Fig. 6: Conceptual model of potential block/matrix strength-inversion scenarios for shallow and deep non-volcanic tremor within a subduction plate boundary shear zone.

The Stage 1 and Stage 2 panels show numerical results from Fig. 2 for mechanical changes associated with this strength inversion, and geological cartoons from Fig. 3 summarising the structural characteristics linked to these numerical models. Shallow environment: Shallow Very Low Frequency (VLF) earthquakes and tremor are proposed to arise when the matrix, originally weaker than its embedded blocks, progressively becomes stronger than the blocks. Several geological processes could lead to this strengthening, including hydrothermal alteration and compaction. Deep environment: Deep non-volcanic tremor is proposed to arise because the compositionally heterogeneous blocks and matrix will undergo metamorphic transformations, in particular, the compositionally-depth-dependent blueschist-eclogite and serpentine dehydration transformations, at slightly different p-T conditions. If the blocks undergo this fluid-releasing transition before their surrounding low-permeability matrix materials, then they could be weakened with respect to surrounding matrix both by the transition and by its associated high-pressure fluid release. See text for further discussion.