Synthetic slip plane, a hybrid kind of calcite twin data in dynamic analysis.

This paper defines a synthetic slip plane as the linear combination of a pair of twinned and untwinned e-planes in a single calcite crystal. This auxiliary slip plane is dependent upon neither the twinned e-plane nor the untwinned e-plane. It can be used together with the twinned e-plane either to further constrain the extents of the compression and tension axes in graphic dynamic analysis or to better estimate the reduced stress in numerical dynamic analysis. Highlights •The linear combination of twinned and untwinned e-planes in a calcite crystal defines the synthetic slip plane.•The synthetic slip plane can be incorporated into graphic and numerical dynamic analysis.

Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra.

Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records.