Neotectonic Setting of the 2004.12.26 Sumatra Earthquake

The epicentre of the Mw 9.0 Indonesia subduction earthquake of 26 December 2004 was located 40 km NW of the island of Simeulue. Both uplift and subsidence are reported from this island and other parts of the area affected by the earthquake and they are widely explained as transient effects controlled by location relative to the megathrust rupture. Vertical movements were noted on Simeulue and several other outer-arc islands after previous earthquakes including the Mw 7.7 event of 1935. 14C dates on Holocene palaeoshore samples collected in 1987 on Simeulue and the neighbouring island of Nias have been recalculated using updated calibration and marine reservoir values. They support the earlier conclusion that over the last 6000 years Simeulue and Nias have undergone northeastward tilting in increments of ~1 m within deformation phases lasting ~1500 yr, the last of which began about AD 1695 and is still in progress, and separated by two periods of quiescence of similar duration. The geometry of uplift indicates coseismic displacement on imbricate faults within the sediment prism of which Simeulue and other outerarc islands are emerged portions supplemented by landward rotation of the prism. The imbricate model is complementary to the concept of a megathrust: faults within the weak sediments of the accretionary prism cannot account for great earthquakes, but by distributing strain along as well as across strike they influence the pattern of surface deformation resulting from major subduction events. They also release stored energy as relatively shallow aftershocks and thus contribute to the diffuse seismicity of the subduction zone. The errors inherent in teleseismic location limit its value for mapping faults within the prism, but some indication of their spacing is given by swath bathymetry obtained by the Royal Navy's HMS Scott soon after the earthquake, and reported at www.pmel.noaa.gov/tsunami/indo20041226/hms_scott.htm, which shows fold-thrust ridges trending NNW some of them 70 km long and of which the youngest are 5-15 km apart, ~500 m high and separated by sediment-filled basins. There are instructive parallels with the Iranian Makran which suggest that deformation may be serial in development. Holocene and bathymetric data may thus be of value in hazard mitigation as indicators of eventual coastal configuration once coseismic deformation has faded and as guides to possible shifts in the locus of shallow seismicity.