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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- Akaike H (1980) Likelihood and the Bayes procedure. Trab Estad Invest Oper 31:143–166View ArticleGoogle Scholar
- Aoi S, Kunugi T, Nakamura H, Fujiwara H (2011) Deployment of new strong motion seismographs of K-NET and KiK-net. In: Akkar S, Gülkan P, van Eck T (eds) Earthquake data in engineering seismology. Geotechnical, geological, and earthquake engineering, vol 14. Springer, Dordrecht, pp 167–186View ArticleGoogle Scholar
- Asano K, Iwata T (2009) Source rupture process of the 2004 Chuetsu, Mid-Niigata prefecture, Japan, earthquake inferred from waveform inversion with dense strong-motion data. Bull Seismol Soc Am 99:123–140View ArticleGoogle Scholar
- Asano K, Iwata T, Irikura K (2005) Estimation of source rupture process and strong ground motion simulation of the 2002 Denali, Alaska, earthquake. Bull Seismol Soc Am 95:1701–1715View ArticleGoogle Scholar
- Bouchon M (1981) A simple method to calculate Green’s function for elastic layered media. Bull Seismol Soc Am 71:959–971Google Scholar
- Burridge R, Knopoff L (1964) Body force equivalents for seismic dislocations. Bull Seismol Soc Am 54:1875–1888Google Scholar
- Chida N (1992) Active faults in Central Kyushu, Southwest Japan—Quaternary faulting along the Median Tectonic Line in Kyushu. Mem Geol Soc Jpn 40:39–51 (in Japanese with English abstract) Google Scholar
- Ekström G, Nettles M, Dziewonski AM (2012) The Global CMT Project 2004–2010: Centroid-moment tensors for 13,017 earthquakes. Phys Earth Planet Inter 200–201:1–9View ArticleGoogle Scholar
- Fukuyama E, Ishida M, Dreger DS, Kawai H (1998) Automated seismic moment tensor determination by using on-line broadband seismic waveforms. J Seismol Soc Jpn 51:149–156 (in Japanese with English abstract) Google Scholar
- Geological Survey of Japan, AIST (2016) 2016 Kumamoto earthquakes: report 4. GSJ Chishitsu News 5:169–174 (in Japanese) Google Scholar
- Hartzell SH, Heaton T (1983) Inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 Imperial Valley, California, earthquake. Bull Seismol Soc Am 73:1553–1583Google Scholar
- Hisada Y, Bielak J (2003) A theoretical method for computing near-fault ground motions in layered half-spaces considering static offset due to surface faulting, with a physical interpretation of fling step and rupture directivity. Bull Seismol Soc Am 93:1154–1168View ArticleGoogle Scholar
- Hoshizumi H, Ozaki M, Miyazaki K, Matsuura H, Toshimitsu S, Uto K, Utsumi S, Komazawa M, Hiroshima T, Sudo S (2004) Geological Map of Japan 1:200,000, Kumamoto, Geological Survey of Japan, AISTGoogle Scholar
- Kennett BLN, Kerry NJ (1979) Seismic waves in a stratified half space. Geophys J R Astron Soc 57:557–583View ArticleGoogle Scholar
- Koketsu K, Miyake H, Suzuki H (2012) Japan integrated velocity structure model version 1. In: Proceedings of the 15th world conference on earthquake engineering, Lisbon, 24–28 September 2012Google Scholar
- Lawson CL, Hanson RJ (1974) Solving least squares problems. Prentice-Hall, Old TappanGoogle Scholar
- Ma K-F, Mori J, Lee S-J, Yu SB (2001) Spatial and temporal distribution of slip for the 1999 Chi-Chi, Taiwan, earthquake. Bull Seismol Soc Am 91:1069–1087View ArticleGoogle Scholar
- Maruyama T (1963) On the force equivalents of dynamic elastic dislocations with reference to the earthquake mechanism. Bull Earthq Res Inst Univ Tokyo 41:467–486Google Scholar
- Nakata T, Imaizumi T (eds) (2002) Digital active fault map of Japan. University of Tokyo Press, TokyoGoogle Scholar
- Nishimae Y (2004) Observation of seismic intensity and strong ground motion by Japan Meteorological Agency and local governments in Japan. J Jpn Assoc Earthq Eng 4(3):75–78Google Scholar
- Okada A (1980) Quaternary faulting along the Median Tectonic Line of southwest Japan. Mem Geol Soc Jpn 18:79–108Google Scholar
- Okada S, Toda S (2016) Report on the emergency field survey of the 2016 Kumamoto earthquake. Report presented at the second emergency meeting, International Research Institute of Disaster Science, Tohoku University, Sendai, 19 April 2016. http://irides.tohoku.ac.jp/media/files/earthquake/eq/IRIDeS_Kumamoto_earthquake_okada_20160419.pdf. Accessed 16 May 2016
- Okada Y, Kasahara K, Hori S, Obara K, Sekiguchi S, Fujiwara H, Yamamoto A (2004) Recent progress of seismic observation networks in Japan Hi-net, F-net, K-NET and KiK-net. Earth Planets Space 56:xv–xxviii. doi:10.1186/BF03353076 View ArticleGoogle Scholar
- Olson AH, Apsel RJ (1982) Finite faults and inverse theory with applications to the 1979 Imperial Valley earthquake. Bull Seismol Soc Am 72:1969–2001Google Scholar
- Ratchkovski NA, Wiemer S, Hansen RA (2004) Seismotectonics of the Central Denali fault, Alaska, and the 2002 Denali fault earthquake sequence. Bull Seismol Soc Am 94:S156–S174View ArticleGoogle Scholar
- Sekiguchi H, Irikura K, Iwata T (2000) Fault geometry at the rupture termination of the 1995 Hyogo-ken Nanbu earthquake. Bull Seismol Soc Am 90:117–133View ArticleGoogle Scholar
- Tanaka M, Asano K, Iwata T, Kubo H (2014) Source rupture process of the 2011 Fukushima-ken Hamadori earthquake: how did the two subparallel faults rupture? Earth Planets Space 66:101. doi:10.1186/1880-5981-66-101 View ArticleGoogle Scholar
- Tsutsumi H, Okada A (1996) Segmentation and Holocene surface faulting on the Median Tectonic Line, southwest Japan. J Geophys Res 101:5855–5871View ArticleGoogle Scholar
- Wald DJ, Heaton TH (1994) Spatial and temporal distribution of slip for the 1992 Landers, California, earthquake. Bull Seismol Soc Am 84:668–691Google Scholar
- Wessel P, Smith WHF (1998) New, improved version of generic mapping tools released. EOS Trans Am Geophys Union 79:579View ArticleGoogle Scholar
- Yarai H, Kobayashi T, Morishita Y (2016) Crustal deformation around the faults revealed by MAI analysis. http://www.gsi.go.jp/cais/topic160428-index-e.html. Accessed 8 May 2016
- Yeats R (2012) Active faults of the world. Cambridge University Press, New YorkView ArticleGoogle Scholar
- Yoshida S, Koketsu K, Shibazaki B, Sagiya T, Kato T, Yoshida Y (1996) Joint inversion of near- and far-field waveforms and geodetic data for the rupture process of the 1995 Kobe earthquake. J Phys Earth 44:437–454View ArticleGoogle Scholar