Date of Award
Doctor of Philosophy (PhD)
This dissertation utilizes multiple techniques of seismic tomography and earthquake location to investigate the upper mantle structure and intermediate-depth seismicity of the Tonga subduction zone, the Lau back-arc basin, and adjacent regions. Data for these studies consist of broadband records from 49 ocean bottom seismographs and 17 island-based seismic stations deployed for one year in 2009-2010, as well as more limited earlier datasets. I conducted tomographic studies of Rayleigh wave velocity and body wave attenuation to examine the thermal variations and the distribution of partial melt in the mantle wedge. The shear-wave velocity structure is first determined using only teleseismic data with the two-plane-wave method, and then jointly inverted from the phase velocities of teleseismic and ambient-noise Rayleigh waves obtained from noise cross-correlation. Additionally, I determine the 3-D P and S wave attenuation structure from t* measurements using local and regional earthquakes. Tomographic results show extremely low velocity and high attenuation within the upper 80-km of the mantle beneath the Lau back-arc basin, suggesting perhaps the lowest shear-wave velocity (VSV = 3.6 km/s) and highest seismic attenuation (QP < 35 and QS < 25) known in the mantle. These anomalies require not only abnormally high temperature but also the existence of partial melt. The inferred melting regions align with the spreading centers at shallow depths of 20-70 km, but shift westwards away from the slab, indicating a passive decompression melting process governed by the mantle wedge flow rising from the west. Assuming that velocity anomalies reflect variations in mantle porosity filled with melt, the mantle porosity is reduced in areas of high mantle water content, implying that the melt segregation and extraction are significantly enhanced by the water released from the subducting slab. The low velocities and high attenuation beneath the northeastern Fiji Plateau and northern Lau Ridge suggest the missing lithospheric root in this region, where the active Taveuni Volcano exists. This, along with the low-velocity anomalies beneath the northwestern Lau Basin, are consistent with a second origin from the deep mantle in addition to the Samoan mantle plume.
In order to investigate water-related slab processes, I precisely locate intermediate-depth earthquakes and associate them with the Global Centroid Moment Tensor solutions. These events form a double seismic zone with a separation of about 30 km in the northern part of the Tonga slab, with a downdip compressional upper plane and a downdip tensional lower plane. The lower termination of the double seismic zone correlates with the convergence rate, extending to 300 km in places, and is consistently deeper than in Japan and other slabs worldwide. Similar trends have been found for a tripe of seismicity at depths of 200-300 km. These observations indicate that the depth of intermediate-depth seismicity is primarily influenced by temperature, implying the importance of the thermally controlled processes, such as serpentine dehydration and fluid-related embrittlement. However, the disappearance of the double seismic zone towards to the south coincides with the change in slab curvature, suggesting that the stress states rather than dehydration reactions control the activity of the lower plane.
Chair and Committee
Douglas A. Wiens
Philip Skemer, Viatcheslav S. Solomatov, Linda M. Warren, Michael E. Wysession,
Wei, Songqiao, "Seismic Studies of the Tonga Subduction Zone and the Lau Back-arc Basin" (2016). Arts & Sciences Electronic Theses and Dissertations. 785.