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.

Why did this earthquake occur so close to the 2010 event? Well... the slip on the fault in 2010 stressed the regions next to it that didn't slip. So, this eq was probably triggered by the previous one, a fairly common scenario, although we don't really understand the time lags.
Both earthquakes occurred on the Enriquillo-Plantain Garden fault, which accommodates part of the ~19 mm/yr of left-lateral strike-slip between the North American and Caribbean plate....
...Here, the sliver plate is bounded by parallel (mostly) strike-slip faults; to the east, it becomes a complex doubly-vergent subduction zone with additional faults to take up obliquity.

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More from @JudithGeology

13 Aug
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
12 Aug
At this point I just assume no one knows anything. (Including myself...) This is especially important when you're working between fields - the same word can mean different things to different people.

Example from #earthquakescience below...
...Fault slip rate = (1) average slip rate recorded by geology, (2) modeled average slip rate from GPS, (3) how fast the fault slips in an earthquake. But somehow, NOT (4) the rate the fault is slipping right now (probably zero)...
...Aseismic = (1) has not generated recorded seismicity, or (2) cannot generate earthquakes. These are very different things!...
Read 4 tweets

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