No abstract

Earthquake data are utilized in conjunction with bathymetry and gravity measurements to study the seismotectonics of the Beaufort Sea area. The epicentre cluster in the Beaufort Sea is confined to the continental slope between the 200 and 2400 m bathymetry contours and falls between the seaward −20 mGal and the landward + 40 mGal contours of an elliptically shaped free-air gravity anomaly. The cluster, which experiences an average of one earthquake magnitude ≥ 4 per year, is shown by epicentre relocation studies to be a distinct zone and not the result of mislocations of earthquakes originating from a spatially confined source. Theoretical calculations of the stress field under the region where the gravity anomaly is most pronounced show that the horizontal component of the stress field is deviatoric tension normal to the axis of the continental margin; focal parameters of the June 14, 1975, mb 5.1, Ms 4.2 Beaufort Sea earthquake indicate deviatoric tension in the same (east–west) direction at a depth of 40 km. The relatively small surface waves from this and other Beaufort Sea earthquakes, compared to other on-shore Arctic earthquakes, are probably due to this deeper focus at the continental margin. The horizontal component of the deviatoric compression of this earthquake is north–south; a horizontal compressive stress from the north may be transmitted through the Arctic Ocean lithosphere from the Nansen–Gakkel spreading ridge. It is suggested that these stresses are acting on localized zones of weakness.

In this paper, we used detailed bathymetry, earthquake distribution and focal mechanisms to study the phenomenon of active westward exten sion of the Okinawa trough in the northern Taiwan area. We found a dis tinguishable subsiding and collapsing area on the continental shelf edge and the continental slope on the northern side of the Okinawa trough. This area extends westwards to at least 121.5°E and includes several morpho logical units related to the existence and formation of three major canyons. The canyons and the morphological units are still evolving through the sedi ment transport and through the subsidence and collapse of material due to the formation of the Okinawa trough. According to the degree of develop ment, we found that these morphological units have developed from the east to the west. There are two parallel E-W trending central graben at the westernmost part of the Okinawa trough, with each corresponding to a narrow shallow seismic belt. The widths of the central graben are 10-15 km. There is geophysical and geological evidence that the formation of these central graben has been extended westwards to the onland area of Taiwan. Focal mechanisms of earthquakes and the topographic features show that the formation of the Okinawa trough is associated with the down-dip ex tensional stress along the subducting slab of the Philippine Sea plate, and most of northern Taiwan and all the northeastern offshore area of Taiwan are under tensional stress. New portions of the Okinawa trough have been forming across the whole width at its western end through subsidence in the continental shelf, the continental slope and the traditionally recognized area of the Okinawa trough in the northeastern Taiwan area, to make the Okinawa trough develop gradually and extend westwards.