Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls

[ 1] Large uplifts and tilts occurred on the Sumatran outer arc islands between 0.5 degrees and 3.3 degrees S during great historical earthquakes in 1797 and 1833, as judged from relative sea level changes recorded by annually banded coral heads. Coral data for these two earthquakes are most complete along a 160-km length of the Mentawai islands between 3.2 degrees and 2 degrees S. Uplift there was as great as 0.8 m in 1797 and 2.8 m in 1833. Uplift in 1797 extended 370 km, between 3.2 degrees and 0.5 degrees S. The pattern and magnitude of uplift imply megathrust ruptures corresponding to moment magnitudes (M-w) in the range 8.5 to 8.7. The region of uplift in 1833 ranges from 2 degrees to at least 3.2 degrees S and, judging from historical reports of shaking and tsunamis, perhaps as far as 5 degrees S. The patterns and magnitude of uplift and tilt in 1833 are similar to those experienced farther north, between 0.5 degrees and 3 degrees N, during the giant Nias-Simeulue megathrust earthquake of 2005; the outer arc islands rose as much as 3 m and tilted toward the mainland. Elastic dislocation forward modeling of the coral data yields megathrust ruptures with moment magnitudes ranging from 8.6 to 8.9. Sparse accounts at Padang, along the mainland west coast at latitude 1 degrees S, imply tsunami runups of at least 5 m in 1797 and 3 - 4 m in 1833. Tsunamis simulated from the pattern of coral uplift are roughly consistent with these reports. The tsunami modeling further indicates that the Indian Ocean tsunamis of both 1797 and 1833, unlike that of 2004, were directed mainly south of the Indian subcontinent. Between about 0.7 degrees and 2.1 degrees S, the lack of vintage 1797 and 1833 coral heads in the intertidal zone demonstrates that interseismic submergence has now nearly equals coseismic emergence that accompanied those earthquakes. The interseismic strains accumulated along this reach of the megathrust have thus approached or exceeded the levels relieved in 1797 and 1833.