1] We measured PP-P differential travel times on broadband seismograms of the Incorporated Research Institutions for Seismology (IRIS) network. The measurement was made by cross-correlating the observed PP waveform with the synthetic PP calculated from the observed P waveform. The effect of seawater reverberation on the measurement is taken into account if the PP bounce point is under ocean. The PP-P times measured after low-pass filtering is systematically shorter than those measured on raw broadband seismograms. We show that low-pass filtering tends to merge the precursory and postcursory phases into the main PP phase so that the measured PP-P times are artificially short. A cutoff frequency as high as 0.5 Hz is required to isolate the main PP. With such a high cutoff frequency, we obtained a total of about 7000 PP-P times. The residuals relative to a reference P velocity model are consistent with those calculated for the aspherical model WEPP2 [Obayashi et al., 1997] when the PP bounce points are located in the region well resolved by WEPP2. The PP-P times, combined with 2 million International Seismological Centre (ISC) first arrival times, were inverted for new model WEPP2 0 . This model renders a variance reduction of PP-P residuals of 63% without compromising the fit to P residuals used originally in WEPP2. In WEPP2 0 , the low velocity anomaly beneath Hawaii is limited in depth down to the uppermost part of the lower mantle. The high velocity anomaly of the Russian continental lithosphere is underlain in the mantle transition zone by a body of low velocity anomaly.INDEX TERMS: 7207 Seismology: Core and mantle; 7218 Seismology: Lithosphere and upper mantle; 8120 Tectonophysics: Dynamics of lithosphere and mantle-general; KEYWORDS: seismic tomography, plume structure, lithosphere structure Citation: Fukao, Y., A. To, and M. Obayashi, Whole mantle P wave tomography using P and PP-P data,
S U M M A R YWe present an extension to the coupling scheme of the spectral element method (SEM) with a normal-mode solution in spherical geometry. This extension allows us to consider a thin spherical shell of spectral elements between two modal solutions above and below. The SEM is based on a high-order variational formulation in space and a second-order explicit scheme in time. It combines the geometrical flexibility of the classical finite-element method with the exponential convergence rate associated with spectral techniques. In the inner sphere and outer shell, the solution is sought in terms of a modal solution in the frequency domain after expansion on the spherical harmonics basis. The SEM has been shown to obtain excellent accuracy in solving the wave equation in complex media but is still numerically expensive for the whole Earth for high-frequency simulations. On the other hand, modal solutions are well known and numerically cheap in spherically symmetric models. By combining these two methods we take advantage of both, allowing high-frequency simulations in global Earth models with 3-D structure in a limited depth range. Within the spectral element method, the coupling is introduced via a dynamic interface operator, a Dirichlet-to-Neumann operator which can be explicitly constructed in the frequency and generalized spherical harmonics domain using modal solutions in the inner sphere and outer shell. The presence of the source and receivers in the top modal solution shell requires some special treatment. The accuracy of the method is checked against the mode summation method in simple spherically symmetric models and shows very good agreement for all type of waves, including diffracted waves travelling on the coupling boundary. A first simulation in a 3-D D -layer model based on the tomographic model SAW24b16 is presented up to a corner frequency of 1/12 s. The comparison with data shows surprisingly good results for the 3-D model even when the observed waveform amplitudes differ significantly from those predicted in the spherically symmetric reference model (PREM).
a b s t r a c tA large number of shallow low frequency events were recorded after the 2011 Mw 9.0 Tohoku-oki earthquake by the cabled network of broadband ocean bottom seismometers (DONET) deployed in the eastern part of the Nankai trough. This low frequency event activity was intense for the first few days after the great earthquake and gradually decreased. Signals of the events are most clearly visible at the frequency range around 2-8 Hz. Some of the events are accompanied by a very long frequency (VLF) signal, which is clearly observed at around 0.02-0.05 Hz. The magnitude and source duration estimated by waveform analysis for one of the largest very low frequency earthquakes (VLFEs) was 3.0-3.5 and 17 s. This source duration is extremely long compared to ordinary earthquakes of comparable magnitude. These newly detected VLFEs are likely to be normal fault earthquakes located at shallow depths within the accretionary prism, in contrast to the previously reported VLFEs that were explained by a low angle thrusting along the decollement zone. On the other hand, the low frequency events with no clear VLF signal were previously regarded as being low frequency tremors (LFTs). We show that events with and without the VLF signal likely represent the same phenomenon, and the VLF signal is only observed when a large magnitude event occurs near the station. The waveforms of VLFEs are characterized by the coexistence of long source duration and high-frequency radiation of signals, and such features were previously explained by the co-occurrence of shear failure and hydrofractures under the influence of fluid brought into the decollement zone. Our result indicates that the stress state and the mechanical environment, which promote the occurrence of VLFEs, exist not only along the decollement zone but also in the shallower part of the accretionary prism.
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