MULTIPLE ANIMATED COLLADA MODELS OF THE TONGA SUBDUCTION ZONE AND AMERICAN SAMOA TEAR FAULT: AN EXAMPLE OF THE EMERGENCE OF RESEARCH FROM TEACHING

The M 8.1 tsunamigenic earthquake near Samoa on September 29, 2009 drew worldwide attention to the complex tectonic setting of the western Pacific, so we decided to create instructional resources for the region in Google Earth. The main tectonic features are the NNE-trending Tonga and Lau arcs and their northward termination in the WNW-trending American Samoa Tear Fault. The area has long served as a type locality for trench roll-back and back-arc spreading.

To create a realistic COLLADA model of the slab in Google Earth, we accessed data from Syracuse & Abers (2006) using www.GeoMapApp.org. These data were exported as a KML file and imported into Google SketchUp. Using linear interpolation, same-depth data points were connected to form contour lines, and a 3D wireframe of the slab’s topography was assembled. The seafloor from Google Earth was added as a top texture in SketchUp. The resulting COLLADA model is a realistic lithospheric slab and tear fault that can be lifted above sea level and rotated for a 360° view. Different elements – arc, slab, mantle, etc. – can be selectively shown or hidden.

The concept of eastward rollback of a westward dipping subduction zone is difficult for students to grasp, therefore we devised ways to animate a simplified COLLADA model using Javascript controls and the Google Earth API. It soon became clear that three scenarios needed to be compared, each with a different combination of plate motion and tear point migration. In scenario 1, the Pacific plate movings rapidly (7mm/year) westward whilst the tear point and trench migrate eastward at a slower, independent velocity. In scenario 2, the rollback velocity is numerically greater than the plate velocity, and in scenario 3, no rollback occurs and the tear point is stationary. Because the fault line in the tear region is unusually complicated, with different structural elements across its width, each scenario results in different slip regimes, which can be compared with real focal mechanism data. When the detailed history of back-arc spreading is considered, a fourth model emerges involving rapid foundering of the back-arc around 6 m.y. ago. and resulting in a shallow slab currently at current mid-mantle level. Thus a model initially intended as for teaching with preconceived outcomes evolved into a research project with open-ended outcomes.