Use of anisotropy of magnetic susceptibility (AMS) in the study of tsunami deposits: Application to the 2004 deposits on the eastern coast of Banda Aceh, North Sumatra, Indonesia

We studied emplacement dynamics of the December 26, 2004 tsunami deposits in artificial shallow ponds and in shallow lagoons, along the northern coast of Sumatra in Banda Aceh area. Successive tsunami waves inundated topographic lows, which acted as sediment traps and favored preservation of deposits. Individual waves led to the deposition of fining upwards sedimentary sequences. This multiple normal grading deposit with a thickness ranging from 2 to 80 cm constitutes a site of major interest for the study of the deposits emplaced by a tsunami wave train. The main interest lies in the characteristics of the deposits that can be correlated to the behavior of the waves attested by numerous testimonies. In this setting, we aimed at testing the applicability of the AMS (Anisotropy of Magnetic Susceptibility) technique as a proxy of unconsolidated sediment fabric. The combination of this proxy and traditional sedimentary analysis helped reconstructing the hydrodynamic conditions — particularly the flow direction, prevailing during sedimentation. We applied the AMS technique on eight sections stretching inland from 1.5 km to 2.2 km from the pre-tsunami coastline. Grain-size and AMS data from these deposits provide similar and partially complementary information. Traction dominates in the basal parts of the deposits and occurs mainly for the basal layers of the sequences whereas settling dominates for the upper layers of these sequences. The vertical evolution of the AMS parameters within the deposits is related to the arrival of the successive waves with variations in the traction and decantation processes during deposition. Variations of current strength during sedimentation are related to the wave's momentum and to the progressive infill of topographic depressions. The vertical change of the fifth coarsest percentile vs. the median grain sizes of the deposits suggests strong variations of the energy of the currents, and confirms the evolution of the AMS parameters. In the present study, the fabric of the deposits, which developed exclusively during the uprush phase of the cycle, also allows the reconstruction of flood directions that are both normal and parallel to the shoreline, with variations related to flow dynamics, mean grain size and local topographic control. Due to the absence of backwash in between the waves and low velocity of the final backwash, information about the sedimentation conditions of the tsunami deposits appears to be better preserved in topographic depressions in areas located remote from the seashore. The AMS proxy appears to be a promising tool for further studies of recent- and paleo-tsunami deposits.