Source rupture processes of the 2016 Kumamoto, Japan, earthquakes estimated from strong-motion waveforms
© The Author(s) 2016
Received: 1 July 2016
Accepted: 19 September 2016
Published: 3 October 2016
The detailed source rupture process of the M 7.3 event (April 16, 2016, 01:25, JST) of the 2016 Kumamoto, Japan, earthquakes was derived from strong-motion waveforms using multiple-time-window linear waveform inversion. Based on the observations of surface ruptures, the spatial distribution of aftershocks, and the geodetic data, a realistic curved fault model was developed for source-process analysis of this event. The seismic moment and maximum slip were estimated as 5.5 × 1019 Nm (M w 7.1) and 3.8 m, respectively. The source model of the M 7.3 event had two significant ruptures. One rupture propagated toward the northeastern shallow region at 4 s after rupture initiation and continued with large slips to approximately 16 s. This rupture caused a large slip region 10–30 km northeast of the hypocenter that reached the caldera of Mt. Aso. Another rupture propagated toward the surface from the hypocenter at 2–6 s and then propagated toward the northeast along the near surface at 6–10 s. A comparison with the result of using a single fault plane model demonstrated that the use of the curved fault model led to improved waveform fit at the stations south of the fault. The source process of the M 6.5 event (April 14, 2016, 21:26, JST) was also estimated. In the source model obtained for the M 6.5 event, the seismic moment was 1.7 × 1018 Nm (M w 6.1), and the rupture with large slips propagated from the hypocenter to the surface along the north-northeast direction at 1–6 s. The results in this study are consistent with observations of the surface ruptures.
The main target of this study is the M JMA 7.3 event (hereafter called the M 7.3 event) that occurred at 01:25 JST on April 16, 2016 (16:25 UTC on April 15, 2016). This event caused strong ground motions that were felt throughout Kyushu, with maximum seismic intensity of 7, the largest intensity on the Japan Meteorological Agency (JMA) scale, and maximum peak ground acceleration (PGA) over 1000 cm/s2. The observations of surface ruptures, the spatial distribution of aftershocks, and the geodetic data, which will be mentioned later, suggest that the rupture of this event occurred on multiple fault planes along the Hinagu and Futagawa fault zones. Therefore, the use of a single fault plane model, which has been adopted in many source-process analyses, is unsuitable to analyze the source process of this event and a more realistic fault model needs to be used. In this study, we propose a curved fault model based on the observations of surface ruptures, the spatial distribution of aftershocks, and the geodetic data. Using the curved fault model, we estimate the source process of the M 7.3 event and compare it with the hypocenter distribution of aftershocks, the distribution of observed surface ruptures, and the result of back-projection analysis with high-frequency seismic waves. We also check the contribution of the fault ruptures to the strong ground motions. In addition, we conduct another source inversion with a single fault plane model to demonstrate the significance of using the curved fault model by comparing their results.
We also estimate the source process of the first large event (M JMA 6.5; hereafter called the M 6.5 event) that occurred at 21:26 JST on April 14, 2016 (12:26 UTC on April 14, 2016), and caused strong ground motions with maximum seismic intensity of 7 and maximum PGA over 1000 cm/s2. We compare the source model with the hypocenter distribution of events between the M 6.5 event and the M 7.3 event and the distribution of seismicity in 2000. We also discuss the relationship between the fault rupture and the strong ground motions.
The M 7.3 event (April 16, 2016, 01:25, JST)
Curved fault model
Method and data
The source process is estimated by the multi-time-window linear waveform inversion method (Olson and Apsel 1982; Hartzell and Heaton 1983), which has been applied to source-process analyses of many earthquakes (e.g., Sekiguchi et al. 2000; Suzuki et al. 2010). For a detailed description of the methodology employed, we refer the reader to the aforementioned studies. The curved fault model is divided into 28 subfaults along the strike direction and 12 subfaults along the dip direction, each with a size of approximately 2 km × 2 km (Fig. 2a). The slip time history of each subfault is represented by 13 time windows, each with a width of 0.8 s, with a lag of 0.4-s lag. Thus, the allowed slip duration for each subfault is 5.6 s. The first time window starting time is defined as the time prescribed by a circular rupture propagation with the constant speed of V ftw. The slip rate of each time window at each subfault is derived by minimizing the difference between the observed and synthetic waveforms normalized for each station by the observed maximum amplitude of the three components. To stabilize the inversion, the slip angle is allowed to vary within ±45° around the central rake angle using the nonnegative least-squares scheme (Lawson and Hanson 1974). The central rake angle is set to −142°, which is the rake angle of the F-net moment tensor solution (Fukuyama et al. 1998). In addition, a spatiotemporal smoothing constraint on the slip (Sekiguchi et al. 2000) is imposed. We performed inversions using several combinations of V ftw and weight of smoothing constraint. The weight of the smoothing constraint for inversion with a certain V ftw value is determined based on Akaike’s Bayesian information criterion (Akaike 1980) following previous studies (e.g., Sekiguchi et al. 2000), and the inversion solution that gives the minimum misfit among those with different V ftw is selected as the best model.
Source process of the M 7.3 event
Another region with large slips (>1.6 m) is found at the depth of 5–10 km above the rupture starting point. These slips were caused by a rupture at 2–6 s propagating toward the surface from the rupture starting point. After it reached the near surface, the rupture propagated toward the northeast along the near surface at 6–10 s with large slips. This rupture occurred on the Takano-Shirohata segment of the Hinagu fault zone and the southern part of the Futagawa segment of the Futagawa fault zone.
The field surveys discovered surface ruptures with a length of approximately 30 km along the surface traces of the Hinagu and Futagawa fault zones after the M 7.3 event (e.g., GSJ/AIST 2016; Kumahara et al. 2016; Shirahama et al. 2016). They reported that the surface ruptures near the epicenter were not very large (<0.5 m), and that large surface ruptures of more than 1 m were observed from approximately 5 km to approximately 30 km northeast of the epicenter along the Futagawa fault zone. The extent of the large near-surface slips in our source model (Fig. 4) is roughly consistent with the extent of the observed large surface ruptures.
Figure 4a, b shows the hypocenter distribution of the aftershocks following the M 7.3 event. Most of the events are located deeper than 5 km, and there are few aftershocks in the shallow part of the large slip region (>2.4 m). Some events do occur in the deep part of the large slip region, which includes the largest aftershock following the M 7.3 event (April 16, 2016, 01:45, JST; M JMA 5.9). The seismicity is high near the hypocenter and south of the hypocenter.
Pulido (2016) applied a seismic back-projection analysis (Pulido et al. 2008) to this event with strong-motion waveforms in the period of 5–10 Hz. Based on his result, high-frequency seismic waves were radiated mainly from the region around the hypocenter. However, in our source model, the main rupture with large slips occurred more than 10 km northeast of the hypocenter, and the minor rupture occurred near the hypocenter (Figs. 4, 5a). This difference suggests the possibility that the seismic radiation of the M 7.3 event had a frequency-dependent spatial variation: The rupture near the hypocenter had modest slips and a strong high-frequency seismic radiation, whereas the rupture 10–30 km northeast of the hypocenter had large slips and a weak high-frequency seismic radiation.
The M 6.5 event (April 14, 2016, 21:26, JST)
Fault model, method, and data
The source process of the M 6.5 event was estimated with almost the same methodology as that of the M 7.3 event. Hereafter, we mention only the differences from the analysis for the M 7.3 event. For the fault model, we assume a 22 km × 14 km rectangular plane with a strike of 212° and a dip of 89° based on the F-net moment tensor solution. The rupture starting point is set at the hypocenter location, 32.7417°N, 130.7994°E, and depth of 12.49 km, determined in the same way as we did for the M 7.3 event. This near-vertical fault is consistent with the hypocenter distribution of events just after the M 6.5 event (Fig. 2c). The top depth of the fault model is approximately 1.5 km; its extension to the surface corresponds to the surface trace of the Hinagu fault zone (Fig. 3b). This fault model corresponds to the Takano-Shirohata segment of the Hinagu fault zone. Although the M 7.3 event also had a rupture along the Takano-Shirohata segment of the Hinagu fault zone, the dip angle inferred from the hypocenter distribution differs between these events (Fig. 3c). The causative fault of the M 6.5 event is considered to differ spatially from that of the M 7.3 event on the Takano-Shirohata segment of the Hinagu fault zone.
The fault plane model is divided into 11 subfaults along the strike direction and 7 subfaults along the dip direction, each with a size of 2 km × 2 km. The slip time history of each subfault is represented by 5 time windows with a width of 0.8 s, each with a lag of 0.4 s. Thus, the allowed slip duration for each subfault is 2.4 s. The central rake angle is set to −164°, which is the rake angle of the F-net moment tensor solution.
We use three-component strong-motion waveforms at 16 stations within an epicenter distance of approximately 50 km: 5 K-NET stations with ground surface observation, 8 KiK-net stations with borehole observation, 2 KiK-net stations with ground surface observation, and 1 F-net station with observation in a vault (Fig. 3b). The velocity waveforms at these stations are band-pass filtered between 0.1 and 1.0 Hz and resampled to 10 Hz. The time window of the observed waveforms begins at 1 s before the S-wave arrival, and its length for each station varies from 7 to 10 s to avoid the effect of the local event just after the M 6.5 event. For the station closest to the fault, KMMH16, we use a weight that is two times larger than those for the other stations
Source process of the M 6.5 event
Figure 8b also shows the hypocenter distribution of events in the period between the M 6.5 event and the M 7.3 event. Most events are located deeper than 5 km; there are few aftershocks in the shallow large slip region. The seismicity in the large slip region around the hypocenter is relatively low compared to that in the surroundings. Many events, including the largest earthquake in the period (April 15, 2016, 00:03, JST; M JMA 6.4), occurred north and south of the major slip region.
The M JMA 5.0 earthquake (June 8, 2000, 09:32 JST) and the aftershocks had occurred near the source area of the M 6.5 event. These events were located south of the major rupture region of the M 6.5 event and within the southern part of the high-seismicity area after the M 6.5 event (Fig. 8b).
Kumahara et al. (2016) and Shirahama et al. (2016) reported that small surface cracks were observed by some residents along the Takano-Shirohata segment of the Hinagu fault zone just after the M 6.5 event. The occurrence of the near-surface large slips in our source model for the M 6.5 event is consistent with the appearance of the small surface cracks.
For the M 7.3 event, we also conducted another source inversion using a single rectangular fault plane 56 km × 24 km with a strike of 226° and a dip of 65° (Fig. 2a). The fault plane was divided into 2 km × 2 km subfaults. The inversion settings, such as the station distribution and the smoothing constraint, were the same as for the analysis with the curved fault model. A comparison between the synthetic waveforms from the curved fault and the single fault plane models (Fig. 6) demonstrates that the use of the curved model leads to improved waveform fit at the stations south of the fault (KMMH14, KMMH11, SIB, and KMMH07). Figure 7b shows the observations and the synthetic waveforms radiated from the southern part of each fault model at KMMH14, KMMH11, SIB, and KMMH07. This figure indicates that the synthetic waveforms at these stations radiated were mainly from the southern part of each fault model and that the difference of synthetic waveforms at these stations shown in Fig. 6 is caused mainly by the fault geometry of the southern part. The stations of KMMH14, KMMH11, and SIB are located along the direction of the southwestern extension of the southern part of the curved fault model, that is, the maxima direction of S-wave radiation pattern of the right-lateral strike-slip fault. In contrast, the stations are not located along the maxima direction of S-wave radiation pattern in the case of the single fault plane model. This positional relationship between the stations and the curved fault model leads to improved waveform fit at these stations, demonstrating the importance of using a curved fault model for analysis of the M 7.3 event.
We estimated the source processes for two large events of the 2016 Kumamoto earthquakes (the M 7.3 event at 1:25 JST on April 16, 2016, and the M 6.5 event at 21:26 JST on April 14, 2016) from strong-motion waveforms. To analyze the source process of the M 7.3 event, we developed a realistic curved fault model. The source model for the M 7.3 event had two significant ruptures: One rupture with large slips propagated toward the direction of the northeastern shallow region at 4 s after rupture initiation and continued to approximately 16 s. This rupture caused the large slip region with a peak of 3.8 m that is located 10–30 km northeast of the hypocenter and reached the caldera of Mt. Aso. There were few aftershocks in the shallow part of the large slip region, although some aftershocks occurred in the deep part. The contribution of the large slip region to the seismic waveforms was significant at many stations. The other rupture propagated toward the surface from the hypocenter at 2–6 s and then propagated toward the northeast along the near surface at 6–10 s. This rupture largely contributed the seismic waveforms at the stations south of the fault and close to the hypocenter. A comparison with the results obtained using a single fault plane model demonstrates that the use of the curved fault model led to improved waveform fit at the stations south of the fault. A comparison between our source model and a back-projection result with high-frequency seismic waves suggested the possibility that the seismic radiation of the M 7.3 event had a frequency-dependent spatial variation. The source model obtained for the M 6.5 event had large slips in the region around the hypocenter and in the shallow region north-northeast of the hypocenter. Both regions had a maximum slip of 0.7 m. The rupture of the M 6.5 event propagating from the hypocentral region to the region north-northeast could have caused the strong ground motions due to the forward directivity effect at KMMH16 and surroundings. The seismicity in the large slip areas of this earthquake was relatively low compared to that of the surroundings. The source-inversion results of this study were consistent with the field survey observations of surface ruptures. The source models estimated in this study are available at http://www.kyoshin.bosai.go.jp/inversion/.
HK analyzed the data, interpreted the results, and drafted the manuscript. WS also analyzed the data and interpreted the results. SA and HS participated in the design of the study and the interpretation of the results. All authors read and approved the final manuscript.
We thank Prof. Ralph J Archuleta and the anonymous reviewer for their helpful comments. We used the unified hypocenter catalog determined by JMA and the 10-m mesh DEM published by the Geospatial Information Authority of Japan. We also used Generic Mapping Tools (Wessel and Smith 1998) to draw the figures.
The authors declare that they have no competing interests.
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