Last time on Disaster Geology, we saw how hurricanes keep geologists busy. In this edition, we're going to zoom in on Hurricane Harvey, and observe some of the profound effect hurricanes can have on the earth beneath us, as well as on our cities and citizens. We're also going to see how humans are, in some ways, like a hurricane ourselves.

The Day the Earth Got Squished

Hurricane Harvey dumped a staggering amount of water on southern Texas when it stalled near the coast. The Jet Propulsion Laboratory determined that the area hit was depressed about two centimeters by the enormous amount of rain that fell. It's hard to imagine that rain can cause that much geomorphological change – especially when we measure the uplift of mountains caused by colliding continents in just millimeters per year. Imagine the force it takes to change the elevation of a large area by almost an entire inch!

Why this would happen is simpler than you might think. A gallon of water weighs about 8.34 pounds. And by one estimate, Harvey dropped 33 trillion gallons of water across the area it hit. So that’s roughly 275 trillion pounds.

And it turns out that scientists have measured the effects of loading a bunch of water onto land many times. For example, a 2012 study of the Himalayas detected a seasonal flux in the height of the mountains as water fell, and then ran off those mountains into Asia’s rivers.

 

But it's not just a simple mass=depression calculation:

There are some caveats, Milliner later explained. Some of the change could come from the soil underneath the GPS stations becoming compacted by the water’s weight. But because some stations located on bedrock also experienced the depression, he believed that the key mechanism was crust deformation. It’s also possible, he elaborated, that some areas outside Houston were pushed up by the way the water squished the Houston crust.

That's a heck of a lot of geomorphological change happening in the course of a few days! Ad if your mind isn't completely blown yet, the article goes on to show how humans can have the same effect on the landscape as a hurricane. We're one of the few mammals who can have such a dramatic impact on our lived environment (pocket gophers are another).

You can see the extent of the flooding on these NASA maps and USGS Landsat images. The USGS recorded record-high flow on nearly two dozen rivers, streams, and bayous. And you can see how hurricanes are part of the geologic processes that shape our world by viewing the stark before-and-after photos here.

Humans + Hurricanes = Extra Disaster

Hurricanes have a huge impact on people, but we're also having an impact on hurricanes – and it's not a good one.

Anne Jefferson has analyzed the effect humans might have had on Hurricane Harvey in two hard-hitting articles. In the first, she assesses the evidence for climate change, and makes a strong case that our pumping greenhouse gasses into the atmosphere caused Harvey to be stronger and more destructive than it might have been otherwise.

In the second, she turns to her specialty, hydrogeology, and determines what impacts humans and our construction had on the flooding.

Urban development that encroaches on natural water storage areas – floodplains and wetlands – makes flooding worse elsewhere. (And more to the point, it puts people and property in harm’s way.) If you fill in the low areas, install drainage pipes to shunt water elsewhere, or build levees to keep water off the floodplain, then that water has to go somewhere, and in fairly flat areas like Houston, that somewhere will become everywhere.

As Anne points out, it's a complicated story. But it's well past time we got better about reducing our greenhouse gas emissions and building safer, more sustainable cities.

 

Hurricanes and Earthquakes, Oh My

David Bressan has some fascinating facts about the relationship between hurricanes and earthquakes:

According to some preliminary research published in 2013, hurricanes can displace enough mass and change the local tectonic stress field, increasing the risk of smaller earthquakes. After Hurricane Irene hit Virginia in 2011, more small earthquakes were registered in the region. Just one week earlier, a 5.8 earthquake was felt in the eastern US and southeastern Canada, among the largest events to occur in this region in the last century. The 5.8 earthquake possibly ruptured various smaller faults in the area. As air pressure changes caused by Irma put additional stress on the activated faults, they slipped more easily, triggering minor aftershocks.

Hurricanes can also be detected with seismometers, used in earthquake detection. On Tuesday, when Hurricane Irma was upgraded to a Category 5 storm, the noise produced by 175 mph fast winds and waves hitting the coast, was picked up by seismometers on Guadeloupe, an island in the southern Caribbean Sea.

Yep. For a meteorological phenomenon, hurricanes surely do have quite the impact on geology.

Satellite image of Hurricane Havey. The storm has a tight center with a well-defined eye, but its rain bands look thin and diffuse. It's a huge storm looming over the Gulf and the Texas coast.
Hurricane Harvey on August 25th, on the verge of making landfall on the Texas coast. Credit: NOAA Satellites