Complex migration pattern and size-dependent stress drops of foreshocks of 2014 Mw 6.2 Northern Nagano earthquake

Abstract:

The 2014 M_w 6.2 (M_JMA 6.7) Northern Nagano, central Japan, earthquake occurred on 22 November, 2014 which broke a northern part of the Itoigawa-Shizuoka Tectonic line active fault system. One of the features of this earthquake is the occurrence of foreshock sequence from four days before the mainshock (about 50 events in JMA catalogue), which were captured by a dense permanent seismic observation. In particular, a 632-m deep borehole station (Hakuba of NIED Hi-net) located about 5 km west of the foreshock region provides us an unprecedented opportunity to investigate the detail of the foreshock sequence. The double-difference relocation of events listed in the JMA catalogue reveals that the foreshocks were located at a depth of 3-4 km and distributed on a NNW dipping 1 km x 1 km plane (plane A) distinct from the aftershock distribution. By a visual identification of running spectra at the Hakuba station, we newly detected about 400 foreshocks. We determined their locations and magnitudes on the basis of a waveform cross-correlation between newly detected foreshocks and DD relocated events. Our new catalogue indicates that the foreshock sequence started at the deeper part of the plane A, migrated to the shallower part, and then jumped to a N-S striking vertical plane (plane B) 1 day before the mainshock. The feature is also supported by the foreshock focal mechanisms determined by P-wave polarities as well as body-wave amplitudes. It is noted that the plane B is consistent with one of nodal planes of the P-wave first-motion mechanism of the mainshock. We further estimated stress drops of the fore- and aftershocks based on the Multi-Window-Spectral-Ratio method, showing that both sequences have stress drops ranging approximately from 0.1 to 10 MPa. The foreshock sequence shows the size-dependent stress drops, while the aftershocks have scale-invariant stress drops as often seen in ordinary earthquakes. This may suggest different rupture properties in the foreshocks, compared with the aftershocks and other ordinary earthquakes.

Acknowledgements: Seismograph stations used in this study include permanent stations operated by NIED Hi-net, JMA, ERI, and DPRI.