Seismic structure of the Tonga Arc and Lau Backarc Spreading Center from joint inversion of local and teleseismic body wave arrivals

The Lau Backarc Spreading Center and Tonga Arc provide an excellent environment in which to the study the dynamics of melt production and migration. The Lau Backarc Spreading Center is comprised of three segments, Central Lau Spreading Center, Eastern Lau Spreading Center, and the Valu Fa Spreading Center. The segments show very different spreading and magma production rates as a function of their distance to the arc and slab, which decreases from north to south. Geochemical studies also indicate a systematic change in magma composition along strike of the Lau Back-Arc Spreading Center with more water and fluid-mobile elements in the south.

From 2009-2010, 16 broadband seismometers and 51 ocean bottom seismometers (OBS) were installed for one year across the Lau backarc spreading centers and in Fiji and Tonga to image the along-strike structural variation of the subduction zone and the dynamics of the melt production region. We present preliminary results from the joint inversion of P and S wave arrivals from local and teleseismic events recorded by this network. Arrivals from local events were picked manually, while travel time residuals for teleseismic arrivals were determined via the adaptive stacking method developed by Rawlinson and Kennett (2004). Travel times for local events and residuals for teleseismic events are inverted using TOMOG3D (Zhao et al., 1994) to solve for three dimensional velocity structure along strike of the Lau Backarc Spreading Center and Tonga Arc.

Preliminary results show a clearly defined westward-dipping low velocity region beneath the Eastern Lau and Valu Fa Spreading Centers, which is most pronounced at depths less than 150 km. The low velocities beneath the Eastern Lau and Valu Fa Spreading Centers are more pronounced relative to those associated with the Tonga Arc.