Intermittent and lateral varying ULVZ structure at the northeastern margin of the Pacific LLSVP

Abstract

Thin patches with ultralow velocities have been proposed to exist at the core‐mantle boundary (CMB). The detection and mapping of ultralow velocity zones (ULVZs) are difficult, in part, because of limited source‐receiver geometries of seismic phases used in ULVZ modeling. Here we develop a new approach that simultaneously utilizes ScS precursor and postcursor energies to investigate the CMB region for ULVZ structure. We stacked source‐deconvolved ScS waveforms within 1.5° geographic bins to extract ScS precursor and postcursor energies, if present, with ScS effectively removed from waveforms. We investigate the CMB beneath the central Pacific Ocean, and evidence for ULVZs is clearly apparent. Geographic bin stacks possessing similar ScS precursor‐plus‐postcursor behavior are grouped by using cluster analysis to produce more robust waveforms by enhancing the signal‐to‐noise ratios. Synthetic seismograms that demonstrate the amplitude and timing of the ULVZ arrivals are sensitive to ULVZ thickness and internal velocities. To pursue local ULVZ properties we processed 13,850 1‐D synthetic models with various ULVZ thicknesses and internal properties, using the identical ScS‐stripping method as with the data. A best fitting model was found for each geographical bin cluster by using an amplitude‐sensitive cross‐correlation algorithm. While limitations exist due to 1‐D modeling, strong lateral variations are clearly apparent in ULVZ thickness and properties across the large low shear velocity province (LLSVP) margin in our study area. Inside hypothesized LLSVP edges, ULVZs appear to distribute unevenly, suggesting 3‐D variations of convection currents.

Slab Control on the Northeastern Edge of the Mid‐Pacific LLSVP Near Hawaii

“…This PKP-inferred boundary from Luo et al's (2001) study is near the southern edge of a circular ULVZ with radius of~500 km and height of~20 km, proposed to explain late arrivals observed in S wave-diffracted coda (Cottaar & Romanowicz, 2012). Other localized ULVZs (shapes labeled 3-5 in Figure 1b) have been reported using PcP/ScS precursor or postcursors (Avants et al, 2006;Courtier et al, 2007;Hutko et al, 2009;Kohler et al, 1997;Lay et al, 2006;Mori & Helmberger, 1995;Revenaugh & Meyer, 1997;Zhao et al, 2017) and ScS2/ScS travel times (Liu et al, 2011). However, mostly layered ULVZ structures of the northeastern edge of the mid-Pacific LLSVP have been assumed.…”

“…Such features involving late PKP ab arrival, reduced amplitudes, and waveform complexity have been observed for the eastern most mid-Pacific structure (Sun et al, 2007). Other localized ULVZs (shapes labeled 3-5 in Figure 1b) have been reported using PcP/ScS precursor or postcursors (Avants et al, 2006;Courtier et al, 2007;Hutko et al, 2009;Kohler et al, 1997;Lay et al, 2006;Mori & Helmberger, 1995;Revenaugh & Meyer, 1997;Zhao et al, 2017) and ScS2/ScS travel times (Liu et al, 2011). Other localized ULVZs (shapes labeled 3-5 in Figure 1b) have been reported using PcP/ScS precursor or postcursors (Avants et al, 2006;Courtier et al, 2007;Hutko et al, 2009;Kohler et al, 1997;Lay et al, 2006;Mori & Helmberger, 1995;Revenaugh & Meyer, 1997;Zhao et al, 2017) and ScS2/ScS travel times (Liu et al, 2011).…”

“…The cyan dashed line 1 displays the boundary inferred by Luo et al (2001). The shaded gray region inferred ULVZs: 3 (Liu et al, 2011), 4 (Avants et al, 2006Courtier et al, 2007;Hutko et al, 2009;Kohler et al, 1997;Lay et al, 2006;Mori & Helmberger, 1995;Revenaugh & Meyer, 1997), and 5 (Zhao et al, 2017). The shaded gray region inferred ULVZs: 3 (Liu et al, 2011), 4 (Avants et al, 2006Courtier et al, 2007;Hutko et al, 2009;Kohler et al, 1997;Lay et al, 2006;Mori & Helmberger, 1995;Revenaugh & Meyer, 1997), and 5 (Zhao et al, 2017).…”

“…The dashed polygon 2 area of strongly varied D″ structure . The shaded gray region inferred ULVZs: 3 (Liu et al, 2011), 4 (Avants et al, 2006Courtier et al, 2007;Hutko et al, 2009;Kohler et al, 1997;Lay et al, 2006;Mori & Helmberger, 1995;Revenaugh & Meyer, 1997), and 5 (Zhao et al, 2017). The large ULVZ of Cottaar and Romanowicz (2012) as magenta circle 6.…”

Lateral Variations of Shear‐Wave Velocity in the D″ Layer Beneath the Indian‐Eurasian Plate Collision Zone

“…The ScS ‐S difference time is a robust seismic observable and often used to study the seismic structure of the deep mantle. It is less strongly influenced by seismic structure in the upper mantle and by earthquake mislocations than the absolute travel times (e.g., He et al, 2015; Russell et al, 1998; Wysession et al, 2001; Zhao et al, 2017). We measure the arrival times of ScS and S by cross‐correlating the band‐pass filtered recorded and computed waveforms to account for all propagation effects on the S and ScS waveforms.…”