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The 2016 Kumamoto–Oita earthquake sequence: aftershock seismicity gap and dynamic triggering in volcanic areas
© The Author(s) 2016
Received: 22 July 2016
Accepted: 29 October 2016
Published: 15 November 2016
Tectonic faults often lie near volcanoes (Azzaro 1999; Nishigami 1997), and therefore, large inland earthquakes sometimes strike volcanic regions (Ando and Okuyama 2010; Moran et al. 2002). The elucidation of the influence of volcanic areas, in addition to the fault geometries and other properties, on earthquake generation is essential for better understanding of the seismo-volcano tectonics as well as for improvement in the accuracy of natural hazard evaluation.
This earthquake sequence broke out in the Kumamoto Prefecture at 21:26 on April 14, 2016 [hereafter all times are written using the Japan Standard Time (UTC + 9)] by an M w 6.2 event (hereafter “event α”), which gave rise to devastating ground motion, followed by an M w 6.0 event (“event β”) at 0:03 on April 15, 2016, located at a distance of only 10 km south from the event α (Fig. 1). An even larger and, so far, the largest event (M w 7.0; “event γ”) struck at 1:25 on the 16th near the event α and caused further damage through the strong ground-shaking in the surrounding area. The hypocenters of all three events were located along the Futagawa and Hinagu fault systems. Following the event γ, the seismicities in two volcanic areas, the northern Aso and Yufuin–Beppu areas, became active. In addition, seismicity gaps are also observed between the Futagawa fault system and the northern Aso area (we refer to this gap as the Aso gap), and between the northern Aso and the Yufuin–Beppu areas.
This earthquake sequence raises several questions: Why were these faults ruptured not by a single large earthquake but by three successive ones? What caused the seismic gaps? Toward addressing these questions, phenomenological understanding of this earthquake sequence is essential. This paper presents following seismological observations: the underground fault geometry of the Hinagu and Futagawa fault systems inferred from distribution of aftershocks refined by an earthquake relocation analysis; the relationship between the Aso gap and the rupture process of the largest shock, event γ, inferred from a finite fault slip inversion using strong-motion data; and a detection and quantification of a dynamic triggering event in the Yufuin–Beppu area, Oita prefecture. Based on these observations, we will show a perspective to solve the abovementioned questions.
Seismic velocity structure used in seismological analyses
One-dimensional seismic velocity and attenuation structure
V p (km/s)
V s (km/s)
Geometries of underground faults in Futagawa and Hinagu fault systems
We investigated the geometries of faults over which the three large events occurred, based on relocated hypocenters in the entire region shown in Fig. 1. The relocation was conducted with the hypoDD program (Waldhauser and Ellsworth 2000), using 282,999 and 253,335 arrival time differences for the P- and S-waves, respectively. These arrival times are picked for 2770 events that occurred between 14 and 21 of April 2016, and listed on the Japan Meteorological Agency (JMA) Unified Earthquake Catalog.
For calculating double-difference of arrival times, we allow that each event have up to 15 neighbors within 10 km according to the JMA catalog. Each pair must have 8 or more of travel time difference data from either P- or S-arrivals from stations within 100 km from event locations. We gave half of weights for S-arrival times than P-arrival times. Finally, we successfully relocated the 2370 events, reducing the weighted root-mean-square of the residual times from 0.1436 to 0.0510 s (Additional file 1: Figure S1).
The result implies that the three major events ruptured the fault planes of different geometries. The spatial distributions of small earthquakes before and after the event γ are clearly different. The event α was initiated on the vertical fault (E–E′ cross section in Fig. 3b), as also implied by the first-motion focal mechanism (Fig. 1a) estimated from seismograms of KiK-net (Okada et al. 2004), the HASH code (Hardebeck and Shearer 2002), and the velocity structure (Fig. 2; Table 1). The event β was initiated on the steeply dipping fault rather than vertical one, and the nucleation of the event γ occurred on another vertical fault. Several earthquakes preceded the event γ around its hypocenter (E–E′ cross section in Fig. 3b). This activity started right after the occurrence of the event α. We do not see increase in seismicity rate of this activity. On the basis of a catalog constructed with a matched filter technique, Kato et al. (2016) showed migrations of seismicity following the event α, which also propagated to the nucleation point of the events β and γ.
Slip distribution of the largest event and Aso Gap
Geometry of the fault model for the slip inversion analysis
We express the slip direction between those corresponding to the pure normal faulting and the pure right-lateral strike-slip faulting by their summation. On the fault planes, we placed one-dimensional spline functions (“triangular functions”) every 2.5 km in space and every 2 s in time to represent the spatiotemporal distribution of the fault slip velocity. The time windows on the faults are open for 12 s from the time when a hypothetical rupture front with the velocity of 3.0 km/s arrives from the hypocenter.
We inverted the displacement waveforms obtained by integrating the original acceleration data twice after the removal of the offset and bandpass-filtering between 0.025 and 0.25 Hz. Moreover, we resampled the data every 0.4 s.
We calculated the Green’s functions using the one-dimensional seismic velocity model (Fig. 2; Table 1) by the reflection-transmission matrix method (Kennett and Kerry 1979) and wavenumber integral method (Bouchon 1981) while accounting for anelastic attenuation (Takeo 1985).
We aligned the data and the Green’s functions by S-arrivals to suppress bias of unmodeled lateral variation of velocity structure. Actually, we performed the alignment from P-arrival of the event γ, and S-P time of a collocated event (M j 4.3) that occurred at 4:51 on April 16, 2016, was measured using Hi-net or KiK-net data at the same boreholes. This is because we found it difficult to pick the S-arrivals for the event γ due to the overlapping P waves,
We inverted the data using the nonnegative least square algorithm (Lawson and Hanson 1995) with a spatial smoothing constraint. The intensity was objectively set in such a way as to minimize the Akaike’s Bayesian Information Criterion (Uchide and Ide 2007; Yabuki and Matsu’ura 1992).
The waveform fits by the estimated model (Figs. 3a, 4c, d) are shown in Fig. 4b. The variance reduction (ratio of the variance of the residual to that of the data) was 71%, which is quite reasonable. The estimated model yields a seismic moment of 5.1 × 1019 Nm (M w 7.1) and implies that the fault rupture extends below the northern part of the Aso caldera, rupturing the shallow portion of the Aso gap by the oblique slip with a peak slip of around 6 m. Few fore- and aftershocks occurred in the large slip areas. This result has an overall similarity to a model proposed by Yagi et al. (2016) inferred from teleseismic data. The existence of the large slip area penetrating the Aso caldera is also supported by the field observation of the surface rupture (Shirahama et al. 2016) and the InSAR analysis (Geospatial Information Authority of Japan 2016).
Dynamic triggering in Yufuin–Beppu area
The amplitude ratios of the accelerograms high-pass-filtered at 16 Hz range from 3 to 13 with a median of 5 (Fig. 5). This inter-station variation in the amplitude ratios is due to many factors including the differences in source locations and focal mechanisms. Therefore, we choose the median value as the high-frequency amplitude ratio of these two events. The moment ratio of the triggered event to the M w 5.1 event is then estimated to be 125, implying that the M w of the triggered event was 6.5. It should be noted that this estimation is very rough because it strongly relies on the validity of the assumption of the ω 2 model and the self-similar scaling law. Therefore, we conservatively conclude that the triggered event was a mid-M w 6 event.
Discussion and conclusions
First, we have revealed the geometries of the faults ruptured by three distinct large earthquakes. The complex fault system may have prevented the simultaneous rupture of all these faults (Anderson et al. 2003), and instead, large earthquakes occurred successively. However, it should be noted that complexities in fault geometry do not always stop earthquake rupture. For example, in the 2012 off Sumatra earthquake, multiple strike-slip faults were ruptured by a single earthquake (Meng et al. 2012; Satriano et al. 2012; Wang et al. 2012; Yue et al. 2012). In the Nankai and Tonankai megathrust earthquakes, some M8-class events ruptured one of these regions, and some events ruptured both regions simultaneously (Ando 1975). Further investigations are needed to confirm how the irregularities of fault geometries contributed the earthquake generation.
Next, we have shown that the Aso gap corresponds to the large slip area by the largest event (event γ). Owing to the small amount of stress remaining for aftershock production, it is often observed that fewer aftershocks occur in the fault segments with a large slip area (Mendoza and Hartzell 1988). However, it is quite unusual that fewer aftershocks in the Aso gap are observed even below the large slip area. While a rigorous discussion on the origins of the Aso gap requires further investigation of the volcanic structure and resultant fault behaviors, a high temperature due to the geotherm of Mt. Aso must play an important role. Three-dimensional magnetotelluric surveys (Asaue et al. 2012; Hata et al. 2016) identified low electric resistivity in this area. Although the interpretation of the low resistivity is still ambiguous, one can interpret the low resistivity as indication of high temperature area, because, relatively high geothermal gradient (40–90 K/km around 1 km of depth) was observed in Mt. Aso (Tanaka et al. 2004a, b). Moreover, Curie-point depth (Okubo et al. 1985) in the Aso gap is shallower than that in the rupture area of the event γ. The magma chamber of Mt. Aso located west of the active crater and east of the Aso gap (Sudo and Kong 2001) will contribute the high temperature in the Aso gap. The fact that earthquakes before the Kumamoto earthquake distribute in the very shallow portion of the Aso gap (Fig. 3c) may reflect an effect of the geotherm. These observations will be a key to understand the influence of the volcanic geotherm on the formation of the Aso gap. Our fault slip model further suggests that the seismicity in the northern Aso can be interpreted as usual aftershocks of the event γ.
On the dynamic triggering event, we hypothesize that the triggered rupture reached Beppu from Yufuin, which is supported by the following empirical relation and observations: The scaling law for the fault length (Wells and Coppersmith 1994) implies a fault length of 10–20 km for this mid-M w 6.0 triggered earthquake; the seismicity was activated immediately after the event γ (Fig. 1b) in the elongated zone with a length of almost 20 km; and the In-SAR image from ALOS-2 also indicates a surface offset of up to 6 cm for 10 km or longer along the surface trace of a pre-existing fault (Geospatial Information Authority of Japan 2016). This dynamic triggering mechanism produced an apparent gap in seismicity between the northern Aso and Yufuin–Beppu areas, which is different from the case in the Aso gap described above.
The features of the earthquake sequence we revealed give rise to new questions. It is important to understand the mechanism of the formation of the complex system of the Futagawa and Hinagu faults. The fault slip model strongly suggests that large stress accumulation is possible at shallow depths near the volcanos, where the accumulation mechanism remains an open question. Furthermore, the formation mechanism of the Aso gap is not yet fully elucidated. Here, detailed information on the structure of Mt. Aso should advance our understanding. While, in this paper, we discuss the influence of volcanic areas on earthquake generation, the effect of this earthquake sequence on the volcanic activities should also be investigated. These studies will contribute to the improvement in the accuracy of the scenarios of potential disaster in the seismo-volcano tectonics, which are used for evaluating the natural hazards posed by these events.
TU performed all data analyses with help from HH, RM and KI, and drafted the manuscript. MN produced the movie to display the hypocentre distribution in three-dimensional space. NS provided geological interpretation. RA provided comments from the viewpoint of earthquake source dynamics. All authors read and approved the final manuscript.
We thank Nobuo Matsushima for providing his trace data of the caldera rim of Mt. Aso. We also thank Maki Hata, Yoshiki Shirahama, Chen Ji, one anonymous reviewer, and the Editor for their constructive comments. We used the phase data from the JMA Unified Earthquake Catalog (based on seismic data from JMA, NIED, and Kyushu University) and seismograms from Hi-net, KiK-net, and K-NET of NIED (Okada et al. 2004).
The authors declare that they have no competing interests.
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