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Crustal Deformation Model of the Beppu-Shimabara Graben Area, Central Kyushu, Japan, Based on Inversion of Three-Component GNSS

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Crustal Deformation Model of the Beppu-Shimabara Graben Area, Central Kyushu, Japan, Based on Inversion of Three-Component GNSS Mochizuki and Mitsui Earth, Planets and Space (2016) 68:177 DOI 10.1186/s40623-016-0550-x LETTER Open Access Crustal deformation model of the Beppu Shimabara graben area, central Kyushu, Japan, −based on inversion of three‑component GNSS data in 2000–2010 Kazuma Mochizuki and Yuta Mitsui* Abstract The 2016 Kumamoto earthquakes, including an Mw-7 right-lateral earthquake on April 15 (UTC), occurred along faults within the Beppu Shimabara graben in central Kyushu, Japan. Previous studies showed that the graben area was under heterogeneous− stress conditions with north–south T-axes and spreading in a north–south direction. Here, we construct a detailed crustal deformation model using three-component Global Navigation Satellite System data in 2000–2010 and considering the distribution of geological fault traces in this area. Our inversion analysis suggests that the strain accumulation rate for the right-lateral seismic slip segment (corresponding to the Futagawa fault), where the largest of the 2016 Kumamoto earthquakes ruptured, was several times smaller than the other segments in the Beppu Shimabara graben. Furthermore, we observe distinct subsidence along the Beppu Shimabara graben. Our base model− attributes the subsidence to deflation of magma reservoirs beneath volcanoes,− but the observed vertical velocities are poorly fit. In order to improve the fitting results for the vertical deformation, we need more sophisticated volcano-deformation model (such as a sill-like deformation source for Mt. Aso) or graben model. Keywords: The 2016 Kumamoto earthquakes, Kyushu, GNSS, Crustal deformation, Beppu Shimabara graben, Volcano, Block-fault model, Nucleation − Introduction Tada (1984, 1985) investigated crustal deformation The 2016 Kumamoto earthquakes, including an Mw-7 of the Kyushu region using repeated triangulation sur- earthquake on April 15 (UTC), struck a central part of veys for 1892–1982 and found that the Beppu−Shima- the island of Kyushu, southwest Japan. These earthquakes bara graben, going across the central Kyushu region in occurred along faults within the Beppu−Shimabara an east–west direction, was spreading in a north–south graben area. The graben area includes some volcanoes, direction about a rate of 15 mm/year. Tada interpreted notably Mt. Aso with a large caldera whose area is about the Beppu−Shimabara graben as a terminal part of 400 km2 (Fig. 1). Around the source region of the 2016 the Okinawa Trough (a back-arc basin) extending events, several M6 class historical earthquakes occurred southward. in 1895, 1894, 1889, 1848, 1723, 1625, 1619 (Utsu 1979; On the basis of horizontal displacement data observed Usami 2013). This area is under heterogeneous stress by Global Navigation Satellite System (GNSS) for 1996– conditions associated with both right-lateral faults and 2002, Nishimura and Hashimoto (2006) constructed a normal faults (Matsumoto et al. 2015), whose T-axes block-fault model for southwest Japan including rotation trend north–south. of rigid blocks and elastic strain accumulation on inland faults and subduction zone interfaces. Their model *Correspondence: [email protected] included a block boundary along the Beppu−Shima- Department of Geosciences, Shizuoka University, Suruga‑ku, bara graben. Loveless and Meade (2010) also made a Shizuoka 422‑8529, Japan © The Author(s) 2016. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Mochizuki and Mitsui Earth, Planets and Space (2016) 68:177 Page 2 of 9 three-component (both horizontal and vertical) GNSS data in 2000–2010. Methods We use time-series data of the F3 solution (Nakagawa et al. 2009) observed in a GNSS array by Geospatial Infor- mation Authority of Japan, GEONET. We examine posi- tions at 65 stations from January 2000 to January 2010, located in a range of 129.8°E–132°E and 31.9°N–34°N. We express displacements relative to a fixed station, Maebaru, numbered 0450, at 130.251°E and 33.536°N. We estimate velocities at all the stations as follows. First, we remove the offsets related to antenna replace- ment. Next, we calculate median values for each month (121 months during the above period). Then we exclude effects of non-stationary deformation due to nearby large events from the median time series, using fault models in previous studies: the 2001 Geiyo earthquake (Geospa- tial Information Authority of Japan 2001), the 2003–2004 Fig. 1 Epicenter locations of the 2016 Kumamoto earthquakes in Bungo slow slip events (Geospatial Information Author- Kyushu island, southwest Japan. The beach-ball symbols illustrate ity of Japan 2004), and the 2005 Fukuoka earthquake moment tensor solutions (magnitude 4) in April 2016, using (Nishimura et al. 2006). Finally, we obtain the observed ≥ the F-net broadband seismograph network, provided by National velocities per year (annual velocities) by linear regression Research Institute for Earth Science and Disaster Resilience. The of the processed time-series data with standard errors. red lines show geological fault traces from active fault database by National Institute of Advanced Industrial Science and Technology. The Table 1 compiles the observed annual velocities and pink triangles represent active volcanoes. The purple oval approxi- their uncertainties (95% confidence intervals), and Fig. 2 mately shows the area called “Beppu Shimabara graben” (e.g., Tada shows the annual velocities during 2000–2010. Since − 1985) the uncertainties of the linear regression are sufficiently small, at most 0.48 mm/year, we do not illustrate the uncertainties in Fig. 2. The horizontal velocities repre- block-fault model for Japan, and their result showed that sent the effects of westward subduction of the oceanic inland faults across the central Kyushu region in the E–W Philippine plate on the east side and the north–south direction opened around 2–9 mm/year in the N–S direc- divergence along the Beppu−Shimabara graben. From tion and slipped right-laterally about 6–9 mm/year as the vertical velocities, we find subsidence in the graben boundaries of crustal blocks. Similar block-fault models area of the central Kyushu region, particularly near the have been developed (Nishimura and Takada 2015; Taka- volcanoes. hashi and Hashimoto 2015). Using the velocity field and the distribution of the The above previous GNSS studies mainly focused on geological fault traces (red lines in Figs. 1, 2), we define the current tectonics for the whole of or southwest Japan. five blocks ([1]–[5]) and block boundaries (N1–N2, C1– However, in order to investigate strain accumulation for C5, U1, H1) on Kyushu island, as shown in Fig. 3. Each each fault, a more detailed model reflecting surface fault block rotates about its Euler pole, with relative rota- traces is preferable. tion between adjacent blocks giving the slip rate on the Here, we focus on the strain accumulation states boundary. Shallow part of several block boundaries (N1, C1–C5) works as sources of elastic strain accumulation around the Beppu−Shimabara graben area prior to the 2016 Kumamoto earthquakes. We employ verti- (dislocation sources) as “slip deficits” (e.g., Matsu’ura cal deformation as well as the horizontal deformation et al. 1986) leading to earthquakes. We set their geom- in the graben area. The vertical deformation may reflect etries on the basis of recent seismicities and the infor- characteristics of the graben area, especially around the mation about the fault traces by National Institute of volcanoes. We present a detailed model for the crus- Advanced Industrial Science and Technology (available tal deformation around the central Kyushu region con- online at https://gbank.gsj.jp/activefault/index_e_gmap. sidering the three-dimensional surface motion within html). The dip angles of the dislocation sources are a framework of spherical block rotation, elastic strain also based on the same information. We assume that accumulation on faults, and volcanic deformation, using the upper and lower edges of the dislocation sources Mochizuki and Mitsui Earth, Planets and Space (2016) 68:177 Page 3 of 9 Table 1 Observed annual velocities and their uncertainties by linear regression at 65 GNSS stations Lat. (°) Lon. (°) Vel. (NS) Vel. (EW) Vel. (UD) Unc. (NS) Unc. (EW) Unc. (UD) (mm/year) (mm/year) (mm/year) (mm/year) (mm/year) (mm/year) 33.73 130.48 0 0 0 33.46 131.69 1.1 2.4 2.3 0.12 0.25 0.33 − − 33.33 130.97 5.3 14.0 2.4 0.20 0.12 0.31 − 32.92 131.88 2.2 6.0 0.6 0.12 0.14 0.29 − − 33.48 129.85 4.5 18.9 3.9 0.29 0.25 0.29 − 32.55 130.65 0.9 1.2 1.0 0.14 0.07 0.24 − 32.45 131.63 4.3 5.5 0.6 0.14 0.12 0.25 − − − 32.01 130.19 3.3 14.9 1.7 0.17 0.14 0.25 − 33.54 130.25 4.8 1.7 0.6 0.33 0.42 0.37 − − 33.47 130.83 2.4 4.9 0.3 0.14 0.13 0.20 − − 33.21 130.56 0.5 3.5 1.7 0.12 0.13 0.18 − − − 33.27 130.27 0.2 1.5 2.9 0.06 0.12 0.33 − 33.10 130.09 0.5 0.8 1.0 0.10 0.12 0.22 − − 32.87 130.27 3.9 2.4 0.2 0.24 0.13 0.20 − − − 32.64 130.15 3.6 2.5 0.0 0.10 0.12 0.19 − − 32.93 130.55 1.1 2.8 1.6 0.17 0.16 0.24 − − − 32.84 130.76 2.2 5.3 0.3 0.16 0.16 0.20 − − − 32.74 131.10 1.9 10.4 0.0 0.15 0.17 0.24 − − 32.33 129.99 4.8 1.3 0.3 0.10 0.14 0.20 − − 32.30 130.51 5.1 4.3 0.3 0.14 0.16 0.24 − − 32.24 130.80 5.2 5.6 0.8 0.14 0.15 0.25 − − 33.67 131.56 5.1 8.4
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