Satoru Tanaka

Very broadband analysis of a swarm of very low frequency earthquakes and tremors beneath Kii Peninsula, SW Japan

[1]xa0We have conducted a temporal broadband seismic observation in Kii Peninsula, southwest Japan, and detected a swarm of 110 very low frequency earthquakes (VLFs) and deep low frequency tremors. During three days of the swarm activity, VLFs and tremors occur concurrently in two localized regions separated by ∼10 km. Stacking analyses are also employed to detect VLF signals of a period longer than 50 s, whose focal mechanisms are determined for the first time from data and shown to be consistent with the subducting plate motion. Evaluation of the VLF seismic moment implies that a substantial portion of the SSE seismic moment is released as a VLF swarm.

Upper mantle tomography in the northwestern Pacific region using triplicated P waves

We conducted delay time tomography of the upper mantle beneath the northwestern Pacific using P data from NorthEast China Extended SeiSmic Array, F-net, and nearby available stations. To improve resolution and accuracy in the vicinity of mantle discontinuities, we extracted traveltimes of both initial and secondary phases observed at triplication distances by using a waveform fitting technique. Compared with the model obtained by using only the initial phase, the resolution just above the 410 km discontinuity is especially improved, and low-velocity anomalies beneath the Changbai Volcano are clearly observed down to the 410 km discontinuity. Compared with previous models, low-velocity anomalies atop the 410 are more pronounced. The results of this study together with the previous receiver function analysis provide further support that we have hot material beneath the Changbai Volcano.

Constraints on the core-mantle boundary topography from P4KP-PcP differential travel times

[1]xa0P4KP-PcP differential travel times are examined to infer the core-mantle boundary (CMB) topography. A total of 362 P4KP-PcP times are measured with a measurement error of 0.5 s. The travel times are corrected for the Earths hydrostatic ellipticity and mantle heterogeneity using a P-wave tomographic model. Spherical harmonic expansion up to degree 4 is adopted for model parameterization. The P-wave velocity heterogeneity is then estimated in the lowermost 150 km of the mantle to overcome the problem of underestimation of the velocity perturbation at the base of the mantle in the global 3-D P-wave mantle model. Subsequently, the CMB topography is inferred using the residues of the above processes. Since the odd-degree components of the CMB topography are insensitive to the P4KP-PcP times, only the components of degrees 2 and 4 are solved for. The resultant features indicate that the maximum amplitude of the CMB topography does not exceed ±2 km, with an uncertainty of less than 0.5 km. A numerical test confirms that the pattern of degree 4 is more reliable with less amplitude recovery. The obtained degree 4 pattern shows an amplitude of less than ±1 km and indicates the presence of depressions under the circum-Pacific, the central Pacific, and South Africa.

Preface for the article collection of “Multidisciplinary Researches on Deep Interiors of the Earth and Planets”

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One-dimensional modeling of multiple scattering in the upper inner core: Depth extent of a scattering region in the Eastern Hemisphere

Attenuation of PKP(DF) in the Eastern Hemisphere is examined in terms of multiple scattering to simultaneously explain a puzzling relationship, a relatively fast velocity anomaly corresponding to strong attenuation. Reflectivity synthetics with one-dimensional random velocity fluctuations are compared with observations of PKP(DF)/PKP(Cdiff) amplitude ratios and differential travel times of PKP(Cdiff)-PKP(DF) for the equatorial paths. A Gaussian distribution of P-wave velocity fluctuations with the standard deviations of 5%, 6%, and 7% in the uppermost 200 km of the inner core is superimposed on the velocity structure that is slightly faster than the typical structure in the Eastern Hemisphere, which is likely to explain both the travel time and amplitude data as far as only the one-dimensional structure is considered. Further examinations of the statistic characteristic of scatterer distribution in two and three-dimensions are required to obtain a realistic conclusion.