Focal process of the South Sandwich Islands earthquake of May 26, 1964

Abstract

A source study is made, primarily on the basis of long-period surface-wave data, on a large intermediate-depth earthquake that occurred in the South Sandwich Islands region on May 26, 1964. Long-period multiple Love and Rayleigh waves (G3 and R3) are obtained for a period range 100 to 400 s from the long-period records of WWSSN stations around the source. All the seismograms are equalized to a uniform propagation distance of 5 2 π. These observed seismograms are compared with synthetic seismograms for wave form, radiation pattern and asymmetry to estimate various source parameters. Wave forms of Rayleigh waves are very sensitive to depth, so that the source depth is determined to be 120 ± 20 km. A double-couple force system explains the observed data of surface and body waves. The aftershock hypocentres are confined to a remarkably thin zone which elongates northward from the origin of the main shock. The geometry of the aftershock area fits the geometry of one of the nodal planes of the double-couple solution. This strongly suggests that the earthquake occurred in the form of a shear faulting. This large-scale faulting is presumably due to the extensional force exerted by a sharp bending of the sinking lithosphere. The following source parameters are determined: force system, double-couple; source-time function, step function; fault plane, dip direction N 70° W, dip angle 48°; auxiliary plane, dip direction N 55° E, dip angle 57°; rupture length, 60 to 100 km; rupture velocity, 2 to 3 km/s towards the north; seismic moment, 6.2 × 10 27 dyne · cm; average slip dislocation, 3.5 m; stress drop, 120 b; strain drop, 1.7 × 10 −4; released strain energy (residual strain is assumed to be zero), 5.3 × 10 23 erg. In these calculations, the fault area and the rigidity are assumed to be 2.5 × 10 3 km 2 and 0.7 × 10 12 dyne/cm 2, respectively. The stress drop, 120 b, is about 100 b higher than stress drops for large shallow earthquakes. This may suggest that an effective cohesive strength of the mantle material increases with depth.