- Express Letter
- Open Access
The 2018 Hokkaido Eastern Iburi earthquake (MJMA = 6.7) was triggered by a strike-slip faulting in a stepover segment: insights from the aftershock distribution and the focal mechanism solution of the main shock
- Kei Katsumata1Email authorView ORCID ID profile,
- Masayoshi Ichiyanagi1,
- Mako Ohzono1,
- Hiroshi Aoyama1,
- Ryo Tanaka1,
- Masamitsu Takada1,
- Teruhiro Yamaguchi1,
- Kazumi Okada1,
- Hiroaki Takahashi1,
- Shin’ichi Sakai2,
- Satoshi Matsumoto3,
- Tomomi Okada4,
- Toru Matsuzawa4,
- Shuichiro Hirano5,
- Toshiko Terakawa6,
- Shinichiro Horikawa6,
- Masahiro Kosuga7,
- Hiroshi Katao8,
- Yoshihisa Iio8,
- Airi Nagaoka8,
- Noriko Tsumura9,
- Tomotake Ueno10 and
- the Group for the Aftershock Observations of the 2018 Hokkaido Eastern Iburi Earthquake
© The Author(s) 2019
- Received: 24 December 2018
- Accepted: 23 April 2019
- Published: 20 May 2019
- The Hokkaido Eastern Iburi earthquake
- Strike-slip fault
- Reverse fault
- Aftershock distribution
- Focal mechanism solution
- Local seismic network
- Stepover segment
The Hokkaido Eastern Iburi earthquake occurred on September 6, 2018, and the parameters of the hypocenter given by the Japan Meteorological Agency (JMA) are as follows: origin time = 03:07:59.3 JST, epicenter = (42.691°N, 142.007°E), depth = 37.0 km, and MJMA = 6.7. This earthquake is located in the Hokkaido corner region, where the northern Japan island arcs connect to the Kurile Islands. The Pacific plate subducts beneath Hokkaido Island on the North American or the Okhotsk Sea Plate in this region (e.g., Takahashi et al. 1999; Katsumata et al. 2002). In addition, the Kurile island arc is moving toward the southwest and colliding with the northeastern Japan arc (e.g., Kimura 1996). This region is called the arc–arc-type Hidaka collision zone (HCZ). The depth to the upper surface of the subducting Pacific plate is approximately 100 km around the focal area (Katsumata et al. 2003); therefore, this earthquake obviously occurred in the North American plate or the Okhotsk Sea plate, i.e., this earthquake was a shallow intraplate earthquake in the inland area of Japan.
The first purpose of this study is to re-evaluate the depth and distribution of the main shock and aftershocks accurately. The hypocenters of main shocks are usually shallower than 20 km for shallow intraplate earthquakes in the inland area of Japan (Omuralieva et al. 2012), whereas the depth of this sequence was obviously deeper than 20 km according to the preliminary report from the JMA. Therefore, we relocated the hypocenters of the main shock and aftershocks in this study by considering a local complex velocity structure of the crust.
The second purpose of this study is to investigate the reason for the mismatch between the centroid moment tensor (CMT) solution and the focal mechanism solution determined by using the first-motion polarities of P waves at individual stations. The CMT solutions of the main shock were reported by four groups: JMA (2018a), the National Research Institute for Earth Science and Disaster Resilience (NIED 2018a), the United States Geological Survey (USGS 2018), and the Global CMT project (Dziewonski et al. 1981; Ekström et al. 2012). All CMT solutions of this earthquake are consistent in showing that the earthquake resulted mainly from reverse faulting. However, the focal mechanism of this earthquake was mainly strike-slip faulting (JMA 2018b; NIED 2018b). We estimated the focal mechanism solution by adding data from seismic stations in the focal area and investigated a model to explain qualitatively the discrepancy between the CMT solutions and the focal mechanisms by referring to the relocated aftershock distribution.
To relocate hypocenters, we first determine hypocenters of earthquakes with an assumption of a 1-D velocity structure using the hypomh algorithm (Hirata and Matsu’ura 1987). We then used the hypocenters as the initial locations of earthquakes and carried out relative hypocenter relocation using double-difference tomography (tomoDD) (Zhang and Thurber 2003) with a fixed 3-D velocity structure. The initial locations of hypocenters were calculated with an assumption of a 1-D structure for the velocity of the P wave (Vp) based on Iwasaki et al. (2004) as shown in Fig. 1c: 3.2 km/s at 0.0 km depth, 4.8 km/s at 3.0 km, 5.78 km/s at 7.0 km, 4.65 km/s at 10.0 km, 6.95 km/s at 22.5 km, 7.2 km/s at 35.0 km, and 8.06 km/s at 50.0 km. The velocity of the S wave (Vs) was calculated at each depth as Vp/1.73 with the assumption that the Vp/Vs ratio is 1.73. Recently several authors presented a 3-D tomographic model of Vp and Vs in this region (Katsumata et al. 2006; Yoshida et al. 2007; Kita et al. 2012; Shiina et al. 2018); thus, we assumed a 3-D velocity structure as shown in Additional file 1. The study area was divided into grids every 10 km in both the longitude and latitude directions on each plane at depths of 0, 5, 10, 22.5, 35, and 50 km. Values of Vp and the Vp/Vs ratio at depths shallower than 10 km were based on Yoshida et al. (2007), and those at depths equal to and deeper than 10 km were based on Shiina et al. (2018).
To support the results obtained by the method described above, we determined the hypocenters by using a homogeneous station method. Seven seismic stations were selected among the 35 stations, and their epicentral distances from the main shock ranged from 14 to 38 km. Seventy aftershocks were relocated for the 4 h immediately after the main shock between 2018-09-06 03:15 and 07:18. The arrival times of both P and S waves were picked at all seven stations for all 70 aftershocks; thus, hypocenters have no variations due to differences in the combination of seismic stations. We used the hypomh program (Hirata and Matsu’ura 1987) to determine hypocenters, and the 1-D Vp structure was assumed to be as follows: 2.6 km/s at a depth of 0 km, 3.4 km/s at a depth of 3 km, and 5.8 km/s at a depth of 7 km. These values are based on Iwasaki et al. (2004). The Vp structure at depths deeper than 7 km was assumed to be as follows: V1 km/s at a depth of (7 + H) km, V2 km/s at a depth of (7 + H) + 5 km, and 9.0 km/s at a depth of (7 + H) + 5+370 km. The four parameters, V1, V2, H, and the Vp/Vs ratio, were determined using a grid search technique. V1, V2, H, and the Vp/Vs ratio ranged from 5.9 to 7.9 km/s with every 0.1 km/s, from V1 + 0.1 to 8.5 km/s with every 0.1 km/s, from 1 to 50 km with every 1 km, and from 1.60 to 1.80 with every 0.01, respectively. The optimal solution for the four parameters was determined under the condition that the residual of the 70 aftershocks reaches a minimum.
To determine the focal mechanism solution of the main shock, we used the HASH method (Hardebeck and Shearer 2002), which is a grid search program using the first-motion polarity of the P wave. P wave first-motion polarities were read manually by careful inspection at 53 seismic stations with epicentral distances from 9 to 114 km. No amplitude data were used.
In the case of the homogeneous station method, the optimal solutions for the four parameters are as follows (Fig. 1d): V1 = 5.9 km/s, V2 = 7.1 km/s, H = 18 km, and Vp/Vs = 1.70. By using the optimal 1-D velocity structure, we located the epicenter of the main shock at (42.681°N, 142.004°E), and the depth was 34.3 km. The spatial patterns described above were also obtained in the homogeneous station method analysis, as shown in Fig. 2. The different analyses presented similar results. Therefore, the two noteworthy spatial patterns obtained in this study, i.e., (1) the deep distribution of aftershocks and (2) the aftershock area consisting of three segments, seemed to be reliable.
Extraordinarily deep aftershock distribution
In this study, we relocated the main shock and the aftershocks of the 2018 Hokkaido Eastern Iburi earthquake and found that almost all earthquakes were concentrated at depths from 20 to 40 km. This range is extraordinarily deeper than other intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Table S1 in Additional file 3 shows that in the case of the 1995 Hyogoken–Nanbu earthquake (MJMA 7.2), the depth range of the aftershock area is from 0 to 20 km (Hirata et al. 1996); the 2000 Western Tottori (MJMA 7.3), 2–13 km (Shibutani et al. 2005); the 2004 mid-Niigata (MJMA 6.8), 2–13 km (Okada et al. 2005); the 2005 West Off Fukuoka (MJMA 7.0), 2–16 km (Shimizu et al. 2006); the 2007 Chuetsu-oki (MJMA 6.8), 5–22 km (Nakahigashi et al. 2012); the 2007 Noto Hanto (MJMA 6.9), 1–13 km (Sakai et al. 2008); the 2008 Iwate-Miyagi Nairiku (MJMA 7.2), 2–8 km (Okada et al. 2012); the 2011 Iwaki (MJMA 7.0), 2–15 km (Kato et al. 2013); the 2016 Kumamoto (MJMA 7.3), 2–18 km (Shito et al. 2017); and the 2016 Tottori (MJMA 6.6), 5–15 km (Ross et al. 2018). On the other hand, the aftershock area is deep in and around the HCZ, Hokkaido. For example, in the cases of the 1970 MJMA = 6.7 Hidaka earthquake and the 1982 MJMA = 7.1 Urakawa-oki earthquake, the aftershocks were located at depths from 20 to 30 km and from 18 to 35 km, respectively (Moriya 1972; Moriya et al. 1983). The 2018 Hokkaido Eastern Iburi earthquake is also located in the HCZ and its deep distribution of aftershocks is very similar to those of previous earthquakes in this region.
The hypocenter of the 2018 earthquake is located near the bottom of the aftershock area. This fact is consistent with a model in which the stress accumulates on a seismic fault in the upper crust due to a weak zone in the lower crust (Iio et al. 2002). The P and S wave velocities, the resistivity, and the pore pressure in and around the aftershock area provide clues for further discussion.
A model of the fault ruptures during the main shock
The rupture process including a stepover segment is well known in the case of strike-slip faulting, e.g., the 1992 Mw 7.3 Landers earthquake (Sieh et al. 1993) and the 2016 Mw 7.8 Kaikoura, New Zealand earthquake (Hamling et al. 2017). Many numerical simulations and theoretical studies on the physical mechanism of rupture propagation consider a stepover segment between strike-slip faults (e.g., Bai and Ampuero 2017). However, little is known about the rupture process with a stepover segment in the case of reverse thrust faulting. In this sense, the 2018 Hokkaido Eastern Iburi earthquake is an unusual earthquake.
We relocated hypocenters of the main shock and aftershocks immediately after the 2018 Hokkaido Eastern Iburi earthquake (MJMA = 6.7) by using two different methods: the hypoDD method with an assumed 3-D velocity structure and a homogeneous station method. As a result, we found that the main shock was located obviously deeper than 30 km, and almost all aftershocks were located at depths between 20 and 40 km. These depths are extraordinarily deeper than those for other intraplate earthquakes in the inland area of Japan. The CMT solution of the main shock is inconsistent with the focal mechanism solution determined by the first-motion polarities of the P wave. This discrepancy is probably explained by the model proposed in this study in which the rupture started as a small left-lateral strike-slip fault in the stepover segment, and afterward, two large reverse faults were triggered in the northern and southern segments.
We thank Takuji Yamada and an anonymous reviewer for valuable comments. We used waveform data from seismic stations maintained by JMA, and we also used waveform data from Hi-net (High-Sensitivity Seismograph Network of Japan) and F-net (Broadband Seismograph Network) maintained by NIED. We used observation equipment at some seismic stations supported by the Earthquake Research Institute Joint Usage/Research Program. GMT-SYSTEM (Wessel and Smith 1991) was used for mapping data. MICAP-G (Naito and Yoshikawa 1999) was used to calculate changes in the Coulomb failure stress.
A list of individual members of the Group for the Aftershock Observations of the 2018 Hokkaido Eastern Iburi Earthquake follows: Kei Katsumata1, Masayoshi Ichiyanagi1, Mako Ohzono1, Hiroshi Aoyama1, Ryo Tanaka1, Masamitsu Takada1, Teruhiro Yamaguchi1, Kazumi Okada1, Hiroaki Takahashi1, Shin’ichi Sakai2, Koji Miyakawa2, Shin’ichi Tanaka2, Miwako Ando2, Satoshi Matsumoto3, Tomomi Okada4, Toru Matsuzawa4, Naoki Uchida4, Ryosuke Azuma4, Ryota Takagi4, Keisuke Yoshida4, Takashi Nakayama4, Satoshi Hirahara4, Toshiko Terakawa5, Yoshiko Yamanaka5, Yuta Maeda5, Shinichiro Horikawa5, Shuichiro Hirano6, Hiroki Miyamachi6, Hiroshi Yakiwara6, Masahiro Kosuga7, Takuto Maeda7, Hiroshi Katao8, Yoshihisa Iio8, Airi Nagaoka8, Noriko Tsumura9, Masahiro Shimazaki9, Tomotake Ueno10, and Youichi Asano10, where 1 Hokkaido University, 2 University of Tokyo, 3 Kyushu University, 4 Tohoku University, 5 Nagoya University, 6 Kagoshima University, 7 Hirosaki University, 8 Kyoto University, 9 Chiba University, and 10 National Research Institute for Earth Science and Disaster Resilience.
This study was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program. This study was also partly supported by MEXT KAKENHI Grant JP18K19952.
MI read all arrival times of P and S waves and determined hypocenters using the tomoDD method. KK performed the homogeneous station method analysis, the HASH analysis and was a major contributor in writing the manuscript. MO, HA, RT, MT, TY, KO, and HT are the technical staff members who maintain the seismic stations of Hokkaido University. SS, SM, TO, TM, SH, TT, SH, MK, HK, YI, AN, NT, TU, and members of the Group for the Aftershock Observations are the assistant staff to maintain the seismic stations. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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