Crustal origin of trench-parallel shear-wave fast polarizations in the Central Andes
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Slab
Shear wave splitting
Seismic anisotropy
Shear wave splitting
Asthenosphere
Seismic anisotropy
continental collision
Collision zone
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Tsunami earthquake
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Lithospheric flexure
Hydrostatic equilibrium
Radius of curvature
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Large shallow earthquakes of the Kuril Trench are distinct from those of the adjacent Aleutian Trench in two important ways: in the case of the Kuril events, (1) the rate of occurrence of events near or oceanward of the trench axis with tensional focal mechanisms is markedly lower and (2) the average apparent stress of 14 shallow events is almost an order of magnitude higher. An explanation of these observations is that the oceanic lithosphere forming the Kuril Trench-Hokkaido Rise system is acted upon by a horizontal compressive stress of several kilobars in a direction normal to the Kuril Trench axis. Calculations based on simple models of elastic deformation support this explanation. Unless the Kuril oceanic lithosphere has a strong nonelastic response on a time scale of a few million years, a horizontal compressive stress of approximately several kilobars is necessary to maintain the Kuril Trench-Hokkaido Rise bathymetry. The oceanic lithosphere in this region has an effective (elastic) thickness of 30–40 km and is capable of supporting deviatoric compressive stresses of several kilobars. The elastic thickness of the Kuril oceanic lithosphere agrees well with the seismic thickness of the lithosphere descending beneath the Tonga arc.
Pacific Plate
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Slab width plays a major role in controlling subduction dynamics and trench motion. However, observations on natural narrow subduction zones do not show any correlation between slab width and trench velocities, indicating that other factors may have a greater impact. Here, we use 3D numerical subduction models to evaluate the effect of slab width, strength of slab coupling to the lateral plate and overriding plate thickness on trench kinematics. Model results show that slab width has little influence on trench migration rates for narrow subduction zones, but that the thickness of the overriding plate plays a major role, with trench velocities decreasing as the thickness increases. These results explain trench velocities observed in natural narrow subduction zones showing no relation with slab width but an inverse dependence on overriding plate thickness. Finally, we find that the overriding plate thickness also significantly affects the trench shape.
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<p>We used more than one decade of core-refracted teleseismic shear (SKS) waveforms recorded at more than 160 broadband seismic stations across the Iranian plateau and Zagros to investigate seismic anisotropy beneath the region. Splitting analysis of SKS waveforms provides two main parameters, i.e., fast polarization direction and split delay time, which serve as proxies for the trend and strength of seismic anisotropy beneath the stations. Our observation revealed a complex pattern of splitting parameters with variations in the trend and strength of anisotropy across the tectonic boundaries. We also verified the presence of multiple layers of anisotropy in conjunction with the lithosphere deformation and mantle flow field. Our observation and modeling imply that a combined system of lithosphere deformation and asthenospheric flow is likely responsible for the observed pattern of anisotropy across the Iranian Plateau and Zagros. The rotational pattern of the fast polarization directions observed locally in Central Zagros may indicate the diversion of mantle flow around a continental keel beneath the Zagros. The correlation between the variation in lithosphere thickness and the trend of anisotropy in the study area implies that the topography of the base of lithosphere is also a determining factor for the pattern of mantle flow inferred from the observations.</p>
Seismic anisotropy
Shear wave splitting
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In the southernmost Kuril Trench, the tsunami source regions vary their along-trench extent even among earthquakes occurring within the same segment. Recent studies suggest that the tsunami source of the 1952 Tokachi-oki earthquake (M 8.1) differs from but partially overlaps with that of the 2003 Tokach-oki earthquake (M 8.0). Furthermore, the along-trench extent among the earthquakes seems to differ between deep and shallow portions of the subduction interface. A seismic gap has been recognized along the deep subduction interface between the sources of the 1952 and 1973 earthquakes. We propose that the gap is now larger, including both shallow to deep portions of the interface between the 1973 and 2003 earthquakes. Variability in spatial extent of large subduction earthquakes in both along-trench direction and trench-normal direction makes it difficult to forecast future earthquakes in the southernmost Kuril Trench.
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