Shearing instabilities accompanying high-pressure phase transformations and the mechanics of deep earthquakes

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   Fig. 1.

   Earthquake depth distribution. (a) Semilog plot of global earthquake frequency per 10-km-thick depth interval, showing a bimodal distribution. All earthquakes below ≈50 km are in subduction zones, the coldest parts of the mantle. The boundary between the mantle transition zone and lower mantle in subduction zones is at ≈700 km. No earthquake has ever been detected in the lower mantle. Modified from ref. 35. (b) Cartoon of subduction zone and earthquake distribution. Lithosphere (speckled) at right, with uppermost layers altered to antigorite (serpentine), is subducting beneath lithosphere at left. Earthquakes in olivine-dominated upper mantle are shown as red dots in serpentine and white diamonds. In the mantle transition zone, olivine is hypothesized to remain present despite being no longer thermodynamically stable and to slowly react away to spinel (wadsleyite or ringwoodite) during descent, occasionally generating earthquakes (black dots) by the process discussed in the text. Note volume reductions accompanying phase transformations at 410 and 660 km. Modified from ref. 36.

   
   
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   Fig. 2.

   Serpentine faulting experiments. (a) Phase diagram showing stability field of antigorite serpentine and its break-down products. Circles, squares, and triangle show experimental conditions of ref. 4. (b) En echelon faults and anticracks decorated by solid reaction products (bright) in dark antigorite matrix. (c) Sketch illustrating fault segments (heavy lines) and anticracks (light, wavy lines). (d) Region of relict olivine with antigorite inclusion showing mode I cracks produced during dehydration of antigorite; several early formed cracks are marked only by arrays of water bubbles after crack healing. Modified after ref. 4. (Scale bars: 20 μm.)

   
   
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   Fig. 3.

   Anticracks: their nature, appearance, and explanation. (a) Cartoon showing idealized shape and orientation of anticracks in the stress field that caused them. Extremely rapid nucleation of a more dense phase in a less dense one leads to nanocrystalline lenses of the daughter phase that orient themselves normal to maximum compressive stress and develop very large compressive stresses at their tips. See text for discussion of analogy to fluid-filled cracks. (b) Anticracks filled with nanocrystalline spinel produced in olivine. (c) Thermodynamic explanation for anticrack formation (see text), modified from ref. 10.

   
   
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   Fig. 4.

   Cartoon showing active Tonga subduction zone and fossil slab floating above it. Original figure is modified after ref. 26. Yellow and orange stars and circles were added in ref. 28.

   
   
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   Fig. 5.

   Mapped areas of petrologic anomaly in mantle transition zone beneath Fiji showing anisotropy only where earthquakes are occurring (a). Black dots in sector I show hypocenters of outboard earthquakes; sector II has markedly higher seismic velocities in transition zone that slow to the south; sector III is normal mantle transition zone. (b) Shear-wave splitting measurements. Note that anisotropy ends abruptly at the boundary between sectors I and II, coincident with jump in seismic velocities and cessation of earthquakes. [Reproduced with permission from ref. 27 (Copyright 2003, American Geophysical Union).]