Characteristics of relocated hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its aftershocks with a three-dimensional seismic velocity structure
KeywordsRelocations of hypocenters The Hidaka collision zone Hokkaido Seismic structures
In this area, large inland earthquakes with magnitudes of approximately M7 have occurred repeatedly at intervals of approximately 40 years, including the 1931 Mj 6.8, the 1932 Mj 7.0 (Utsu 1999; Ichikawa 1971), and two recent large events—one in 1970 and one in 1982—at depths of 20–30 km for the 1970 Mj 6.7 Hidaka earthquake (Ichikawa 1971; Moriya 1972) and 18–35 km for the 1982 Mj 7.1 Urakawa-oki earthquake (Moriya et al. 1983) (Fig. 1a). These focal depths are anomalously larger than those of typical inland crustal earthquakes in the Japanese islands (~ 15 km Omuralieva et al. 2012).
In the determination of the hypocenter by the JMA, the routine one-dimensional seismic structure is usually applied to seismic data. In the present study, I relocated hypocenters using the three-dimensional seismic structure to examine the focal depth of mainshock, its relationship to previous large events, the detailed collision zone process and the tectonic setting of Hokkaido, including active faults.
Data and method
In total, 5109 events were relocated. The average root mean square value of the DDs for each event was reduced by this procedure from 1.29 to 0.14 s. Using observations with random noises corresponding to picking errors (standard deviations of 0.10 s for P waves and 0.20 s for S waves), the mean errors were estimated to be 0.65 km in depth and 0.54 km in the horizontal direction. The results for relocated hypocenters are shown in Fig. 3. The hypocenters parameters of the main shock and its aftershocks are shown in Additional file 2. After relocation, the depth of the mainshock became 35.4 km with estimated errors of 0.23 km in depth and 0.26 km in the horizontal direction. Comparing the depths of the relocated and initial hypocenters at depths of less than 30 km and those at depths of more than 30 km, the depths of the relocation results were, respectively, 4.2 km and 5.1 km (on average) shallower than those of the initial hypocenters.
Figure 3 shows horizontal and vertical slices of the hypocenter location. The relocated mainshock is also plotted on the tectonic setting presentation figure (Fig. 1a). The mainshock and most of the aftershocks are relocated eastward of the Ishikari Lowlands (Ishikari-teichi-toen) fault zone. In Fig. 3, the relocated aftershocks and mainshock are generally found at depths of 10 to 40 km, striking in the north–south direction and inclined eastward at less than 60°. The relocated aftershocks and mainshock are also plotted on the seismic velocity structure (Kita et al. 2012) shown in Figs. 4a, g, and 5. The aftershocks and mainshock are located in the portions of Vp < 7.0 km/s and Vs < 4.0 km/s, parallel to high-velocity-material (Vp > 7.6 km/s and Vs > 4.2 km/s) intrusion at depths of 30 to 40 km (Figs. 4a, g, 5h, i, k, l). The aftershocks at depths of 10 to 20 km are also located parallel to high-velocity materials and the tectonic line near the Ishikari-teichi-toen fault zone (Fig. 5a–f). The mainshock and aftershocks are also plotted on the seismic attenuation structure (Kita et al. 2014) shown in Fig. 6. The aftershocks at depths of 7.5 to 22.5 km are clearly located at the western edge of the high-attenuation zone (Qp = 300), which approximately corresponds to the Ishikashi-teichi-toen fault zone (Fig. 6a, b). The mainshock lies in the moderate attenuation portions (Fig. 6c, e, f).
Ishikari-teichi-toen fault zone and the Hidada collision zone
Kita et al. (2012) indicated that a broad low-velocity zone of P and S waves, with velocities consistent with those of crustal rocks (Vp < 7.2 km/s, Vs < 4.2 km/s), was revealed west of the Hidaka main thrust at depths of 35–90 km (Figs. 4, 5). The Vp/Vs ratio values in this low-velocity zone ranged from 1.60 to 1.71, suggesting that the low-velocity zone is not serpentinized mantle wedge material, because Vp/Vs values for serpentine are generally larger than 2.0 (Christensen 1996). In Fig. 4c through f, the low-velocity zone (crust materials) exists at depths of more than 40 km and contacted the upper surface of the subducting Pacific plate beneath the westward area of the Hidaka main thrust, whereas thickened crust materials exist at depths of less than 40 km in Figs. 1b, 4a and g, including the 2018 mainshock and the aftershocks. These figures showed that the aftershocks and mainshock are located on the northwestern edge of the thickened crust materials related to the Hidaka collision zone beneath the NE Japan arc. The mainshock and aftershocks also lied near the high-velocity-material intrusion (Vp > 7.6 km/s and Vs > 4.2 km/s) (Figs. 4a, g, 5h, i, k, l), which corresponds to uppermost mantle materials (e.g., serpentine and/or peridotite (Christensen 1996).
The relocated hypocenters of the mainshock and aftershocks are generally located at depths of 10 to 40 km (Fig. 3b, c), striking in the north–south direction (Fig. 3a), inclined eastward at less than 60° (Fig. 3b) and parallel to the high-velocity-material intrusion (Figs. 1b, 5g, h, k, l). Kita et al. (2012) found a broad low-velocity zone of P and S waves with velocities consistent with those of crustal rocks and also found that several small-scale high-velocity zones are located at depths of 0–35 km inclined northeastward at 40°–60° (Fig. 4). All of these anomalous high-velocity zones are located at deeper extensions of Neogene thrust faults (Fig. 4b–d and h–j, especially Fig. 4d and i). Moreover, the fault planes of the 1970 M 6.7 Hidaka earthquake and the 1982 M 7.1 Urakawa-oki earthquake occur in the crust materials beneath the NE Japan arc, and are located near the sharp boundaries of this anomalous high-velocity zone and this low-velocity zone (Figs. 1c, 4d in the present study, and Fig. 11 of Kita et al. (2012). Kita et al. indicated the possibility that these two events occurred through Neogene thrust faults, which penetrate from the shallower portion to the deeper portion (at least 40 km) in the collision zone (Fig. 1b). They thought that these Neogene thrust faults were important to the collision process. On the other hand, the cross-sectional view in Fig. 3b shows that the 60° inclined hypocenter sequence appears to be located in the deeper extension of the Ishikari-teichi-toen fault zone. The seismic attenuation structure at depths of 10, 20, and 30 km by Kita et al. (2014) also seems to support that the Ishikari-teichi-toen fault zone is a highly inclined fault (Fig. 6a). The highly inclined fault indicated in the present study is not inconsistent with the 74° inclined fault estimated by the InSAR data analysis (Geospatial Information Authority of Japan 2018a, b; Kobayashi 2019). Since the dip of the fault is approximately the same as the dip of the sharp material boundaries in central Hidaka indicated by Kite et al., the present study suggests that the 2018 M 6.7 event occurs by means of old fracture, such as the Ishikari-teichi-toen fault zone. Geographical studies, however, reported that the fault plane of the Ishikari-teichi-toen fault zone has a low inclined dip (Earthquake Research Committee 2010). Since their study depth only covered the portion near the surface, the dip of the fault zone at a shallow depth would not be the same as the dip indicated by the present study. Gravity anomaly research reports did not show a large difference between the regions eastward and westward of the Ishikari-teichi-toen fault zone (Earthquake Research Committee 2010; Komazawa 2014), which does not clearly suggest a large previous dip slip activity. Based on a geological study, the Neogene (Miocene) thrust faults in the Hidaka collision zone, including the Ishikari-teichi-toen fault zone, were originally strike faults, having a long history of strike faulting from 25 Ma before the arc–arc collision started (12 Ma) (Maeda 1990; Kimura 1996; Kusunoki and Kimura 1998; Jolivet et al. 2018). Moreover, based on the geochemical analysis of zircon, a petrological study (Kemp et al. 2007) shows the intrusion of gabbro into the accretionary prism of the Cretaceous in the Hidada metamorphic belt occurred by the magmatism due to an asthenospheric upwelling related to the backarc spreading in the Miocene (19 Ma) (Kimura 1996). The effect on the gravity anomalies by the gabbro intrusion and strike slip faulting can be greater than that by the dip slip of the high-angle fault. The accretionary prism of the Cretaceous (130 to 100 Ma) consists of fractions of oceanic crust materials and thrusts. The Miocene marine sedimentary rocks are reported in the surface geology of the Eastern Iburi earthquake region in the Ishikari Lowland (Geological Survey of Japan AIST 2006) and seem to be located over the Cretaceous accretionary prism therein (e.g., Figure 1 of Ueda 2005). Geographical studies reported that the fault plane within the sedimentary rocks, but the relationship of detailed locations of the sedimentary rocks and the Cretaceous accretionary prism remains unknown. Therefore, I consider the possibility that the 2018 M6.7 event occurs by means of a fault in the crustal materials related to the Cretaceous accretionary prism beneath the Ishikari-teichi-toen fault zone. The fault possibly connects to the Ishikari-teichi-toen fault zone at deeper depths (Fig. 1b). Near the Ishikari-teichi-toen fault zone, other small faults are also located in the deeper extension of the aftershocks hypocenters of the 2018 event. Using data from the Daichi-2 satellite, the Geospatial Information Authority of Japan shows that the pattern of the InSAR clearly changed beneath the eastern region of the Ishikari-teichi-toen fault zone (westward of the epicenter of the mainshock of the 2018 event) in the NNW–SSE direction (Earthquake Research Committee 2018), although the problem of less sensitivity of INSAR for a deeper part of the rupture must not be neglected. The gravity anomaly pattern (Komazawa 2014) is also changed at the boundary of the InSAR pattern, which corresponds to locations of the Atsuma fault (Kimura 2019) and the Atsuma syncline reported by Matsuno and Ishida (1960). A recent reflection study reports the fault plane of the Atsuma fault inclined eastward and the dip was ~ 60° at depths of ~ 1 km and ~ 20° at depths of more than 2 km (Kozuka et al. 2002). The location of the Atsuma fault is shown in Fig. 3a. Further discussion of the 2018 event along local geoscientific characteristics is intended as an area for future study.
Based on not only the geological evidences, but also the results of geodetic studies using triangulation/trilateration data during several decades (e.g., Tada and Kimura 1987), Kimura (1986, 1996) proposed that the Kuril forearc sliver in the southeastern Hokkaido is moving southwestward and has been colliding with the NE Japan arc at the Hidaka collision zone since 12 Ma. Recently, our understanding of crustal deformation before, during and after large earthquakes has been greatly improved by Global Navigation Satellite System (GNSS) observations with the improved temporal resolution from conventional geodetic observations such as triangulation/trilateration. Interseismic north-westward deformation of the forearc regions in Hokkaido recently observed by GNSS has been mainly explained by deformations due to locking between the continental and Pacific plates (Ito et al. 2000; Suwa et al. 2006; Hashimoto et al. 2009; Itoh et al. 2019). A block model assuming the Kuril forearc sliver as well as interplate locking did not fit the recent GNSS data well, either (Loveless and Meade 2010). The Kuril forearc sliver migration indicated by the geological studies could be presently small enough to be hidden by deformation due to the interplate locking, which may be related to the reason why large gravity anomalies are not reported beneath the Eastern Iburi earthquake region.
Figure 5 shows the horizontal sections of seismic structures, indicating also that the hypocenter sequence of the mainshock and aftershocks was located in the anomalous deep forearc crust materials. The hypocenter sequence was also located parallel to the sharp boundary of the crust and mantle materials at depths of 30 to 35 km (green ellipse in Fig. 5h, i, k, l), and also located parallel to high-velocity crust materials at depths of 15 to 25 km (green ellipse in Fig. 5b, c, e, f, g, j). Although the high-velocity-material intrusion with velocities of uppermost mantle materials westward of the hypocenter sequence is very small, the restoring resolution test (Zhao et al. 1992) by Kita et al. (2012) has shown that this intrusion structure of high-velocity materials is believable (Fig. 5 in Kita et al. 2012). The western edge of the low Qp (Qp = 300) in the Hidaka collision zone corresponds to the Ishikari-teichi-toen fault zone at depths of 10 and 20 km, at which the aftershocks of the 2018 events are located (Fig. 6a, b). Therefore, the main shock and aftershocks occurred in the crust material, but using the sharp material boundaries between the uppermost mantle and crustal materials (Fig. 1b). This kind of relationship between the hypocenters and material distributions is the same as that of the 2018 M6.7 event, and the 1970 and the 1982 events in the Hidaka collision zone. Oman ophiolite studies have shown that dunite of several hundred meters in width, which is easily deformed compared to other peridotite rocks due to the weakest crystal structure among them, is always reported in the Moho transition zone of the Oman ophiolite (e.g., Boudier and Nicolas 1995; Jousselin et al. 1998). The velocities of the uppermost mantle material intrusion near the fault of the Iburi eastern earthquake are slightly smaller than those of dunite (Christensen 1996), but strain may accumulate on the material boundaries of uppermost mantle and crust materials near the main shock before the beginning of the rupture of the mainshock, which could be a trigger of the mainshock.
Cause of the great depth and heat flow studies
The mainshocks of the 2018 M6.7 event and the 1970 and the 1982 events in the Hidaka collision zone have a greater depth than typical inland earthquakes in the Japan archipelago. Since the greater depth limit of inland earthquake is generally related to weakening the temperature of quartz, a locally low temperature beneath and north-westward of the Hidaka collision zone could cause anomalous deep large-magnitude events.
Commonality with damaging inland crustal earthquakes in Hokkaido
Damaging inland crustal earthquakes at depths of 10 to 30 km in Hokkaido reported by Utsu (1999) and Kita et al. (2014) are shown in Fig. 6. Most of these events are located at the edge of the low-Q zone (Qp < 166). The 2018 event itself is located in a mid-Qp zone, but close to the low-Qp part of the Hidaka collision zone. The surrounding areas of the low-Qp zone may experience future damaging earthquakes.
I relocated the hypocenters of microearthquakes, including the aftershocks and mainshock of the 2018 M6.7 inland type event in south-central Hokkaido, using a high-resolution three-dimensional seismic velocity structure. The results of the relocation show that the aftershocks are generally relocated at depths ranging from 10 to 40 km, and the focal depth of the mainshock changed from 37.0 km to 35.4 km after the relocation. The relocated hypocenters of the mainshock and aftershocks were located in the anomalous deep forearc crust materials beneath the northwestern edge of the Hidaka collision zone. Like the previous two large events in the collision zone, the focal depth of the mainshock is located in the crust materials parallel to the uppermost mantle intrusions beneath the NE Japan forearc and is anomalously larger than the average inland crustal earthquakes beneath the Japanese archipelago. These large events have a role in the ongoing collision zone process. These anomalously large focal depths were caused by the locally low temperature reported in south-central Hokkaido.
The fault plane was approximately 60° inclined and appears to be located in the deeper extension of the Ishikari-teichi-toen fault zone. The aftershocks of the 2018 event also lie in the western edge of the high-attenuation zone, parallel to the Ishikari-teichi-toen fault zone. Like the previous two events, the 2018 M6.7 event occurred using one of material boundaries, which could be related with one of old thrust (dip slip) faults like the Ishikari-teichi-toen fault zone, though the detailed relationship between the thrust fault reported on the surface and the 2018 event has been still unknown.
The previous seismological studies have thought that the anomalously deep crustal materials, in which the 2018 M6.7 and previous two events occurred, are mainly formed by the arc–arc collision process. Not only the collision process itself but also the geological processes before the collision could be related with the formation of the thickened crust. Consideration of this geological history is needed for understanding the present observation results of seismological, geodetic, geological, petrological and geographical studies and revealing the overall of the Hidaka collision zone process.
I would like to thank Yuji Itoh, Asuka Yamaguchi, Gaku Kimura, Eiichi Takazawa, Makoto Matsubara, Akira Hasegawa and Toru Matsuzawa for the helpful discussions. I also thank the anonymous reviewers for beneficial and constructive comments and editors for the review process of my paper.
This paper has only one author, Saeko Kita, who processed the seismological data, determined hypocenters, made all figures and wrote this paper.
This study was also supported by the Japan Society for the Promotion of Science (JSPS) through Grants-in-Aid for Scientific Research 17K05637, 25800243 and 23740329.
The author declares that they have no competing interests.
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