• Shear wave splitting and mantle flow in Alaska

      McPherson, Amanda M.; Christensen, Douglas; Tape, Carl; Freymueller, Jeffrey (2018-08)
      We explore the nature of mantle anisotropy and flow under Alaska by presenting 2389 SKS shear wave splitting observations from 547 earthquakes recorded at 384 broadband stations deployed in Alaska since 2010. We expand upon the results of Seismic Anisotropy under central Alaska from SKS splitting observations by Christensen and Abers (2010) and Insights into mantle structure and flow beneath Alaska based on a decade of observations of shear wave splitting by Perttu, Christensen, Abers, and Song (2014) to better understand the effect of flat slab subduction on mantle anisotropy and flow. Shear wave splitting is a common tool to investigate anisotropy in the upper mantle, which is often assumed to be caused by mantle flow or preexisting fabrics in the lithosphere. In Alaska, splitting appears to be controlled by the absolute plate motion of the North American and Pacific plates, and the interaction between the two plates. In particular, the subducting Pacific plate acts as a barrier to flow. Directly north of the slab, fast directions are oriented along the strike of the slab with large δts, and are caused by along strike flow in the mantle wedge. Stations further to the north, outside of the influence of the mantle wedge, gradually see fast directions parallel to the absolute plate motion direction of the North American plate. South of the slab, fast directions depend on the geometry of the subducting plate. South of the Alaska Peninsula, the fast directions are parallel to the trench, which represent along strike flow under the Pacific plate. To the east, however, flat slab subduction dominates. Here the fast directions are perpendicular to the trench (parallel to the absolute motion of the Pacific plate) and are indicative of entrained flow from the motion of the Pacific plate. Fast directions near the Fairweather- Queen Charlotte transform system are parallel to the faults, and are likely caused by the deformation associated with large lithospheric blocks moving past each other. The region between the inferred east end of the Pacific plate and the transform boundary is dominated by the collision and accretion of the Yakutat terrane. The tectonics of this region are still in debate and the fast directions are difficult to interpret.
    • Spatial and temporal variations in slip behavior beneath Alaska-Aleutian subduction zone

      Li, Shanshan; Freymueller, Jeffrey T.; Christensen, Douglas; Tape, Carl; West, Michael (2018-08)
      Characterizing spatial and temporal variations of slip behavior observed along subduction faults is of great significance for understanding the dynamics of subduction zones, features of great subduction zone earthquakes and deformation patterns across the subduction plate boundary through the seismic cycle. The Alaska-Aleutian subduction zone is one of the most tectonically active margins in the world. Great earthquakes and slow slip events recorded in this area are closely related in space. An increasingly dense array of Global Positioning System (GPS) receivers measures surface deformation at sites with high accuracy and provides a perfect tool for estimating the slip distribution on the plate boundary. GPS observations show that the motion of the Earth is not entirely linear: the long-term steady motion is interrupted by events like earthquakes, slow slip events (SSEs) and deformation of volcanoes, etc. Two long-term SSEs were detected in Lower Cook Inlet, Alaska (1992.0-2004.8 and 2009.85-2011.81) by inverting the slip distributions from GPS site velocities. The occurrence of SSEs based on the estimated slip distribution patterns provides strong evidence for the transition from stick-slip behavior to episodes and continuous aseismic creep on the subduction plate interface. Coulomb stressing rate changes (CSRC) due to the two detected long-term SSEs indicate that regions in the shallow slab (30-60 km) that experience significant increase in CSRC show an increase in seismicity rate during SSE periods. The modified quantitative rate/state stress transfer model suggests that the SSEs increase stress on surrounding faults, thereby increasing the seismicity rate even though the ratio of the SSE induced stressing rate to the background stressing rate is small. The SSEs were shown to cause significant stress changes in the seismogenic zone. This highlights the importance of exploring the relationship between SSEs and earthquakes, as well as how this relationship impacts the strain accumulation in the subduction zone. A repeat survey of the existing campaign GPS sites combined with continuous GPS sites provided a > 20 year time span for estimating the interseismic velocities of the Alaska Peninsula. From this I inferred a more precise model for the location and spatial extent of the change from locked to creeping behavior across the Alaska Peninsula. Given this more detailed distribution of the slip behavior, the results suggest that slip behavior correlates with the pre-existing plate fabric on the downgoing plate, seismic behavior, the reflection character of the slab interface itself and the rupture history of past great earthquakes.