Abstract
Abstract We deployed an extremely dense temporal seismic network in the source region of the 2004 mid-Niigata Prefecture Earthquake (thrust fault) on October 23, 2004, Japan. The seismic network consisted of 145 temporary seismic stations within a 30 km squared and had been kept within approximately a month after the mainshock. High accurate hypocenters of 708 events were determined by inverting the arrival times using double-difference earthquake location algorithm. The aftershocks along the mainshock (M w = 6.6) and the largest aftershock (M w = 6.3) rupture zones are distributed on two 60° westward-dipping planes, located approximately 5 km apart. Conversely, the Oct. 27 aftershock (M w = 5.8) occurred on an eastward dipping plane with a dip angle of 25° that was conjugate to the mainshock fault plane. Most of aftershocks at both northeastern and southwestern edges occurred at shallow depths with eastward-dipping planes. Epicenters of aftershocks in the southwestern region are aligned along N15°E, and rotate approximately 20° counterclockwise from the strike of the mainshock fault. This rotation of the aftershock alignments coincides with the rotation of anticline axes in the southwestern area of the source region. Furthermore, distributions of station corrections for a one-dimensional velocity model suggest that the seismic velocity at the western side of the Muikamachi-fault is lower than that at the eastern side. It is also inferred that the velocity structures in the hangingwall vary along the fault strike. The average velocity in the mainshock rupture area is higher than the periphery in the hangingwall, especially compared with the southwestern side of the hypocenter.
Highlights
The 2004 mid-Niigata Prefecture Earthquake of Mw 6.6 occurred in the backarc area of the main Japanese Island as a shallow inland earthquake at 17:56 (JST = UT + 9 hours) on October 23, 2004, causing serious seismic damage and landslides in and around the source region
This rotation of the aftershock alignments coincides with the rotation of anticline axes (HA and TA in Fig. 5) in the southwestern area of the source region
The depth sections of aftershocks reveal that the mainshock and the largest aftershock occurred on two 60◦ westward-dipping planes, located approximately 5 km apart (Fig. 6)
Summary
High-resolution hypocenters of 708 events are determined by the double-difference relocation algorithm (Waldhauser and Ellsworth, 2000) using differential arrival times obtained by both the manually-picked and the waveform cross-correlation method. We used differential arrival times obtained by the manually-picked and waveform correlation method In this analysis, we used the 708 aftershocks listed in the JMA catalog (for the period between Oct. 27 and Nov. 23) and observed by the very dense seismic network. The threshold criteria for calculation of the double difference data; the normalized cross-correlation coefficient (Cc: timedomain) Cc >85% After applying this cut-off, we were left with a data set of more accurate differential arrival times that contained 199,261 P-wave observations and 104,202 S-wave observations for use in hypoDD algorithm. Relative location errors in the horizontal and vertical directions are averaged to be 40 m and 80 m, respectively
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