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Dr. Judith Hubbard Profile picture
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14 Jan, 5 tweets, 4 min read
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
This is kind of a weird tectonic zone: to the north, the Sumatran Fault is a major strike-slip fault running along the volcanic arc in Sumatra. That fault ends here, near where this new earthquake occurred. Related? Maybe. 4/5

doi.org/10.1002/2016JB… Image
Fortunately, damage is expected to be low, despite the fact that >40 million people probably felt light shaking! #Jakarta

earthquake.usgs.gov/earthquakes/ev…

If you live in the area, report your experience here; this data is used to improved damage estimates:

earthquake.usgs.gov/earthquakes/ev…

5/5 Image

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More from @JudithGeology

Dr. Judith Hubbard Profile picture
14 Jan
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
15 Nov 21
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
13 Nov 21
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
Dr. Judith Hubbard Profile picture
11 Oct 21
An #ophiolite is a rock with a secret: it tells the story of an ocean that lived and died.

Ophiolites are pieces of crust and mantle that formed at #spreadingcenters below an ocean. Why do we find these rocks (black dots) in mountain belts (red)? 🧵

doi.org/10.1016/j.gsf.…
The #WilsonCycle describes how tectonic plates break apart, forming an ocean basin that grows around a spreading center. But the oceanic lithosphere is dense, and it eventually breaks and sink into the mantle. #Subduction closes the basin and the plates on either side collide.
Rocks that form at a #spreadingcenter have a distinctive sequence: sediments on top, then basalts that erupted underwater, then denser rocks crystallized from melted mantle, grading into mantle. You might find this sequence on land (an #ophiolite), but it formed under the ocean.
Read 5 tweets
Dr. Judith Hubbard Profile picture
14 Aug 21
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.

#haitiearthquake
Read 6 tweets
Dr. Judith Hubbard Profile picture
13 Aug 21
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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