Geoscience Reference
In-Depth Information
Fig. 5.11
Message issued by the
Pacific Tsunami Warning Center
following the Papua New Guinea
earthquake of July 17, 1998
Subject: Tsunami Information Bulletin
Date: Fri, 17 Jul 1998 09:46:05 GMT
From: TWS Operations <ptwc@PTWC.NOAA.GOV>
To: TSUNAMI@ITIC.NOAA.GOVTSUNAMI
BULLETIN NO. 001
PACIFIC TSUNAMI WARNING CENTER/NOAA/NWS
ISSUED AT 0943Z 17 JUL 1998
THIS BULLETIN IS FOR ALL AREAS OF THE PACIFIC BASIN EXCEPT CALIFORNIA,
OREGON, WASHINGTON, BRITISH COLUMBIA, AND ALASKA.
. . THIS IS A TSUNAMI INFORMATION MESSAGE, NO ACTION REQUIRED . .
AN EARTHQUAKE, PRELIMINARY MAGNITUDE 7.1 OCCURRED AT 0850 UTC
17 JUL 1998, LOCATED NEAR LATITUDE 2S LONGITUDE 142E
IN THE VICINITY OF NORTH OF NEW GUINEA
EVALUATION: NO DESTRUCTIVE PACIFIC-WIDE TSUNAMI THREAT EXISTS.
HOWEVER, SOME AREAS MAY EXPERIENCE SMALL SEA LEVEL
CHANGES.
THIS WILL BE THE ONLY BULLETIN ISSUED UNLESS ADDITIONAL
INFORMATION BECOMES AVAILABLE.
. . . NO PACIFIC-WIDE TSUNAMI WARNING IS IN EFFECT . . .
At Aonae, dump deposits several tens of centimeters thick
were observed around obstacles and a sheet of poorly
sorted sediment and debris was deposited throughout the
village (Fig.
5.10
). This material rarely was transported
inland more than 200 m. Internal, seaward-dipping bed-
ding in some deposits indicates that upper flow regime
antidunes formed. Where it first made landfall, the second
tsunami cut parallel grooves up to 40 cm deep across the
foreshore. Erosion also occurred with the formation of
turbulent vortices around obstacles and in channelised
backwash. Flow velocities interpreted from these sediment
features agree with those interpreted from structural dam-
age to buildings.
past, because a thin buried sand layer sandwiched within
muds exists in the Arop area. At 6:49 PM (08:49 UTC) on
July 1998, an earthquake with an M
s
magnitude of 7.1
shook this coast (Kawata et al.
1999
). 20 min later a
moderate aftershock with a moment magnitude, M
w
, of 5.75
jolted the coastline. Later analysis indicates that this second
event was preceded by 30 s of slow ground disturbance.
The location of the epicenter is still indeterminate; but the
spread of aftershocks indicates that the earthquake was most
likely centered offshore of Sissano lagoon, on the inner wall
of the New Guinea trench that forms a convergent sub-
duction zone where the Australian Plate is overriding the
North Bismarck Sea (Tappin et al.
2001
). The Pacific
Tsunami Warning Center detected the first earthquake and
issued an innocuous tsunami information message about an
hour later (Fig.
5.11
). In the meantime, a devastating tsu-
nami with a tsunami magnitude, M
t
, of 7.5 had already
inundated the Sissano coastline shortly after the main
aftershock. Tsunami flow depth averaged 10 m deep along
25 km of coastline (Fig.
5.12
), reaching a maximum 17.5 m
elevation. The wave penetrated 4 km inland in low-lying
areas. In places, the inundation of water was still 1-3 m
deep 500 m inland. The wave also was measured at Wutung
on the Indonesian border, where it reached a height of
5.4.4
Papua New Guinea, July 17, 1998
Historically, the Sissano coast of northwest Papua New
Guinea (PNG) has been no more at risk from tsunami than
any other South Pacific island in a zone of known seismic
activity. Two previous earthquakes in 1907 and 1934—
neither of which appears to have generated a tsunami of any
note—appear responsible for the formation of Sissano
lagoon (Tappin et al.
2001
). Tsunami have occurred in the
