Weak, frictionally unstable input sediments explain shallow seismogenesis at the north Sumatran subduction zone

The cause of unexpectedly shallow seismic slip during the 2004 Sumatra-Andaman earthquake, which resulted in a devastating tsunami, is still under debate. One hypothesis is that diagenetic strengthening of décollement-forming input sediments prior to subduction allows shallow seismogenic behavior. We tested this hypothesis by measuring the frictional behavior of input sediments, sampled offshore northern Sumatra during International Ocean Discovery Program (IODP) Expedition 362, in single-direct shear experiments. The input sediments that have been correlated with the north Sumatra proto-décollement horizon are frictionally unstable, indicating a seismogenic shallow décollement. These sediments are also frictionally weak, which deviates from expected friction systematics but means that diagenetic strengthening is not required. Our observations indicate that threshold concentrations of amorphous silica (≥11 wt%), palagonite rims on glass fragments, and feldspar interspersed within weak, smectite-bearing sediments entering the north Sumatra subduction zone may cause the incoming sediments to be frictionally unstable before subduction. This instability led to the unexpectedly shallow coseismic slip during the 2004 Sumatra-Andaman earthquake and may be valuable information for hazard assessments of other subduction margins.