Improving seismic resolution of outermost core structure by multichannel analysis and deconvolution of broadband SmKS phases

Details Improving seismic resolution of outermost core structure by multichannel analysis and deconvolution of broadband SmKS phases Improving seismic resolution of outermost core structure by multichannel analysis and deconvolution of broadband SmKS phases

Apr 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006pepi..155..104e&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 155, Issue 1-2, p. 104-119.

Physics

9

Scientific paper

The presence of a thin (≤12 km) layer atop the outermost core has important implications for the geodynamic and geomagnetic nature of the Earth, but detecting such a layer is difficult. SmKS seismic phases, which traverse the mantle as an S-wave and reflect m - 1 times from the underside of the core mantle boundary (CMB), offer a means to test proposed models since they are extremely sensitive to the velocity structure of the outermost core. To improve seismic resolution in this region, we have developed and validated a modified vespagram (slant stack) technique that makes use of phase-independent amplitude-envelope traces. Confidence limits on arrival-time and slowness estimates are obtained by bootstrap resampling. In order to mitigate the effects of waveform complexity and facilitate comparison between observed and modeled waveforms, we have used a simple deconvolution method similar to techniques used in exploration seismology. We have applied these methods to high-quality broadband recordings of three deep-focus earthquakes recorded by seismic arrays in Canada. Some of our measurements have turning depths <190 km in the outer core, a region of sparse coverage in previous global compilations of SmKS measurements. Based on the analysis of travel times, our data show that the P-wave velocity of the outermost core is PREM-like until the outer ˜150 km or less, where it becomes much slower than PREM and most standard Earth models. Additionally, amplitudes of S3KS/S4KS phases relative to SKKSac are significantly larger than those predicted by standard models. Using reflectivity synthetic seismograms we show that no previous Earth model explains both our travel-time and amplitude anomalies. The superposition of a 12-km high-velocity and low-density layer at the top of the core, as proposed by Helffrich and Kaneshima [Helffrich, G., Kaneshima, S., 2004. Seismological constraints on core composition from Fe O S liquid immiscibility. Science 306, 2239 2242], improves the fit of waveform amplitudes regardless of the reference model.

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