Seafloor Changes Offshore Northern Sumatra from 1997-2009 Bathymetric Data

Abstract:

Following the 2004 Sumatra-Andaman giant earthquake (Mw 9.1), multiple geology and geophysical surveys were conducted to gain better understanding of the behavior and mechanism of the tsunamigenic event. We present the first regional study of seafloor depth changes offshore of northern Sumatra, covering from the northwest of Northern Sumatra to west of the Simeulue Island, 1-6.5°N and 93-96.5°E. The oldest bathymetric dataset was from Dec 1997-Jan 1998 and the newer datasets were from the multiple surveys between Jan-Feb 2005 and Jan-Mar 2009. Using a re-sampled cell size of 115 m and sample interval of 100 m, we focused our observation on five 30-50 km long, SW-NE transects comparing the 1997/8 data with later surveys of the toe of the accretionary prism. The depth errors are the standard deviation for each dataset. For the five regions discussed, from N to S, we observe for crossing: (1) an average shoaling of ~120 ±31 m between 1997 and 2005 adjacent to and on the seaward flank of a fold closest to the Sunda Trench; (2) a landward shift of the fold closest to the Trench with an average shoaling of ~29 ±9 m between 1997 and 2005 on the landward flank of this fold; (3) the second fold landward of the Trench shows an average deepening of ~43 ±16 m between 1997 and 2005 proximal to a mass-wasting feature and a consequent shoaling of ~44 ±3 m on landward flank of the next fold seaward; (4) a seaward shift and a shoaling of ~38 ±14 m between 1997 and 2005 on the fold closest to the Trench; and (5) a slight landward shift between 1997 and 2005. While uncertainties remain, the 10s of m vertical and 100s of m horizontal changes likely reflect co-seismic motion on the décollement (e.g. crossing 4), post-seismic motion on landward vergent thrusts (e.g. 2), and/or slumping (e.g. 1,3).