A ‘seiche’ wave can outpace a tsunami, and both can be triggered by meteorites and earthquakes



File 20190403 177184 r6mkdz.jpg?ixlib=rb 1.1
Waves can be generated in lakes and other bodies of water when seismic energy travels through land.
Leo Roomets / Unsplash, CC BY

Craig O’Neill, Macquarie University

A catastrophic event occurred on Earth 66 million years ago. A huge meteorite struck our planet in what is now Mexico, triggering mass extinctions of the dinosaurs and most other living creatures.

A new paper shows the first recorded victims of this impact were fish and other marine animals, stranded by a wave that left them high and dry in an ancient river in North Dakota, at a site called Tanis.

For scientists unpacking the evidence around the event, a full picture of the cataclysm has involved looking into the details of planetary surface physics during giant impacts.

But beyond the first layer of fascinating results – little glass impact beads stuck in the gills of fish, for example – one really interesting aspect of this work is around how water behaves when it’s exposed to extreme forces.

If you’ve never heard of a form of wave called a seiche, this is your chance to catch up.

This is a seiche – a standing wave – in a swimming pool, during a large earthquake in Nepal.

Waves of damage

The Chicxulub meteorite crater in coastal Mexico is strongly associated with the mass extinction of the dinosaurs (and 75% of all species), 66 million years ago.

The first victims were right at the site. Any marine creatures close to the point of impact would have been instantly vaporised (sadly leaving no fossil record), along with much of the surrounding rock.

Around the periphery, the energy of the impact melted and ejected tonnes of molten rock, which together with condensing rock vapour, formed little glass beads (“impact spherules”) that can be found in a layer around the world at this time.

The shock wave itself pulverised the adjacent rock enough to metamorphise it, forming features like “shocked quartz” – fractured quartz indicative of enormous pressures. It carried the energy equivalent of a magnitude 11 earthquake – 1,000 times more energy than the 2004 Boxing Day quake which killed almost 230,000 people.

Vast inland sea now gone

North Dakota is more than 3,000km away from the Chicxulub crater, and was a similar distance at the time of the meteorite impact event.

Separating them back then, however, was a vast inland sea that covered much of midwest USA, from Texas up to the Dakotas. Feeding into that inland sea was a river system upon which the Tanis site in North Dakota was formed. This site has preserved the earliest recorded deaths of the Chicxulub impact.

Different views of the Tanis site. A: Tanis (starred) within a regional context (large map) and on a national map (inset). B: Photo and interpretive overlay of an oblique cross-section through Tanis. C: Simplified schematic depicting the general deposits at the site (not to scale). Most fish carcasses were found at point 3.
Robert A DePalma and colleagues

The site itself is unusual. The deposition of sediments can tell us about the flow of water in the river.

Most ripples (or flame structures) indicate a southerly flow of the river before and after the Tanis deposit. However, these flow indicators point the wrong way during the time the Tanis unit formed. Water was flowing upstream, fast.

At the site are also found the fossilised remains of species, like sharks and rays, that occupied brackish water, rather than the freshwater of the stream. These had to be brought inland from the sea by something, and left to die, smothered in sediment, on a riverbank.

Stranded in Dakota

The obvious candidate is an impact tsunami. Perhaps the impact of the meteorite hitting the ocean generated a huge wave that carried fish from the inland sea, and against the flow of fresh water, to leave the creatures stranded in Dakota?

But there are problems with this hypothesis. The tiny impact spherules that formed in Chicxulub can be found throughout the deposit (many clogging the gills of fish), and pockmarks in the sedimentary layers means rocks were still raining down. This means the surge of water occurred within around 15 minutes to two hours of the impact itself.

For a tsunami to travel the 3,000km from the point of impact, to the Tanis site across the inland sea, would have taken almost 18 hours. Something else killed these creatures.

The seismic waves from the impact would have travelled through the Earth much faster than a tsunami travelled across water – and arrived near Tanis between 6-13 minutes later. The authors of the Tanis study suggest these seismic waves may have triggered an unusual type of wave in the inland sea, called a seiche.

Standing waves

Seiches are standing waves in bodies of water, and are often found in large lake systems during strong winds. The winds themselves cause waves and water displacement, which can have a harmonic effect, causing the water to slosh side to side like an overfull bathtub.

However, earthquakes are also known to cause seiches. Particularly dramatic seiches are often seen in swimming pools during large quakes. The interaction of the seismic wave’s period (the time between two waves) with the timescale of waves sloshing in a pool can amplify their effect.

But seiches can affect larger bodies of water too.

During the 2011 Tohuku earthquake in Japan, seiches over 1m high were observed in Norwegian fjords more than 8,000km away. With an energy more than 1,000 times greater, the Chicxulub event could quite conceivably have generated bigger than 10 metre swells in the North American inland sea – the scale implied by the deposition of the Tanis site.

These waves in Norwegian fjords were created by seismic waves from the 2011 Tohoku earthquake in Japan.

Given a seiche can be driven by seismic waves, it’s conceivable that one drove the surge that stranded marine creatures at Tanis, resulting in the short time between the impact debris and the surge deposit.

Still lots of questions

But a lot remains unclear regarding exactly what did happen 66 million years ago.

Could the fish stranding have been driven by the first seismic activity to appear at Tanis (the P and S waves in science parlance, which travel through the interior of the Earth, arriving at Tanis 6 and 10 minutes after impact, respectively), or the more destructive but slower surface waves at the top of the Earth’s crust, which arrived 13 minutes after impact?

How might seiche waves have interacted with global hurricane-strength wind storms caused by the impact?

Would the period of sloshing of a seiche be consistent with the scale of the inland sea? (The inland sea was much larger than most lakes seiches are traditionally observed in – and may or may not have been open to the ocean). Given so little is really known about the dimensions of the inland sea, this is hard to constrain.

The Tanis site has given us an incredible window into the first few hours of a mass-extinction. But it has also highlighted how little we have probed into the fatal surface physics of these extreme events.The Conversation

Craig O’Neill, Director of the Macquarie Planetary Research Centre/Associate Professor in Geodynamics, Macquarie University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

How to find a meteorite that’s fallen to Earth


Phil Bland, Curtin University

A bright fireball lit up the night sky around Kati Thanda (Lake Eyre South) in South Australia on November 27, 2015.

But how to find the impact site of that meteorite? And how can we know where in the solar system the object came from?

Thankfully, a new meteorite tracking system we’ve installed in Australia has enabled us to answer these questions, helping us better understand the history and composition of our solar system.

Meteorites are the oldest rocks in existence. They contain a unique physical record of the formation and evolution of the solar system, and the processes that led to terrestrial planets.

They sample hundreds of different heavenly bodies, a compositional diversity that spans the entire inner solar system.

But the most basic piece of data – context – is absent. In almost all cases, meteorite researchers have no idea where their samples came from.

What they need are orbits and the ability to track meteorites back to their place of origin in the solar system. The goal of the Desert Fireball Network is to provide that data.

A network of ‘eyes’

This is a project that started in 2012 and since then we’ve installed a network of 32 automated observatories in remote areas of Australia. They are capable of operating for 12 months without maintenance, storing all imagery collected over that period.

The locations of some of the automated camera stations.
Desert Fireball Network (clickable map available), Author provided

Although they are high resolution intelligent imaging systems, they cost around A$5,000 each, which is only a fraction of the cost of previous systems. We’ve completely automated data reduction, so we can potentially scale up the system to arbitrary size without needing hordes of poor PhD students doing manual labour.

And members of the public can contribute by sending in their own reports via a smartphone app that we’ve developed called Fireballs in the Sky.

Trying to track an object moving at many kilometres a second, from the edge of the Earth’s atmosphere to the surface, isn’t easy. You have to account for everything from minor distortions in the camera lenses, to the effect of winds blowing the object off course when the light has gone out.

We would only know that it worked when we found a rock on the ground.

One of the automated cameras keeping watch on the sky.
Desert Fireball Network, Curtin University, Author provided

A green flash in the sky

When that fireball lit up the skies above South Australia in November, it was imaged by five Desert Fireball Network automatic observatories. The stations sent alerts to our server in Perth, attaching thumbnails of the fireball image.

With data from just a couple of cameras, we could tell pretty quickly that we had a meteorite on the ground. First, we had to get out to South Australia to pick up additional data from cameras that weren’t online, so that we could precisely triangulate the fireball.

We took a light aircraft flight from William Creek, which showed us that there was a feature on the surface that might be where the rock plunged into the mud. Now we had to get out on the lake.

Some of our team set to work pulling together all the data. The more accurately we could pinpoint the fall position, the easier any search would be. Their analysis showed that the object came in at a very steep angle, with a velocity of 50,000km/h, and punched down low in the atmosphere, still visible as a fireball at 18km altitude.

When it entered the atmosphere, it was about 80kg. At the end of the fireball it had more likely been whittled down to between 2kg and 6kg.

Alongside the effort to work all this out, we were putting together logistics for the trip. We knew we had to get there quickly. There had already been rain. Much more of it and any trace of the rock might be wiped away.

In addition, Kati Thanda has spiritual significance for the Arabana people. We would need their permission before we could go out on the lake. But the Arabana understood the urgency, and gave consent almost immediately. The Arabana guides, Dean Stuart and Dave Strangway, who came with us on the trip were a huge help.

The search is on

We got to the lake shore on December 29. But the lake doesn’t have a firm surface; it’s thick mud. We had to pick our way out to the fall site – almost at the centre of the lake – trying to find a route that would support a quad bike. Eventually, we found a way in.

Next day we got to the site, and searched the area, but didn’t find any trace of the feature that we’d seen a couple of weeks before from the air. Time was running out. Rain was coming in. We figured we might have just have one more day left.

Professor Phil Bland and PhD student Robert Howie digging the meteorite out of the mud in the middle of Kati Thanda (Lake Eyre) South.
Jonathan Paxman, Desert Fireball Network, Author provided

So we decided to double down: one of our team would fly over the site, while two of us would search on the ground. If they saw anything from the air they would radio, circle the spot, and we could check it immediately.

It was overcast and drizzling as we headed out to the shore, but heavy rain held off long enough for us to get to the fall site. For an hour, the plane just circled.

Then we got a call that they’d seen it. We ran to the spot, and found the last remnant of the feature that our friend had seen a couple of weeks before. The meteorite had punched a deep hole in the mud.

Digging down through that pipe my fingers eventually touched a rock. We’d found our meteorite. The rock is 1.6kg in weight, a bit lighter than we’d expected, and it’s probably an ordinary chondrite, the most common type of meteorite. But we need to do some analyses to tell for sure.

The 1.6kg meteorite close up.
Desert Fireball Network, Curtin University, Author provided

An unexpected surprise

We didn’t know it when we built the network, but it turns out it can do a lot more than we ever expected. We can track satellites, space debris and rocket launches. We’ve even tested systems that will let us do fundamental astronomy. And, with a minor upgrade, we’ll have a facility that can spot supernovae and optical counterparts to gamma ray bursts.

But it’s the potential for planetary research that still gets us excited. Already, we’ve seen more fireballs than have ever been recorded up to now, giving us a unique window on what’s hitting the Earth.

As we recover more rocks, we will gradually build a geological map of the inner solar system. If we can link a meteorite to an asteroid, then we essentially have a sample-return mission to near-Earth asteroids, without the need for spacecraft.

This first rock we’ve recovered is just the start. In itself, it’s a research gold mine. But it also proves that our system works so there should be many more.

The Conversation

Phil Bland, ARC Laureate Fellow, Curtin University

This article was originally published on The Conversation. Read the original article.