Forty-four new focal mechanisms from shallow-focus earthquakes in the Indonesian-Philippine region have been determined from P and S waves recorded on long-period instruments. The dominant modes of deformation are thrust and normal faulting rather than strike-slip faulting. Along the Sunda and Philippine arcs the most active zone of shallow focus activity occurs between the ocean trench and the line of active volcanoes. Mechanism solutions from earthquakes in this zone are all of the thrust type with the sense of motion on the shallower dipping of the two nodal planes consistent with underthrusting beneath the island arc. Seismic slip vectors strike in a northeasterly direction along the convex side of the western Sunda arc and strike in a westerly direction along the Philippine arc and its extension along the eastern margin of the Celebes Sea. Normal faulting mechanisms from one earthquake under the Java Trench and another under the Philippine Trench may result from extension in the upper surface of the lithosphere as it bends beneath the island arc. The curvature of the Sunda arc may have caused the inclined seismic zone beneath the arc to shoal toward the northwest. There is a correlation between negative isostatic gravity anomalies, maximum water depth in the trench and the length and shape of the seismic zone beneath the Sunda arc. Shoaling of the inclined seismic zone beneath the nearly linear Phillippine arc can be explained by a decrease in the slip rates from south to north along the arc. Complex regions such as western New Guinea, the Sulu Spur and lesser Philippine islands may include a number of small plates of lithosphere.

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