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A (late!) coda to this thread. Did you know that even after 60 years, the Earth is still 'feeling' the effects of this massive earthquake? [1/12] https://twitter.com/Allochthonous/status/1264198073007382529
Plate motions are slow and continuous, but faults are not perfectly smooth. There is a stop-start cycle of slowly building elastic strain that is suddenly released in abrupt, energetic earthquakes once enough strain has built up to overcome friction on the fault surface. [2/12]
320 years since the last big Cascadia quake, and GPS data shows it building up for the next one. The subduction thrust is locked by friction, so the coast on the overriding North American plate is being pushed inland as it moves with the subducting Juan de Fuca plate. [3/12]
![320 years since the last big Cascadia quake, and GPS data shows it building up for the next one. The subduction thrust is locked by friction, so the coast on the overriding North American plate is being pushed inland as it moves with the subducting Juan de Fuca plate. [3/12]](https://pbs.twimg.com/media/EY3uAUUXsAEJCD4.jpg "320 years since the last big Cascadia quake, and GPS data shows it building up for the next one. The subduction thrust is locked by friction, so the coast on the overriding North American plate is being pushed inland as it moves with the subducting Juan de Fuca plate. [3/12]")
When the earthquake finally happens, the strain is released and the deformed part of the overriding plate rapidly heads back seawards, as we see here for the M9 Tohoku earthquake in 2011. [4/12]
But here's where it gets interesting: that seaward motion doesn't stop when the earthquake does. Here's GPS motions for the three months following the 2011 magnitude 9 Tohoku earthquake (from https://www.nature.com/articles/nature10227). Cm rather than m, but still in the same direction. [5/12]
![But here's where it gets interesting: that seaward motion doesn't stop when the earthquake does. Here's GPS motions for the three months following the 2011 magnitude 9 Tohoku earthquake (from https://www.nature.com/articles/nature10227). Cm rather than m, but still in the same direction. [5/12]](https://pbs.twimg.com/media/EY3ul4fWoAAiDyw.jpg "But here's where it gets interesting: that seaward motion doesn't stop when the earthquake does. Here's GPS motions for the three months following the 2011 magnitude 9 Tohoku earthquake (from https://www.nature.com/articles/nature10227). Cm rather than m, but still in the same direction. [5/12]")
And here's data for a year after the 2004 magnitude 9.2 Sumatran earthquake which shows similar seaward motion (from https://www.sciencedirect.com/science/article/abs/pii/S1367912014001485) [6/12]
![And here's data for a year after the 2004 magnitude 9.2 Sumatran earthquake which shows similar seaward motion (from https://www.sciencedirect.com/science/article/abs/pii/S1367912014001485) [6/12]](https://pbs.twimg.com/media/EY3wKdmXkAET_J0.jpg "And here's data for a year after the 2004 magnitude 9.2 Sumatran earthquake which shows similar seaward motion (from https://www.sciencedirect.com/science/article/abs/pii/S1367912014001485) [6/12]")
Some of this motion is due afterslip - further motion on the fault surface itself. Aftershocks are largely caused by this, but some weaker - or weakened - parts of the ruptured surface will creep aseismically as well. [7/12]
But the massive stresses imparted by these earthquakes also affect the deeper warmer parts of the Earth that flow - rather than fracture - in response. Afterslip effects die away quickly with time, so after a few years this 'visco-elastic' response dominates. [8/12]
And this flow can continue for a very long time from a human's (or an earthquake's!) perspective; which is where the 1960 Chilean quake comes in. GPS data (from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2007GC001721) show that the inner coastal region is *still* moving seaward rather than landward. [9/12]
![And this flow can continue for a very long time from a human's (or an earthquake's!) perspective; which is where the 1960 Chilean quake comes in. GPS data (from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2007GC001721) show that the inner coastal region is *still* moving seaward rather than landward. [9/12]](https://pbs.twimg.com/media/EY3xfVjXQAAUI2B.png "And this flow can continue for a very long time from a human's (or an earthquake's!) perspective; which is where the 1960 Chilean quake comes in. GPS data (from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2007GC001721) show that the inner coastal region is *still* moving seaward rather than landward. [9/12]")
Eventually, the arrows further inland will change direction too, and the whole region will start moving landward, as it is in Cascadia. But it hasn't got there yet. The Earth has a long memory. [10/12]
Reiterated by the canonical example of this kind of response: the isostatic rebound still affecting the bits of the Northern hemisphere which were under thick ice sheets until about 10,000 years ago. http://www.antarcticglaciers.org/glaciers-and-climate/sea-level-rise-2/recovering-from-an-ice-age/ [11/12]
This figure (from https://www.nature.com/articles/nature11032 - an excellent overview of this stuff) allows a direct comparison of Cascadia, Sumatra, and Chile at different points in their earthquake cycle, but note that there are actual (red arrows) and modelled (blue arrows) data on here. /FIN
