Subduction styles/EQ magnitude

Strong Earthquakes in Latin America can be attributed to a greater release of seismic energy associated with flat slab vs.steep slab segments. The geometry of these flat slab regions allow for more stress to build up between the plates (Gutscher, et al.,2000).

Flat Slab segments lack the arc volcanoes which are prominent in western America from subducting young oceanic lithosphere, as depicted in the image above of Normal slab subduction. Normal slab subduction is characterized by maintaining a distinct low viscosity and high temperature melt in the boundary between the slab and overriding plate. Flat subduction occurs when a distinct decrease in the dip angle of the slab results the slab tip flattening underneath the plate overriding it. The wedge corner is subject to high stresses as the slab-tip is pushed into the mantle thereby constricting the corner flow above the slab.

Flat slab segments are attributed to the high magnitude earthquakes in LA, and one possibility behind the differing slab geometry is the buoyancy between the two slabs with the younger buoyant slab resisting subduction. The subduction of young buoyant lithosphere can be modified when encountering denser older lithosphere changing the thermal structure at the margin and displacing the asthenosphere away from the trench. The cooler temperature at the forearc predicts a larger locked zone therefore increasing the risk of great interplate earthquakes.



While Earthquakes occur all along the subduction zone, those in the ‘Seismogenic zone are most destructive. This is because they occur near the surface. The earthquake cycle in the Seismogenic Zone can be said to occur in two stages, the Interseismic Period and the Coseismic Period as described below:

The Interseismic Period Time between earthquakes: (10’s to 100's of years) where plate convergence continues at approximately 8cm a year (between Nazca and S. American plates) with the two plates are locking over a portion of the subduction plate boundary. This results in not only uplift, but also a horizontal shortening of the overlying plate margin.

The Coseismic Period/Earthquake rupture occurs over a few minutes. Once the accumulating stress exceeds the strength of the fault,  failure occurs in the locked zone and great earthquake occurs. During this episode stored elastic strain is released which in turn results in subsidence and horizontal extension where slow uplift and horizontal shortening had previously accumulated. In addition, the underwater displacements can cause tsunamis. Once the stress is relieved, the cycle resets and stress begins to build again.


In Latin America there is a combination of Steep slab/Flat slap segments, with the greater earthquakes occurring along the zones of flat slab subduction.  (Fig. below shows a run down of where earthquakes of greater magnitude occur as opposed to those of lesser magnitude).

Note: I couldn't find a comprehensive enough image to suit my purpose for this blog post so I created my own from a variety of figures (upper two images of steep slab/flat slab), with the principle source being from Gutscher, 2002.





Sources:
http://gsc.nrcan.gc.ca/geodyn/eqcycle_e.php
Gutscher, MA., 2002. Andean subduction styles and their effect on thermal structure and interplate coupling. Journal of South American Earth Sciences 15: 3
http://www.geo.arizona.edu/geo5xx/geos577/projects/flesch/The_Ecuador_Peru_Gap.html
http://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_55-56/010021658.pdf





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My View of things

For lack of anything worthwhile posting about (that isn't time consuming), I thought I would share with you my view of things regarding what I see when I look at the view outside my neighbourhbood. A few months ago I was in a class and the topic somehow shifted to that of dangerous areas. I cannot recall offhand the particulars on what generated the topic, but it circled around geologic hot spots and the dangers therein. I usually stay out of these conversations unless I am in a geology class at the time, of which this was not, so just sat back and let my mind wander as I waited for the topic to return to the class at hand. At one point a student expressed their opinion on how they could not understand why anyone would live in California because of all the earthquakes. This normally would not grab my attention, but considering the area in which we currently reside (including said student) consists of several large volcanoes I sat up in my chair to listen more intently to what this student had to say. When they were finished with their diatribe, I said, "You do realize that Mt. Rainier is a volcano, right?" Upon the blank stare I received in return, I came to a surprising realization. Other people do not look at that mountain in the same way I do. When I first saw it from the ground my breath was taken away. I had seen it many times from the air on route home to Alaska via SeaTac, but I had never truly explored the area outside of the airport, thus not getting the full magnitude. For me, to truly appreciate it I had to see it from the ground. And this only occurred recently, so I don't just see a mountain; I see the forces behind the mountain. Sometimes I forget not everyone thinks volcano when they look at the gorgeous view out their backyards.

So coming back to the student with the blank stare, I realized she had no clue as to the potential dangers of living in such close proximity to said "mountain". (I have trouble just referring to it as a Mt., because it just epitomizes a volcano in my mind). I forget too, that most of these kids weren't even born yet when Mt. St. Helens blew. I still recall that moment with vivid clarity, and I lived quite a distance away in the remote wilderness of Alaska. (There are no access roads to where I grew up, and only one TV channel at the time). I have images in my mind from the news showing people walking with scarves over their faces as they walked through what appeared like a nuclear winter. Ash was dropping like snow and the sun was a strange pink color through the hazy sky. No, I don't suppose the majority of people who look at the mountain that towers over the area as anything other than a beautiful, majestic piece of eyecandy.

While the likelihood of an explosion in the near future is probably non-existent, I would have to weigh the risk factor for living under the dome of volcano quite a bit more substantial than living in California and dealing with earthquakes. Mainly because California is smart about how they have built their cities, and have outstanding educational outreach programs teaching the general public on how to react in the event of an EQ. While I think if something were to occur here, it (Mt. Ranier) would pretty much wipe out the whole area , if not from the explosion itself, but from the pyroclastic flows and/or lahars that would soon follow. That aside, if any forewarning of and impending "event" were provided, the surrounding cities and towns would most probably be paralyzed. I say this because there are no clear exit (evacuation) strategies for such an event- at least as far as I have seen in the 7mos I have resided here.

But all in all, volcanoes are pretty fascinating to examine. I once wrote a paper titled: 'The Base Jumpers of Geology'. It takes a certain brave individual to walk across a crusted over lava field when they know that at any moment there is a possibility of breaking through, yet they do it anyway. My paper pretty much focused on Harry Glicken, who would have been the vulcanologist who died when Mt. St. Helen's blew, but was in California for a graduate interview and David Johnston had taken his watch. Harry later died in an unfortunate incident (along with Maurice and Katia Krafft) when they misjudged the path a pyroclastic flow would take and were overtaken. I wish more were written about Harry Glicken, as I admire a lot of his attributes. His specialty, per se, was debris avalanches, and after his work on Mt. St. Helens, debris avalanches (as defined by his work) were recognized around volcanoes globally.

I suppose I have digressed a bit (as is my habit) from my original intent of this post- I mainly was just going to feature a "before"  and "after" picture of the view I have from my neighbourhood expressing what others see vs what I see when looking at that behemoth.



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