Systematic difference between first-motion and waveform-inversion solutions for shallow offshore earthquakes due to a low-angle dipping slab

Shunsuke Takemura,Takeshi Kimura,Katsuhiko Shiomi,Tatsuhiko Saito

doi:10.1186/s40623-016-0527-9

Shunsuke Takemura, Takeshi Kimura + Show 2 more

Open Access

https://doi.org/10.1186/s40623-016-0527-9

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Abstract

Systematic difference between first-motion and waveform-inversion solutions for shallow offshore earthquakes was examined by using the seismograms of the 2016 Off Mie (Mw 5.8) earthquake occurred at a depth of 14 km southeast off of the Kii peninsula, central Japan. Observed seismograms illustrated first arrivals with an apparent velocity of 7.2 km/s, which is faster than crustal P waves. The apparent velocity and polarization pattern of the first arrivals were reproduced by a finite-difference method simulation incorporating the three-dimensional Philippine Sea slab. The first arrivals consist of P waves radiated downward from the source, passing the oceanic Moho as head waves. Thus, a first-motion analysis, assuming a one-dimensional structure, causes incorrect estimations of the focal mechanisms and hypocenter depths, which tend to be deeper than the actual ones. Our result possibly indicates that the seismicity above the oceanic Moho was underestimated in the previous catalogs.

Highlights

The Philippine Sea slab (PHS) is subducting beneath southwestern Japan along the Nankai Trough at a rate of 2–6 cm per year (e.g., Seno et al 1993; Heki and Miyazaki 2001)
Using Hi-net waveforms during the 2016 Off Mie earthquake, we propose that the misestimation of focal mechanisms and depths for first-motion polarization analysis is caused by subsurface structure related to the geometry of the low-angle dipping slab
We conclude that the systematic difference between firstmotion and waveform-inversion solutions for shallow offshore earthquakes is mainly caused by the subducting PHS, which generates a PPHS phase with an apparent velocity of 7.2 km/s and causes the misestimations of the takeoff angles and hypocenter depths

Summary

Introduction

The Philippine Sea slab (PHS) is subducting beneath southwestern Japan along the Nankai Trough at a rate of 2–6 cm per year (e.g., Seno et al 1993; Heki and Miyazaki 2001). Using Hi-net waveforms during the 2016 Off Mie earthquake, we propose that the misestimation of focal mechanisms and depths for first-motion polarization analysis is caused by subsurface structure related to the geometry of the low-angle dipping slab.

Results

Conclusion