Source process of the great 1971 Solomon Islands doublet

Large underthrusting earthquakes in the Solomon Islands tend to occur as multiplets, separated by a few hours to several days in time and 30—100 km in space. The largest sequence, a doublet on July 14, 1971 (M~= 8.0) and July 26, 1971 (M~= 8.1), is particularly unusual in that it occured at the junction of two trenches, with the July 14 event in the Solomon Islands Trench preceding the July 26 rupture in the New Britain Trench. The location and large moment release of this doublet as well as an incompatibility of published body wave and surface wave focalmechanisms for the July 14 event, motivated a detailed study of the source process of the 1971 sequence. To satisfy both the surface wave and body wave data, we explored the possibility of a change in the faulting geometry during rupture for the July 14 event. A fault model that changes orientation from a northwest—southeast striking, shallow-dipping plane, similar to the mechanisms of neighboring underthrusting events in the Solomon Islands Trench, to a more north—south striking plane — 70 s after rupture initiation is consistent with the observed Rayleigh and Love wave amplitude patterns. This change in mechanism is suggested by systematic variations in the mechanisms of smaller earthquakes near the cusp in the Solomon Islands plate. A simultaneous inversion of WWSSN P wave seismograms for the time, location, seismic moment and focal mechanism of individual subevents gives results in agreement with the surface wave model, although the precise change in mechanism is not well resolved. The spatiotemporal distribution of subevents reveals two major pulses of moment release. The first occurred on the initial fault near the epicenter at the origin time of the earthquake. The second occurred on the north—south striking fault, 50—70 km northwest of the epicenter — 70 s after the origin time. The change in mechanism appears to reflect contortion of the slab near the trench junction. Inversion of WWSSN P wave seismograms for the rupture history of the July 26 event reveals a westerly propagating rupture extending — 60 km along the New Britain Trench. For both of the 1971 earthquakes, regions of high moment release are located in the vicinity of the Solomon Islands—New Britain Trench junction. Contortion of the subducting lithosphere as it bends around the arc—arc junction may have enhanced the degree of mechanical coupling between the subducting and overriding plates, concentrating stress in this region, and in turn, promoting efficient triggering across the trench junction.