Abstract
Seismic wave propagation from shallow subduction-zone earthquakes can be strongly affected by 3D heterogeneous structures, such as oceanic water and sedimentary layers with irregular thicknesses. Synthetic waveforms must incorporate these effects so that they reproduce the characteristics of the observed waveforms properly. In this paper, we evaluate the accuracy of synthetic waveforms for small earthquakes in the source area of the 2011 Tohoku–Oki earthquake (MJMA 9.0) at the Japan Trench. We compute the synthetic waveforms on the basis of a land–ocean unified 3D structure model using our heterogeneity, oceanic layer, and topography finite-difference method. In estimating the source parameters, we apply the first-motion augmented moment tensor (FAMT) method that we have recently proposed to minimize biases due to inappropriate source parameters. We find that, among several estimates, only the FAMT solutions are located very near the plate interface, which demonstrates the importance of using a 3D model for ensuring the self-consistency of the structure model, source position, and source mechanisms. Using several different filter passbands, we find that the full waveforms with periods longer than about 10 s can be reproduced well, while the degree of waveform fitting becomes worse for periods shorter than about 10 s. At periods around 4 s, the initial body waveforms can be modeled, but the later large-amplitude surface waves are difficult to reproduce correctly. The degree of waveform fitting depends on the source location, with better fittings for deep sources near land. We further examine the 3D sensitivity kernels: for the period of 12.8 s, the kernel shows a symmetric pattern with respect to the straight path between the source and the station, while for the period of 6.1 s, a curved pattern is obtained. Also, the range of the sensitive area becomes shallower for the latter case. Such a 3D spatial pattern cannot be predicted by 1D Earth models and indicates the strong effects of 3D heterogeneity on short-period ({ lesssim };10 {text{s}}) waveforms. Thus, it would be necessary to consider such 3D effects when improving the structure and source models.
Highlights
Seismic wave propagation from shallow subductionzone earthquakes can be strongly affected by threedimensionally heterogeneous structures, such as oceanic water and sedimentary layers with irregular thicknesses (e.g., Okamoto 2002; Nakamura et al 2012, 2015)
We investigate the accuracy of synthetic waveforms for small suboceanic earthquakes in the Japan Trench area, in connection with the 2011 Tohoku–Oki earthquake (MJMA 9.0: Fig. 1a), as the effect of 3D heterogeneities on the waveforms would be different depending on the source location on the vast fault plane of the 2011 Tohoku–Oki earthquake
Results of first-motion augmented moment tensor (FAMT) Analysis We find that the best origin time correction to the Japan Meteorological Agency (JMA) origin times is 0 s for both events (Additional file 1: Fig. S1)
Summary
Seismic wave propagation from shallow subductionzone earthquakes can be strongly affected by threedimensionally heterogeneous structures, such as oceanic water and sedimentary layers with irregular thicknesses (e.g., Okamoto 2002; Nakamura et al 2012, 2015). FAMT analysis uses the initial part of the short-period waveforms in addition to the long-period full waveforms in order to augment the information based on the body wave first motions.
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