Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle

Abstract

Small-scale heterogeneities in the mantle can give important insight into the dynamics and composition of the Earth's interior. Here, we analyse seismic energy found as precursors to PP, which is scattered off small-scale heterogeneities related to subduction zones in the upper and mid-mantle. We use data from shallow earthquakes (less than 100 km depth) in the epicentral distance range of 90 • -110 • and use array methods to study a 100 s window prior to the PP arrival. Our analysis focuses on energy arriving off the great circle path between source and receiver. We select coherent arrivals automatically, based on a semblance weighted beampower spectrum, maximizing the selection of weak amplitude arrivals. Assuming single P-to-P scattering and using the directivity information from array processing, we locate the scattering origin by ray tracing through a 1-D velocity model. Using data from the small-aperture Eielson Array (ILAR) in Alaska, we are able to image structure related to heterogeneities in western Pacific subduction zones. We find evidence for ∼300 small-scale heterogeneities in the region around the present-day Japan, Izu-Bonin, Mariana and West Philippine subduction zones. Most of the detected heterogeneities are located in the crust and upper mantle, but 6 per cent of scatterers are located deeper than 600 km. Scatterers in the transition zone correlate well with edges of fast features in tomographic images and subducted slab contours derived from slab seismicity. We locate deeper scatterers beneath the Izu-Bonin/Mariana subduction zones, which outline a steeply dipping pseudo-planar feature to 1480 km depth, and beneath the ancient (84-144 Ma) Indonesian subduction trench down to 1880 km depth. We image the remnants of subducted crustal material, likely the underside reflection of the subducted Moho. The presence of deep scatterers related to past and present subduction provides evidence that the subducted crust does descend into the lower mantle at least for these steeply dipping subduction zones. Applying the same technique to other source-receiver paths will increase our knowledge of the small-scale structure of the mantle and will provide further constraints on geodynamic models.

Seismic Detections of Small-Scale Heterogeneities in the Deep Earth

“…The PP precursor wavefield shows evidence for symmetric reflections off the upper-mantle discontinuities (Shearer 1990) and for asymmetric reflections off the surface or uppermost mantle (Wright 1972;King et al 1975;Weber and Wicks 1996). Asymmetric off-azimuth reflections off slab material arriving as PP precursors have been identified beneath the Mariana and Izu-Bonin subduction zones (Rost et al 2008;Bentham and Rost 2014), outlining deep subduction beneath these subduction zones and the transport of oceanic crust far into the lower mantle. Other phases such as PcP have been used to image lower mantle structure through the analysis of P-to-P and S-to-P scattering (Braña and Helffrich 2004).…”

Origin and significance of deep earthquake clusters surrounding a pronounced seismic gap in northeast China

“…And the observed variations in focal mechanism types investigated by Myhill (2013) are attributable to slab buckling. Beyond the hinge line the subducted lithosphere is broken into fragments, and fragments that penetrate into the lower mantle form a discontinuity or thin layer at about $1000 km depth (Bentham and Rost, 2014;Li and Yuan, 2003;Niu, 2014). The fragmentation indicates the subducting slab is weak, which helps explain the shallow depth of cessation of seismicity that we observed.…”

A Sequence of up to 11 Seismic Discontinuities Down to the Midmantle Beneath Southeast Asia

“…In any case, compositional alteration of the mantle material is mandatory in order to explain the observed velocity discontinuities. A recent study utilizes array techniques to detect seismic energy scattered off subducted oceanic crust in the western Pacific subduction zones which image fragments down to 1,880 km (Bentham & Rost, ). Their study does not include the area sampled in our analysis; however, it corroborates the assumption that lithospheric fragments can exist at that depth below subduction zones.…”

“…Such discontinuities can be explained by different origins. These include variations of the anisotropic properties in response to dynamic stress (e.g., Bostock, 1997;Revenaugh & Jordan, 1991), the existence of partial melt (e.g., Revenaugh & Sipkin, 1994;Song et al, 2004), changes in the chemical composition of the mantle material (e.g., Bentham & Rost, 2014;Kawakatsu & Niu, 1994), or mineral phase transitions (e.g., Bagley & Revenaugh, 2008;Helffrich & Wood, 2001;Vacher et al, 1998) which are usually related to specific pressure and temperature conditions but do not involve chemical alteration. Thus, the presence and depth level of seismic discontinuities can provide important constraints on the thermal and compositional state of the mantle.…”