Rob Porritt (UTIG)
Fri, September 22, 10:30am - 11:30am
Host: Thorsten Becker
Abstract: Passive-source seismic imaging provides valuable constraints on Earths layered structure, relative thermal and compositional anomalies, and deformation and flow field. These constraints are derived from several increasingly well standardized and innovative imaging methods and a growing pool of passive seismic deployments, such as the IRIS GSN, USArray, and Wavefields projects. Here, I present a tour of recent results focusing on Texas, the Gulf Coast, the Eastern North American Margin, and abroad. In Texas, the Llano Uplift and Ouachita-Marathon Front are recognized in continental-scale body-wave and surface-wave imaging. Their underlying mantle structure suggests these surficial features are linked to ongoing mantle processes. Along the Gulf Coast, imaging the entire crustal column has been difficult with passive methods due to the thick low velocity sediments. However, emerging receiver function results indicate a mid-crustal base to the sediments at ~15-20 km depth and locally over-thickened crust in several areas. Ambient noise results for the Eastern North American Margin indicate a structural transition across the passive margin, coincident with gravity and magnetic anomalies. This relatively smooth and simple structure is in marked contrast with the active margin in the Banda arc where the Indian Ocean Plate and Australian Plate collide with the young Banda Plate; this margin is characterized by over-thrusting of shelf deposits and exhumation of subduction assemblages in Timor Leste and eastern Indonesia. The subducting plate in this incidence is shallow in the collisional zone and then dives steeply under the Banda Sea. The fate of this slab is uncertain below the 670km discontinuity, but the interplay of slabs and the mantle transition zone remains an important area of research. This uncertainty is particularly relevant in the Izu-Bonin islands where a M8 class earthquake occurred at 682 km depth. Long period receiver function evidence suggests the base of the slab in this region reaches 690 km depth; while this does maintain that slabs are required for deep earthquakes, it forces questions about how the transition zone can be pushed 20 km deeper than its nominal base. This tour highlights the efficacy of passive seismic tools to fundamental earth science questions. As more data becomes available and new tools are developed, our understanding will improve and new questions will emerge.