Marine Geology 357 (2014) 384–391

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Marine Geology

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Ambiguous correlation of precisely dated coral detritus with thetsunamis of 1861 and 1907 at Simeulue Island, Aceh Province, Indonesia

Shigehiro Fujino a,⁎, Kerry Sieh b,1, Aron J. Meltzner b,1, Eko Yulianto c, Hong-Wei Chiang d,1

a Active Fault and Earthquake Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Site C7 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japanb Tectonics Observatory, California Institute of Technology, Pasadena, CA 91125, USAc Research Center for Geotechnology, Indonesian Institute of Sciences, Bandung, Indonesiad High-precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan, ROC

⁎ Corresponding author at: Graduate School of LifeUniversity of Tsukuba, 1-1-1 Ten-noudai, Tsukuba, Ibarak

E-mail address: shige-fujino@geol.tsukuba.ac.jp (S. Fu1 Now at: Earth Observatory of Singapore, Nanyang Te

Singapore.

http://dx.doi.org/10.1016/j.margeo.2014.09.0470025-3227/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 March 2014Received in revised form 19 September 2014Accepted 28 September 2014Available online 22 October 2014

Communicated by J.T. Wells

Keywords:tsunami depositcoral boulderSimeulue IslandAceh Province

Precise U–Th dates from coral detritus in two pre-2004 tsunami deposits on Simeulue Island in Aceh Provinceallow us to correlate the deposits with historically documented tsunamis in the recent few centuries, but becauseof potential discordance between the death dates of the corals and deposition of the sand layers, ambiguity in thiscorrelation remains. Pits at coastal lowland sites exposed sand layers beneath the 2004 tsunami deposit atBusung and Naibos on southern Simeulue Island. The layers share sedimentological characteristics with the de-posit of the 2004 tsunami, and are interpreted as pre-2004 tsunami deposits. Historical accounts document earth-quakes and tsunamis in 1907 and 1861 and suggest that the 1907 tsunami was larger locally than any othershistorically. Nonetheless, U–Th analyses of coral boulders in the younger of two pre-2004 deposits at Busungand in the lone deposit at Naibos yielded dates of death that overlapwith 1861, but there were no tsunami layersthat could be directly dated to 1907. The younger pre-2004 sand deposit can be attributed to both the 1861 and1907 events, if the dated corals were killed by uplift in 1861 and subsequently entrained and deposited by the1907 tsunami. A piece of coral in the older of the two pre-2004 sand layers at Busung dated to around AD1783, and was deposited by an unknown tsunami that occurred after AD 1783, or possibly by the 1861 tsunami.The nearly equatorial latitude of the study sites minimizes potential for geological confusion between tsunamiand storm. Our results show the difficulty inmatching known events and geological records when using limitingmaximum ages from allochthonous fossils, even with high precision radiometric dates.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Howwide can gaps be between the true age of a layer and ages fromits accompanying allochthonous fossils? Researchers commonly useages of plant or animal fossils in sediment layers to estimate the timingof events such as earthquakes and tsunamis, and to correlate thoseevents with events at other sites, or with events from the historical re-cord. However, in many cases allochthonous fossils may have died de-cades or centuries before their deposition. For example, leaf fragmentsfrom pre-2004 tsunami deposits at Phra Thong Island in southernThailand gave radiocarbon ages that differed from one another by hun-dreds of years, despite their occurrence in the same layer (Jankaewet al., 2008). Similarly, Nelson (1992) reported that various materialsfrom the same stratigraphic horizon can differ in radiocarbon age bymany hundreds of years. On the other hand, using in-situ plant fossils

and Environmental Sciences,i 305-8572, Japan.jino).chnological University, 639798

killed by coseismic subsidence, Nelson et al. (1995) significantlyreduced the uncertainty in matching the age of dated material to anearthquake.

In this study, we compare ages of coral detritus entrained in tsunamideposits with ages of historical earthquakes, by using uranium–thoriumtechniques withmuch greater precision thanwould have been affordedby radiocarbon techniques. Our pit excavations exposed pre-2004tsunami deposits from the past few centuries at two study sites onSimeulue Island, Aceh Province, Indonesia. Although both sites lie atopthe 2005 rupture patch, they are exposed to tsunamis from both the2004 and 2005 patches (Fig. 1).

Large tsunamigenic fault ruptures occur frequently along the Sundamegathrust. Off the coast of northern Sumatra, in the region of the 2004MW 9.2 and 2005 (MW 8.6) ruptures, earthquakes potentially ofM ~7.5or greater occurred in 1861 and 1907 and resulted in tsunamis knownto have affected the Acehnese coast (Newcomb and McCann, 1987).The 1861 earthquake caused heavy shaking at Nias Island and wasaccompanied by a tsunami that affected more than 500 km of theSumatran coastline; it is regarded as an analog of 2005 in size andspatial extent (Meltzner et al., 2012b) (Fig. 1). Seafloor displacementof the 1907 earthquake caused a tsunami that devastated Simeulue

B

97°E96°E

3°N

2°N

Simeulue Is.

Banyak Is.Busung

Naibos

0 20km

B

Ph

Me

Ni

15 N

10 N

5 N

0

95 E

100 E2004

2005, 1861Sumatra

1941

1881

1847

A

1907

Su

nd

am

eg

at h

r u s t

AndamanSea

IndianOcean

50

100

50

100

2005patch

150

50

100150

2004patch

Fig. 1. Selected ruptures since the 19th century along the Sundamegathrust (A), and loca-tions of the study sites (B). Rupture zones are fromBilhamet al. (2005), Briggs et al. (2006)and Meltzner et al. (2010). The 1907 location is speculative. Ph, Phra Thong Island; Me,Meulaboh; Ni, Nias Island. Dotted lines show 2004 and 2005 uplift contours in centime-ters, from Meltzner et al. (2012a).

385S. Fujino et al. / Marine Geology 357 (2014) 384–391

Island and extended over 950 km along the Sumatran coast (NewcombandMcCann, 1987;McAdoo et al., 2006; Yogaswara and Yulianto, 2006;Yulianto et al., 2010). Recent analysis of old seismograms suggests thatthe 1907 earthquakewas a tsunami earthquake,with amagnitude ofMS

7.8 ± 0.25 (Kanamori et al., 2010).

2. Pre-2004 sand layers at Busung

The Busung site is located in southwestern Simeulue Island. It is asmall strand plain developed on a coral reef basement, and located atthe head of an embayment, Teluk Gosong (Gosong Bay). Because of itslocation inside the bay, protected behind a sandbar that has emergedto become a small islet following tectonic uplift, the Busung site

Psh

Former sandbar (now an island due to uplift in 2005)

Pre-2005 seacliffAlluvial apron

2004 and 2005 tsunami sand

Trees killed by pre-2005 subsidence

Post-2005 shoreline

Pit F

Stream

A B

Water buffalos

N

Quarry

Fig. 2. (A) Aerial photo of the Busung study site, taken inMay 2005. The coral reef flat emerged2005 subsidence stand on the emerged reef. White patches on the grassy coastal plain are sanboulders at Busung. Note that the orientation is different from (A). The former sandbar (post-2

experiences lower wave energy than open coasts. The Busung reef flatsubsided gradually before the 2005 Nias–Simeulue earthquake butrose more than 100 cm during the 2005 earthquake to be emergedabove high tide (Briggs et al., 2006) (Fig. 2). Interseismic subsidencein the decades prior to the earthquake gradually eroded the beachberm on the strand plain, but the coseismic uplift left the erodedbeach berm sequestered from further wave action on the emergedreef flat (Fig. 2). Palm trees that had grown on the reef flat followingan earlier uplift were killed by the pre-2005 subsidence, but are nowemerged as snags above high tide (Fig. 2). Both the 2004 and 2005 tsu-namis surged through Gosong Bay, though the 2005 tsunami was nothigh enough to fully inundate the flat across the berm, according tolocal eyewitnesses. The 30–40 m wide plain behind the beach bermwas a swampy flat before the 2004 tsunami but changed to grasslandbecause of the 2005 uplift and deposition of the 2004 tsunami sand.An aerial photo taken just after the 2005 earthquake shows whitesand deposited on the swampy flat by the tsunamis (Fig. 2A).

Many coral boulders lie on the plain behind the beach berm. Theywere buried by the 2004 tsunami sand but still project above the groundsurface (Fig. 3). Some of them were transported from the reef by the2004 tsunami, though others could have been transported by one ormore prior tsunami. In addition, some limestone boulders derivedfrom an alluvium apron behind the plain now lie atop the plain itself.These limestone boulders are re-crystallized and therefore can bedistinguished from corals transported by a tsunami.

Shallow pit excavations and a trenchmade for a port construction atBusung exposed two bioclastic sand layers (Units B1, B2) inmuddy sed-iments beneath the 2004 tsunami deposit (Pits A and F, Figs. 2, 4, 5).Fieldwork included stratigraphic logging of the pit walls, geochronolog-ic sample collection and GPS mapping.

Approximately 1 m of sediment lies atop the mid-Holocene coralreef basement and consists of layers of coral rubble, blue sandy silt,the lower bioclastic sand layer, black organic-rich peaty silt with abun-dant rootlets, the upper bioclastic sand layer, more of the black organic-rich peaty silt, and finally the 2004 tsunami sand, in ascending order(Figs. 4, 5). The older bioclastic sand layer (Unit B1) separates organic-rich silt and underlying blue sandy silt, and the younger one (Unit B2)is bounded both above and below by organic-rich silt.

The coral rubble is composed of pieces of corals, articulated anddisarticulated bivalves and calcareous algae. The blue sandy silt containsfragments of shell and calcareous algae, and has abundant burrows.Rootlets are observed in the sandy silt but are much less commonthan in the peaty silt. Some or all of the rootlets in the sandy silt grewdownward from horizons in the peaty silt to penetrate Unit B1. Thesandy silt is massive and does not show any laminations. The presence

Quarry

0 20m

N

Stream

Pit A

Pit F

Limestoneboulders

Alluvialapron

old coral boulder(Fig.3B)

Fig. 3

2004 breach

ost-2005 oreline

Emerged reef flat

Pre-2005 seacliff

Swampy flat

Road

Pre

-200

5 be

rm

due to uplift in the 2005 Nias–Simeulue earthquake. Palm tree that had been killed by pre-d from the 2004 (and, if any, 2005) tsunami. (B) Locations of excavated pits and selected005 island) in the foreground in (A) is just off the map in (B).

Pre-2005 seacliff Goats

RoadEmerged reef flat

Pit A2004 breach

Detail in B

A

B

Old Coral Boulder

Modern coral Boulders

Tree

Fig. 3. (A) Viewof the study site at Busung (looking north). The pre-2005 beachbermbehind the emerged reefflatwas breached by the 2004 tsunami. The 2004 tsunami left coral boulderson the field. (B) A coral boulder projecting above the 2004 and 2005 tsunami deposit. This was not dated and its stratigraphic relationship to the paleotsunami deposits was not checked,but it must be older than the 2004 tsunami, given that a tree has grown on the boulder.

386 S. Fujino et al. / Marine Geology 357 (2014) 384–391

ofmarinematerials and the direct contactwith the underlying coral clastlayer suggest that the sandy siltwas deposited in a sub-tidal or inter-tidalcalm environment that arose probably due to the formation of the sea-ward beach berm. The peaty silt just under the 2004 tsunami sand isrich in rootlets and plant fragments. No marine materials such as fora-minifers, coral fragments, and calcareous algae fragments were observedin the peaty silt. This peaty silt, therefore,was probably deposited under asubaerial environment that was similar to the swampy flat just beforethe 2004 and 2005 earthquakes and tsunamis. Diatoms are absent

Coral platform basemen

Sand (Unit B2)

Unit

Silt Coral ru

Dirt ber

A

B

Seaward

2004 and

Sampled for dating (BSG-F1)

S

Unit

Sand (Unit B1)

Detail in B

Fig.6C

Fig. 4. (A) Photo of thenorthern face of Pit F at Busung. Two tsunami deposits (Units B1 and B2)tsunami deposits are outlined by blue lines. (B) and (C) Close-up views of the northern face of

in the entire sedimentary succession of Pit F at Busung. The reasonfor this is unknown, but diatoms may have dissolved because ofsome diagenetic effects such as high ground water temperature.

Unit B1 and the 2004 tsunami deposit are laterally continuous in PitF (Fig. 4), whereas Unit B2 is patchy. All three units are also observed atthe same stratigraphic positions in Pit A, ~50m away from Pit F (Fig. 5),implying their lateral consistency at the study site.

The 2004 tsunami sand and the bioclastic sand layers (Units B1, B2)share some sedimentary features. They are all composed of multiple

Organic-rich siltCoral rubblet

B2

bble

1 m

Ground surfaceGround surfacem

C

Landward

2004 and 2005 tsunami deposits

2005

ampled for dating (BSG-F2)

B1

Detail in C

and associated coral boulders are traced by red and yellowdashed lines. The 2004 and 2005pit F.

Sandy Silt

Organic-richsilt

Unit B1

Unit B2

2004 and 2005 tsunami deposits

Pit bottom

10 cm

Burrows

Detail in B

Detail in C

2004 and 2005 tsunami deposits

Unit B1

Unit B2

Organic-richsilt

Sandy SiltPit bottom

Burrows

Ground surfaceGround surface

10 cm

A B

C

Coral bouldersampled for dating(GSG06-3)

Fig.6AFig.6B

Fig. 5. (A) View of Pit A at Busung. The coral boulder (GSG06-3) is outlined in yellow. (B) Southern face of Pit A. The thick sand layer at the top is the deposit from the 2004 and 2005tsunami. A bioclastic sand layer (Unit B2, yellow dashed line) is within the organic-rich silt of a back-berm swamp. Another bioclastic sand layer (Unit B1, red dashed line) is above asandy silt that was probably deposited in an inter-tidal or sub-tidal environment and below the organic-rich silt. (C) Eastern face of Pit A. The coral boulder that gave an age of AD1865.9 ± 18.5 (GSG06-3) is lying on Unit B2.

387S. Fujino et al. / Marine Geology 357 (2014) 384–391

sub-layers, rich in marine materials and accompanied by coral boulders(Fig. 6). The 2004 tsunami deposit is composed of alternating sub-layersof sand and silt (Fig. 6A), and accompanied by coral boulders as men-tioned above. Units B1 and B2 each have a sharp erosional basal contactand contain sub-layers (Fig. 6B, C). Both units are rich in calcareousalgae, coral fragments and other calcareous debris, indicating a seaflooror beach source. Coral boulders up to a few decimeters in diameter areabundant in Units B1 and B2 (Figs. 4, 5).

3. Pre-2004 sand layers at Naibos

TheNaibos site lies on a strand plain ~25 kmnorth of the Busung site(Fig. 1). The 2005 earthquake uplifted the Naibos site more than 0.5 m(Briggs et al., 2006), expanding the coastal flat by a few tens of metersseaward (Fig. 7A). As is the case at the Busung site, coral boulderswere left at the site. A 2 m coral boulder sits on a beach face in front ofthe study site (Fig. 7A). Another 2 m coral boulder and multiple smallerones lie partly buried in a paddy field ~250 m inland. These were not

dated but they were already in their present positions prior to the2004 and 2005 tsunamis, according to locals. Another old coral boulderis on a beach ridge ~40 m landward of the pre-2005 sea cliff; a pit wasexcavated beside it, exposing the boulder in profile (Fig. 7A).

The sediment above the beach ridge sand is composed of sandy silt, apre-2004 bioclastic sand layer (Unit N), peaty organic-rich silt and the2004 (or 2005, or both) tsunami sand, in ascending order. The peatyorganic-rich silt has abundant rootlets and plant debris. As seen underpreliminary microscopic observation, freshwater diatoms are abundantin the peaty organic-rich silt, though they were not counted. The sandysilt between the beach ridge sand and the sand layer (Unit N) has lessplant debris and rootlets than the upper peaty organic-rich silt. Diatomsand other bioclasts are absent in the sandy silt. Given that both thesandy silt and the peaty organic-rich silt were lying atop the beachridge, these were likely deposited under subaerial environments. If thebeach ridge had ever been below high-tide level and exposed to waveand tidal action, either during or after sedimentation of the silt layers,it is likely to have been eroded and not preserved.

Fine- to medium-sand 10cm

Organic-richsilt

Ground surfaceGround surface

Fine- to medium-sand with calcareous algae

Fine- to medium-sand

Sandy silt

Sandy silt

Sandy silt

Fine sand

2004 and 2005tsunami deposits

Silt

Organic-rich silt

Fine- to medium-sand with calcareous algae

Fine- to medium-sand with calcareous algae

Silty fine sand

Silty fine sand

Unit B1

Coral rubble

10cm

Organic-richsilt

Medium- to coarse-sand with calcareous algae

Medium- to coarse-sand with calcareous algae

Silty fine sand

Unit B1

Unit B2

Organic-richsilt

10cm

A

B

C

Fig. 6. (A) Close-up view of the 2004 (and, if any, 2005) tsunami deposit in Pit A at Busung. The deposit is composed of alternating sub-layers of sand and silt. See Fig. 5 for its position.(B) Close-up view of Unit B2 in Pit A at Busung. The deposit is composed of alternating sub-layers of silty fine sand and calcareous medium to coarse sand. See Fig. 5 for its position.(C) Close-up view of Unit B1 in Pit F at Busung. The deposit is composed of alternating sub-layers of silty fine sand and calcareous fine to medium sand. Unit B1 separates overlyingorganic-rich silt and underlying bluish-gray silt. See Fig. 4 for its position.

388 S. Fujino et al. / Marine Geology 357 (2014) 384–391

The pre-2004 bioclastic sand layer (Unit N) separates peaty organic-rich silt and underlying sandy silt (Fig. 7B, C). The unit is rich in marinematerials and shows internal structure (Fig. 7C). Coarser sub-layers con-sist of shells of calcareous algae mixed with fine to medium sand; thefiner sub-layer is composed of fine sand and is sandwiched betweenshell-rich coarser sub-layers (Fig. 7C). The lateral extent of Unit N isnot well determined, since the deposit could not be recognized inother pits at the site.

The old coral boulder rests directly atop Unit N (Fig. 7D, E), suggest-ing simultaneous deposition of the layer and boulder. An alternative hy-pothesis, that the coral boulder and the underlying paleotsunami layerwere from different tsunamis, is unlikely: it is improbable that a secondtsunami scoured down and left the boulder on exactly the same horizonwith the initial tsunami sand layer, without leaving a second sand layer.A portion of the coral boulder was chiseled off for dating.

4. Dating

Coral boulders from within the pre-2004 bioclastic sand layers(Units B1, B2, N) were dated with U–Th techniques to constrain the

ages of past tsunami events. We sampled coral boulders from Pits Aand F at Busung and from the pit at Naibos (BSG-F1, BSG-F2, GSG06-3,NBS-B; Figs. 4, 5, 7,Table 1).

In each case, there are two possible age relationships between a coralboulder and the deposit within which it was entrained: either the coralwas killed by the event (such as a tsunami) that resulted in the boulder'stransport and ultimate deposition, or the coral died previously (possiblyfromuplift in an earlier earthquake) and its outer surface predates the de-position. Given that the coral boulders' surfaces were all eroded to somedegree and that their outer growthbands sustained substantial bioerosionby boring organisms, the sampled boulders could have been older, by un-known amounts of time, than the deposit withwhich they are associated.

Samples for dating were removed from each boulder, and the boul-ders were sliced and X-rayed to enable the counting of annual bandsoutward from each geochronologic sample to the outer edge of theboulder. The dating results are summarized in Table 1.

U–Th sample ages suggest that the outermost of thepreserved annu-al bands of the coral boulders from Unit B2 date to AD 1865.9 ±18.5 years (GSG06-3; Pit A; Fig. 5) and AD 1521.8 ± 63.4 (BSG-F2; PitF; Fig. 4). Likewise, the outer preserved band of the boulder from Unit

Breach

Str

eam

bank

Road

Pre-2005 seacliff

Beach-ridge crest

Inter-ridge swale

2 m coral boulderin beach face

2 m coral boulder c.a. 250 m inland

1 m coral boulder

0 20m

Pit

N

Coral boulder

Ground surface

Ground surface

Dirt berm

Sand of beach ridge

Sand(Unit N)

Chiseled off for dating(NBS-B)

Detail in E

Sand of beach ridge

2004 tsunami sand

Sand (Unit N)

Coral boulder

Ground surfaceGround surface

Pit bottom

Scale (1 m

)

Detail in C

Organic-rich silt

Sandy silt

Fine- to medium-sand with calcareous algae

Fine- to medium-sandwith calcareous algae

Fine sand

10 cm

Dirt berm

A

B

C

D

Coral boulder

Sandy silt

Interspace filled byorganic-rich silt

Sand (Unit N)

E

10 cm

Fig. 7. (A) Location of the excavated pit at Naibos. (B) Photo of the northern face of the pit. (C) Close-up view of the pre-2004 tsunami sand layer (Unit N) in the northern face of the pit.The layer is separated into lower and upper coarser parts and amiddlefiner part, showing sub-layer structure. The coarser parts are rich in bioclasts. (D) Photo of thewestern face of thepit.The topof the tsunami sand layer corresponds to the bottomof the coral boulder. (E) Close-up viewof the contact between the sand layer (Unit N) and bottomof the coral boulder (NBS-B).The top of the sand layer corresponds to the bottom of the coral boulder. The gap between the layer and part of the boulder was filled by organic-rich silt probably deposited after theboulder was emplaced.

389S. Fujino et al. / Marine Geology 357 (2014) 384–391

B1 (BSG-F1; Pit F; Fig. 4) dates to AD 1783.4 ± 5.6. The outer preservedband of the Naibos boulder (NBS-B; Fig. 7) dates to AD 1871.0 ± 12.7.

5. Discussion and conclusions

5.1. Identification of the pre-2004 sand layers as tsunami deposits

The existence of sub-layer structure, abundance of marinematerials,lateral consistency and coral boulder accompaniment of the sand layersat Busung and Naibos (Units B1, B2, N) indicate that the layers weredeposited by past tsunamis. Sub-layer structure is often observed inmodern tsunami deposits including the 2004 tsunami deposit at

Table 1Summary of dating on coral boulders.

Sample Position Locality name Pit Age of sample (B.P.) Chemistry dat

GSG06-3 Unit B2 Busong Pit A 153.0 ± 18.5 5/22/2007BSG-F2 Unit B2 Busong Pit F 503.0 ± 63.2 10/24/2007BSG-F1 Unit B1 Busong Pit F 228.5 ± 5.5 11/26/2007NBS-B Unit N Naibos – 141.8 ± 12.7 10/24/2007

Busung, and past tsunami deposits (e.g. Benson et al., 1997). Such struc-ture is attributed to sedimentation from distinct phases of flow, such asfrom a sequence of tsunami inundations (e.g. Kon'no et al., 1961;Nanayama and Shigeno, 2006; Naruse et al., 2010). Abundant marinematerial in the sand layers reflects sediment transport from the sea,and excludes the possibility that a river flood or other processes thatsupply terrestrial sediments deposited the sand layers. The sand layerswere observed in limited number of pits but seem to be distributed con-sistently in the strata at Busung: they are laterally continuous in the 15-m-wide pit face (Pit F) and are also observed in a pit 50 m away (Pit A).This lateral continuity is a common feature of modern tsunami deposits(Morton et al., 2007).

e Date of sample (AD) Annual bands aftersample

Date of the outermost preservedannual band (AD)

1854.4 ± 18.5 11.5 ± 1.0 1865.9 ± 18.51504.8 ± 63.2 17.0 ± 5.0 1521.8 ± 63.41779.4 ± 5.5 4.0 ± 1.0 1783.4 ± 5.61866.0 ± 12.7 5.0 ± 1.0 1871.0 ± 12.7

390 S. Fujino et al. / Marine Geology 357 (2014) 384–391

Conceivably, sand layers and coral boulders can also record excep-tionally large storms, but the study sites are located well within thelow-latitude Coriolis minimum, where cyclone development is limited.Tropical storms can strike Simeulue Island, but they are rare and maynever be strong. The only documented tropical storm in the vicinity ofSimeulue was Tropical Storm Vamei, which formed off Borneo inDecember 2001 and passed just north of Simeulue. Although Vameihad reached minimal typhoon status when it initially made landfall onthe Malay Peninsula, just north of Singapore, it weakened quickly as itcrossed the Malay Peninsula (Juneng et al., 2007). Its sustained surfacewinds never exceeded 30 knots (55 km/h) as it crossed Sumatra orpassed north of Simeulue (Joint Typhoon Warning Center, 2002), andit was small and compact. Indeed, although storms with intense rain,wind, and waves form frequently in this region, they do not result inlarge sea-level fluctuations: at such low latitudes, it is not possible todevelop a large pressure gradient that could sustain a significant seasurface height differential (Koh Tieh-Yong, personal communication,2014).

5.2. Correlation with historical documents

The U–Th dates of coral boulders from Unit B2 in Pit A at Busung(GSG06-3) and from Unit N at Naibos (NBS-B) substantially overlapthe 1861 uplift and tsunami. Although sample BSG-F2 embedded inUnit B2 in Pit F is much older than sample GSG06-3 embedded in UnitB2 in Pit A 50 m away (Fig. 2, Table 1), we have high confidence in thestratigraphic correlations (Figs. 4, 5). The age discrepancy could easilybe explained if BSG-F2 was a coral clast that had been long dead beforebeing entrained and deposited in Pit F. We therefore take the age of theyounger GSG06-3 as the best approximation (but still a maximumbound) for the age of Unit B2.

Meanwhile, we do not have age data that overlap the 1907 tsunamiat either study site, though it is inferred from historical records that the1907 tsunami inundated both sites (NewcombandMcCann, 1987). Pos-sible causes of the absence of direct geological evidence for the 1907tsunami include (1) post-depositional processes may have completelyerased the 1907 tsunami trace, or (2) Unit B2may have been depositedby the 1907 tsunami instead of in 1861. In the first hypothesis, the 1907tsunami deposit may have been thin and subtle andwas later destroyedby bioturbation or erosion. In the second hypothesis, the 1861 earth-quake uplifted and killed corals on the reef seaward of the study site,but the associated tsunamiwas not big enough to leave a robust deposit.In turn, the larger 1907 tsunami entrained corals killed by emergence in1861 and transported them to the coastal plain. The second hypothesisis supported by the observation that the outer surface of coral boulderGSG06-3 was somewhat eroded, suggesting that the coral was deadand exposed for some time (several years or longer) before being de-posited. Furthermore, uplifted coral microatolls in southern Simeulueand northern Nias gave ages corresponding to the 1861 earthquakeand suggest that the 1861 earthquake was comparable to that in 2005(Meltzner et al., 2012b). The Busung site was uplifted more than100 cm by the 2005 earthquake (Briggs et al., 2006), but, based on eye-witness accounts, the associated tsunami was small and scarcelyovertopped the beach berm there. Given that the 2005 earthquakeappears to be an analog of 1861, it is plausible that the 1861 tsunamileft little deposit, if any, on the coastal plains at Busung and Naibos.

The coral boulder from Unit B1 at Busung provided an age of AD1783.4 ± 5.6 years. The change in sedimentary facies across Unit B1from bioturbated sandy silt upward to organic-rich peaty silt impliesan environmental change due to uplift concurrent with the tsunami.However, at present, coral microatoll studies have not found consistentevidence for coseismic uplift of southern Simeulue around the date ofthe boulder from Unit B1. If there really was no crustal deformationaround AD 1783, the dated coral boulder fromUnit B1may have alreadybeen dead when entrained by the tsunami to give an age older than theactual deposition of Unit B1. In this scenario, sedimentation of Unit B1

can be attributed to the 1861 tsunami or an unknown tsunami afterAD 1783. It is worth noting that the change in sedimentary faciesthrough Unit B1, from a sub-tidal or inter-tidal sandy silt up to asupra-tidal peaty silt, would be consistent with uplift in 1861 at thestudy site (Meltzner et al., 2012b).

Though we cannot definitively attribute sand layers at Busung andNaibos to specific tsunamis, U–Th dating on entrained coral bouldersgives improved age constraints for the timing of tsunamis in recent cen-turies. But because of potential discordance between the death dates ofthe corals and deposition of the sand layers, ambiguity in this correla-tion remains. Our results show the difficulty in matching known eventsand geological records when using limiting maximum ages fromallochthonous fossils, even with high precision radiometric dates.

Acknowledgments

We dedicate this paper to the memory of our friend Adi RahmanPutra, who assisted in fieldwork. We thank Katherine Whitlow for thefieldwork and Yuki Sawai for the diatom analysis. This study was sup-ported by JSPS and LIPI under the JSPS-LIPI Joint Research Program.This manuscript benefitted from the constructive comments by BrianAtwater and an anonymous reviewer. This is Earth Observatory ofSingapore contribution 67.

Appendix A. Supplementary data

Supplementary data associated with this article can be found in theonline version, at http://dx.doi.org/10.1016/j.margeo.2014.09.047.These data include Google map of the most important areas describedin this article.

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