T sunam i h istory from the peat layers
I f the se q u en ce o f m ud and peat layers is sam p le d c are­
fully, and e x a m in e d for traces o f tsunam i sand, it will
reveal the history o f m a jo r tsunam is on this coast, for at
least the last 1000 years and possibly for longer ( i f the
layers below 4 m arc sam pled). Each m a jo r tsu n a m i will
be represented by a sand layer and the sand w ill be
m ixed in with the peat. By dating the peat, w e can learn
the age o f the tsu n am i sand.
Provided that the p eople o f the M alo l-A rop-W arapuSissano area agree, a sm all diesel-p o w ere d drill rig will
be brought to the old W arapu village site in A ugust. The
rig will be o p erated by U nited Pacific D rilling, M adang.
It will be shipped to A itape by Lutheran S hipping, arriv­
ing 9 A ug u st, then taken to W arapu by p o w e re d dinghy.
The basic plan is to drill tw o holes 20-40 m w est o f the
O tto m outh to a m a x im u m depth o f 20 m . I f the results
are interesting, and if representatives o f all villages
agree, the p ro g ram could be extended to drill a third or
fourth hole, b e c a u se the m ore holes that are drilled the
m o re reliable will be the results. A ll w o rk should be
co m pleted, a n d the drill rem oved back to A itape for
shipping to M a d an g , w ithin a w eek. T h e drilling itse lf
should o ccu p y 2-4 days.
T h e drill c o re s will be ex a m in e d at the drill site, and
sam p le s collected for dating. I f it is physically possible,
the drill c o re s will be split lengthwise. H a lf th e core will
be left w ith the B arupu p eople as a p e rm a n e n t record,
and the o ther h a lf taken to U niversity o f P N G fo r further
study. At the U niversity the p eat sam p le s w ill be dried,
then sent to W aikato N Z , for dating.
R e p o r tin g the results
R esults will be reported to the M alol-A rop-W arapuS issano p e o p le , the prov in c ia l authorities, a n d the
N ational D isaster M a n a g e m e n t O ffice in N ovem berD e c e m b c r this year.
R eferences
S ynolakis, C., B orrcro, J., D avies, H., Grilli, S., Okal,
E., Silver, E., S w eet, S., Tappin, D., a n d Watts, P., 2001.
T he slu m p origin o f the 1998 Papua New· G u in e a tsuna­
mi. Proceedings o f the Royal Society (in press).
Tappin, D.R., M a ts u m o to , T., W'atts, P., Satakc, Κ.,
M cM urtry, G .M ., M a tsu y a m a , M ., Lafoy, Y., Tsuji, Y.,
K a n a m a ts u , T., L.us, W., Iw a b a c h i, Y., Yeh, H.,
M a tsu m o to , Y., N a k a m u ra , M ., M oihoi, M ., Hill, P.,
C ro o k , Κ., A nton, L., and Walsh,
J.P., 1999. S e d im e n t s lu m p likely
c a u s e d the 1998 P a p u a New
G u in e a
ts u n a m i.
EOS
T ra n s a c tio n s o f the A m e ric a n
G e ophysical U n io n 80, pp. 329,
334, 340.
T a p p in ,
D .R .,
W atts,
P..
M cM urtry, G .M ., Lafoy, Y., and
M a ts u m o to ,
T.,
2001.
The
S is s a n o , P a p u a New· G u in ea,
tsunam i o f J u ly 1998 - offshore
e v id e n c e o f the source m ech a­
nism. M arine Geology.
S m all d r i ll in g su c h as w o u ld be
u se d for th e d rillin g a t old Warapu.
C o ntent and prod uctio n by Division
o f G eoscien ces, U niversity o f Papua
N e w Guinea.
D esign and layout by R eligious
Television A sso ciation o f Papua New
Guinea.
Financial support pro vided by
A usA ID .
A u thorised by N ational Disaster
M anagem ent O ffice, Port M oresby.
Printed by: Post Printing
THE CAUSE OF THE 1998 AITAPE TSUNAMI
A m ajo rity o f the investigating scientists are n o w a g reed that
the 1998 A ita p e tsunam i w a s c a u s e d b y an u n d e rw a te r land­
slide that occu rred 2 9 km northeast o f the A ro p -W arap u coast
in a w a te r depth o f 1500 m. T h e landslide h ap p e n e d at 7.02 pm
local tim e, 13 m in u tes afte r a m a g n itu d e 7 earthquake.
In tro d u ctio n
T h e N e w G u in e a T rench is a geological plate b o u n d ary that lies
3 0 -4 0 km offshore from, and parallel to, the A itape-V anim o
coast, and e x ten d s w est-n o rth w e st a long the West Papua coast
(Fig. 1). A t the plate boundary, the rocks that fo rm the floor o f
the B ism arck Sea (the B ism a rc k Sea Plate) are carried southw e stw a rd b en e a th the rocks that form the island o f New' G u in ea
(the A u stra lia n Plate), at a rate o f 11 cm p er year. The m o v e ­
m en t o f the tw o plates causes earth q u ak es. T h e earth q u ak es, in
turn, can c a u se tsunam is.
T h e specialists are n o w co n v in ce d that the earth q u ak e
its e lf did n o t c a u se the tsunam i, but rather the tsunam i
w a s ca u se d by a su b m a rin e landslide o r slu m p , that wras
triggered by the shak in g o f the earth q u ak e. T h is c o n ­
clu sio n wras pu b lish ed in p relim in ary form in in 1999,
in a jo u rn a l o f the A m e ric a n G eo p h y sic al U nion, EOS,
has been p u b lish e d this year, in m o re co m p lete form , in
the the international jo u rn a l M arin e G eology, and will
b e p u b lish e d in the Pro cee d in g s o f the Royal S ociety
later this year.
E v i d e n c e t h a t t h e A i t a p e t s u n a m i w a s c a u s e d by
a s u b m a r in e la n d slid e
T h e re are several lines o f e v id en ce that the tsu n am i was
cau se d b y a landslide, and that the landslide happ en ed
a t 7.02 p m , 13 m in u te s a fter the first m a jo r earthquake.
T h e se are:
141 ®E
1) T h e size o f the w av e. The w;av e w as to o big to have
b een ca u se d b y m o v e m e n t on a geological fault. There
is a k n o w n relationship betw een the strength o f an
e a rth q u ak e, the a m o u n t o f m o v e m e n t on a fault, and the
size o f a tsu n am i w a v e that can be g en era ted by that
m u c h m o v e m e n t. O n all counts, the A ita p e tsu n a m i w a s
m u c h too big to h av e been cau sed by the m a g n itu d e 7
earth q u ak e. T h e earth q u a k e could h av e ca u sed a w ave
1-2 m high, no more.
1996 (7.9)
0 °<
2) T h e tim in g o f th e w ave. I f the tsu n am i w'ave had
been caused b y the first m a jo r e a rth q u a k e (w h ich hap­
p en ed at 6.4 9 p m local tim e), it sh o u ld h av e reached the
c o ast b e fo re 7 pm . In fact the w a v e reach ed the nearest
part o f the c o a st at 7.08 o r 7.09 pm . It arrived 9 o r 10
m in u te s ’late'.
N e w G u in e a T re n c h an d the e a rth q u a k e s o f 1996 an d 1998
In 1996, a m a g n itu d e 7.9 earth­
q u a k e on the West P apua p art o f this
plate b o u n d ary caused a tsunam i
w ith w-aves as high as 7 m , that
killed m o re than 100 p eo p le o n B iak
Islan d . In 1998, a m a g n itu d e 7
e a rth q u a k e n e a r A itap e cau se d a
tsu n am i, w ith weaves as high as 15
m, that killed m o re than 2 0 0 0 p e o ­
ple o n the A itap e West coast.
Tsunam i specialist scientists agreed
that the 1996 B iak tsunam i w as
ca u se d by m o v e m e n t o n a g e o lo g i­
cal fault on the sea floor at the tim e
o f the earthquake. T h e w a v e w a s the
right size for such an earthquake.
H ow ever, this w a s n o t true fo r the
A itap e tsunam i.
T h e w a v e was
m u c h to o b ig to h av e been caused
by m o v e m e n t on a fault that caused
a m a g n itu d e 7 earthquake.
A rh& ld R iv e t
Lum ber
M ill
~Serai
'A san o V illages
W arapu ©
A r o p V illages
"x.
M a l o l V illages
S issa n o L a g o o n
A itape
T h e large star la b elled 08.4 9 is the lo c atio n o f th e first e a rth q u a k e , an d th e sm a ll stars
la belled 0 9 .0 9 an d 0 9 .1 0 a re the loc atio n s o f th e d o u b le e a rth q u a k e a fte rsh o c k tha t
o c c u rre d 2 0 m in u te s later. T h e sta r m a rk e d 0 9 .0 2 is th e lo c atio n o f th e la n d slid e as d e te r­
m in e d fro m T -w av es, a n d the large circle is the location o f th e la n d slid e as d e te rm in e d by
m a p p in g the sea floor. T h e large circle is w ith in th e e rr o r e llip s e o f the 9 .0 2 e v e n t. T im e s
are in U n iv e rs a l T im e (U T ): 0 8 . 4 9 U T = 18.49 local tim e an d 9 . 0 2 U T = 19.02 local tim e.
3) T h e ev id e n c e from T -w aves. W h e n an earth q u a k e
occurs, it se n d s o u t seism ic w a v e s that y o u and I can feel
(the g ro u n d is m o v in g up and d o w n , o r sidew ays), and
that can be reco rd ed a t seism ic stations around the globe.
W hen the seism ic w a v e reaches the ocean, it travels
w ithin the o cean as an ord in a ry sound w ave. This is
k n o w n as a T -w ave or Tertiary-w ave. W h en a T -w ave
rea ch e s land it is c o n v erte d into a no rm al seism ic w ave
again.
,
T aiw an
Enm o
Pelropavlovsk
Guam
Van Inlet
WAKE
WK31
M o n te re y
H aw aii
A dm iralty
W h a t do the u n d e r w a te r rock e xp osu r es tell us?
H O W FREQ UENT ARE TSU N A M IS ON THE
W E W A K -V A N I M O C O A S T ?
T sunam i w a v e speed varies w ith the de p th o f the water.
B eca u se w e k n o w the shape o f the sea floor o f f this
coast, w e can calculate h o w long it w ill take for a tsu n a ­
mi w a v e to reach the shore, fro m an y likely source area.
F ro m a n y so u rc e betw een the N e w G u in e a T rench and
the coast the w a v e w o u ld reach the shore in not m o re
than 8 m inutes. W orking back w ard s, w e can say that, if
the ts u n a m i w a v e reached the coast a t 7.10 p m , then it
m u st h av e o rig in ated at 7.02 p m , o r later.
P h o to g ra p h taken fro m the J a p a n e s e research su b m a rin e
s h o w s a fr e sh ly -d e v e lo p e d crack, a b o u t a m e tre a cro ss, in
the sea flo o r s ed im e n ts. T h e c ra c k m a y h a v e o p e n e d a s the
lan d slid e started to m ove.
5) T h e e v id e n c e from c o m p u te r m odelling.
Putting all these leads together, the investigators then set
o ut to test the possibility that a landslide in this valley on
the sea floor, at 7.02 p m , co u ld have ca u se d the tsunam i.
T h e y entered all o f the inform ation into a c o m p u te r the shape o f the sea floor and the tim e and location o f the
land slide - a n d a sked th e c o m p u te r to calcu late w hat
sort o f w a v e w o u ld be generated, w h a t part o f the coast
w o u ld it hit, at w h a t tim e w ould the w ave arrive, and
h o w high w ould the w a v e b e? T h e c o m p u te r told them
that the w a v e w o u ld be 10-15 m high, w ould be focussed
on the A ro p -W a ra p u coast, and w o u ld arrive at the coast
at 7.09 o r 7.10 pm.
T h e plate b o u n d a ry that is m a rk e d b y the N e w G uinea
T rench e x ten d s from j u s t w e s t o f W ew ak to Aitape,
V anim o and beyond. M a n y ea rth q u a k e s o c c u r alo n g this
plate boundary, and a n y o f these earth q u ak es m a y cause
a tsu n am i, either by d isp lacin g part o f the sea floor on a
geological fault, o r b y trig g erin g landslides in sea floor
sedim ents. T sunam is that w e k n o w w e re linked to m o v e ­
m e n t on this plate b o u n d a ry are the B iak tsunam i o f
1996 and the A itap e tsu n am i o f 1998.
But j u s t h o w dan g ero u s is this coast? D o big tsu n am is
h ap p en frequently at, say, 5 0 -y e a r intervals,
or infrequently, at intervals o f several h u n ­
dreds o f years? I f w e can get an a n s w e r to
this q uestion, then w e are part w a y d ow n
the track o f estim atin g w h e n the next m a jo r
tsunam i m ight occur. Will it be this decade,
this century, or at so m e d istan t tim e in the
future?
Oral histories tell us there h av e been d a m ­
a g in g tsunam is on this coast in the past but
d o not tell us w h e n they occu rred . W ritten
h is to ry tells us n o d a m a g in g ts u n a m i
occurrcd on this coast in the period 18961998, but d oes not tell us w h a t happ en ed in
th e years and centuries before 1896.
T h e vegetable m aterial that form s each peat lay er ac cu ­
m u lated in a fre sh -w a te r sw am p , at sea level or clo se to
sea level. So, the b o tto m -m o s t p e a t layer rep resen ts a
s w a m p that w a s at sea level 1000 years ago, b u t has
sincc sub sid ed and been b u ried b y later m ud an d peat
layers. This s h o w s that this co ast has su n k 4 m in 1000
years. The su b sid en ce has not been a steady, slo w m o v e ­
m en t but, rather, has been a jerky m o v e m e n t, subsiding
10-20 cm at a tim e then rem ain in g stable fo r so m e
d ecad es or centuries. Each su b sid en ce event p robably
c o in c id e d w ith an earth q u ak e, like the s u b sid en ce that
form ed Sissano L ag o o n in 1907.
%
·.. 2500
F ortunately there is a third w a y to find the
history o f tsunam is. T h is is in the layers o f
sand and m ud that underlie the present land
P rofessor D avies co llects a sam ple o f peat from the lo w erm o st c liff lace, Septem ber
surface.
C h ris tm a s
Islands
T u a m o tu
1999. T he peat layers show as d ark layers in the c liff face and the m ud as light lay­
found to be 1000 y ears old.
T -w a v e sig n a ls rec e iv e d at six statio n s a ro u n d th e P acific
T su n am i record in old sed im e n ts
ers. T h e peat w a s
In S e p t e m b e r 1999 J o c e ly n a n d H u g h
D avies d ived at the O tto m o u th to investigate stories that
cracks w e re a p p e a rin g in the lagoon floor. Wzhat they
found w a s three small cliffs, in w h ic h w e re ex p o sed the
sed im e n ts that underlie the p re se n t land surface. T he
cliffs had form ed b y erosion, as currents sw ept in and
o u t o f the lagoon w ith the tide. T h e topm ost cliff, that
ex te n d e d from 2 m to 3 m below surface, w a s m a d e up
o f grey sticky m ud, w ith tu b e-lik e holes and brow n
stains w h e re roots o f trees had o n ce p en etrated the mud.
tell us that a la n d slid e o c c u rre d at 7.0 2 p m ( 0 9 .0 2 U T ) at a
locatio n o ffsh o re from the A ro p -W a ra p u coast.
A ro u n d the P acific there are listening p osts that ro utine­
ly record T-waves. Six o f these stations reco rd e d a m ajor
e v en t that originated in the A ita p e area at 7.02 p m , 13
m in u tes after the initial earthquake. T h e re cording did
not look like a signal from an e a rth q u ak e, w h ich w ould
have a strong initial burst o f ene rg y and then the energy
tails off. R ath er it looked like a signal from a landslide,
starting w ith low energy, then increasing, then tailing o ff
again. A lso, the signal lasted longer (44 sec o n d s) than
w o u ld be e x p ected from a sm all earthquake.
4) T h e e v id e n c e fro m m a p p in g o f the sea floor. The
m a p s o f the sea flo o r p r o d u c e d by the Ja p a n e s e
research ers early in 1999 rev ea led a basin -sh ap ed valley
in the sea floor, 29 km north e ast o f the W arapu-A rop
coast, in w h ic h there a p p ea r to have been landslides on
the sea floor. W h e n this area w as ex am in e d , u sing small
s u b m arin es, the o b servers saw cra c k s in the sedim ents
on the sea floor that su g g e ste d the sedim en ts had m oved
recently. T h is w a s in a w ater depth o f 1500 m.
C o m p u t e r m o d e l o f the w a v e t h a t w o u ld be g e n e ra te d b y a
lan d slid e in the sea flo o r v alley 2 9 km fro m th e A ropW arapu coast.
In o ther w o rd s, th e w a v e predicted by the c o m p u te r
m a tc h e d a lm o s t ex a c tly the w a v e that h a ppe n e d o n the
night o f 17 July 1998, both in the size and focus o f the
w a v e and in the tim in g o f the wave.
We can c o n c lu d e th a t the m o st likely cause o f the tsu n a ­
mi w a s a landslide, o r slu m p in g o f se dim ents, on th e sea
floor, in a sea floor v a lley 29 km northeast o f the A ropW arapu coast, that occu rre d at 7.02 pm . This is the c o n ­
clusion that will be publish e d in the Proceed in g s o f the
Royal Society.
The next tw o cliffs e x p o sed m o re grey sticky m ud, but
interlayered w ith in the m u d w e re six layers o f peat.
(Peat is organic matter, leaves and sticks, that has
b e c o m e black as it starts to c h a n g e into coal.) The
seq u en ce o f interlayered m u d and p eat w a s at 3 m to 4 m
b elow surface. O ne o f the p eat layers co n tain ed a lot o f
sand - p o ssib ly sand from an a n c ie n t tsunam i. This was
difficult to s am p le, b ecau se the divers had no equ ip m en t
to collect loose sandy m aterial. H o w ev er, th e y w e re able
to sa m p le the b o tto m -m o st peat layer, w h ic h w a s m ore
coherent. Later, this sa m p le w a s d ated by c a r b o n - 14
m e th o d in a laboratory a t the U n iv e rsity o f W aikato in
N e w Z e alan d , and w a s fo u n d to be 1000 years old.
T h u s the peat layers tell the story o f a repeated cycle o f
events as follows:
- A s w a m p form s, and v egetable m atter a cc u m u lates on
the sw a m p y ground;
- T h e re is an earth q u a k e and the land subsides,
the s w a m p sin k s and dies, a lagoon form s and the
vegetable m atter is slow ly b u ried b y m ud;
- W hen sufficient th ick n ess o f m u d has accum ulated,
an o th e r s w a m p form s, v eg etab le m a tte r accum ulates
on the g round, there is a n o th e r earth q u ak e, and so on.
swai
sea
swar
subsides
new swamp
Hops
mud layer
peat layer