11:00 AM - 11:15 AM
[SSS30-08] Analysis of foreshock sequence of the 2014 Mw6.2 Northern Nagano earthquake: Implications for slow-slip transient and unusual source property
Keywords:2014 Northern Nagano earthquake, foreshock, source property, slow slip
In order to investigate the foreshock sequence in more detail, we analyzed seismograms recorded at Hakuba Hi-net station, which is a 632-m deep borehole station located about 5 km west of the foreshock region. By a visual identification of running spectra at the Hakuba station and S-P time, we newly detected 384 foreshocks, which are nearly seven times more than those in the JMA catalogue. We determined their locations and magnitudes on the basis of waveform cross-correlations and amplitude ratios, respectively, between newly detected foreshocks and DD relocated events. Our new catalogue delineated another plane with a N-S striking vertical plane (plane B), which is consistent with one of nodal planes of the P-wave first-motion mechanism of the mainshock. The spatial and temporal distribution of our new catalogue indicates that the foreshock sequence started at the deeper part of the plane A, migrating to the shallower part, and then jumped to the plane B, migrating to the mainshock hypocenter. The migrating speed is less than a few km/day, implying a possible slow-slip transient. A hypothesis is that the foreshock sequence is driven by aseismic slip, which causes stress loading at the mainshock hypocenter and triggers the mainshock. We further determined source parameters of the foreshocks to investigate their fault properties. We applied Multi-Window-Spectral-Ratio method (Imanishi and Ellsworth, 2006) to the foreshocks and aftershocks using the deep borehole data. The estimated corner frequencies of aftershocks decrease with magnitude and indicate constant stress drop. In contrast, the estimated corner frequencies of foreshocks are almost constant over nearly two orders of magnitude. The constant corner frequency suggests that fault dimension is the same regardless of magnitude or stress drop increases with magnitude under an assumption of scale-invariant rupture velocity. It is noted that the same observation was reported for the foreshock sequence of the 1999 Mw7.6 Izmit earthquake, Turkey (Bouchon et al., 2011), which may indicate that the constant corner frequency or the size-dependent stress drop is a common specific property of foreshocks.
Acknowledgements: Seismograph stations used in this study include permanent stations operated by NIED Hi-net, JMA, ERI, and DPRI.