Seismic Imaging Of Interplate And Intraplate Areas: New Insights Into Seismogenesis And Volcanism

Earthquakes and volcanic eruptions are two main geological processes which transfer energy and materials through the Earth systems, and they are also the most serious natrual hazards on Earth. In this work, we used seismological methods, including seismic velocity and attenuation tomography, and multi-channel seismic data migration imaging, to study seismic structure of the seismogenic and volcanic zones in several interplate and intraplate regions including the Manila, Cascadia, South-Kuril and Northeast Asia subduction zones, as well as the New Madrid intraplate seismic zone. Our main findings are summarized as follows.1) We studied the detailed seismic structure of the incoming plate along the northern Manila trench by integrating 21 multi-channel seismic lines and 6 OBS profiles. The seismic imaging revealed crustal heterogeneities, crustal thickness, sediment thickness and basement depth of the incoming plate. The segmentation of the incoming plate correlates well with the deformation pattern of the accretionary wedges and seismically active regions, implying that pre-existing heterogeneities inherited from the rifting process of the continental margin may have an important effect on carving seafloor morphology and seismicity at the precollision zone.2) We investigated the detailed crustal and upper-mantle structure of the Cascadia subduction zone by conducting tomographic inversions using a starting velocity model which includes the high-velocity subducting Juan de Fuca slab as a priori information. The lateral velocity variations revealed by our tomographic images indicate different degrees of slab dehydration and forearc mantle serpentinization. The segmentation in episodic tremor and slip (ETS) is also spatially coincident with the velocity heterogeneities, suggesting that the ETS occurrence and recurrence interval are controlled by fluid activity in and around the mantle wedge corner.3) We obtained detailed 3-D velocity and attenuation tomography of the crust and upper mantle beneath Hokkaido, Japan. Low-velocity and high-attenuation anomalies are revealed beneath active arc volcanoes, which reflect arc volcanism caused by a combination of slab dehydration and hot upwelling flow in the mantle wedge. However, such anomalies are absent beneath the volcanic gap area in southern Hokkaido, indicating weak or none flux melting at the arc-arc collision zone.4) We determine a 3-D P-wave velocity model of the crust and upper mantle of the New Madrid Seismic Zone. Our results show that, beneath this intraplate seismic zone, a significant low-velocity (low-V) zone exists in the upper mantle down to 200 km depth. This low-V zone may have relatively low shear strength and act as a viscously weak zone embedded in the lithosphere, being apt to concentrate tectonic stress and transfer stress to the seismogenic faults in the upper crust, leading to the large intraplate earthquakes in the New Madrid Seismic Zone5) We obtained an updated P-wave tomography of the Pacific subduction zone under Northeast Asia, targeting at the mantle transition zone beneath the vicinity of the Changbai volcano. Our result shows that a focused mantle upwelling originates from the hinge of the stagnant slab beneath the Changbai volcano. The origin and localized concentration of the asthenospheric upwelling may result from a combined effect of deep dehydration of hydrous minerals, additional fluids released through deep earthquakes and stronger mantle circulation near the hinge of the stagnant Pacific slab.