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[SSS06-P06] Frequency dependence of waveguide/anti-waveguide effects in a heterogeneous slab: A numerical investigation
Keywords:Numerical simulation of seismic wave propagation, Waveguide/Anti-waveguide
The high-frequency seismic waves are trapped within the subducting slab because of the heterogeneous structure of seismic wave velocity in the slab, which is called as a waveguide effect. Due to the waveguide effect, coda waves at the frequency of above 2 Hz and wave dispersion are observed. On the other hand, the seismic wave velocity distribution inside the slab also has the effect of driving the seismic waves out from the slab, called anti-waveguide (e.g., Furumura and Kennett, 2005). For example, in the December 2020 Okhotsk deep-focus earthquake (M6.7, 619 km), anomalous seismic zones were observed at stations on the Pacific side of Hokkaido. In addition, a waveform with abnormally large amplitude was observed at a station near Wakkanai (located in the northern part of Hokkaido), which is close to the epicenter distance. The large amplitudes dominant at a frequency of less than 1 Hz suggest that both the waveguide and the anti-waveguide did in effect. However, it is not well understood whether the contribution of waveguide or anti-waveguide is larger.
This study investigates the frequency dependence of waveguide and anti-waveguide by a numerical simulation of 2D P-SV seismic wave propagation by means of the finite difference method in a randomly heterogeneous medium. The simulation range covers 300 km in the horizontal direction and 400 km in the vertical direction, respectively. We used a horizontally stratified structure, assuming the velocities at the mantle wedge, the slab, and the mantle from the shallow part. We locate the seismic source that radiates waves isotropically in the layer corresponding to the slab. We superimposed a velocity fluctuation described by the von Karman function with a velocity fluctuation RMS of 10% and a correlation length of 10 km along with the slab and 0.5 km in the orthogonal direction. We set receivers in two different directions: (1) horizontally in the slab layer at intervals of 30 km for studying the distance dependence of the waveguide effect in the slab, and (2) concentric circles of 15 degrees each at equidistant from the epicenter in the mantle wedge layer for studying the angular dependence of the anti-waveguide effect on the slab.
We observed the differences of the waveforms and spectrograms at each station between directions (1) and (2). For the direction (1), waves at the frequency of lower than 1 Hz propagate faster than those at the frequency of higher than 2 Hz in both radial and transverse components. This dispersive characteristic became stronger with distance from the epicenter. In addition, the amplitude of the coda wave was particularly large at frequencies higher than 2 Hz. On the other hand, for direction (2), a dispersive effect related to the head wave at the frequency of lower than 1 Hz was observed at an angle of 15 degrees with the slab, but no such feature was observed at other stations. The amplitude of the high-frequency coda wave was larger at stations with an angle of less than 45 degrees with the slab. However, at the other stations with an angle of more than 45 degrees, there was no significant difference between the amplitudes at the low and high frequencies. As a result of the angular dependence of the anti-waveguide effect on the slab in direction (2), we observed that at stations close to the slab, high frequencies contributed more to the large amplitude than low frequencies. In contrast, at stations far from the slab, both low and high frequencies contributed to the large amplitude.
This study investigates the frequency dependence of waveguide and anti-waveguide by a numerical simulation of 2D P-SV seismic wave propagation by means of the finite difference method in a randomly heterogeneous medium. The simulation range covers 300 km in the horizontal direction and 400 km in the vertical direction, respectively. We used a horizontally stratified structure, assuming the velocities at the mantle wedge, the slab, and the mantle from the shallow part. We locate the seismic source that radiates waves isotropically in the layer corresponding to the slab. We superimposed a velocity fluctuation described by the von Karman function with a velocity fluctuation RMS of 10% and a correlation length of 10 km along with the slab and 0.5 km in the orthogonal direction. We set receivers in two different directions: (1) horizontally in the slab layer at intervals of 30 km for studying the distance dependence of the waveguide effect in the slab, and (2) concentric circles of 15 degrees each at equidistant from the epicenter in the mantle wedge layer for studying the angular dependence of the anti-waveguide effect on the slab.
We observed the differences of the waveforms and spectrograms at each station between directions (1) and (2). For the direction (1), waves at the frequency of lower than 1 Hz propagate faster than those at the frequency of higher than 2 Hz in both radial and transverse components. This dispersive characteristic became stronger with distance from the epicenter. In addition, the amplitude of the coda wave was particularly large at frequencies higher than 2 Hz. On the other hand, for direction (2), a dispersive effect related to the head wave at the frequency of lower than 1 Hz was observed at an angle of 15 degrees with the slab, but no such feature was observed at other stations. The amplitude of the high-frequency coda wave was larger at stations with an angle of less than 45 degrees with the slab. However, at the other stations with an angle of more than 45 degrees, there was no significant difference between the amplitudes at the low and high frequencies. As a result of the angular dependence of the anti-waveguide effect on the slab in direction (2), we observed that at stations close to the slab, high frequencies contributed more to the large amplitude than low frequencies. In contrast, at stations far from the slab, both low and high frequencies contributed to the large amplitude.