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| Titel |
A global study of the lowermost mantle using scattered PKKP waves (PKâKP) |
| VerfasserIn |
Daniel Frost, Sebastian Rost, Neil Selby |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250073463
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| Zusammenfassung |
| The short-period (1 Hz) seismic wavefield shows strong evidence for scattered energy
from the interior of the Earth. This energy mainly arrives in the coda following
major seismic phases; however, several ray configurations exist in which seismic
energy from the lowermost mantle arrives as precursors to main phases, allowing
analysis of energy undisturbed by crustal interference. Here we use the phase PKKP
to infer small-scale structure at the Core-Mantle Boundary (CMB) and in the D′′
layer.
PKKP back-scattered at, or above, the CMB (PK-KP where the - represents the location
of scattering) is observed in a time window starting about 1720 s after origin (for a surface
focus) and can be observed from 0° to about 60° epicentral distance. PK-KP is not closely
related to the parent phase PKKP since it arrives in a time and distance range where only
PKKPdf could be observed; nonetheless, due to the high attenuation in the inner core
PKKPdf is rarely detected. The time window used is free from other major and minor arrivals
thus allowing the identification of the scattered PKKP energy, despite its relatively low
amplitude. The ray path of PK-KP is complicated with scattering occurring off great-circle
path, thus avoiding the attenuating inner core. Due to the scattered energy travelling
the major arc of the great-circle path (similar to PKKP), PK-KP waves sample
regions of the Core-Mantle Boundary inaccessible to most other scattering probes
using similar source-receiver combinations. Observations of 1-2 Hz PK-KP energy
indicate that the scatterers are discrete heterogeneities with a scale length of ~10
km.
Here we use the dense, small to medium aperture arrays of the International Monitoring
System of the CTBTO to extract the small amplitude PK-KP from seismic noise. Arrays
increase signal-to-noise ratio and give directivity information allowing the determination of
the scattering location in the lowermost mantle through ray-tracing. We use the
frequency-wavenumber (fk) analysis in conjunction with the F-statistic coherency measure,
commonly used in forensic seismology, to greatly increase the slowness vector resolution of
the small aperture arrays dominating the International Monitoring System. The dataset
consists of ~2800 earthquakes, with magnitudes larger than 6.0, allowing an unprecedented
coverage of the Core-Mantle Boundary, especially in the southern hemisphere, greatly
increasing our knowledge of small-scale (~10 km) heterogeneities in the lowermost
mantle. These results will be essential for our understanding of mantle processes and
dynamics. |
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