地震　第2輯

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We analyze GPS high sampling records of GEONET and a continuous stress record of the Tono Research Institute of Earthquake Science in Japan observed from the 2011 M_w9.1 Tohoku Earthquake. The most dominant feature in the GPS high sampling records is a main pulse with amplitude of a few meters and a width of around 100 s, the overall feature of which is elucidated by synthetics in a half space with a rectangular fault model with a moment of 2.8×10²³ Nm (M_w8.9), starting around 35 s after the JMA origin time. The next dominant feature is a sub-pulse with amplitude of up to a meter and a width of 25-35 s, within the wavetrain of the main pulse, appearing in a transverse component around 25 s after the onset of the main pulse. Record sections of the transverse component reduced by S wave velocity of 3.9 km/s along N30W-N150W directions clearly indicate that the sub-pulse was an SH wave that radiated off the coast of Miyagi Prefecture within the source region of the 2011 Tohoku earthquake. Matching the sub-pulse with synthetics by trial and error, we obtain an extraordinary sub-event model of a left lateral strike slip fault, striking along the N145E direction, which we call a super sub-event. The slip velocity and seismic moment for the super sub-event are around 10 m/s and (4-5)×10²² Nm (M_w8.4), respectively, while the slip and area of faulting cannot be separately solved because of a trade-off between them.

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