Journal
EARTH PLANETS AND SPACE
Volume 68, Issue -, Pages -Publisher
SPRINGEROPEN
DOI: 10.1186/s40623-016-0527-9
Keywords
Focal mechanism; First-motion polarization; Head wave; Nankai Trough; Philippine Sea slab
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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.
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