Finite frequency whole mantle P wave tomography: Improvement of subducted slab images

Abstract

[1] We present a new whole mantle P wave tomographic model GAP_P4. We used two data groups; short-period data of more than 10 million picked-up onset times and long-period data of more than 20 thousand differential travel times measured by waveform cross correlation. Finite frequency kernels were calculated at the corresponding frequency bands for both long-and short-period data. With respect to an earlier model GAP_P2, we find important improvements especially in the transition zone and uppermost lower mantle beneath the South China Sea and the southern Philippine Sea owing to broadband ocean bottom seismometers (BBOBSs) deployed in the western Pacific Ocean where station coverage is poor. This new model is different from a model in which the full data set is interpreted with classical ray theory. BBOBS observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments.

Seismic evidence for a steeply dipping reflector—stagnant slab in the mantle transition zone

“…A weak velocity anomaly was apparent in their regional tomography studies that appears to correspond to the reflector in location 1. A significant improvement in resolution of this area was shown recently in Obayashi et al (2013) and . By combining ray theory and finite frequency kernels for broadband waveform data, they were able to image slabs in the MTZ with unprecedented clarity.…”

“…Only location '1' fits all observations. the dedicated deployment of broadband ocean bottom seismometer networks supported by the Japanese project 'Stagnant Slab Project' (Miyamachi et al 2009;Shiobara et al 2009) and the development of new tomographic methods Obayashi et al 2013). So, after this forced hiatus, we present here our original findings, which still constitute key observations due to the scarcity of deep large events in this region, augmented by new data from two additional events recorded at additional arrays (Section 4.1) and several new figures , which show that the results from our array analysis, using both P and S waves in contrast to previous publications, provide a location for a stranded slab that is in good agreement with recent high-resolution tomography.…”

Effects of shallow-layer reverberation on measurement of teleseismic P-wave travel times for ocean bottom seismograph data

“…Next, we dealt with the relative P-wave travel times among stations in a seismic array, which have often been measured and used for recent seismic tomography (Wolfe et al 2009(Wolfe et al , 2011Obayashi et al 2013Obayashi et al , 2016. We examined the effects of shallow-layer reverberation on the relative travel times and methods for correcting the effects of reverberation in the study of P-wave tomography for the mantle structure.…”

“…Furthermore, filtering out tidal noise complicates the interference of teleseismic body waves with the shallow-layer reverberation, which could result in a waveform distortion and a bias in travel time measurements by waveform cross-correlation of teleseismic body waves (Blackman et al 1995). This bias is frequency dependent, which may cause a bias in mantle images obtained by finite-frequency tomography using BBOBS data (Obayashi et al 2013(Obayashi et al , 2016. Obayashi et al (2004) addressed the effects of shallow-layer reverberation on PP-phases in PP-P differential time measurements.…”

“…In particular, we estimated the effects on P-wave travel times measured with waveform cross-correlation and their frequency dependence by calculating synthetic seismograms using realistic shallow-layer models. In addition, we estimated the shallow-layer effect on the measurement of relative travel times of teleseismic P-waves recorded by an oceanic seismic array, which is a common method to extract the travel time information of teleseismic P-waves from BBOBS data (e.g., Wolfe et al 2009Wolfe et al , 2011Obayashi et al 2013Obayashi et al , 2016. We were then able to propose a simple method to correct the relative travel times for shallow-layer reverberation.…”