Deep structure of the Ou mountain range strain concentration zone and the focal area of the 2008 Iwate-Miyagi Nairiku earthquake, NE Japan—seismogenesis related with magma and crustal fluid
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The 2008 Iwate-Miyagi Nairiku earthquake (M 7.2) occurred on June 14, 2008 in a zone of concentrated deformation along the mountain backbone of Tohoku encompassing SW Iwate and NW Miyagi Prefectures, NE Japan. This earthquake was a shallow intraplate earthquake with the reverse-type focal mechanism. Regular seismic activity is high in this concentrated deformation zone. We have performed regional-scale seismic tomography in and around the focal area and found that the distinct low-velocity regions are continuously distributed from the mantle wedge to the lower crust just below the focal area and active seismic zones. This low-velocity zone can be interpreted to be a region of partial melting, suggesting that crustal fluids separating and upwelling from deeper portion may be closely related with the occurrence of both the active seismic zones and the 2008 earthquake.
Key words2008 Iwate-Miyagi Nairiku earthquake intraplate focal area crustal fluids upwelling low-velocity belt
In the Tohoku region (northeast (NE) Honshu), NE Japan, the Pacific Plate subducts beneath the overiding plate. Since the boundary between the Pacific Plate and the overriding plate is coupled in the shallow region, the continental crust (island arc crust) is deformed under compressive stress in the direction of relative motion of the plate, which is approximately east-west. Deformation of the island arc crust due to this compressive stress can be observed from global positioning system (GPS) data. Figure 1 shows the east-west strain distribution obtained from GPS data acquired by the Geographic Survey Institute and Tohoku University between 1997 and 2001 (Miura et al., 2004). Blue indicates regions of compressive strain and red shows regions of tensile strain. A prominent region of compressive strain can be identified along the Ou Mountain Range (Miura et al., 2002). The existence of this a region can also be confirmed from the distribution of horizontal crustal deformation velocity over the last 100 years (Hasegawa et al., 2000). Several earthquakes have occurred in this zone of concentrated deformation along the Ou Mountain Range, including a number of large ones (for example, the 1896 Rikuu earthquake), as shown in Fig. 1(a).
Hasegawa et al. (2005) hypothesized on the possible source of such a concentrated deformation of the crust in NE Japan. The distribution of S-wave velocity perturbation at a depth of 40 km in this region is shown in Fig. 2(a) (Nakajima et al., 2001). In a subduction zone such as Tohoku, the hydrated minerals in the subducting oceanic slab (Pacific Plate) undergo dehydration and decomposition as the temperature and pressure of the slab increase with subduction, and the water thus released migrates into the overlying mantle wedge. This water is eventually entrained in an upwelling sheet flow within the mantle wedge, penetrating into the crust just beneath the volcanic front (i.e., the Ou Mountain Range). Figure 2(a) depicts a distinct low-velocity belt that corresponds to the upwelling flow that reaches the Moho along the mountain range. As the seismic velocity is lower in regions of high temperature and large amounts of fluid, the upwelling flow, as well as the magma and crustal fluid, can be imaged as regions of lower S-wave velocity (Fig. 2(b)). When the melt within the upwelling flow intrudes into the crust and cools, water released from the solidified melt migrates upward. Through this process, water from the subducted Pacific slab is transported to the shallow crust beneath the mountain range.
The source region of the 2008 Iwate-Miyagi Nairiku earthquake is indicated by the solid red rectangle in Figs. 1 and 2. This earthquake was an intraplate earthquake that occurred in the concentrated deformation zone along the Ou Mountain Range. In the study reported here, we examined the hypocentral region in more detail with the aim of determining the relationship between the concentrated deformation zone, including the hypocentral region, of the 2008 Iwate-Miyagi Nairiku earthquake and the upwelling fluid that can be imaged as a seismic low-velocity zone.
2. Data and Method
We performed regional-scale tomography using dense temporary seismic networks in and around the focal area of the 2008 Iwate-Miyagi Nairiku earthquake, determining b the three-dimensional seismic velocity structure and relocating hypocenters simultaneously using double-difference tomography (Zhang and Thurber, 2003). Figure 3 shows the data set used in this study. Travel time data were obtained from the Japanese universities joint seismic observation in the Tohoku Backbone range (1997–1998) and the e aftershock observation for the 2003 Northern Miyagi earthquake (Okada et al., 2003; Umino et al., 2003). We also used data from routinely operated stations of Tohoku University, JMA, and Hi-net in the period from 1997 to 2007. Note that we have included the events (blue) within the subducting Pacific slab in the tomography inversion in order to image the lower crust structure. The initial velocity structure is taken from Hasegawa et al. (1978). The total number of earthquakes is 25844. We used a grid net with an interval of about 6 km (0.5 degrees). The depth of the interval was also 6 km. The numbers of P- and S-wave arrival times are 504362 and 381719, respectively, and those of DD for P-and S-wave arrival times are 2668428 and 1885441, respectively. The travel time residual varies from 0.24 to 0.09 s.
3. Results and Discussion
3.1 Relation between shallow seismic activity and deep crustal structure
Low-velocity regions have been recognized just below the source regions of the 1962 North Miyagi earthquake (M 6.2), the 1996 Akita-Miyagi earthquake (M 5.9) (Nakajima and Hasegawa, 2003; Asano et al., 2004) (Fig. 7(d)), the 1970 South Akita earthquake (M 6.2) (Umino et al., 2000) (Fig. 7(b)), and the 2003 North Miyagi earthquake (M 6.4) (Okada et al., 2003; Umino et al., 2003) (Fig. 7(e)). It thus appears that fluid supplied from a deeper portion contributed to the occurrence of these earthquakes (e.g., Miller et al., 2004).
For example, in the cases of the 2003 North Miyagi earthquake, the source fault was found to have steep dips. It has therefore been inferred that the source was reactivated through the compressive inversion of normal faults formed during the opening of the Japan Sea (Okada et al., 2003; Umino et al., 2003). For this kind of inversion of a steeply dipping normal fault (as a reverse fault) to occur, there would have to be high pore-fluid pressure to reduce the frictional strength acting on the fault (Sibson, 1990). The low-velocity region in the source region has been identified as a fluid source, and it has been inferred that the source region itself has high pore-fluid pressure (Okada et al., 2007).
The focal area of the 2008 earthquake is located where these two seismic zones and the seismic low-velocity zone in the lower crust are closely located. The presence of a low-velocity region just beneath the source region of the main shock of the 2008 earthquake (see Fig. 7(c)) similarly suggests that fluid supplied from depth contributed to the occurrence of this earthquake.
However, the resolution in our study may not be fine enough to discuss the shallow structure that shows the path of fluid in the mid-to upper crust separated from the possible partial melt in the lower crust, which is imaged as a low-velocity zone in this study. A more detailed description of the seismic velocity structure in and around the fault of the 2008 earthquake based on dense aftershock observation data (e.g., Okada et al., 2009) is necessary for further discussion of this subject.
3.2 Upwelling magma flow to the volcanoes and its relation with the fault of the 2008 earthquake
The 2008 Iwate-Miyagi earthquake occurred in an area of concentrated deformation along the Ou backbone mountain range of Tohoku, in the vicinity of several active volcanoes, including Mts. Kurikoma and Yakeishi-dake and the Onikobe and Naruko volcanoes. We have found a distinct seismic low-velocity zone just beneath the concentrated deformation zone and the focal area of the 2008 Iwate-Miyagi earthquake by seismic tomography using regional seismic observation data. The low-velocity zone extends from the uppermost mantle to each active volcano at the surface. These observations suggest that the high-temperature upwelling partial melt and the crustal fluid released from it are closely related with the concentration of crustal deformation and the occurrence of the 2008 Iwate-Miyagi earthquake.
We used data from JMA and Hi-net/NIED as well as from JNES (Japan Nuclear Energy Safety Organization). We thank Prof. Cliff Thurber and Dr. Haijang Zhang for providing their programs and valuable discussion. We also thank R. Sibson, S. Miura, J. Nakajima, N. Uchida, Y. Ohta, T. Iinuma, M. Ohzono and T. Matsuzawa for fruitful discussion. We acknowledge T. Sato, S. Hori, K. Tachibana, T. Kono, T. Nakayama, S. Hirahara and S. Suzuki for their efforts during the seismic observations. We would like to thank the editor (Takashi Iidaka), and the reviewers (Shinji Toda and an anonymous reviewer) for helpful comments. This work was conducted with the support of Grant-in-Aid for Special Purposes, MEXT, Japan.
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