Seismotectonics of the Indonesian Region
The Indonesian region is one of the most seismically active zones of the earth; at the same time it has a leading position from the point of view of active and potentially active volcanoes. It is a typical island-arc structure with its characteristic physiographic features, such as a deep oceanic trench, a geanticline belt, a volcanic inner arc and a marginal basin.
In most subduction zones, motion of the subducted plate is nearly perpendicular to the trench axis. In some cases, for example Sumatra, where the motion is oblique to the axis, a strike-slip fault zone is seen, and is lying parallel to the volcanic chain.
In the subduction zone southwest of Sumatra, the Sunda trench axis strikes approximately N 37°W. The Indian Ocean crust is moving in an azimuth of approximately N 23°E relative to Southeast Asia, giving an angle of obliquity of 60°. Eastern Indonesia, forming the southeastern extremity of the Southeast Asian lithospheric plate, crushed between the northward-moving Indo-Australian and the westward-moving Pacific plates, is certainly the most complex active tectonic zone on earth. The rate of subduction is some centimeters per year; for example, it is 6.0 cm per year in the West Java Trench at 0°S 97°E (azimuth 23°); 4.9 cm per year in the East Java Trench at 12°S 120°E (azimuth 19°); and 10.7 cm per year in New Guinea at 3°S 142°E (azimuth 75°).
Frequent volcanic eruptions and frequent earthquake shocks testify to the active tectonic processes which are currently in progress in response to the continued movement of these major plates. The distribution of small ocean basins, continental fragments, remnants of ancient magmatic arcs and numerous subduction complexes which make up the Indonesian region indicate that the past history of the region was equally tectonically active.
Abridged from Southeast Asia Association of Seismology and Earthquake Engineering, Series on Seismology, Volume V - Indonesia, June 1985.