Subduction process of a fracture zone and aseismic ridges — the focal mechanism and source characteristics of the New Hebrides earthquake of 1969 January 19 and some related events

摘要

The subduction of the D'Entrecasteaux fracture zone-seismic ridge system in the New Hebrides island arc is investigated on the basis of the focal process of the New Hebrides earthquake of 1969 January 19 (m_b= 6.4, h= 107 km), mechanisms of some related events, seismicity and regional tectonics. A notable feature of this island arc is the discontinuity of the New Hebrides Trench in the central New Hebrides where the ridge-fracture zone is subducting and intersecting the arc. The 1969 New Hebrides earthquake occurred along the subducted portion of the fracture zone and is characterized by unusual waveforms with remarkably large excitation of long period waves, rare for earthquakes of comparable mb and depth; the P-wave records show anomalously long duration and complexity indicating that the earthquake was a multiple event with a source duration of at least 30 s. P-wave first motions, seismicity data and synthetic body-wave seismogram indicate that the earthquake represents transverse left-lateral motion on a nearly vertical, E—W trending fault plane with a slip vector subparallel to the down-dip direction of the Benioff zone. The total seismic moment is found to be 5 × 10^(26) dyne cm by matching the amplitudes and waveforms between observed and synthetic surface waves. The location and mechanism of this earthquake suggest that the D'Entrecasteaux fracture zone structurally extends to the east of the trench. This structural boundary at depth seems to be reflected in the spatial distribution of two earthquake swarms which are bounded sharply at the latitude of 15.2°S. At the extension of the ridge-fracture zone, the activity of intermediate depth earthquakes, which are characterized by a very consistent pattern of down-dip extensional mechanism, is much higher and their depths are systematically shallower than in the adjacent regions. These features can be interpreted as a consequence of subduction of a buoyant ridge and the resultant increase in the extensional stress at the intermediate depths of the sinking slab. Fault-plane solutions of 22 earthquakes suggest that the subduction of aseismic ridges in the New Hebrides is characterized by high-angle thrusts. The lithospheres on the two sides of the D'Entrecasteaux fracture zone under the arc subduct more or less independently and generate alternating left-lateral and right-lateral earthquakes along the subducted portion of the fracture zone.