This is a combined active and passive seismic experiment along the Eastern Lau Spreading Center to test the following hypotheses. 1. Circulation in the mantle wedge is dominated by slab driven flow. 2. Interaction of the arc and backarc magma production controls the character of the ridge by influencing melt flux, petrology, and geochemistry. 3. Variations in the mantle melt supply control ridge crest features such as morphology, thermal structure, and hydrothermal venting. The passive experiment consists of 55 broadband ocean bottom seismographs and five land seismographs deployed for 10 months to image the larger-scale structure of the melt production region and the mantle flow pattern. The active source experiment consists of 100 ocean bottom seismographs deployed along a 250 km section of the spreading center extending from the inflated Vala Fa region to the magma-starved northern Eastern Lau Spreading Center where the axial melt lens is absent.
In broader terms this experiment addresses a first order problem in ridge dynamics in a unique subduction zone setting and is essential for the stated goals of R2K program. Graduate students will be supported by all four PIs.
This project was one component of a collaborative study to image the deep earth beneath the Lau Basin in the western Pacific. We performed computer simulations of slow flow in the mantle that is driven by subduction of the Pacific plate at the Tonga trench. Deformation of the minerals in that slowly deforming mantle cause seismic waves generated by subduction earthquakes to travel faster in one direction than another. Our models isolated the effects of tectonic plate speed and the geometry of the trench relative to volcanic centers behind it on the size of this difference in seismic wave speed. We found that signals in the southern part of the basin should be different than those in the north if plate speed and geometry are the main factors in controlling this ‘seismic anisotropy’. Part of the project entailed revising/testing software, to allow the effects that differences in mantle properties on the computed flow pattern to be determined. Part of the work required experiment planning and going to sea to help recover ocean floor seismometers that had been deployed in the basin for a year to record earthquakes that serve as the sound source for the seismic imaging. Another aspect was graduate student training- a Scripps Institution of Oceanography student learned about computer modeling, trouble shooting when results did not look right the first time, signal processing techniques, and visualizing results. Her results for this project form one chapter of her PhD thesis. As the seismic data comprising the main part of the collaborative study are becoming available in late 2012, comparison of our model predictions with the observations will be possible. We expect to learn whether some parameters in our models need to be updated to better match the ‘real world’.
