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You might hear a geologist say: "We can't predict earthquakes."

Well, WHY NOT???



#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

Read 23 tweets
A #volcano just erupted near #Tonga - but why was it there in the first place?

Tonga sits on top of the #TongaKermadec #subductionzone, where the #PacificPlate sinks below the #AustralianPlate.

The subducting #PacificPlate carries the culprit into the mantle: #water. 🧵1/3 Source: https://source.wust...
Some people think that volcanoes at subduction zones are due to frictional heating from the two plates rubbing against each other - NO!

In this case, melting is because of the addition of water, sort of like how adding salt can make ice melt. 2/3

Is this the only volcano in the region? Absolutely not. Volcanoes parallel the plate boundary, a string of conical mountains 2000 m high, occasionally poking above the sea. Usually quiet, but built over millions of years through thousands of eruptions. 3/3

#tectoplot Image
Read 3 tweets
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…
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… Image
Read 5 tweets
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… 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… Image
Read 4 tweets
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.


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:
Read 9 tweets
A catastrophic #earthquake in 2010 on this fault system in #Haiti killed ~250k people. It just ruptured again, this time to the west. Hopefully the lower population density in this region, further from Port-au-Prince, will mitigate the impact. 😧

#tectoplot #haitiearthquake
The updated focal mechanism for the earthquake from GFZ indicates the rupture was on land, and oblique thrust - similar to the overall 2010 event, which combines a mostly strike-slip mainshock with a cluster of smaller thrust earthquakes.

#tectoplot #haitiearthquake
Scattered aftershocks from the 2010 earthquake were still occurring up through this year, although none were close in magnitude to the mainshock.

Read 6 tweets
The #SouthSandwichIslands are a wonderful example of #platetectonics in miniature. The #SouthAmericanPlate is subducting west at ~7 cm/yr below the #SandwichPlate; this #subductionzone hosted a Mw7.5 #earthquake yesterday (Aug 12). 1/4

#tectoplot Image
The depth of the #earthquake is still poorly constrained. GFZ puts it shallow, above the plate interface, dip 11°. USGS puts it deeper, within the slab, dip 26° and non-double-couple. Historical events of this scale in the region are old so not much help - 1929, 1933, 1964. 2/4 ImageImage
Given the curvature of the #subductionzone, it would certainly be reasonable to have some intra-slab deformation, and fracturing could be complex, leading to non-double-couple. The closest large event (1964) was apparently quite deep (125 km). 3/4
Read 4 tweets

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