Two Big quakes today, one grabbing headlines in Alaska and one drowning in obscurity in Argentina. Both about the same magnitude and about the same impact (nil to none) and people wonder two things, why the one gets headlines and why neither is a problem.
Both are interesting being 6+, a 6.8 and a 6.7. The Alaska one would have been a disaster almost anywhere except in a sparsely populated and mostly water part of America. The other so deep that you wonder how it could happen or how the earth’s crust could plunge so deep into the mantle. To me the Argentina quake is more interesting as it tells us more about how our planet and tectonics works and the shape of the subducted pacific plate under the South American continent.
The Alaska quake was close to the surface, and big. If it had happened at say Seattle there would have been a panic and people running for the hills. It would have toppled buildings, caused the interstate to fall into the sea and would have created a damaging tsunami. In Japan it would have been directly under Tokyo and killed thousands. But in Alaska it probably disturbed the salmon run and shook up some lobsters. So why does it get headlines? Well it’s in the US and the news there is always looking for the next disaster. After the east coast quake and the hurricane didn’t produce the mayhem they seem to require and since it is a full magnitude higher it must be causing some type of disaster so it gets the news.
The Argentina quake though is, well, not in the US or a major supplier of the consumable junk the US lives for like Japan and most news people in the us couldn’t find it on a map anyway so they ignore it. But the Argentina quake is more interesting. It’s so deep and there is no direct connection from the solid crust and the subducted crust that energy transfer is not complete that while it would have caused some shaking on the surface there is a lot of distance and energy transfer is diminished by square of distance.
But if you look at the history maps for the Argentina quake you will notice that really deep quakes in this area form a fairly neat straight line on a north south heading. something is happening at that depth and it is a parallel line of the subduction zone to the west. If you plot the quakes from the subduction zone to this quake by depths you will notice that they form a deep curve and a line along the surface. The surface quakes are from the Andes mountains being built as south america compresses itself while trying to subduct the pacific. The curve going down is the subducted pacific plate that is curving and then plunging into the planet. This goes from a mild slide under the Andes but continues to curve as it plunges. At 750 km from the fault the quake was at 592 km deep. At 490 km the earthquakes are about 130 km deep. At the pacific coast line they are 10 km deep. There are no quakes between 130 and the line of 500 or so km quakes 100 km inland from the 130 quakes except surface quakes from 10 to 60 or so km deep.
Plotting this shows a cluster of quakes right about 500 km from the subduction zone off shore, then nothing until another almost 300 km. (Plot is degrees west vs depth.) That is one strange graph and completely not what I was expecting. What is happening down there, is it breaking up, boiling, running into something?
In other places that have these deep quakes like near Fiji the quakes happen from 0 to 500 km in a constant and almost straight line. Japan, the same thing. But under the Andes it gets weird. Not a lot happens there but it’s so weird it’s interesting.
Now at Alaska not so interesting either, it just goes down to about 200 km and quits. It melts off or whatever. Sure it gets the headlines but Argentina has more interesting quakes.
- Major 7.1 quake hits Alaska, local tsunami warning issued (windsorstar.com)