Search

Using PKIKP and PKiKP travel times and amplitude ratios, we examine the velocity and attenuation anisotropy structures in the uppermost 100 km of the inner core beneath the central Pacific region. Our results indicate a clear change from approximately 180°E to 190°E longitude. Below the western Pacific, velocity anisotropy is weak and attenuation is high (Qp ~200), whereas below the eastern Pacific, velocity anisotropy is strong (3.3%) and attenuation is low (Qp ranges from 400 to 150). Furthermore, the velocity and attenuation anisotropies are correlated with each other. Our results indicate that the growth of the inner core may be coupled with core mantle boundary thermal heterogeneities. The heterogeneities may affect directional heat flow or deformational processes in the uppermost inner core, cause different iron crystal alignments, and result in lateral variations in anisotropy structures. Plain Language Summary: The inner core solidified from the liquid outer core, so the structure at the top inner core is closely linked to its formation process. In this study, based on seismic data availability, we choose the uppermost 100 km of the inner core beneath the Pacific region and investigate in detail its velocity and attenuation differences in different directions, or anisotropy structures. Our results not only depict a transition boundary between the eastern and western Pacific but also show different anisotropy characteristics of these two regions, in which the western/eastern Pacific region shows weak/strong anisotropy in both velocity and attenuation. Moreover, the velocity and attenuation anisotropies are correlated. These results indicate that the inner core growth may be coupled with the core mantle boundary heterogeneity, which may cause different texture alignment through different deformation or heat flow, and result in different physical properties in the inner core. Key Points: Both velocity and attenuation anisotropy in the uppermost 100 km of the inner core beneath the central Pacific are investigatedWe find clear variations in both velocity and attenuation anisotropy structures, and there exist good correlation between themThe inner core growth may be coupled with core‐mantle boundary heterogeneity and different deformation processes [ABSTRACT FROM AUTHOR]

Abstract: The Xinfengjiang Water Reservoir (XWR) in Guangdong, China, is one of the reservoirs that have triggered earthquakes of magnitudes greater than 6. Numerous earthquakes have occurred since the impoundment of the reservoir, making it one of the most active seismic zones in Guangdong. However, due to the lack of seismic stations, the detailed seismic structures and earthquake mechanisms within XWR have not been resolved, and the significance of XWR as a typical protracted earthquake location is not well judged. In this study, by collecting waveform data from both permanent and temporary stations from 2012 to 2015, we relocated 1,528 earthquakes and inverted both Vp and Vs structures from traveltimes of these earthquakes. Using waveform data, we also investigated focal mechanisms of earthquakes with magnitude greater than 1.5 in this region. Our results reveal fine crustal structure that has never been shown before and show complicated crust structure with several low‐velocity zones extending to 5–10 km depth under the major faults. Earthquake focal mechanisms show more dip‐slip faults than strike‐slip faults, and the two types of earthquakes are roughly divided by the reservoir boundary. The direction of principle stress of the earthquakes is northwest‐southeast, consistent with the direction of tectonic principal stress. Combining the above results, and investigation of historical earthquakes and water level change, we suggest that water loading cycle and diffusion play important role in XWR seismicity. They increase the pore pressure, make the earthquakes migrate to deeper depth, and change the type of earthquakes. [ABSTRACT FROM AUTHOR]