Discover and read the best of Twitter Threads about #subductionzone
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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
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
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
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
#tectoplot
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
#tectoplot
Fortunately, a Mw6.6 is much, much smaller than a Mw9.1 - 5000x less energy! 2/5
earthquake.usgs.gov/education/calc…
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…
earthquake.usgs.gov/earthquakes/ev…
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
#tectoplot
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
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
