Focal Mechanism of Earthquakes on Island Arcs in the Southwest Pacific Region

Abstract

The focal mechanisms of 33 earthquakes that occurred along island arcs from New Guinea to the Fiji Islands have been determined from the long-period seismograms of the World-Wide Standardized Seismograph Network and some other observations. The observed first motions of P waves show a quadrantal distribution for most of the earthquakes. The solutions for some earthquakes are also supported by the source amplitudes of P waves, which have been corrected for the effects of propagation and instrument. The polarization angles of S waves for several earthquakes suggest the double-couple type mechanism. The axes of the maximum pressures inferred from a number of the solutions are oriented almost perpendicularly to the trend of the island arcs in some cases, but nearly parallel in other cases. The plunge of the pressure axis for most of intermediate and deep earthquakes appears to be parallel to the dip of the spatial distribution of foci.