Source rupture processes of the foreshock and mainshock in the 2016 Kumamoto earthquake sequence estimated from the kinematic waveform inversion of strong motion data
© The Author(s) 2016
Received: 17 May 2016
Accepted: 29 July 2016
Published: 31 August 2016
The 2016 Kumamoto earthquake sequence started with an MJMA 6.5 foreshock occurring along the northern part of the Hinagu fault, central Kyushu, Japan, and the MJMA 7.3 mainshock occurred just 28 h after the foreshock. We analyzed the source rupture processes of the foreshock and mainshock by using the kinematic waveform inversion technique on strong motion data. The foreshock was characterized by right-lateral strike-slip occurring on a nearly vertical fault plane along the northern part of the Hinagu fault, and it had two large-slip areas: one near the hypocenter and another at a shallow depth. The rupture of the mainshock started from the deep portion of a northwest-dipping fault plane along the northern part of the Hinagu fault, then continued to transfer to the Futagawa fault. Most of the significant slip occurred on the Futagawa fault, and the shallow portion of the Hinagu fault also had a relatively large slip. The slip amount on the shallowest subfaults along the Futagawa fault was approximately 1–4 m, which is consistent with the emergence of surface breaks associated with this earthquake. Right-lateral strike-slip dominated on the fault segment along the Hinagu fault, but normal-slip components were estimated to make a significant contribution on the fault segment along the Futagawa fault. The large fault-parallel displacements recorded at two near-fault strong motion stations coincided with the spatiotemporal pattern of the fault slip history during the mainshock. The spatial relationship between the rupture areas of the foreshock and mainshock implies a complex fault structure in this region.
Keywords2016 Kumamoto earthquake Source rupture process Kinematic source inversion Strong motion data Futagawa and Hinagu faults
This earthquake sequence occurred along the Futagawa fault zone and the northern part of the Hinagu fault zone in central Kyushu. The Futagawa–Hinagu fault system is one of the major active fault systems on Kyushu Island and is a right-lateral strike-slip fault system. This fault system is thought to be part of the western extension of the Median Tectonic Line, which is the longest active right-lateral strike-slip fault in Japan (e.g., Chida 1992; Okada 1980; Yeats 2012). The average horizontal slip rate of the Futagawa–Hinagu fault system has been 0.88 mm/year in the late Quaternary (Tsutsumi and Okada 1996). Surface breaks caused by the mainshock were found along the Futagawa–Hinagu fault system by emergency field surveys (e.g., Geological Survey of Japan 2016; Okada and Toda 2016). The crustal deformation and surface rupture along the Futagawa–Hinagu fault system were also identified by multiple-aperture interferometry (MAI) analysis using ALOS-2/PALSAR-2 data (Yarai et al. 2016). These observational results imply that the fault rupture is associated with the Futagawa fault and northern part of the Hinagu fault.
This paper focuses on the source rupture processes of the two significant events during the 2016 Kumamoto earthquake sequence based on kinematic waveform inversion analyses using strong motion data. The obtained spatiotemporal source models were compared with reported surface breaks, displacement time histories observed at near-fault strong motion stations, and the seismic activity during this earthquake sequence.
Summary of settings in waveform inversion analysis
Rake angle variation
−164° ± 45°
−142° ± 45°
−142° ± 45°
1 km × 1 km
2 km × 2 km
Number of subfaults
Number of time windows
Duration/time-shift of time window
1.0 s/0.5 s
1.0 s/0.5 s
Target frequency range
Resampling of data
Number of stations
We used strong ground motion data observed by K-NET, KiK-net, and F-net, which are nationwide observation networks operated by NIED (Okada et al. 2004; Aoi et al. 2011), and strong motion data from the JMA seismic intensity observation network (Nishimae 2004). Records from downhole sensors were used for the KiK-net stations. K-NET and JMA have sensors at the ground surface, and F-net sensors are installed in a vault. Original acceleration data were numerically integrated into velocity in the time domain except at the F-net stations, which had a velocity-type strong motion seismograph installed. The S-wave portion of three-component velocity waveforms was used in the analysis. Figure 1 shows a map of the strong motion stations used in the analyses.
Results and discussion
Source rupture process of foreshock
The duration of moment release was approximately 2 s at most subfaults (Fig. 4b), and the total duration of the rupture was approximately 8 s (Fig. 4c). Right-lateral strike-slip dominated the rupture area. A large slip was found near the hypocenter or rupture starting point. The rupture mainly propagated upward and northeastward (Fig. 4c), and another large-slip area ruptured approximately 4 s after the origin time at a depth of 5 km, close to the northeastern end of the Hinagu fault beneath the town of Mashiki, where a seismic intensity of 7 was recorded. The rupture propagation toward the southwest was not significant during this foreshock. Instead, another MJMA 6.4 earthquake occurred at 00:03 JST on April 15, 2016, about 2.5 h after the foreshock at a depth of 6.71 km, close to the southwestern end of the foreshock’s rupture area (see the top-right panel in Fig. 2). The synthetic waveforms reproduced the observed waveforms well (Fig. 4d).
Source rupture process of mainshock
As stated in the Introduction, the acceleration time histories on the ground surface were recorded at two near-fault stations. The locations of these two stations (93048 at Nishihara village hall and 93051 at Mashiki town hall) are indicated in the snapshots in Fig. 6 and the map in Fig. 1. These two stations were located within 2 km of the surface trace of the Futagawa fault. Thus, the fault-parallel motion at these stations can be considered to resemble the nearby fault motion. We calculated the displacement waveforms from the observed acceleration records by double integration in the time domain with the appropriate baseline correction but did not perform any filtering of these data. Figure 6 shows the comparison of the particle motions on the horizontal plane obtained from these displacement waveforms with the rupture snapshots for each timeframe. Because the timing and amount of the fault-parallel displacements coincided with the spatiotemporal slip history estimated by the kinematic waveform inversion of the strong motion data, we confirmed that the obtained source model represents the spatiotemporal slip history during this event.
Spatial relationships among foreshock, mainshock, and aftershocks
The large slip of the foreshock and small events immediately after this foreshock also seem to have had a complementary distribution in space (Fig. 8b). Although the fault plane of the foreshock was partially overlain with that of the mainshock, the exact locations of large slips at shallow depth were not the same as those of the mainshock.
Fault segment #1 of the mainshock spatially overlapped the rupture area of the foreshock but had a different dip angle from the fault plane of the foreshock. That is, the rupture of the mainshock was initiated on another fault plane closely parallel to the fault plane of the foreshock along the Hinagu fault, as expected from the difference in spatial patterns of the hypocenters before and after the mainshock (Fig. 2), and continued to be transferred to the Futagawa fault. This implies a complex fault structure along the Futagawa–Hinagu fault system. In order to examine this hypothesis, further studies on aftershock relocation and reflection surveys to image the complex structure of fault planes in the source region are necessary.
The MW 7.9 Denali earthquake on November 3, 2002, was an inland crustal earthquake along the Denali fault system, Alaska, and was preceded by the MW 6.7 Nenana Mountain earthquake on October 23, 2002. The aftershocks of the Nenana Mountain earthquake formed a vertical plane along the Denali fault system. The rupture of the Denali earthquake started on the Susitna Glacier fault, which is a splay fault south of the McKinley strand of the Denali fault system, where the Nenana Mountain earthquake occurred. It then propagated eastward along the main strand of the Denali fault system. (e.g., Ratchkovski et al. 2004). The spatial and temporal relationships between the foreshock and mainshock of the 2016 Kumamoto earthquake sequence look similar to those of the 2002 Denali earthquake sequence.
The source rupture processes of the foreshock and mainshock in the 2016 Kumamoto earthquake sequence were estimated by kinematic waveform inversion of strong motion data. The foreshock was a right-lateral strike-slip event occurring on a nearly vertical fault plane along the northern part of the Hinagu fault, and two large-slip areas were found near the hypocenter and at shallow depth. The rupture of the mainshock started from the deep portion of a northwest-dipping fault plane along the Hinagu fault. Then, it continued to be transferred to the Futagawa fault and propagated northeastward and upward to generate significant slips with surface breaks. The peak slip of the mainshock was 5.1 m, including the normal component of the slip, and the duration of the rupture was approximately 20 s. The slip amount of the shallowest subfaults along the Futagawa fault was approximately 1–4 m, which is in rough agreement with the emergence of surface breaks associated with the mainshock. The large fault-parallel displacements at two near-fault stations coincided with the spatiotemporal pattern of the fault slip history during the mainshock. The spatial relationship between the rupture areas of the foreshock and mainshock implies a complex fault structure in this region. The central and southern parts of the Hinagu fault were not ruptured during this earthquake sequence.
KA analyzed the data and drafted the manuscript. TI participated in the design of the study and processed the displacement waveforms. Both authors read and approved the final manuscript.
Strong motion data from K-NET, KiK-net, and F-net were provided by the National Research Institute for Earth Science and Disaster Resilience, Japan. The strong motion data from the seismic intensity observation network were released by the Japan Meteorological Agency and Kumamoto prefectural government. The JMA unified earthquake catalog is produced by JMA in cooperation with the Ministry of Education, Culture, Sports, Science and Technology (MEXT). We thank two anonymous reviewers and guest editor Haruo Horikawa for their helpful comments and suggestions on improving the manuscript. Generic Mapping Tools (Wessel and Smith 1998) was used to draw the figures. This study was supported by a Grant-in-Aid for Special Purposes (16H06298, PI Prof. Hiroshi Shimizu) from MEXT and by the Earthquake and Volcano Hazards Observation and Research Program of MEXT.
The authors declare that they have no competing interests.
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