2003年11月20日
Went to a seminar with the ESS this afternoon, and MY GOD!! I was like the only Asian + the only girl + the only one who is below 40 years old at the seminar!! The average age for the audiences was like 45~50 years old, and mainly prof-level. Now I feel so honored + proud of myself to join the seminar. :)
It's a relatively interesting subject, (okok~~ I know all of you think I'm a freaky geek in this area. >.<") and got a chance to look deeply into how we testing the Earth as well as learning some fancy ESS terms.
Interesting!! =======================================================
Why Subduction Zone Earthquakes?
A Deep Relationship with Metamorphism
Bradley R. Hacker, University of California, Santa Barbara
New thermal-petrologic models of subduction zones show a correlation between the patterns of intermediate-depth seismicity and the locations of predicted hydrous minerals: earthquakes occur in subducting slabs where dehydration is expected, and they are absent from parts of slabs predicted to be anhydrous. A subducting oceanic plate can consist of four petrologically and seismically distinct layers.
1) The hydrated, glassy and fine-grained basaltic upper crust is dehydrating under equilibrium conditions and producing earthquakes driven by dehydration embrittlement; earthquakes in this layer are restricted to rocks with hydrous minerals. Once the rocks have transformed to nearly anhydrous eclogite, seismicity in this layer stops.
2) The coarse-grained gabbroic lower crust is generally anhydrous, but contains local areas of substantial hydration produced either at mid-ocean spreading centers or during bending at the outer rise. Hydrous rocks in this layer undergo dehydration under equilibrium conditions and produce earthquakes, but the bulk of the gabbro layer transforms aseismically to eclogite at depths well beyond equilibrium because of the kinetic hindrance of solid-solid transformations and the reduced hydration state. Seismicity in upper seismic zones thus receives some contributions from the lower crust, but comes dominantly from the upper crust. Because the thermal gradient in the upper seismic zone is inverted in most subduction zones, and therefore the potential for dehydration increases upward, the seismicity begins at the top of the slab and then descends slowly toward the slab Moho with increasing subduction depth.
3) The uppermost mantle is locally hydrated down to depths of ~40 km in cold slabs as a result of bending at the outer rise; it could also be hydrated by H2O supplied from dehydration reactions at greater depth. The hydrous portions dehydrate under equilibrium conditions and produce earthquakes. Because the temperature in the lower seismic zone increases downward, and therefore the potential for dehydration increases downward, the seismicity begins at modest depth in the slab and then ascends slowly toward the slab Moho with increasing subduction depth.
4) The remaining, anhydrous mantle transforms sluggishly and aseismically from spinel- to garnet-bearing assemblages.
This hypothesis explains several important features of intermediate-depth seismicity:
i) the earthquakes occur at locations in the slab that correspond to predicted dehydration reactions;
ii) the observed low-velocity waveguides that persist to depths greater than the equilibrium transformation of gabbro ? eclogite exist because equilibrium transformation occurs in the upper crust, while the coarse, dry lower crusts persists metastably to greater depth;
iii) the upper and lower zones of double seismic zones result from dehydration reactions in the crust and mantle, respectively;
iv) the gap between the upper and lower zones of double seismic zones reflects slower and/or later reaction in the cold core of the slab.
posted by Biochemie on 9:34 下午
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