No abstract

A huge earthquake of magnitude M 9.0 occurred at 00:58 (UT), December 26, 2004, in the sea off the west coast of northern Sumatra, Indonesia, followed by a huge tsunami that hit almost all coasts facing the Indian Ocean. We conducted a field survey in the residential area of Banda Aceh, the town of the severest damage by the tsunami, on the west coast of the northernmost point Sumatra, Sigli City, about 80 kilometers east of Banda Aceh three-four weeks after the event. In Banda Aceh, almost all houses in the residential area about 2 km from the coast were swept away, while houses more than 3 km rarely were. Inundation continued about 5 to 6 km from the shoreline. In Lhoknga and several villages on the west coast of Sumatra Island near Banda Aceh, where tsunamis 15 to 30 meters high hit coastal villages, nobody survived. Along the valley about 1 km north of the cement plant, seawater rose to a height of 34.8 m above MSL, which is the highest recorded inundation measured in our survey.

The 1998 Papua New Guinea earthquake of M w 7.0 occurred near the Wewak trench where the North Bismarck plate is subducting beneath the Australian plate. Its mechanism is thrust-type, and one of the nodal planes is almost parallel to the plate interface. To determine which of the two nodal planes of the main shock is the fault plane, we relocated the main shock and aftershocks using a method of modified joint hypocenter determination. We combined and employed two types of data in this study. Firstly, we used data reported by the National Earthquake Information Center (NEIC) of the U.S. Geological Survey (USGS), which includes three stations at the northeastern edge of Irian Jaya and one station in northern Papua New Guinea, from which the epicentral distances are less than 2 degrees. Secondly, in addition to the above permanent-station data, we used data from temporary aftershock observations near the epicentral area around the Sissano Lagoon carried out by TSUJI et al. (1998). Using three-component seismometers, they carried out observations from August 2 to October 2, 1998 at three sites. Although the network did not record the main shock and immediate aftershocks, the data obtained by temporary observation sites can clearly assist in identifying their absolute locations, since it is possible to apply the joint hypocenter determination (JHD) method. Hypocenters were relocated between the coastline and the Wewak trench, distributed along a nodal plane dipping shallowly to the southwest. Therefore, we can conclude that this nodal plane is the main shock fault and that the 1998 Papua New Guinea earthquake was an interplate earthquake between the North Bismarck and Australian plates.