bar in the Cuyahoga River, downtown Kent. (Photo by A. Jefferson, March 2015)

If I’ve learned one thing from thinking about the interweaving of human history and rivers, it’s that things never stay the same for very long. As we contemplate what can seem like insurmountable problems of politics, infrastructure, environmental degradation, or life, I think we would be wise to spend time by the river, flowing endlessly on while the trappings of our society come and go in its course.

If you like the intersection of history and geology, you might also enjoy this post from 2009 on Landsford Canal in South Carolina.

Categories: by Anne, geomorphology, outcrops, photos, society

A year of Anne’s reading…reporting from 2 months in

A post by Anne JeffersonDespite January 1st being the middle of the school year, academics can’t resist making New Year’s Resolutions. “I will read more papers” seems to be a perennial one for many folks. Jacquelyn Gill vowed to read a paper a day in 2015 and started the hashtag #365papers to track her efforts and those others daring enough to join her. I *know* I won’t read 365 papers this year, but I thought I’d use the tag to catalog my reading for the next 12 months. I’ll be updating the storify below irregularly throughout the year, so that I have a catalog of my reading and you can follow along at home. So far it’s shaping up be a bit of an eclectic mix of urban hydrology, geomorphology, and social science!

I also promised myself I’d read 12 actual, honest-to-goodness books in 2015, and I’m a few books into that quest as well. Those are being interspersed in the storify too. Recently, Jacquelyn (instigator of all my memes, apparently) and Kate Clancy shared this article “I challenge you to stop reading White, Straight, Cis Male Authors for One Year.” Since the first two books I read in 2015 were by women, and the one I’m currently enjoying is also woman-authored, I decided to hop on the bandwagon and make my 12 books a litany of lady literature.

Enough with the preamble, here are the papers and books I’ve read in the first two months of the year.

A reverse chronological order listing of my 2015 adventures in academic reading.

Categories: academic life, by Anne