Phase transformations of metastable olivine might trigger deep-focus earthquakes (400 to 700 kilometers) in cold subducting lithosphere. To explore the feasibility of this mechanism, we performed laboratory deformation experiments on germanium olivine (Mg2GeO4) under differential stress at high pressure (P = 2 to 5 gigapascals) and within a narrow temperature range (T = 1000 to 1250 kelvin). We found that fractures nucleate at the onset of the olivine-to-spinel transition. These fractures propagate dynamically (at a nonnegligible fraction of the shear wave velocity) so that intense acoustic emissions are generated. Similar to deep-focus earthquakes, these acoustic emissions arise from pure shear sources and obey the Gutenberg-Richter law without following Omori’s law. Microstructural observations prove that dynamic weakening likely involves superplasticity of the nanocrystalline spinel reaction product at seismic strain rates.
Microstructure of the transformed fault zone. Electron micrographs of sample D1253, which failed at 5 GPa. (A) Full view of the sample (s1 is vertical). Horizontal fractures are likely to have been caused during decompression. The red box corresponds to the location of (B). (B) High-magnification view of one of the fracture planes. The white grains are Ge-enstatite (MgGeO3), and the gray grains are Ge-olivine, the grain size of which are <10 and 150 mm, respectively. The red box corresponds to the location of (C). (C) A narrow band of light gray material highlights the fault. The red line indicates the location of the FIB section (D). (D) Close-up TEM view of the fault zone. Dashed lines highlight the main fault, only 100 nm thick. The gouge consists of fully crystalline nanometric material. The fracture walls are intensely deformed. (E) Electron diffraction patterns of the gouge and adjacent wall rock display bright olivine spots and numerous spinel spots emanating from the gouge itself.
AE magnitude catalog and GR distribution. (A) Amplitude and duration of 5 AEs of relative magnitude equal to ≈0, ≈1, ≈2, ≈3 and ≈4, respectively, from bottom to top. The reference event MAE = 1 is shown. (B) Statistical distribution of the relative magnitudes within four magnitude bins (0 < MAE < 1, 1 < MAE < 2, 2 < MAE < 3, and 3 < MAE < 4) and for experiments D1247 (4 GPa) and D1253 (5 GPa). The moment magnitude completeness of each catalog is MAE > 0. A slope of 1 for the b value is displayed for reference.
A. Schubnel, F. Brunet, N. Hilairet, J. Gasc, H.w. Green II, “Deep-Focus Earthquake Analogs Recorded at High Pressure and Temperature in teh Laboratory,” Science, (2013):341 (6152), 1377-1380, DOI: 10.1126/science.1240206