[S-CG43] スラブ内地震とその発生メカニズム

[SCG43-07] Transformational faulting at high pressure and temperature in Fe-rich olivines: Implications for deep-focus earthquakes

★Invited Papers

*Timothy Officer^1,2、Richard A Secco²、Man Xu¹、Tony Yu¹、Lupei Zhu³、Amanda M Dillman⁴、David L Kohlstedt⁴、Yanbin Wang¹ (1.Univ. of Chicago、2.Univ. of Western Ontario、3.Saint Louis Univ.、4.Univ. of Minnesota)

キーワード：deep-focus earthquakes, transformational faulting, multi-anvil apparatus, high pressure, acoustic emission, olivine

One of the major unresolved questions in earth science is how earthquakes can nucleate, initiate and propagate under high pressure/high temperature (HPHT) conditions > 350 km below the surface. This is because brittle failure is strongly inhibited by high pressure, which prevents crack growth, while elevated temperature promotes creep and flow. This suggests ductile deformation should occur before the fracture stress is reached. Nevertheless, deep-focus earthquakes are routinely observed in the cold lithospheric cores of subduction zones. One hypothesis is they occur as a result of metastable olivine transforming to its high pressure polymorphs wadsleyite and/or ringwoodite. While this mechanism has been shown to operate in the olivine/ringwoodite analogue Mg₂GeO₄ below ~4 GPa, there is a lack of experimental evidence confirming it conclusively in silicate olivine at higher pressures. To test this hypothesis, experiments were performed on Fe-rich olivines undergoing the olivine to ringwoodite (spinel structure) transformation. Samples were deformed in multi-anvil apparatuses at HPHT while monitoring acoustic emission (AE) activity using piezoelectric transducers. By arranging the transducers in an array surrounding the sample, the locations of events could be determined using arrival time inversion techniques commonly employed for earthquake location. In the Kawai apparatus, deformation of fayalite (Fe₂SiO₄) in the spinel stability field generated AE events that locate within or within 1σ of the sample in five experiments defined by the P,T envelope P = 3.9–8.4 GPa and T = 748–923 K. Optical and scanning electron microscopy of the recovered specimens displayed conjugated faulting associated with transformation from olivine to spinel including the presence of spinel in the form of “anti-cracks” and “nano-shear bands” morphologically equivalent to those associated with HPHT faulting in Mg₂GeO₄. However, a key question that remains is what effect the addition of Mg cations will have on the faulting process. As part of an ongoing study, experiments on (Mg_0.5,Fe_1.5)SiO₄ and (Mg₁,Fe₁)SiO₄ were carried out in the D-DIA and D-DIA-30 apparatuses respectively, which allow for controlled deformation of the sample independent of confining pressure. In addition to AE monitoring, these experiments employed synchrotron X-ray diffraction and radiography for in situ determination of stress and strain. Preliminary results demonstrate AE activity in the spinel stability field as well as macroscopic faulting in CT scans of the recovered samples. In contrast, samples were acoustically quiet and remained intact when confined to the olivine stability field. The results of these studies suggest seismogenic fracture is associated with the olivine/spinel transformation implying that transformational faulting could generate deep-focus earthquakes in the mantle. In particular, this has implications for earthquakes that occur at depth in exceptionally cold subducting slabs, such as Tonga, where metastable olivine may transform directly to ringwoodite throughout most of the transition zone.