Imaging Shallow Subduction Zones With Surface Waves - fig. 2
Imaging Shallow Subduction Zones With Surface Waves - fig. 2
Credit:
Michael H. Ritzwoller, Nikolai M. Shapiro • University of Colorado at Boulder/IRIS Consortium
Description
Figure 2. Similar to Figure 1, but for northern Sumatra and the Andaman Sea. The blue surface (+1.2%) is oceanic lithosphere subducted at the Sunda and Andaman trenches and the red surface (-1.5%) reflects low velocities beneath the Andaman spreading center, relative to a regional average at each depth. Brown lines show the active plate boundaries, blue arrows are relative plate motion, and yellow arrows indicate extension in the Andaman Basin.
Intermediate and long period surface waves can image relatively old (>40 Ma) subducting lithosphere above ~250 km. These images are complementary to teleseismic body wave images, which are best below ~200 km depth. Examples are shown here of the upper mantle beneath the Aleutian and Sumatra-Andaman subduction zones. These are interesting regions to contrast as the incoming plate transitions from convergence to oblique incidence. The Pacific Plate moves obliquely to the western Aleutians west of about 173°E and the Indian Plate moves similarly compared to the Andaman Sea. Beneath the western Aleutians (Figure 1) there is a “slab portal” in the region of oblique incidence. The lateral termination of the high shear wave speeds, which are diagnostic of the slab, coincides with a gap in deep seismicity. In contrast, high speed shallow subducting lithosphere is imaged along the entire Sunda Arc (Figure 2), from the Andaman Sea, where oblique incidence occurs, to Sumatra and Java. The age of the subducting lithosphere varies appreciably along the arc, being youngest (~40 Ma) off northern Sumatra near the initiation of the great Sumatra-Andaman Islands earthquake of Dec 26, 2004, and its largest aftershock that occurred on Mar 28, 2005. Very high temperatures are observed in the supra-slab mantle wedge in the Andaman back-arc. The thermal structure of the mantle from the Andaman Sea to Java may have contributed to the location of the initiation of rupture of the great earthquake, the strength of seismic coupling, and the differential rupture speed between the northern and southern segments of the earthquake.
Levin, V., N.M. Shapiro, J. Park, and M.H. Ritzwoller, Seismic evidence for catastrophic slab loss beneath Kamchatka, Nature, 418, 763-767, 2002.
Levin, V., N.M. Shapiro, J. Park, and M.H.Ritzwoller, The slab portal beneath the Western Aleutians, Geology, 33(4), 253-256, 2005.
Ritzwoller, M.H., N.M. Shapiro, and E.R. Engdahl, Structural context of the great Sumatran-Andaman Islands earthquake, submitted 2005.
Photographer / Contributor: Michael H. Ritzwoller, Nikolai M. Shapiro • University of Colorado at Boulder
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