Fault source model for the 2016 Kumamoto earthquake sequence based on ALOS-2/PALSAR-2 pixel-offset data: evidence for dynamic slip partitioning
© The Author(s) 2016
Received: 21 July 2016
Accepted: 8 October 2016
Published: 22 October 2016
The Erratum to this article has been published in Earth, Planets and Space 2016 68:196
Previous geological and geodetic observations pointed out the presence of Beppu-Shimabara rift system across the Kyushu island (Ehara 1992; Matsumoto 1979; Tada 1993), along which the NE–SW trending seismicity of the 2016 Kumamoto earthquake sequence was distributed. The northeastern edge of the rift system is located at the most western part of the Median Tectonic Line that is the longest fault system in Japan (Ikeda et al. 2009). Meanwhile, the northernmost end of Okinawa trough reaches the southwestern edge of the rift system (Tada 1984, 1985). The rift system contains geothermal area and active volcanic system including Kujyu, Aso and Unzen volcanoes. Based on geodetic observations by the first-order triangulation survey (Tada 1984, 1985) and GPS network (Fukuda et al. 2000; Nishimura and Hashimoto 2006), the central Kyushu region is inferred to be under N–S extension stress regime. Matsumoto et al. (2015) analyzed the focal mechanisms of shallow earthquakes in Kyushu from 1923 to 2013 and showed that Kyushu island was overall under extension stress regime with some modifications due to the shear zones from the Median Tectonic Line (MTL).
In order to understand the mechanism of any seismic event, a variety of sources are available such as seismic wave, tsunami height, Global Navigation Satellite System (GNSS) network and SAR data. The focal mechanisms reported by JMA are based on seismological data that are acquired at distant stations from the hypocenter. Regarding the “main shock” of the Kumamoto earthquake sequences, however, it has been pointed out that another earthquake (M JMA 5.7) occurred 32 s after the main shock at Yufu City, ~80 km to the NW from Kumamoto (JMA hypocenter catalog in Japanese [http://www.data.jma.go.jp/svd/eqev/data/daily_map/20160416.html]). Hence, the estimated mechanism solution based on the far-field and long-period seismic data could be biased due to mixed-up wavelets. In contrast, co- and post-seismic displacements derived by geodetic techniques indicate permanent deformation signals in the near-field around the hypocenters, and thus, the estimated fault model could be less ambiguous, at least, in terms of the location and the geometry of these faults. Over the last two decades, a growing number of fault models have been derived from geodetic data such as GNSS and Interferometric SAR (InSAR). Regarding the 2016 Kumamoto earthquake sequences, Geospatial Information Authority of Japan (GSI) has already reported InSAR (Hanssen et al. 2001; Massonnet and Feigl 1998) and Multiple Aperture Interferometry (MAI, Bechor and Zebker 2006) observation results using the L-band ALOS-2/PALSAR-2 images (http://www.gsi.go.jp/cais/topic160428-index-e.html, last accessed on May 27, 2016). In this paper, we show the three-dimensional (3D) displacements associated with the Kumamoto earthquakes derived by applying offset tracking technique to ALOS-2/PALSAR-2 data. Using the derived pixel-offset data, we develop a fault slip distribution model based on triangular dislocation elements in an elastic half-space and discuss its implication for the rupture processes of the 2016 Kumamoto earthquakes.
Methods and results
PALSAR-2 data in this study
Inc. angle (deg.)
Heading angle (deg.)
Pix. spacing [(Ran., Az.) m]
We can identify three NE–SW trending signal discontinuities in the pixel-offset data (dotted curved lines in Fig. 2), which we attribute to the surface faults due to the earthquake sequences and hereafter denote F1, F2 and F3, respectively. Geologically inferred fault traces by Nakata and Imaizumi (2002) are also shown in Figs. 1 and 2. The longest discontinuity (NE–SW), F1, is located at a part of Futagawa fault system (Watanabe et al. 1979; Okubo and Shibuya 1993), extending from the northwestern area of the Aso caldera to Kumamoto City. Another signal discontinuity, F2, branches off from the southern part of the F1 toward SSW, which seems to be a part of Hinagu fault system (Watanabe et al. 1979). We can point out the other short signal discontinuity, F3, parallel to the F1 segment at the southwest flank of Aso caldera. The F3 segment is situated 2 km away from the longest F1 segment.
Considering the NE–SW striking of the F1 segment, the focal mechanism of the main shock (M w 7.0) suggests mainly right-lateral motion with a near-vertical dip angle. Nonetheless, the northern side of the deformation area indicates significant subsidence by as much as ~200 cm (Fig. 3). This subsidence signal can be explained by the normal faulting on NW-dipping segments, which would be plausible under the N–S extension stress field. In contrast, we could identify few uplift signals at the southern side.
Besides the broadly distributed signals noted above, we can point out more localized signals in places. We can identify the NE–SW trending patchy localized signals at the northern part of Aso caldera in the 3D displacement field (black boxes in Fig. 3). These signals indicate ~200 cm of NNW movement with small vertical displacements. Moreover, the 3D displacements indicate subsidence signals by as much as 50 cm at the western part of Aso caldera, which is located at the eastern part of the deformation area. The detected subsidence signal significantly exceeds the empirical measurement error of pixel-offset which is 0.1 pixel in each image (e.g., Fialko et al. 2001). Moreover, the localized westward displacements can be found between the two discontinuities (Fig. 3a). These signals cannot be explained by the fault model below, and we provide our interpretations later on.
Fault source model
Parameters of fault segments in our model
M w [M o (Nm)]
Ave. slip (m)
Normal + right-lateral
6.77 [1.80 × 1019]
6.62 [1.05 × 1019]
6.46 [0.61 × 1019]
Total release of geodetic moment (GM) and seismic moment (SM)
Component [slip direction]
M o (×1019 Nm)
Dip [normal slip]
Dip [normal slip]
Dip [normal slip]
Remarkably, our fault model indicates that while the strike-slip dominates on F1 and F2, F3 is a pure normal slip fault (Fig. 6; Table 3). The significant normal slip at F3 instead of at F1 could be derived, because the broad subsidence signals are located not only to the north of F1 but also between F1 and F3, and also because the deeper normal slip can explain the broader subsidence signals. Such a configuration of multiple faults is known as slip partitioning, which has been pointed out at active tectonic regions of oblique extension/compression stress regime (e.g., Fitch 1972; Bowman et al. 2003). Our fault model thus suggests that the fault source regions are under oblique extension stress, which is probably due to the combination of the shear stress by the western end of MTL and the extension stress associated with the backarc spreading of Okinawa trough (Nishimura and Hashimoto 2006; Ikeda et al. 2009; Matsumoto et al. 2015).
The patchy localized signals of the NNW horizontal movements (black boxes in Fig. 3), which were masked in the inversion, would not be explained by co-seismic landslides, because directions of the local slope and the horizontal displacements are opposite. We can speculate that these localized signals may suggest the presence of additional small faults, because some faults inside a volcanic caldera are likely to be masked by the thick volcanic ash deposits.
Regarding the localized westward signals between F1 and F3, it is possible to attribute to the left-lateral slip on F3. However, the inferred left-lateral strike-slip on F3 turned out to be insignificant as mentioned before; this may be because we used pixel-offset data for the inversion. Some reports of field-based surface rupture observations indicated left-lateral offsets near the southern end of F3; the strike angles were ~N70W (e.g., http://www.ckcnet.co.jp/pdf/kumamoto_0427.pdf, in Japanese). Given these reports and the modeling results, if the westward signals have their origin in faults, they may suggest the left-lateral faults in between F1 and F3 which strike ~N70W in a conjugate fashion. Although, to our knowledge, there are no reports of such seismic focal mechanisms, they might have occurred at very shallow depth without generating seismic waves. Another possible interpretation would be a co-seismic landslide; the local slop orientation seems to be consistent with this scenario.
Furthermore, the 3D displacement showed ~50 cm subsidence at the western part of Mt. Aso (Fig. 3c), which cannot be explained by our fault model. One possible interpretation of the subsidence signal at Mt. Aso is a consequence by extension of the magma chamber due to the westward movement by the right-lateral earthquakes, which is analogous to the subsidence as reported over the volcanoes on the northeastern Honshu after 2011 Tohoku-oki earthquake (Takada and Fukushima 2013). However, while an effusive volcanic eruption with gas emission was observed after the main shock of the 2016 Kumamoto earthquake sequence, the casual relationship still remains unclear.
The results of offset tracking applied to ALOS-2/PALSAR-2 data indicated three displacement discontinuities that were considered as the surface traces of the main source faults associated with the 2016 Kumamoto earthquake sequence. We thus constructed a fault slip distribution model containing three segments: F1 and F3 belong to the Futagawa fault system, and F2 belongs to the Hinagu fault system. The inferred slip distributions at each segment indicate that while the strike-slip is dominant at shallower depths of F1 and F2, only normal faulting is significant at greater depth of F3, suggesting the occurrence of slip partitioning during the earthquake sequence. Moreover, using our slip distribution, we computed the focal mechanism and the CLVD parameter, which turned out to be quite consistent with those derived from the seismic focal mechanism. Hence, we conclude that the significant non-double couple components in the reported seismic focal mechanisms were due to the dynamic slip partitioning by simultaneous occurrence of both strike-slip and normal slip at the distinct segments.
YH processed PALSAR-2 data and constructed the fault source model. Both YH and MF managed this study and drafted the manuscript. All authors read and approved the final manuscript.
We acknowledge two anonymous reviewers, whose comments were helpful to improve the original manuscript. All PALSAR-2 level 1.1 data in this study are provided from PIXEL (PALSAR Interferometry Consortium to Study our Evolving Land Surface) group under a cooperative research contract with ERI, University of Tokyo. The ownership of ALOS-2/PALSAR-2 data belongs to JAXA. The hypocenter data and focal mechanism have been provided by JMA.
The authors declare that they have no competing interests.
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