1

THE INDIAN OCEAN TSUNAMI 2004: IDENTIFICATION OF TSUNAMI DEPOSITS OFFSHORE IN THE ANDAMAN SEA BY

DIFFERENT PROXIES

KLAUS SCHWARZER1, PETER FELDENS2, DAROONWAN SAKUNA-SCHWARTZ3, SIWATT PONGPIACHAN4, YVONNE MILKER5, DANAI

TIPMANEE6

1. Sedimentology, Coastal and Continental Shelf Research, Institute of Geosciences, Kiel University, 24118 Kiel, Germany. kls@gpi.uni-kiel.de.

2. Institute of Geophysics, Kiel University, 24118 Kiel, Germany. pfeldens@geophysik.uni-kiel.de.

3. Oceanography Unit, Phuket Marine Biological Center, P.O. Box 60, Phuket 83000, Thailand. daroonwans@gmail.com.

4. NIDA Center for Research and Development of Disaster Prevention & Management, School of Social and Environmental Development, National Institute of Development Administration, Bangkok, Thailand. pongpiajun@gmail.com.

5. Institute of Geophysics and Geology, Leipzig University, 04103 Leipzig, Germany. Yvonne.milker@uni-leipzig.de.

6. Faculty of Technology and Environment, Prince of Songkla University, Phuket Campus, Phuket, Thailand. dtipmanee@hotmail.com.

Abstract: It was doubted for a long time that in wave dominated coastal areas the impact of tsunami waves on shoreface deposits can be preserved. Following high resolution mapping with different hydroacoustic methods, positions for grab sampling and coring were identified, where tsunami deposits were supposed to occur. The sampled material was analyzed using a wide range of sedimentological, geochemical, micropalaeontological, chemical, and physical methods. Storm and tsunami event layers could be identified and distinguished. Individual layers, ranging from 12 - 39 cm in thickness, were interpreted as tsunami deposits. Run-up and backwash deposits could be distinguished. Based on foraminiferal transfer functions and textural analyses re-suspension of sediment during run-up seemed to be restricted to about 20 m water depth. On the other hand it could be shown by using PAHs as a chemical proxy that the loaded backwash extends up to 25 km offshore.

Introduction

Contrasting onshore deposits, impacts of tsunami events, whatever the genera-ting source is, have been poorly investigated in the marine realm and the interac-tion of tsunami waves with the seafloor is poorly understood. Recently, investi-gations of sedimentary phenomena caused by tsunami action in near- and off-shore regions became new, developing research topics. When a tsunami is gen-erated, the influence to the seafloor can be of different origin. Gravity flows can be generated (Ikehara et al. 2014); an earthquake as the source might cause

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movements of the seafloor destabilizing sedimentary deposits by liquefaction (Goto et al. 2012). Another possibility is the interaction of tsunami waves with the seafloor when passing the shelf and shallow marine environment (Haraguchi et al. 2013; Paris et al. 2010; Noda et al. 2007) and the influence of the loaded backflow (Sugawara et al. 2009). Most information about tsunami induced sea-floor erosion is derived from studies of onshore deposits containing marine microorganisms (Dawson 2007; Uchida et al. 2010). Direct observations of the seafloor immediately after a tsunami are rare and require pre-tsunami investiga-tions to assign changes to the tsunami. Chavanich et al. (2005) and Seike et al. (2013) report about geomorphological changes in the range of meters and sedi-mentological changes from fine to coarse grained sediment; however, they could not distinguish between run-up and backwash effects.

We focus on the influence of tsunami waves in the Andaman Sea, which were generated by the 2004 Sumatra-Andaman momentum magnitude (Mw) 9.3 earthquake (Titov et al. 2005; Clieh et al. 2007), far away from the area where they hit the coastline. Existing data of offshore tsunami influences to the sea-floor and deposits generated by the run-up and backflow are scattered and insuf-ficient to establish key criteria for their clear identification (Shanmugam 2006, 2008). This lack of data originates from the limited potential of preservation and recognition as reworking and transport by waves, tides, currents (Weiss and Bahlburg 2006) and bioturbation (Noda et al., 2007; Wheatcroft and Drake 2003) are active in shallow water.

The impacts of the recent tsunamis, 2004 Indian Ocean (Bell et al. 2005; Tsuji et al. 2006) and 2011 Tohoku (Goto et al. 2014), led to most detailed investiga-tions in tsunami research (Ikehara et al. 2014). Our objective is to identify the impact of tsunami waves on offshore sediments which were transported and deposited by the 2004 Tsunami. We are searching for fingerprints of tsunami deposits and aim to elaborate criteria to distinguish these deposits from other event layers created by storms, as it is often difficult to distinguish a storm layer from a tsunami layer only from the sedimentological record. With this information we enhance knowledge about the recognition of offshore tsunami deposits, their preservation potential and, applied to the sedimentological record, return periods (Monecke et al. 2008). In general this information supports the possibility to understand the sediment architecture of shelf environments which are influenced by events of different origin.

Investigation Area

The Andaman Sea Shelf adjacent to the Malay Peninsula is narrow and slightly inclined, reaching 50 m water-depth approximately 7 km offshore Phuket and about 30 km offshore Phang Nga Province (Fig. 1). It is mainly a sediment

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starving shelf (Rodolfo 1969; Schwab et al. 2012). The coastline consists of beaches in front of lowlands alternating with rocky cliffs. Longshore currents and sediment transport are mainly directed from south to north generating com-mon coastal structures like spits (Fig. 1). Mud patches are common in water depth from 5 to 15 m north and south of Pakarang Cape (Feldens et al. 2012).

Fig. 1. Position of sediment samples and sediment cores based on a side-scan sonar mosaic and high resolution shallow seismic.

Granite outcrops are scattered along the inner shelf at water depth of 5 - 10 m and on the mid shelf at a water depth of about 30 m (Sakuna-Schwartz et al.

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2014). At Pakarang Cape, a 3 km long fringing reef complex and palaeoreefs nearby form a platform extending to approximately 10 - 12 m water depth. The platform is covered by numerous boulders, with diameters often exceeding 1 m and increasing in number towards offshore. A channel network, with up to 2 m depth of the individual channels and between 30 - 100 m in width is intersecting this platform Fig 1, cutout B). Tin mining down to 50 m water depth took place here, leaving pits with up to 100 m in diameter and 7 m depth behind (Feldens et al. 2012).

From December to February this area is dominated by the dry NE-monsoon while the SW-monsoon with moderate to heavy rainfall prevails from May to September. During the monsoon wind induced waves can reach heights up to 5m (Choowong et al. 2009). The tide is mixed semidiurnal with a tidal range varying from 1.1 m to 3.6 m due to neap- and spring tide. This region is poorly affected by tropical cyclones. Typhoons can occur but their frequency is low (Phantuwongraj and Choowong 2012). Along this coast, the tsunami runup height varied from < 3 m on Kho Khao Island to almost 10 m at Nham Kem and to > 15 m at Pakarang Cape (Siripong 2006). Low river discharge (Jankaew et al. 2008; Brill et al. 2011) increases the potential to observe and sample tsunami and storm induced deposits even a few years after the event. Besides numerous onshore surveys (Szczuciński et al. 2012b), only few investigations have been carried out offshore after the tsunami (Di Geronimo et al. 2009; Sugawara et al. 2009; Feldens et al. 2012; Sakuna et al. 2012; Milker et al 2013; Sakuna-Schwartz et al. 2014).

Methods

Prior to post-tsunami sampling, detailed knowledge of seafloor conditions regarding bathymetry and sediment-distribution patterns is indispensible. Hydro-acoustic surveys, underwater video observations and sediment sampling cam-paigns were carried out during 3 cruises each of about 3 - 4 weeks duration from 2007 – 2010 to catalogue geomarine effects of the tsunami. High resolution side-scan sonar mapping, shallow reflection seismic surveys and bathymetric mapping with a multibeam echosounder were performed (Feldens et al. 2009, 2012) resulting in approximately 2000 nautical miles of hydroacoustic profiles. An area of about 1000 km² was mapped covering the shoreface from 5 to 90 m water depth. For detailed investigations a full coverage side-scan sonar mosaic of 105 km² was recorded offshore Pakarang Cape in water depths ranging from 5 to 35 m. Based on these data, 156 grab samples and 60 sediment cores up to 2 m in length were retrieved from the seafloor (Fig. 1).

A multi-method and multi-proxy approach was chosen to identify offshore event deposits. Besides processing of hydroacoustic data (Feldens et al. 2012),

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laboratory treatment of sediment samples include grain size analyses with a laser based particle sizer device (range: 0.04 - 2000 μm) and micropalaeontological analyses to identify provenances of sediment components (Mamo et al. 2009; Uchida et al. 2010; Milker et al. 2013). For decades, PAHs (Polycyclic Aromatic Hydrocarbon) have been used as chemical proxy to distinguish anthropogenic from biogenic sources in marine sediments (Cantwell et al. 2007). We used PAH analyses from 70 grab samples to discriminate typical marine sediments from tsunami backwash deposits and to identify the backwash path (Tipmanee et al. 2012). Analysis of variance (ANOVA), Gaussian distribution, hierarchical cluster analysis (HCA), principal component analysis (PCA) and Redundancy Analysis (RDA) have been applied to the different data sets. Further details are described in Sakuna et al. (2012), Milker et al. (2013) and Pongpiachan et al. (2013).

Sediment cores underwent multi-sensor core logging (MSCL) to collect basic sediment properties like bulk density, gamma-ray attenuation, magnetic suscep-tibility and p-wave velocity. X-ray fluorescence (XRF) core scanner analysis was applied to semi-quantitatively assess sediment composition and element analysis to differentiate terrigenous material from sediments of marine source (Weltje and Tjallingii 2008). X-ray photos (Sakuna et al. 2012) help to clearly recognize internal sedimentary structures and unconformities. 210Pb has been widely used to assess sediment accumulation rates of unconsolidated, fine grained sediments since the onset of the industrialization (Nittrouer et al. 1979). 210Pb activity was measured to identify the 2004 tsunami deposits, to estimate changes in sediment accumulation rates and to detect the presence of major erosional events and event layers (Sakuna et al. 2012). As 137Cs activity was mostly below detection limits, these data were not used.

Results

Between the 2004-tsunami and our investigations no big storm nor anthropogen-ic activities affected this region creating new event layers or eroding older layers. By revisiting some areas of the upper shore-face, a natural shift of borders in the range of tens of meters between different sediment facies could be measured, which is typical for sandy wave dominated coastal environments due to seasonal changes. This observation is supported by episodes of erosion and sedimentation indicated by sedimentary structures observable in x-radiographs of core material (Fig. 2 and 3) reflecting sediment-dynamics. Thus, an impact of the tsunami on the surface of the seafloor could not be identified over large areas neither in the morphology nor in sediment distribution patterns, grain size composition or specific morphological features, as these attributes were already rearranged naturally.

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Results from high resolution subbottom profiling show channel structures in an ancient reef platform (Fig. 1, cutout A) and depressions seawards of granitic outcrops. Both are filled with different seismo-stratigraphical units. Coring in these structures reveal below a few centimeter thick cover of fine to muddy sand, tsunami deposits with a thickness ranging from a few cm up to 39 cm (Tab. 1). These deposits, appearing above an erosional unconformity, contain abundant shell fragments, clay clasts, gravel, pieces of laterite, are poorly sorted and show particulate cross lamination. This unit hosts partly onshore compo-nents but derives mainly from the nearby vicinity where similar sediments are found further on- and offshore. This unit is also highlighted as similar 210Pb activities are measured in the layers above and below but an anomalously low 210Pb activity is measured in samples taken from this unit (Fig. 3), supporting its event origin.

Tab.1: Data of sediment cores taken from the Andaman Sea offshore area (Abbreviation: TL = Tsunami Layer, WT = Water Depth)

Core Date of Coring

Position WT (m)

Distance Offshore (km)

Core Recove-ry (m)

Thickness of TL (cm)

Thickness above TL (cm)

031207-23

03.12.

2007

08°44.879'N

98°01.173'E

57.0 25.5 0.70 --- ---

051207-31+

05.12.

2007

08°47.176'N

98°11.724'E

15.9 7.2 0.70 12.0 0.0

051207-32/2 (c)

05.12.

2007

08°46.725'N

98°11.814'E

14.4

(ch)*

7.2 0.30 5.0 >25.0

030310-C2+

03.03.

2010

08°36.474'N

98°12.454'E

11.5 3.3 0.23 12.0 5.0

030310-C3** +

03.03.

2010

08°38.708'N

98°12.931'E

9.5 3.2 0.97 18.0 22.0

030310-C7+

03.30.

2010

08°41.053'N

98°12.763'E

11.9 2.9 0.65 39.0 3.0

050310-C2

05.03.

2010

08°45.438'N

98°13.186'E

9.8 1.5 0.75 9.0 31.0

050310-C3

05.03.

2010

08°46.352'N

98°13.182'E

11.2 3.9 0.52 13.0 5.0

050310-C4+

05.03.

2010

08°46.659'N

98°12.269'E

15.3 6.3 0.55 13.0 2.0

050310-C6***

05.03.

2010

08°38.761'N

98°12.967'E 9.6 (mp)*

3.1 0.99 18.0 22.0

* ch = channel structure, mp = mud patch, ** see Fig. 2, *** see Fig. 3, + Core shows storm layers below tsunami layer

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Log-ratios of Titanium (Ti) and Calcium (Ca) represent the relative variations of terrigenous versus marine constituents. Ti is abundant in tropical soils where laterite and bauxite are formed and is resistant against diagenetic processes. Calcium reflects the occurrence of CaCO3, which is mainly produced in marine environments. In most of the studied cores the Ti/Ca log-ratio was stable in those layers which were not affected by the tsunami event with generally lower rations in more shell-rich sandy layers. A slightly elevated and variable Ti/Ca log-ratio can be observed in the event layers (Fig. 3). However, not every event-layer shows this signal very clearly. PCA-analyses of XRF-data revealed mixed provenances of the sediments in the event layer supporting as well its event character. Within the event layer of core 050310-C6, runup and backwash deposits could additionally be identified by structural and textural analyses addressing these layers to the tsunami wave train. Analysis of benthic foraminifera from cores 030310-C3 and 050310-C4 (Tab. 1) and from 25 sediment surface samples ranging from 9.5 to 63.4 m water-depth showed distinct bathymetric zonations of most recent species which allowed the development of a transfer function for quantitative palaeowater depth recon-struction. Significant re-deposition of sediments during the tsunami including site specific runup and backwash processes could be observed. Based on the present water depth of 9.5 m for core 030310-C3 (Tab. 1), a net transport from about 20 m water depth could be calculated which resulted in an onshore di-rected distance of about 5 km. For core 050310-C4 only a slightly deeper water depth for particles transported onshore could be estimated but with a higher variability in the calculated depth due to a lack of good analouges in the modern samples compared to fossil samples. Besides the tsunami layers, former storm events represented by sandy layers could be identified with three layers in cores 030310-C3 and 051207-03, 2 layers in core 030310-C7 and one layer in core 050310-C4. A sediment transport from deeper waters to the core location could be shown for storm events as well.

Based on 210Pb activity measurements the deposits above and below the event-layer imply accumulation rates ranging from 1.3 to 5.7 cm y-1. As the grab has a penetration depth of 10 cm, tsunami deposits have partly been traced by grab samples. In some areas below a 5 cm thick cover of muddy fine sand, the sedi-ments contain pockets of fine sand, gravel, pieces of laterite and plant remnants. Such deposits are found down to 18 m water-depth and up to 8 km offshore.

Generally terrestrial derived PAH concentrations in sediments should decrease gradually from onshore to offshore but offshore Pakarang Cape the opposite is measured, PAHs growing with increasing distance from the coastline up to 25 km offshore.

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Fig. 2. X-ray, photo and sediment properties of core 030310-C3. Based on micropalaeontological, sedimentological and geochemical analyses the core is subdivided into 6 subunits (see text).

Fig. 3. X-ray, photo and sediment properties of core 050310-C6. Based on micropalaeontological, sedimentological and geochemical analyses the core is subdivided into 4 subunits (see text).

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By using PAH diagnostic ratios combined with multivariate descriptive statisti-cal techniques it was possible to distinguish between different PAH-sources. The diagnostic PAH isomer ratios provided evidence that road paving asphalt, which was heavily eroded by the tsunami in the Pakarang Cape area, was identi-fied as one of the main sources. Using this proxy it could additionally be shown that the transport direction was mainly just perpendicular to the coastline, which is supported by satellite images (CRISP 2004).

Discussion

Marine sediments seem to be a minor component of the 2011 Tohoku-oki tsunami onshore deposits (Jagodziński et al. 2012, Szczusiński et al. 2012a; Takashimizu et al. 2012). This is in contrast to other areas (Papua New Guinea, 1998, Indian Ocean, 2004) where tsunami deposited a considerable amount of marine sediments onshore (Gelfenbaum and Jaffe 2003; Kokociński et al. 2009). One reason for this might be the geological and sedimentological built up of the offshore environment and the response to tsunami waves when passing over. We showed, that in shallow water offshore Phang Nga area sufficient mobile sedi-ment is available for onshore transport. To understand the impact of a tsunami on a coastal environment, a holistic approach is recommended, requiring com-prehensive on- and offshore studies. According to Shiki et al. (2008) offshore and continental-shelf areas are the environment in which the most characteristic and noteworthy features of tsuna-mi deposits can develop. For this reason, and because of the higher sedimentary preservation potential in this environment compared to onshore and nearshore wave-dominated regions, shallow-marine environments can host tsunami rec-ords. Our investigations showed the opposite. Offshore 20 m water-depth tsu-nami signatures could not be detected in the sediments with the exception of PAHs in grab samples. There is no single set of signatures that could universally be applied to identify clearly tsunami deposits offshore. Most studies carried out after a tsunami are limited to bathymetrical surveys and sediment sampling (Di Geronimo et al. 2009; Sugawara et al., 2009). Haraguchi et al. (2011) added side-scan sonar surveys before sampling. We demonstrate that comprehensive hydro-acoustic surveys including high resolution seismic investigations are necessary to get a clear picture of the architecture of the shelf deposits, before taking any samples. Sample treatment requires a multi-method and a multi-proxy approach to clearly identify tsunami deposits and to separate them from other event deposits like storms. While in sediment cores the impact of the 2004 tsunami could only be shown for an area down to about 20 m water depth (8 km offshore), the

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application of PAHs to detect tsunami deposits was applied for the first time indicating clearly the extension of the backwash to 25 km offshore. A lack of pre-event data often hamper investigations focusing on the impact of tsunami on seafloor conditions as comparisons between pre- and post-tsunami situations are rarely possible (Seike et al. 2013). However, in addition wide-spread absence of pre-event data from continental shelf areas, it is a challenge to provide instruments and shiptime immediately after a tsunami has hit the coast.

Acknowledgements

We thank Phuket Marine Biological Center (PMBC) for providing us with ship-time (RV Chakratong Tongyai and RV Boolert Pasook), the National Research Council of Thailand (NRCT) and the German Research Foundation (DFG, grant: SCHW 572/1) for funding.

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