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Dr. Judith Hubbard Profile picture
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Jan 24 • 3 tweets • 3 min read
The eruption is over but the ground is still shaking - there have been dozens of #earthquakes around the #HungaTonga #volcano (magnitude 4.5-5), continuing today. 1/3

#GoogleEarth #Tonga #earthquake
Many are likely due to slip on existing faults or fracturing of new ones due to changes in crustal stress caused by the eruption. Others may be linked to magma movement, although that usually produces a more persistent signal called #volcanictremor. 2/3

volcano.oregonstate.edu/volcano-sounds…
I haven't seen any focal mechanisms for these events, which would tell us more about the type of #earthquake - it may be difficult to extract that information as these events are M5 or less, and seismic networks are sparse in this region. 3/3

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More from @JudithGeology

Dr. Judith Hubbard Profile picture
Jan 25
You might hear a geologist say: "We can't predict earthquakes."

Well, WHY NOT???

1/n
🧵⬇️⬇️⬇️

#earthquake #earthquakeprediction
First, let's define "prediction". A useful #earthquakeprediction will tell you where, when, and how big a significant #earthquake will be, with a reasonably high success rate.

That's different from #forecasting, #earlywarning, and #aftershocks. 2/n

To date NO ONE has developed an effective way to predict significant earthquakes.

Issue #1: Faults are fractal.

"But we know where the tectonic plates are!"

Yes, mostly, but plate boundaries are complicated - see the maps below comparing plates vs. faults. 3/n

#tectoplot ImageImage
Read 23 tweets
Dr. Judith Hubbard Profile picture
Jan 23
Oh, cool!

Stress is basically how much the rocks are being squeezed, and in which direction. If we can know that, and also know how *strong* the rocks are, we can estimate whether they will break.

But it's super hard to measure... 1/n
When rocks *do* break (#earthquake!), we can use that to estimate stress. If you know the direction of slip, you can do even better. This even works for earthquakes that occurred long ago, if they left scratches on the fault!

rickallmendinger.net/faultkin

2/n
Or, if you have a lot of money and time, you can drill into the Earth and measure the orientation of maximum squeezing based on how the borehole deforms.
#boreholebreakouts

3/n
jgs.lyellcollection.org/content/162/1/… Image
Read 11 tweets
Dr. Judith Hubbard Profile picture
Jan 14
A Mw6.6 #earthquake just occurred below the W tip of #Java, #Indonesia. Here, the Indo-Australian Plate is sinking below the Sunda Plate. To the north, this #subductionzone produced the devastating Mw9.1 2004 Indian Ocean earthquake and tsunami. 🧵1/5

#tectoplot Image
Fortunately, a Mw6.6 is much, much smaller than a Mw9.1 - 5000x less energy! 2/5

earthquake.usgs.gov/education/calc…
The earthquake depth (~35-45 km) is similar to the plate boundary fault, but the focal mechanism shows slip on a steeply dipping thrust fault. This likely represents a hanging wall splay fault, or fracture of the downgoing plate. 3/5

earthquake.usgs.gov/earthquakes/ev… Image
Read 5 tweets
Dr. Judith Hubbard Profile picture
Jan 14
Ever look at global #earthquakes from the top down? The #NorthAmericanPlate and #EurasianPlate seem simple around the Atlantic - they're pulling apart - but if you follow that boundary across the pole to Russia, it gets weird and diffuse. 🧵1/4

#tectoplot Image
#Iceland provides a remarkable view of the plate boundary. Here, the plates are pulling apart over a #hotspot, so the spreading center is on land instead of at the bottom of the sea.

And just look at the result! Pictured here: #LakeThingvallavatn. 2/4

nationalgeographic.org/encyclopedia/r… Image
But follow that plate boundary past the pole and under the ice, and you find yourself in Russia. Suddenly the #earthquakes are scattered and the plate boundaries poorly defined.

There's actually a whole extra baby plate here - the #OkhotskPlate. 3/4

washington.edu/news/2006/05/0… Image
Read 4 tweets
Dr. Judith Hubbard Profile picture
Nov 15, 2021
This is a map of sea level, exaggerated 10,000 times.

Why is it so weird and lumpy? Sea level is not defined by distance from Earth's center - it's an equal-gravity surface.

Understanding the #geoid is key to understanding #climatechange impacts. 1/7

en.wikipedia.org/wiki/Geoid Source: http://doi.org/10.5194/essd-11-647-2019.
The "lumpiness" comes from variations in density and topography. Mountains have gravity, so the #geoid is generally higher in mountainous regions. But inside the Earth there are variations, too - from the different kinds of rocks and the thickness of the crust. 2/7
Elevations on Earth are defined relative to the geoid. So every time you look at a topographic map, there's a secret geoid hidden behind that data! 3/7
Read 7 tweets
Dr. Judith Hubbard Profile picture
Nov 13, 2021
The Earth's most common elevation is sea level - and yet, most of Earth's surface lies below sea level - FAR below sea level, at 4-6 km depth.

Why?

The answer is a combination of #platetectonics and Earth's oceans. 1/9

#tectoplot ImageImage
You might think that the oceans are just parts of the land that are covered with water. Actually, that's really not the point - the oceans are there because the rocks *below* the oceans are fundamentally different from those below continents - and it's all because of magma! 2/9
Below the crust, the mantle is convecting. This is driven by heat given off by radioactive delay deep inside the Earth.

The mantle is solid rock - but every now and then a pocket melts: due to the addition of water, release of pressure, or extra added heat. Magma! 3/9 Source: https://concord.org...
Read 9 tweets

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