Seismic structure near the inner core‐outer core boundary

Abstract

A model of P‐wave velocity of the Earth's core 500 km above and below the inner core boundary (ICB) beneath North America is constructed from travel time analysis and broad‐band waveform modeling of core phases at distances from 130° to 160°. Differential travel times between PKPBC (Cdiff) and PKPDF (TBC–TDF) are about 0.5 sec shorter than those computed from PREM at distances from 146° to 152° and increase with distance to nearly the same as those from PREM at 155°. Differential travel times between PKPCD and PKPDF (TCD–TDF) at distances from 130° to 143° are approximately 0.2 sec shorter than those of PREM. Amplitudes of PKPBC (Cdiff) relative to PKPDF (ABC/ADF) at distances 150° to 156° are about 40% larger than those from PREM. These observations require a smaller P‐wave velocity gradient (0.0005 sec−1) in the lowermost 300 km of the outer core than that from PREM (∼0.00065 sec−1), a slower P‐wave velocity (10.96 km/sec) at the top of the inner core, and a larger velocity gradient (0.0005 sec−1) in the uppermost 300 km of the inner core.

Deep Earth Structure – The Earth’s Cores

“…The modeling of the PKP waveforms gives generally lower v S -values beneath the ICB, from nearly zero (Choy and Cormier, 1983) to 2.5-4.0 km s À1 (Müller, 1973;Häge, 1983;Cummins and Johnson, 1988b). A P-velocity gradient seems to be required in the same depth range (e.g., Kaneshima et al, 1994). All these values are significantly lower than those given by the global Earth models.…”

Seismic structure near the inner core boundary in the south polar region

“…(top) Comparison of our V p model near the ICB (SPR) with other models: PREM, AK135, VMOI [ Kaneshima et al , 1994], S&R [ Souriau and Roudil , 1995], PREM2 [ Song and Helmberger , 1995], and OW+W2 [ Yu et al , 2005]. (middle) Expanded plot of V p profiles in the upper 300 km of the inner core.…”

“…The lowermost outer core and inner core beneath the polar regions have remained poorly constrained because of the limited distributions of earthquakes and seismic stations. Although fine seismic structure near the inner core boundary (ICB) has been examined using body waves by many researchers [e.g., Creager , 1992; Kaneshima et al , 1994; Song and Helmberger , 1995; Tanaka and Hamaguchi , 1997; Wen and Niu , 2002; Yu et al , 2005; Zou et al , 2008], the models presented in those studies were mainly applicable to the seismic structure in the low‐latitude and/or midlatitudes regions (Figure 1a). Recent increase of stations improved data coverage to some extent [ Garcia et al , 2006; Irving and Deuss , 2011] (Figure 1b).…”

Constraints on aspherical core structure from PKiKP‐PcP differential travel times

“…The differencing greatly mitigates the effects of hypocentral uncertainties and lateral variations in crustal and upper mantle structure. The inner core has most often been studied with the PKP AB ‐ PKP DF and PKP BC ‐ PKP DF combinations [e.g., Creager , 1999, and references therein] and more recently with PKP CD ‐ PKP DF [ Kaneshima et al , 1994; Niu and Wen , 2001; Ouzounis and Creager , 2001], while the outer core has been studied with SKS ‐ SKKS differential times [ Souriau and Poupinet , 1990, 1991; Lay and Young , 1990]. A less commonly observed phase pair that is strongly sensitive to core structure is PKiKP ‐ PcP , the direct reflections from the inner and outer core (Figure 1).…”