4. TECTONICS AND THE EARTHQUAKE

　

4.1 Tectonics Backgrounds

　

The Papua New Guinea and neibouring regions are bounded by latitudes 1-12 S and longitudes 130-163E. The earthquake actively results from the collision of the major lithosheric plates Pacific and India- Austalia, except at the extreme central west of the region where the furthest eastward extension of the Eurasia plate extends eastward beyond longtude of 130 E India-Australia/Pacific Plate collision.

　

Tectonics in Northern Sepik region is summarized as follows. The combination of the North Bismarck and (Caroline) Pacific plates north of the Bismarck sea seismic zone obliquely coolide with the India-Australia Plate margin in the region north of the Sepik River. The Pacific Plate is underthrusting the margin, or subducting beneath it. The subduction is not the extension type, in which the ocean plate falls away in front of the margin. The Pacific plate is being obliquely driven into the continental margin. The thick 20 km crust of the Earipik - New Gunear Rise (Den et al, 1971) may be inhibiting the subdution process. However Letz (1985) and Davies (1990) consider that the intermediate depth seismicity beneath the Sepik and central Irian Jaya to be associated with Pacific Palte subduction. The intense shallow seismicity north of the Sepik reflects the deformation of the overlying plate margin as it is dragged WSW by the underthrusting Pacific Plate. The Irian Jaya section of the Papuan Fold Belt seismic zone may represent the southern limit of westward drag on the India-Australia Plate.

　

4.2 Earthquake of 17 July 1998

　

Several research institutes in the world provides quick solution of earthquake mechanism. All of them suggest shallowly or steeply dipping reversed fault with the magnitude of around 7, while a location of estimated epicenter varies from inland to 50 km offshore.

　

From the seismological standpoint, this event baptized as the Sissano earthquake was only of moderate size. Estimates of its conventional magnitudes are mb 6.9 and Ms =7.0. The seismic moment was determined by the Quick CMT algorithm at Harvard to be M0=5.2 x1026 dyn-cm (Mw = 7.1). Mantle magnitude estimates computed at Papeete and Northwestern University were Mm =6.8. The slight discrepancy between body- and surface-wave magnitudes is upheld by the calculation of the estimated energy in the body waves, and of the slowness parameter Q =log10 (E/M0)=-5.5. This indicates that the earthquake source was somewhat deficient in high frequencies, but it did not exhibit the strong character of slowness found in "tsunami earthquakes", such as Nicaragua (1992; Q =-6.30) or East Java (1994; Q =-6.01) by Newman and Okal (1998). This is supported by the observation that the mantle magnitude of the earthquake is stable with frequency, and does not grow with period, as was the case for the tsunami earthquakes. The Sissano earthquake was followed 20 minutes later by an aftershock of mb = 5.6. Careful study shows that the aftershock was itself preceded by a smaller event, 30 seconds earlier, with magnitude mb = 5.3. The aftershock has a mantle magnitude Mm =5.75 and Q=-4.80, indicating that it was not a slow event.

　

The preliminary epicenter of the Sissano earthquake was given by the NEIC at 3.10S; 141.80E, a location significantly inland; however, this epicenter has now been revised to 2.932S; 141.797E which is practically on the coastline, 7km to the West of the Serai lumber mill. The aftershock's location (2.916S; 142.081E) is at sea, 7km due North of Sissano Lagoon.The preliminary characteristics of the source of the earthquake, obtained by Japanese seismologists, suggested a fault area of 30 by 15 km, with a slip of about 2 m. This geometry would be in general agreement with a simple model in which the hypocenter of the main shock would be at the Western end of rupture, and the aftershock would mark the position of the Eastern end. The Harvard CMT mechanism can be interpreted either as shallow angle oblique subduction of the Caroline plate under the Sepik province, or as nearly pure dip-slip on a fault dipping steeply 79 NNE.

　

According to staff of the Port Moresby Geophysical Observatory, there were two strong earthquakes on that Friday evening, one at 6:49 pm and the other at 7:09 pm. The first had a magnitude of 7.0 and the second earthquake was probably slightly weaker. The latest best fix for the location of the epicenter of the first earthquake is 2.9 degrees south latitude and 141.8 degrees east longitude. This a point very close to the coast near the Serra Hills, 35 km west-northwest of Sissano. The location of the epicenter of the second earthquake was not determined.

　

The earthquakes were strongly felt. One of students of University of PNG who was at Aitape describes an undulating movement of the earth that was so strong that she and her friends lay on the ground to avoid falling over. The eyewitnesses obtained in this survey shows modified Mercali Scale (MMS) ranging 5 to 7 which is strong ground shake corresponding to an earthquake with M=7. This indicates that the event would not belong to slow earthquake, which is one of tsunami earthquake.