Abstract
We present the initial results of rapid fault estimations for the 2016 Kumamoto earthquake on April 16 (M j 7.3), and coseismic displacements caused by the two large foreshocks that occurred on April 14 (M j 6.5) and April 15 (M j 6.4) from the GEONET real-time analysis system (REGARD), which is based on a Global Navigation Satellite System (GNSS) kinematic positioning technique. The real-time finite-fault estimate (M w 6.85) was obtained within 1 min and converged to M w 6.96 within 5 min of the origin time of the mainshock (M j 7.3). The finite-fault estimate shows right-lateral strike-slip fault along the Futagawa fault segment, which is consistent with the finite-fault model inferred from post-processed GNSS and InSAR analysis. Furthermore, significant coseismic displacements were observed due to the April 14 and April 15 foreshocks at nearby sites, though these earthquakes were smaller than the pre-assigned system threshold. Our results also demonstrate the potential for the GNSS-based earthquake early warning system for inland earthquakes.
Highlights
The mainshock of the 2016 Kumamoto earthquakes (Mj 7.3) occurred at 01:25 on April 16, 2016 (Japan Standard Time, +9 GMT; hereafter the April 16 mainshock [Mj 7.3]), and two large foreshocks (Mj 6.5, Mj 6.4; hereafter the April foreshock [Mj 6.5] and the April foreshock [Mj 6.4]) occurred around the northern part of the Hinagu fault segment at 21:26 on April (JST) and at 00:03 on April (JST), respectively (Fig. 1; Japan Meteorological Agency [JMA] 2016a)
Real-time fault model inversions were not carried out because their magnitude and maximum displacement were lower than the assigned threshold of the system
The finite-fault estimate with Mw 6.96 was consistent with both the fault models of Yarai et al (2016), which is inferred from static offsets observed by postprocessed Global Navigation Satellite System (GNSS) positioning and InSAR analysis, and the JMA CMT solution (JMA 2016c)
Summary
The mainshock of the 2016 Kumamoto earthquakes (Mj 7.3) occurred at 01:25 on April 16, 2016 (Japan Standard Time, +9 GMT; hereafter the April 16 mainshock [Mj 7.3]), and two large foreshocks (Mj 6.5, Mj 6.4; hereafter the April foreshock [Mj 6.5] and the April foreshock [Mj 6.4]) occurred around the northern part of the Hinagu fault segment at 21:26 on April (JST) and at 00:03 on April (JST), respectively (Fig. 1; Japan Meteorological Agency [JMA] 2016a). These successive earthquakes occurred along the Futagawa–Hinagu fault zone, which is a well-known active fault zone in Kyushu Island of western Japan. The rapid characterization of both hypocenter and source extent is extremely important because earthquakes can result in secondary disasters (e.g., tsunamis, landslides, fires, and large aftershocks), which often occur just above the source area of the earthquake
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