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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Grab a seat, I'll pop the popcorn!

I admit to being a bit mesmerized by the news footage on CNN and so for a few hours today I watched and waited for the tsunami to arrive. It was quite entertaining watching the newscasters become more and more frenzied as the time of estimated arrival drew near. They were like sharks in bloodied waters, switching from  one camera viewpoint to another and snipping at their consultants as they tried to explain the mechanics of tsunamis. In the middle of one particular interview, a man was stammering his explanation when the newscaster interrupted him in a very clipped manner only further compounding his ability to communicate effectively. It wasn't that the man was ill-spoken, most anyone would have stammered when attacked with questions and then interrupted mid-stream. It was as if the poor man was barraged by a tornado of these barbed questions, only because (from what I surmise) he wasn't being sensational enough for the newscaster.

And then it happened. As one of the camera views panned across a beach, I saw a man surfing! I jumped up a bit in my seat to make sure I saw what I thought I saw, and yup- it was a man in the water surfing! At first the newscasters just ignored it, like the proverbial elephant in the kitchen, but as time went by and it just became more and more obvious, one of them finally says, "There is a guy on the beach. Maybe he doesn't have a radio or a TV or computer, so doesn't know about the tsunami. Or, maybe he just doesn't have any sense. I wish I could just shout at that guy." To which I thought to myself, no- don't shout at him. If he can't figure out that there is something wrong when NO ONE IS ON THE BEACH or if he just prefers to ignore all sense in order to get a surfing thrill then he should be left to his own devices. Just keep the camera on him and I'll pop some popcorn- we can then all see how "great" it is to surf in something akin to a flash flood. The dummy.


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Holy Cow!

Before I went to bed last night I heard about the 8.8 Earthquake that hit Chile, but it was too early to get any information on it, so I waited until this morning to read about the damage. From the latest reports, over 147 deaths have been reported and the main road connecting the north from the south is destroyed. A prison reportedly partially collapsed and some inmates escaped only adding to the commotion. Meanwhile, Hawaii has been issued with a tsunami warning and to my utter amazement the news cameras show people along the coast taking pictures of the surf!! What in the heck are these people thinking? Did they not see any footage from the tsunami in Indonesia? Those waves come in with such speed there isn't a chance in heck they can outrun them, yet there they are like dodo birds taking pictures. Crazy. I guess that gives credence to the old adage that you can lead a horse to water but you can't make him drink.

I am impressed with the people of Chile however. There appears to be no looting or any crazy-making idiots out on the streets causing havoc and the buildings seem(from the film footage) to have held up quite well, but then I am not surprised. It is an area that is well aware of the geologic dangers and hence they have prepared accordingly.

Presently the news is reporting that they predict the waves to be about 1-7ft. and would hit at 4pm EST. I think what a lot of people are unaware of is that when the tsunami hits it won't just hit the eastern coastline, but rather it will wrap around the islands hitting both sides. Or, maybe they are well aware of that fact and I am just trying to justify idiotic behavior.

Seismites

 The Dead Sea region has experienced six destructive earthquakes during the last 1000 years, with an average recurrence interval of around 200 years (188 to be exact) beginning with the 1060 AD earthquake. This cluster of events ended with the 1927 earthquake which had a magnitude of 6.2.

The Dead Sea Fault is a Left lateral transform plate boundary, separating the Arabian plate and the Sinai sub-plate and has been active since the Miocene, with movement still occurring in present day. (Fig. 1). (Garfunkel, 1981). This fault zone lies within the Dead Sea graben of which this post will focus upon. More specifically, depositional attributes of the laminated layers that comprise the sedimentary record of the Dead Sea and how those layers can serve as a portal into the past regarding paleo-seismic events.

With the recent drop in lake levels, the banks are quite accessable. By scraping away the halite/sand encrusted sides of these banks, layers of mud and sand/silt are exposed. Instead of being undisturbed laminated layers of sediment, you see beautful swirls and designs where the layers have intermixed. These disturbances, termed siesmites, are not only quite stunning to behold but also have an important significance in regards to providing a geologic record on ancient earthquakes along the Dead Sea transform.

 The lacustrine sediments of the Dead Sea are comprised of alternating layers of aragonite and detritus sediments. The latter of which are composed of dark, silt-clayey size detritus derived from flooding (fluvial events) as suspended material and range in thickness from a couple of centimeters to almost 20 (can vary; these are my measurements). The aragonite layers are intermingled white and dark laminae of silt-clayey sized detritus, and are much thinner in comparison, being in the thicknesses of millimeters. (Bookman, et al., 2004). These layers were originally continuous alternating laminae of aragonite and fine detritus, lying flat on the bottom of the Dead Sea undisturbed. They were later fluidized (brought on by seismic events), disturbing the top of the sediment and causing it to be drawn back into suspension. Deformation of the laminae occurs when the sediment comes to rest after resettling. The event is encompassed by undisturbed sediments above and below.  This mixed layer indicates a disturbance due to a seismic event, and its timing is constrained by the first overlying undisturbed lamina.

Syndepositional faulting in the Dead Sea sediments has been interpreted as when (Marco et al, 2004) a fault offsets a surface creating subaqueous scarp. The top of the sediment is deformed due to liquefaction and suspension during a seismic event, and a mixed layer forms on both sides of fault scarp. After the suspended sediments resettle, the mixed layer in down-thrown block is slightly thicker. As further sedimentation ensues, a thicker sequence accumulates on down-thrown block. The lower mixed layer in the downthrown block is also bent and overlain by folded layers.

The occurrence of seismites and their correlation to historically documented earthquakes has been determined by radiocarbon dating organic material found within the layers of sediment, solidifying the association of fluidizations of sediment and seismic events.







Bookman (Ken-Tor), R., Enzel, Y., Agnon, A. and Stein, M. 2004: Late Holocene lake levels of the Dead Sea. GSA Bulletin 116, 555 71.
Garfunkel Z (1981) Internal structure of the Dead Sea leaky transform (rift) in relation to plate kinematics. Tectonophysics 80:81-108
 Marco, S., and Agnon, A., 1995. Prehistoric earthquake deformations near Masada, Dead Sea graben. Geology, 23: 695-698.


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