Crust and Upper Mantle Structure of Southwest Pacific from Regional Waveform Inversion and Receiver Function Analysis

Yingbiao Xu,, D. A. Wiens, K. D. Koper, and P. J. Shore, Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, yingbiao@izu.wustl.edu

We determine the seismological structure of the Southwest Pacific by inverting complete regional seismograms recorded by the Southwest Pacific Seismic Experiment, a deployment of 11 broadband PASSCAL seismographs in Fiji, Tonga, and Niue Island. The entire 3 component long period (0.01 - 0.1 Hz) seismogram, extending from the P arrival to the surface waves, is used to constrain the structure. The waveforms are inverted using a conjugate gradient method, with partial derivatives computed using an efficient reflectivity calculation code for differential seismograms [Randall, 1994]. Sources and stations are selected to provide regional distance (400-1400 km) propagation paths that lie predominantly within one of the tectonic regions of the Fiji-Tonga region. The inclusion of intermediate depth earthquakes allows the deeper regions of the model to be resolved.

Results show extremely large variability in upper mantle structure in the Southwest Pacific. The North Fiji and Lau basins, containing active back-arc spreading systems, show slow upper mantle shear velocities of 3.9 km/s. The velocity difference between active back-arc basins and old Pacific lithosphere immediately east of the Tonga Trench is maximum at 50 - 90 km depth, with a variation of about 15%. Substantial velocity heterogeneity extends to deeper depth. The difference is about 5% at 200 km and 3% at 350 km between the active back-arc basins and Pacific lithosphere region, indicating that some velocity signature of the backarc activity extends to these depths, consistent with recent tomographic models [Zhao et al., 1996]. Results also show a velocity discontinuity or very large velocity gradient near 220 km depth. The exceptionally low seismic velocities in the back arc regions rank among the lowest upper mantle velocities in the world.

We also investigate the crustal structure of Fiji platform using both linearized and genetic algorithms inversion of receiver functions from local deep events. The receiver function analysis suggests crustal thicknesses beneath the stations of 22 - 28 km. The result is compatible with the regional waveform inversion, which indicates an average crustal thickness about 20 km for the Fiji platform.

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