Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone
© Matsumoto et al. 2015
Received: 27 March 2015
Accepted: 14 October 2015
Published: 23 October 2015
We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes. We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field. Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area. The minimum principal compression stress (σ3), with its near north–south trend, is dominant throughout the entire region. However, the σ 3 axes around the shear zone are rotated normal to the zone. This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data. Conversely, the stress field of the normal fault is dominant in the Beppu–Shimabara area, which is located in the central part of the island. This result and the direction of σ3 are consistent with the formation of a graben structure in the area.
A stress field is an important parameter for understanding the dynamics of the Earth’s crust and earthquake occurrence. Following the development of techniques that can estimate the stress field from the focal mechanism data of earthquakes (Angelier 1979, 1984; Michael 1984), many studies have estimated the relative deviatoric stress fields at various tectonic settings. These studies have concluded that the stress field reveals heterogeneous features that relate to the loading processes acting on the medium. This spatial variation of the stress field is attributed to both non-uniform mechanical forces acting on the outer boundary of the medium and/or the inelastic behavior of the medium interior, such as during faulting and magma intrusion (Matsumoto et al. 2012), both of which are possible causes of spatial variation in the observed seismicity. For instance, the spatial and temporal changes in a stress field can be attributed to the behavior of a shear zone such as the San Andreas Fault zone (Zoback et al. 1987; Hardebeck and Hauksson 2001; Townend and Zoback 2001, 2004; Hardebeck and Michael 2004). Therefore, detailed estimation of the stress field can facilitate the understanding of tectonic processes.
Kyushu Island is located in the southwestern part of Japan. It lies on the overriding plate above the subducting Philippine Sea slab (PHS) to the west of the Nankai Trough subduction zone. A large right-lateral shear zone running through the central part of Kyushu Island divides the island in terms of its geologic features. The shear zone is an extension of the median tectonic line (MTL) that originates at Honshu Island and transects Shikoku. The MTL is the largest tectonic line in southwestern Japan and is mainly composed of metamorphic rocks. In the present-day tectonic setting, geological studies have reported right-lateral strike-slip and extensional movements along the MTL on Kyushu Island (e.g., Okada 1980; Kamata and Kodama 1994, 1999). An area transecting in the EW direction at the central part of Kyushu, which is located to the north of the MTL from the city of Beppu to the Unzen Volcano at Shimabara Peninsula, is called the Beppu–Shimabara graben (hereafter, B–U area) owing to the many normal faults around and between Beppu and Unzen. Itoh et al. (2014) investigated the basin-forming process at the Beppu region, which revealed that the major structure was half-graben under extensional stress in N–S, and PHS convergence strongly affected formation in the region.
Kyushu Island is one of the regions in Japan that hosts significant volcanic activity. The volcanic front runs through the central part of the island from north to south, and a group of volcanoes is also located on a line obliquely crossing the volcanic front. The active volcanoes in this group are Beppu, Kuju, Aso, and Unzen. Their locations are shown in Fig. 1. This figure also shows active faults mapped by the National Institute of Advanced Industrial Science and Technology (hereafter, AIST) as well as the epicenter distribution of micro-earthquakes with depths of less than 30 km. As seen in Fig. 1, the main areas with high seismicity of micro-earthquakes are the B–U area and the northern offshore area of the island. The high activity observed in the northern part corresponds to the aftershock area of the M7.0 2005 Fukuoka earthquake (Shimizu et al. 2006). The seismic activity in the central part of Kyushu Island remains stable and high, thus requiring several intrinsic conditions of stress and/or strength of the medium to explain the occurrence of earthquakes. Townend and Zoback (2006) and Terakawa and Matsu’ura (2010) found that the dominant direction of minimum principal stress (σ 3) generally trends N–S in southwestern Japan. However, the small amount of data from Kyushu Island has only provided a spatial resolution of several tens of kilometers. Therefore, we have attempted to estimate the local stress field at a higher resolution with significant quantities of data on the focal mechanisms in this area.
Focal mechanisms and the stress field at Kyushu Island
We used focal mechanisms to determine the stress condition in the seismogenic layer. Shallow earthquakes with magnitudes greater than 1.0 that occurred at depths ranging from 0 to 30 km from January 1993 to July 2013 were analyzed in this study. The locations of the seismic stations used in the analysis have been plotted in Fig. 1. The stations were installed by Kyushu, Kyoto, and Kagoshima Universities, by Hi-net of the National Research Institute for Earth Science and Disaster Prevention (NIED), and by the Japan Meteorological Agency (JMA); several stations were set up temporarily to determine the focal mechanisms with high accuracy. Most of the temporal seismic stations have been installed since 2005. The hypocenters in this area, with the exception of the Fukuoka earthquake aftershocks, can be determined by using three-dimensional velocity structures obtained through the tomographic inversion of travel-time data (Saiga et al. 2010). The hypocenter for the Fukuoka area was determined by the velocity structure of Hori et al. (2006) because those data provided a finer structure after high-aftershock activity in the area as compared to the data of Saiga et al. (2010). As many as 40,981 hypocenters have been located over approximately 20 years. A focal mechanism was estimated from the polarity data on the onset of the first P wave at eight or more stations by using the HASH algorithm of Hardebeck and Shearer (2002). Only the focal mechanisms characterized by a low misfit of the algorithm (i.e., L < 0.2, where L is the misfit ratio to the total number of polarity data with a weighting factor calculated by the P wave amplitude of the focal mechanism, and S < 0.4, where S is the sum of the weighting factors per the total number of polarity data) were used in this study. A total of 9177 focal mechanisms were adopted for the stress analysis.
where σ 1, σ 2, and σ 3 are the maximum, moderate, and minimum compressional principal stresses, respectively. A relative deviatoric stress has been estimated from the data of the seismic slip direction that coincides with that of the shear traction acting on the pre-existing fault since its absolute value is not able to be obtained from only slip direction data. Hardebeck and Michael (2006) developed a method to estimate the spatial variation of the stress field from parametric data gathered at spatially distributed grid points. In this study, we have adopted their algorithm to reveal the heterogeneous structure of the stress field on Kyushu Island. The relative deviatoric stress tensors were estimated at grid points set at latitudinal and longitudinal intervals of 0.15°. We have assumed a heterogeneous stress field on a scale larger than approximately 15 km. The resolution in this study was, therefore, chosen to be 15 km. Stress in the crust is known to increase with depth because of overburden pressure. Therefore, the relative deviatoric stress field also could vary in the vertical direction. However, the depth distribution of earthquakes is not always uniform; instead, it is typically clustered. Thus, the resolution of the stress inversion reduces with depth, and therefore, a two-dimensional relative deviatoric stress field has been assumed in this analysis. This assumption is applicable for the stabilization of parameter estimations in the inversion.
Stress regimes at Kyushu
As seen in Figs. 3a and 4, the minimum principal compression is stable over the entire island of Kyushu. The dominant σ 3 direction implies that Kyushu Island is undergoing extensional tectonics at present. According to a geological study by Kamata and Kodama (1999), this extensional feature can be attributed to the behavior of the Ryukyu Trench, which is located southeast of Kyushu Island, and this trench might act relatively as an extension force on Kyushu by its roll-back. The behavior of the trench possibly induces seafloor spreading at the Okinawa Trough, northeast of the trench. The direction of the spreading is NNW–SSE, which could also be the reason behind the dominance of σ Hmin at Kyushu Island.
On the basis of the stress field characteristics, the island can be divided into the following three regions: the north area, the B–U area, and the south area. The northern region, which is defined as the area north of 33.3° N, is dominantly a strike-slip regime, and σ Hmin is in the NNW–SSE direction. Most of the active fault traces (purple segments in Fig. 3) are oriented toward a favorable direction for shear faulting. Shear stress is especially large in the Fukuoka earthquake area. The northern B–U area might be bounded by the faults near 33.3° N that strike E–W, and the southern part is bounded by the MTL shear zone. The stress field in the B–U area is represented by σ Hmin in the N–S direction and is dominantly characterized by normal faults. This trend is consistent with the direction of the formation of the graben structure seen in the B–U area. Although the normal faults at the surface only appear in the Hohi volcanic zone (around the Beppu region) and the Unzen area (Kamata and Kodama 1994), the entire B–U area is under the stress regime characteristic of normal faults. In addition, the σ Hmin direction varies from N–S to NNW–SSE toward the shear zone. The southern area lies to the south of the shear zone. In this area, the direction of σ Hmin is either NNW–SSE or NW–SE. The σ Hmin directions at the shear zone tend to be normal to the strike of the active faults.
According to a paleostress study in the southern area (Yamaji 2003), σ Hmin was acting in the WNW–ESE direction during Late Pliocene. The present-day stress has an opposite sense, thus indicating that the extensional tectonics of the Late Pliocene have terminated and changed to compression along the WNW–ESE direction. Yamaji (2003) suggested that a tectonic event, such as a change in the direction of subduction of the PHS from NNW to NW, led to this change. This change then enhanced the collision between Sikoku and Kyushu because the subduction rate of the PHS increased. Seno (1999) investigated the stress field in Kyushu and pointed out that the spatial characteristics of the stress field could not be explained by simple plate interactions. Basically, the stress profile he proposed is similar to the stress field suggested in this study. However, there are some differences from his model, which involve the stress regime of the normal stress in the Beppu area and the strike-slip movement in western Kyushu. According to our results, the normal stress regime in Beppu is still functional. Minimum principal stress is oriented along the N–S or NNW–SSE direction over all of Kyushu Island.
Decreasing shear stress at the MTL on Kyushu
The MTL is a geologic boundary, which has been undergoing right-lateral motion since the Quaternary Period (e.g., Kamata and Kodama 1994). At this geologic boundary, the seismic/aseismic slip could transition to a “shear zone.” We have found that the direction of σ Hmin is normal to the shear zone at the boundary between the B–U and the southern area, which indicates that the shear stress does not act on the faults in the MTL. The σ Hmin gradually changes its direction to normal to the shear zone from both the B–U and the southern area. A possible interpretation for this change is that the shear zone releases shear stress as a weak zone. In addition, there is a difference between the shear strain rate and the stress directions in the southern area, as indicated by the extracted points 1, 2, and 3 in Fig. 5. The shear strains in the extracted points are obliquely oriented to the fault, which could produce shear stresses on the shear zone at present. However, the principal stress directions exhibit no shear stress on the fault. This suggests that the shear zone behaves as a shear stress reduction system. Wallace et al. (2009) estimated that there is a dextral displacement of 7 mm/y at the MTL in Kyushu by using GPS data at the shear zone. Two factors, namely (1) geodetically detected right-lateral motion at the shear zone and (2) perpendicular σ Hmin to the shear zone, provide a reasonable explanation for the interpreted reduction in shear stress at this zone. The shear stress reduction at the zone results in an increase in σ Hmin and decrease in σ Hmax, which could create a stress regime characteristic of normal faults. However, the stress regime at the shear zone is still the strike-slip type. This implies that σ Hmax is higher than σ v even as differential stress is reduced by right-lateral slip at the shear zone. Therefore, we need to consider other mechanisms that may have caused the normal stress regime in the B–U area (i.e., σ v becomes the maximum principal stress).
Stress field in the B–U area
As mentioned earlier, the estimated stress fields in the northern and southern areas are characterized by strike-slip regimes with σ Hmin in the NW–SE or NNW–SSE direction. In contrast, σ Hmin is oriented N–S at the B–U area. In addition, the stress ratio ϕ’ in the B–U area reveals that σ Hmax is equal to or smaller than σ v. The change in stress regime in the B–U from both the northern and southern area requires a declining mechanism of σ Hmax in this area. The shear stress reduction at the shear zone is insufficient to decrease σ Hmax down to σ v as described above.
In Fig. 5, the principal strain directions are similar to the stress directions in the B–U area. In addition, an area with a large contraction in the E–W direction can be found around the Aso and Kuju volcanoes. This large contraction might relate to volcanic activity of the volcanoes and indicates the possibility of the existence of a mechanical anomalous structure. For example, in the case that the area is filled up with material with a low elastic coefficient, the area could behave as a weak zone. This is a possible cause for the decline of σ Hmax. The principal strain-rate-oriented E–W corresponds to the σ Hmax direction. This relation suggests that linear elastic behavior dominates in the surrounding area of the large contraction, although the value of stress cannot directly compare to the strain rate. Therefore, strengths of σ Hmax east and west of the area could be low because of the contraction.
predominantly, σ Hmin is a major feature of the stress field in Kyushu.
the stress condition is a normal fault, or σ Hmax is equal to σ v, in the B–U area, which facilitated the graben formation.
the σ Hmin direction is rotated normal to the MTL shear zone, thus suggesting that shear stress reduction occurs as implied by the strain-rate field.
Further studies will take a quantitative approach to model the tectonic stress and strain-rate field.
We would like to thank Prof. Takenaka and two anonymous reviewers for their valuable comments that helped us to revise our manuscript. We thank all the staff and students of Kyushu and Kyoto Universities for their effort to obtain good-quality seismic data. We used seismic data from Kyushu, Kyoto, Kagoshima, and Tokyo Universities, the JMA, and Hi-net (NIED). This study was supported by the Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions (MEXT) and MEXT KAKENHI (No. 26109004).
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