Identification of candidate paleotsunami deposit from sediment layers drilled in Seubadeh located in South Aceh of Indonesia

This study is conducted to identify prehistoric tsunami deposit (paleotsunami) in Seubadeh village, located in South Aceh Regency, Aceh province, Indonesia. Regional morphologic and seismic characteristics suggest that Seubadeh village is an area that stores tsunami deposits, both new tsunami deposits and prehistoric tsunami deposits. Therefore, in this study, the search for paleotsunami deposits was carried out in Seubadeh village. The tsunami deposit samples were taken at several points in Seubadeh village using a hand drill with a depth of more than 2 m. Sediment classification of the collected samples using Troels-Smith method shows a candidate layer of paleotsunami tsunami deposit was found between two clay deposits. The layer is dominated by plant roots and shell material. In addition, the sediment samples were sieved using a mesh to evaluate the grain size distribution of clay, silt and sand. The results of the grain size distribution and color evaluation of the respective layer are presented. The results of the analysis of the particle size distribution in the sample and the color of the sample indicate that there are 2 layers that show characteristics as a paleotsunami deposit, namely at a depth of about 170-200 cm and 280-292 cm. The paleotsunami deposit found will be further characterized by an emission spectroscopic technique.


1.
Introduction The big Indian Ocean tsunami that occurred on December 26, 2004, devastated most of the countries in the Indian Ocean, including Indonesia, especially in Aceh province.This tragic natural event has killed hundreds of thousands of people, destroyed buildings, and damaged the natural environment on a large scale.One of the causes of the magnitude of the impact caused by these events is because the Aceh area is located along the west coast of Sumatra and is a coastal area directly adjacent to the Indian Ocean.The Aceh tsunami event has passed, but there is still much to be learned and studied as well as a concern for coastal communities and disaster researchers.The large-scale damage and loss of life that occurred in the 2004 tsunami indicates that public knowledge about the tsunami is still inadequate.Detailed knowledge of the giant tsunamis that have hit, both recent and prehistoric tsunamis, is important to study to evaluate vulnerability and preparedness, especially for coastal communities [1] The Aceh tsunami that occurred in 2004 has inspired several parties, including researchers who want to explore the ancient tsunami (paleotsunami) that occurred in Aceh.Since the 2004 Indian Ocean tsunami, the search for evidence of prehistoric tsunamis in Aceh province has been increasingly active, so paleotsunami data can be used to extend the tsunami record and estimate the recurrence of tsunamis in Aceh.Based on previous research, conducted by [2] in the southern part of the island of Simeulue.Meanwhile, in another study in the northern part of Meulaboh, two ancient tsunami layers were identified [3].The first layer is identified as sediment generated by the 1907 tsunami event, and the second layer deposited by the post-1300 tsunami event is a correlative layer.In addition, a highly variable repeat of the tsunami in the 7,400-2600 year range was also found in a coastal cave located in Lhoong, Aceh Besar.Based on data from the cave, Aceh province has been hit by giant tsunamis at least 11 times and the 2004 Indian Ocean tsunami was reported as one of the largest tsunamis since the early Holocene [4][5].Based on traces of research that have been carried out previously, it is hoped that this study can determine the presence of prehistoric tsunami deposits and observe the physical properties of paleotsunami deposits in the form of sedimentary shape, grain size, colour, organic soil, root wood, minerogenic sediments, and fossils in the Seubadeh area, South Aceh, Aceh province, Indonesia.

Material and Methods
In this study, the paleotsunami deposit was collected from Seubadeh village, located in Bakongan Timur sub-district, the western part of Aceh province.Seubadeh Village is approximately 70 km from the center of the South Aceh capital, Tapaktuan.Seubadeh Village is right on the side of the Tapaktuan-Medan road, so Seubadeh village can be easily accessed from Tapaktuan city by vehicle.Seubadeh Village has regional environmental conditions in the form of swamps and rice fields, so based on the environmental conditions Seubadeh Village has the potential to store tsunami sediment layers [6].The sampling location is around Seubadeh village, where the farthest sampling location is approximately 500 m from the beach (Figure 1).

Figure 1. Location of tsunami deposit sample collection in Sibadeuh village
This research begins with finding a location/field survey, a location that has a concave morphology and is protected by coastal currents is a suitable place for tsunami deposits to be deposited.Furthermore, the deposited sample is taken by inserting a hand auger into the ground until it reaches an impenetrable depth as shown in Figure 2(a).After the sample is obtained, a megascopic analysis is carried out to investigate the colour of the sediment, the grain size of the sediment, as well as the units in it such as organic and anthropogenic materials (Figure 2b).

Result and Discussions
Tsunamis occur due to the activity of the earth's tectonic plates which causes a shift in the earth's crust in the sea.This shift can trigger the displacement of water volume in the form of waves in all directions so that the beach sand material will mix with deep-sea mud.If the tsunami is large enough, it will form a tsunami inundation area far inland [7].After the tsunami wave weakens, the seawater returns to the sea at a slow speed so that the sand material carried away will be left behind and settle on the land, this deposit is called a tsunami deposit.Tsunami deposits are used to identify tsunamis in the past (paleotsunami) [8].Physically, the speed of ocean waves can affect the size of grains carried by a tsunami and how far deposits can form on land.The greater the wave speed, the coarser the material that can be carried, conversely the smaller the wave speed, the finer the material.This also applies to the distance between the shoreline and the place where the sediment is formed.[9].
In Figure 3, several colors can be seen, such as brownish yellow, gray, brownish gray, blackish gray, yellow slightly gray and black.Where the black color is paleotsunami sediment which has various organics such as wood roots, shells, fossils of living things from the sea and plants that have been deposited and the black color is also sediment that does not get oxygen from the surface which a b has a very high carbon value so that paleotsunami deposits are formed. .This happened because the tsunami waves brought material from the sea in the form of sand, clay and sea animals.In addition, roots, plants and rock fragments that were buried during the deposition process are sometimes found in tsunami deposits [10].Layers of tsunami deposits can be seen more than once if the tsunami event occurs not only once.The layer of soil beneath the sand deposited by the tsunami is called paleosoil (soil that was formed before the tsunami occurred).While the layer of soil above is a new layer of soil that was formed after the tsunami occurred [11].

-316 cm, h). 316 -353 cm, i). 353 -390 cm
Based on the texture and color of the sediments that have been observed, the sediment layers at a depth of 0 cm -172 cm are dominated by clay sediments, 172 cm -200 cm are dominated by sand sediments and underneath there are clay sediments to a depth of 220 cm.The depth of 220 cm -280 cm is dominated by sedimentary sand and there is a small layer of clay 4 cm thick.The 284 cm -292 cm layer is thought to be a paleotsunami sedimentary layer which is dominated by sand sediments and contains 12 cm thick clay sediments.At a depth of 304 cm -360 cm is a layer dominated by sand sediments.It can be seen in Figure 5 that there is one tsunami layer and one paleotsunami deposit layer, the tsunami deposit layer is at a depth of 172 cm -200 cm and the paleotsunami layer is at a depth of 284 cm -292 cm.There are several things that were found in the samples of this study, starting from roots, wood, charcoal, and different colors in each layer [14].

a b c f e d i h g
In the field, soil texture can be determined by massaging the wet soil between the fingers, while feeling for its roughness, namely feeling the presence of grains of sand, dust and clay [12].In this study, 3 types of sediment were found, namely sand, silt, and clay sediments.Of the three sediments, the most dominant sediment found in the research sample is sand sediment, this is because the grain size of sand is larger than the grain size of silt and clay [13].In the sample weighing process, sedimentary sand is also the type of sediment that is most commonly found because sand sediments are heavier and more abundant than silt and clay sediments.The percentage of sediment types found is calculated based on the results of the process of weighing sifted sediment samples.In Figure 4 sediment samples and research graphs for 10 layers.In layer 1 from a depth of 20 -24 cm it shows that the percent sand (10.58 %), silt (38.04 %) and clay (51.38 %), layer 2 at a depth of 172 -176 cm shows that the percent sand (85 .43%), silt (13.99 %) and clay (0.58 %), layer 3 at a depth of 176 -180 cm shows that the percent sand (83.58 %), silt (15.76 %) and clay (0.66 % ), layer 4 at a depth of 180 -184 cm shows that the percent sand (86.72 %), silt (12.74 %) and clay (0.53 %), layer 5 at a depth of 184 -188 cm shows that the percent sand (82, 73 %), silt (16.58 %) and clay (0.69 %), layer 6 at a depth of 188 -192 cm shows that the percent sand (84.56 %), silt (14.83 %) and clay (0.62 %) , layer 7 at a depth of 192 -196 cm indicates that the percent sand (89.63 %), silt (9.95 %) and clay (0.41 %), and layer 8 is at a depth of 196 -200 cm indicates that the percent sand (84 .83%), silt (14.57%) and clay (0.61 %).

Conclusions
The identification of paleotsunami deposits taken from Sibadeuh Village, located in East Bakongan sub-district, South Aceh Regency has been successfully carried out.The first candidate paleotsunami deposits was found at a depth of 170-20 cm with a thickness of more than 10 cm.While the candidate for the second paleotsunami deposit was found at a depth of 280-292 cm with a thickness of 12 cm.The paleotsunami candidate layer found in this study was characterized by the presence of blackish grey coarse sand material located on top of a dark brown clay layer.There is a sharp erosional contact at the bottom and top of the tsunami deposition layer.In addition, the bottom of the paleotsunami candidate layer contains a lot of shell material such as snails and shells.The characteristics of the paleotsunami deposit layers found in this study are similar to those of the tsunami layers identified in other tsunami areas.It is possible that the 2004 Indian Ocean tsunami sediment layer will be further characterized by geochemical analysis using an optical instrument, namely Laser-Induced Breakdown Spectroscopy (LIBS).

Figure 2 .
Figure 2. a) The deposit sampling process using a hand auger in Sibadeuh village, b).Description of the tsunami deposit sample collected in Sibadeuh village

Figure 4 .
Figure 4. Sediment samples and percentage of sediment types