Evidence for the formation of a new trench in the western Pacific



Shallow intraplate earthquakes, recently detected along an arcuate zone extending from the northern side of the Caroline Ridge across the western Pacific to the southern end of Kiribati (see cover), may portend a northward shift in convergence between the Indo-Australia and Pacific plates. Low-amplitude seafloor structures along this zone indicate recent deformation and, in places, suggest trench-forearc morphology. These earthquakes and seafloor structures may provide evidence for the formation of a new subduction zone: the Micronesian Trench. The western end of the postulated trench seems to be part of a complex triple junction at the concurrence of the Yap, Mariana, and Micronesian trenches. The eastern end joins the Tonga Trench south of Savaii, Western Samoa. Detailed regional studies will be necessary to delineate further the postulated Micronesian Trench. The fact that important earthquakes indicating this structure were not detected by the existing worldwide network of seismic stations suggests the need for a comprehensive reevaluation of oceanic intraplate seismicity.