Establishing the rift valley and the mid-ocean ridge that went all the way around the world for 40,000 miles…You can’t find anything bigger than that, at least on this planet.
Lots of cool science history in this first-person account, but also some less cool stuff. Marie Tharp saw early on what the sea floor was telling us, but was dismissed with casual sexism?
Almost everyone in the United States thought continental drift was impossible. Bruce initially dismissed my interpretation of the profiles as “girl talk.”
Check.
Ideas taken much more seriously when a man starts to advocate for them?
We made profiles of some of the valleys in East Africa and noted the topographical similarities between the valleys in the ocean and on land. Bruce also noticed that the shallow earthquakes associated with the East African Rift fell within the valley walls. He began to endorse the existence of a continuous central valley within the mid-oceanic ridge.
Doc began to get interested at this point. He’d heard of this “gully,” as we called it, and he would pop into our lab from time to time and ask, “How’s the gully coming?”
Check.
Vulnerable people becong collateral damage in a feud between two male egos?
Now our efforts were thwarted by a long-lasting falling-out between Bruce and Doc. There are two sides to that story, but the result was that Doc banned Bruce from Lamont ships and denied Bruce access to Lamont data. He tried unsuccessfully to fire Bruce, who had a tenured faculty position at Columbia, but he did fire me.
Check out @IRIS_EPO's latest science highlight! Scientists discovered numerous previously unknown submarine landslides in the Gulf of Mexico that were triggered by earthquakes that were *super* far away! https://t.co/u62I5HFLVM
— Alka Tripathy-Lang, Ph.D. (@DrAlkaTrip) May 26, 2020
Earthquakes triggering landslides is not a surprise, but surface waves from a magnitude 5.5 earthquake in the Gulf of California triggering a landslide in the Gulf of Mexico (1500 km away) certainly is. This was but one of 65 earthquakes between 2008 and 2015 that were shown to trigger landslides (the landslides were identified and located from their seismic signature). The locations and characteristics of these earthquakes, from figures 4 and S6 of the paper, are shown below. I’m not sure if dominance of strike-slip is anything more than what you’d expect for tectonic situation in the source areas (not just the San Andreas and other inland strike-slip faults, but offshore fracture zones like the Mendecino)
Given that at the distances involved, the dynamic (temporary) stress changes from passing seismic waves will be very small, the local geology is a probably a big factor in amplifying their effects. In the Gulf of Mexico you have:
big piles of loose sediment, which will slow down & amplify passing seismic waves;
weak salt layers deeper down, which could act as easy failure surfaces for the overlying material;
and salt diapirs creating locally steep (and unstable) topography.
We have a region which is primed for submarine landslides. If only a small push is needed, perhaps distant seismic waves are enough to start things moving.
This study also shows the value of deploying (and maintaining) regional seismic networks: look at the cool stuff you find in all of that data!
One important thing to remember about this earthquake is that it happened in the era before we’d fully worked out plate tectonics. We were close, but there was still so much we didn’t quite understand.
Now we know that the deep trenches found off the coast of places like Chile are just the surface manifestations of giant thrust faults that dip very shallowly into the Earth – the subduction zones where the oceanic crust created at the mid-ocean ridges tens of millions of years before is returned to the mantle from whence it came. We didn’t know that then.
Now we know the recipe for giant earthquakes: you need fast plate motions and a long, shallowly dipping fault, which results in a huge surface area where the rocks stay shallow and cold enough to fracture, rather than flow, when stressed. Subduction zones are where these ingredients are present, so are where the Earth cooks almost all of the largest earthquakes. We didn’t know that then, either.
We also didn’t know that because the trenches are where these massive faults reach the seafloor, a large rupture pushes up a massive bulge of water that then rapidly collapses outwards – a tsunami. This amplifies and extends the deadly reach of the giant earthquakes. We – tragically – didn’t understand which earthquakes generate the large tsunami, and why.
This earthquake, and another large subduction earthquake in Alaska 4 years later, provided the key evidence for large horizontal motions on large thrust faults at subduction zones: a building block of what became plate tectonics. But these events also helped us to understand earthquake hazards much more clearly. We still can’t predict when big earthquakes will happen, but now we can understand where they happen, and what they do. This means we can save lives – through informed preparation before, warning of imminent tsunami during, and disaster response after. Today, an event like the Valdivia earthquake would be shocking, and devastating. But, at some level, we can at least comprehend that is it something the planet can do, in places like this.
We’ve come a long way since 1960.
The Earth’s long and drawn out response to a giant earthquake
Did you know that even after 60 years, the Earth is still ‘feeling’ the effects of this massive earthquake? Plate motions are slow and continuous, but the faults are not perfectly smooth. There is a stop-start cycle of slowly building up elastic strain that is suddenly released in abrupt, energetic earthquakes once enough strain has built up to overcome friction on the fault surface
It’s been 320 years since the last big Cascadia quake, and GPS data shows it building up for the next one. The subduction thrust is locked by friction, so the coast on the overriding North American plate is being pushed inland as it moves with the subducting Juan de Fuca plate.
Map of the Pacific North west, showing northwest motion of GPS stations above the locked Cascadia subduction thrust. Source: UNAVCO
When a giant earthquake earthquake finally happens, the accumulated strain is released and the deformed part of the overriding plate rapidly heads back seawards, as we see here for the M9 Tohoku earthquake in 2011.
But here’s where it gets interesting: that seaward motion doesn’t stop when the earthquake does. Here’s GPS motions for the three months following the 2011 magnitude 9 Tohoku earthquake. The motions are much slower – centimetres in months rather than metres in minutes – but they are still in the same direction, suggesting they are a continuation of the process set in motion by the rupture.
Seaward (south-east) motion of GPS stations in Japan in the three months after the 2011 magnitude 9 Tohoku earthquake. Red colours show where there is uplift accompanying the horizontal motion; blue where there is subsidence. Source: Ozawa et al., 2011
This is not an isolated observation: here’s data for a year after the 2004 magnitude 9.2 Sumatran earthquake which shows similar seaward motion.
Arrows show seaward (southwest) motion of GPS stations in Indonesia between early 2005 (just after the December 2004 M9.2 earthquake) and early 2006. Source: Gunawan et al. 2014.
Some of this motion is due to afterslip: further motion on the fault surface itself. Many aftershocks are a response to this, but some weaker – or weakened – parts of the ruptured surface will creep aseismically as well. But the massive stresses imparted by these earthquakes also affect the deeper warmer parts of the Earth that flow – rather than fracture – in response to this change. Afterslip effects die away quickly with time, so after a few years this ‘visco-elastic’ response dominates. Furthermore, this flow can continue for a very long time from a human’s (or an earthquake’s!) perspective. Which is where the 1960 Chilean quake comes in. GPS data show that the inner coastal region is still moving seaward rather than landward.
Orange dotted line shows the transition from GPS stations currently moving landward (east, closer to coast) and stations moving seaward (west, further inland) in region ruptured in 1960 Valdivia earthquake (red star shows approximate epicentre). Source: Wang et al., 2007
This figure from an excellent overview of the “Subduction Earthquake Cycle” allows GPS data from Cascadia, Sumatra, and Chile at different points in their earthquake cycle to be directly compared. The only thing to be careful about is that there are two sets of arrows on these figures: the red arrows show the actual observed motion, and the blue arrows show the results of modelling that includes a visco-elastic component due to flow of the mantle.
Sumatra one year after a giant subduction zone earthquake; Chile after forty years; Cascadia after 300 years. Source: Wang et al 2012.
In Spring 2020, my Watershed Hydrology class transitioned to online in mid-March. This spurred me to create more blog posts and YouTube videos to provide content for the remaining units of the course. This substantial effort added to work I had been doing over the past several years to provide online resources to students in the class. Before we moved to fully online instruction, the goal of my blog posts were to free up class time for hands-on activities, by moving some of the methodological topics online. Obviously, after we moved online, the goal of my materials was to teach all of the content that I thought it was important for students in my class to know.
In this blog post, I provide links to the blog posts I had previously written for teaching purposes on precipitation, evapotranspiration and other topics and a listing of the blog posts I created during Spring 2020, including those with all of my resources on soil moisture and infiltration, streamflow generation, streamflow, and flooding.
How I taught Flooding online in Spring 2020 (this post has everything I did and includes links to the other blog posts under this heading, so if you want one-stop shopping, this is the link to click)
Note: The blog post above contains a number of links to other posts (not listed here) with many case studies of particular floods and flood generating mechanisms.
This post is part of a series in which I provide the details of each unit I taught post-transitioning to online in Spring 2020 in the Watershed Hydrology class at Kent State University. For more context about the course and my perspective on it, please read the introductory post. [I’ve added some bracketed notes about things I’d change up for a future online offering.]
[For 2020, this was the last section of my watershed hydrology course. (Sometimes, I have time for a specific unit on tracers and/or water quality). This is a little bit of a weird unit in that I included some content on analyses you can do with streamflow data before shifting to a focus on floods. This had more to do with evening out content vs. time than any innate splitting of streamflow analysis topics and/or grouping with floods. I do not expect students to do any of the analyses I talk about here, except some basic flood frequency analysis, but I want them to know the many ways that streamflow data are used and that they could explore in advanced class, in graduate research, or in their careers.]
In this unit, we explore:
more hydrological analyses you can do with streamflow data
basic flood definitions
flood frequency analysis (focus of associated problem set)
how we measure floods when there is no USGS gage (or it is wiped out by a flood)
what causes floods, and
how we manage floods
Learning Objectives
List several analyses that can be done with
streamflow discharge data, other than hydrographs and flow duration curves
Discuss how graphical- and tracer-based
hydrograph separations work and the insights gained from each
Identify some of the data inputs required for
watershed hydrological modeling, river forecasting, and hydraulic modeling
Describe the criteria used to distinguish
between National Weather Service flood levels
Demonstrate how to conduct a flood frequency
analysis and interpret the results of one
Identify why discussing floods in terms of
probability and not recurrence interval is considered the best practice
Give examples of situations that require caution
when calculating flood probabilities
Describe two methods that can be used to
estimate flood flows when there is no stream gage
Describe the common ways that meteorological
floods are generated and the spatial and time scales associated with each
Discuss
the role of topography and human land use in modulating flood dynamics
Identify
how climate change may be altering flood frequency and severity
Compare
flood control reservoirs, levees, and natural flood management as approaches
for managing floods
More hydrological analyses you can do with streamflow data
In this video, I talk about some of the ways that stream discharge data can be used to gain insight into processes occurring within a watershed. I talk about graphical and tracer based hydrograph separation, recession analysis, and hydrological modeling for what-if scenarios and river forecasting. You won’t know the details of how to do the analyses after watching the video, but you’ll know what sorts of possibilities exist.
Want to follow along with the slides? They are here (PDF), but I do recommend the video to get the context of the text and images on the slides.
More about river forecasting
The National Weather Service combines weather forecasts, watershed models, and stream gage data to make predictions of future flows at many locations around the US. This video takes you inside a river forecast center to learn more about how they operate.
What is the definition of a flood?
To kick off our course content on floods, start with this video. If you just want the slides, they are here (PDF).
How to do a basic flood frequency analysis
This was the need-to-know content for the last problem set for my students. If you just want the slides, they are here (PDF).
This is a blog post I wrote a while ago about flood frequency. It reinforces some of the things I talk about in my video lecture.
Some important cautionary notes about flood frequency analysis
Now that you know how to do a flood frequency analysis, you shouldn’t just blindly do one with any dataset you can find. Here I talk about some cautions.
In this short blog post, devastating flooding moving downstream in Pakistan’s Indus River watershed is an example of the timescales and effects of flood wave propagation. Flood wave propagation comes up in my cautionary notes video above.
The Red River of the North and its annual ice-jam floods comes up in my cautionary notes on flood frequency analysis video. Here’s a blog post I wrote if you want to read a little bit more about this interesting (and very flat) area.
How do we quantify floods when there is no streamgage?
At the end of my video on cautionary notes about flood frequency analysis, I mentioned some ways we can still estimate flood flows even when there’s no stream gage. The two videos below give you an overview of these important methods.
Hydraulic Models
This video discusses how hydraulic models are used for floodplain mapping and other engineering applications. Hydraulic models, which focus on the mechanics of flow within a channel(or floodplain!) are different than the hydrologic or watershed models I discussed in the “more streamflow analyses” video. Hydrologic models, that focus on the water balance in all parts of a watershed, can be used to provide the input data for a hydraulic model or even be coupled directly into the same computer program.
How to make an indirect measurement of a flood, using Manning’s Equation
In this video, a USGS hydrologist explains the basic theory and measurements needed to make an indirect measurement of flood discharge using the Manning’s equation. Please note that the constant of 1.486 only applies if you use English units. If you use metric units, the constant is 1. Because of course it is.
Note that there are a whole other set of flood generating mechanisms including dam breaks, glacial outbursts, landslide dams, and volcanoes that I don’t even cover here. You’ll just have to stalk my blog after the semester is over to see if I make good on my promise to write about them. Or ask me – I LOVE to talk about this sort of stuff.
Once you’ve read the blog post linked above, you may want to read more about some of the case studies I mention in it. I’ve linked many of them below for your combined reading ease. Note that these are all optional, but you may find them helpful to gain additional details on the concepts in the blog post above.
Optional reading if you want to know about one of the most spectacular floods ever. It will blow your mind.
How do we manage floods?
After all this talk of flooding, you might be asking, what can we do? This last section of the course is designed to help answer that and leave you with a realistic, but maybe hopeful, sense of how hydrologists, engineers, and environmental scientists can help manage floods by working with a watershed’s hydrology and landscape.
This video provides a good introductory overview of several approaches to flood management.
In this blog post, I use my hometown to discuss some of the risks of a heavy reliance on levees to manage flood risks.
How levees can make things worse
How do flood control dams and spillways work?
This video presents an engineering perspective on how flood control dams operate, how spillways work and how they are integral to the design and operation of dams.
One of the important things this video points out is the risk of dam or spillway failure. When such a failure seems like a real possibility, dam operators will do everything within their power to prevent the worst from happening, even if that guarantees that downstream flooding will get worse. This was part of the drama that happened in Houston during Hurricane Harvey.
[Note: I really wish I’d been able to accompany this content on the engineering of dams with some examples of how they change hydrographs in multiple different ways, but, frankly, I ran out of time to prepare that material and couldn’t find an already produced video or written piece that was appropriate. I’d love to know about one, if you’ve got one.]
Natural flood management: a (re)emerging trend
There are lots of opportunities for geologists and environmental professionals to become involved in natural flood management and many other exciting aspects of hydrology. Your training in watershed hydrology could be a launching point for a career managing water resources, protecting people, and improving the environment. But whatever you choose to do after this course is over, I hope that some appreciation for the way water moves through landscapes sticks with you. Thank you for being wonderful students this semester and know that I will be thinking of you for a long time after this semester is over.
Assessment
10 question multiple choice quiz, drawn from a bank of more than 10 questions. Students could take the quiz twice.
Problem set focused on flood frequency and qualitatively assessing uncertainties associated with estimating floods. This year, the problem set had the added bonus of a “record-breaking flood” on the stream we use in the problem set occuring after the end of 2019 water year. That allowed students to construct the flood frequency curve, then look up the 2020 flood discharge, calculate an estimated frequency for it, and then discuss how the assignment would change for next year’s class when that data point was included.
Questions on the final exam, including interpretation of a flood frequency graph.
Please respect my work
This work (my videos and blog posts) are licensed under an Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0). That means that you need to give appropriate credit if you use or modify anything I’ve posted here. It also means that you can’t use the material for commercial purposes. If you want to use other resources I’ve listed above, please respect the rights of the originators. If you want to use my sequencing of topics and resources in your class, by all means, go ahead.
Nice plan for content warnings on Mastodon and the Fediverse. Now you need a Mastodon/Fediverse button on this blog.
For lot's more videos on soil moisture topics, see Drs Selker and Or's text-book support videos https://www.youtube.com/channel/UCoMb5YOZuaGtn8pZyQMSLuQ/playlists
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Nice plan for content warnings on Mastodon and the Fediverse. Now you need a Mastodon/Fediverse button on this blog.