Hypocenter distribution and heterogeneous seismic velocity structure in and around the focal area of the 2008 Iwate-Miyagi Nairiku Earthquake, NE Japan—Possible seismological evidence for a fluid driven compressional inversion earthquake
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We have done seismic tomography in and around the focal area of the 2008 Iwate-Miyagi Nairiku Earthquake (M 7.2) occurred on June 14, 2008 in NE Japan. We used data from temporary aftershock observation network deployed just after the occurrence of the present earthquake. Based on the distribution of aftershocks, the fault plane of the mainshock is inferred to dip to the west. Small immediate foreshocks and preceding seismic activity in 1999–2000 on the fault plane in the vicinity of the hypocenter of the mainshock of this earthquake were observed. Lower-seismic-velocity hanging wall can be imaged in the central and the northern part of the focal area. This possibly suggests the present earthquake is a compressional inversion earthquake. The low-velocity zone in the lower crust extends upward to the upper crust, branches into three portions and reaches each active volcano. This low-velocity region can be seen just beneath the mainshock hypocenter and the whole focal area, suggesting that crustal fluid possibly promote the occurrence of the 2008 earthquake.
Key wordsSeismic velocity structure aftershock foreshock inversion tectonics crustal fluid
A large shallow earthquake (named the 2008 Iwate-Miyagi Nairiku Earthquake) with a Japan Meteorological Agency (JMA) magnitude of 7.2 occurred in the central part of NE Japan on June 14, 2008. Focal area of the present earthquake is located in the Tohoku backbone range strain concentration zone (Miura et al., 2002) along the volcanic front. The focal mechanism or moment tensor shows reverse-type focal mechanism (cf. United States Geological Survey (USGS) MOMENT TENSOR (http://neic.usgs.gov/neis/eq_depot/2008/eq_080613_tfdp/neic_tfdp_fmt.html), National Institute for Earth Science and Disaster Prevention (NIED) MOMENT TENSOR (http://www.fnet.bosai.go.jp/event/tdmt.php?ID=20080613234200&all=&v=&LANG=en&?LANG=en)).
The compressional inversion tectonics is dominated in NE Japan at present (e.g. Sato et al., 2002; Sibson, 2009). In Miocene when Japan-sea was opened, many normal faults are formed under the extensional stress regime. At present these old normal faults act as reverse fault under the present compressional stress regime. In the case of the 2003 northern Miyagi earthquake (M 6.4), which is about 40 km south from the present earthquake, this compressional inversion earthquake shows that high-dip angle of fault plane or aftershock alignment and also shows lower velocity hanging wall (Okada et al., 2007a). The plausible cause of the reactivation of such an unfavorably or badly oriented fault is over-pressurized fluid (Sibson, 1990). In the previous paper (Okada et al., 2010), we have found a distinct low-velocity zone in the lower crust just beneath the focal area. These low-velocity zones might correspond to the area with over-pressurized fluid, which is originated from the upwelling flow in the mantle wedge.
2. Data and Method
We determined three-dimensional seismic velocity structure and relocated hypocenters simultaneously using the double-difference tomography method (Zhang and Thurber, 2003).
We adopted two-step procedure. First, we have done regional-scale tomography to obtain rough and deep image in this area (see Okada et al., 2010 for details). We used the shallow seismicity and intermediate depth seismicity in the subducting slab as a data set. Initial velocity structure is Hasegawa et al. (1978). Total number of earthquake is about 26000. We used a grid net with intervals of 0.05 degrees in north-south and east-west directions and at depths of 0, 6, 12, 18, 24, 36, 48, 60, 80, 100, 120, and 140 km. The resolution is estimated to be about 10 km to 20 km in whole crust.
3.1 Distribution of hypocenters of mainshock, foreshocks and aftershocks using temporary observation data
In the vicinity of the mainshock (Fig. 4(a) 2), the distribution of aftershocks forms a plane that dips to the west at approximately 40°, similar to the angle of the fault plane of the neighboring earthquakes, the 2003 North Miyagi Earthquake (M 6.4) (Okada et al., 2003; Umino et al., 2003) and 1962 North Miyagi Earthquake (M 6.2) (Hasegawa et al., 2005). This westward dipping aftershock alignment is thought to be along the major fault plane of the mainshock, as suggested by the GPS observation (cf. Ohta et al., 2008). Note that “eastward” dipping aftershock alignment can be seen in the vicinity of the main shock (Fig. 4(a) 2).
On June 14, immediately prior to the 2008 earthquake, an M 0.6 foreshock occurred at 8:01 and an M 1.6 foreshock occurred at 8:11 (see Fig. 4). Both are very close to the mainshock.
Ground deformation would caused by this earthquake has been confirmed (e.g. Toda et al., 2008), and location of the deformation belt is closely located with the Mochikorobashi-Hosokura Tectonic Zone (MHTZ). The shallow extension of the aftershock distribution is in approximate agreement with the position of ground deformation (Fig. 4). In the north of the source region, the shallower extension of the aftershock distribution meets roughly near the surface trace of the Kitakami Lowland fault (cf. Sato et al., 2008).
We can see spatial variation of lower limit of aftershock distribution along the focal area shown in Fig. 4(c). In the central part of the aftershock region, in the vicinity of the mainshock, the lower limit of aftershocks is relatively deep (∼10 km), while that the northernmost and southernmost parts tends to be shallower (<5 km) (Figs. 4(b) and (c)). The spatial distribution of the lower limit of aftershocks is expected from the microearthquakes prior to the 2008 earthquake, and represents the regional variation in the seismogenic layer in this region (Hasegawa et al., 2000). The lower limit of the seismogenic layer corresponds to the boundary between brittle and ductile deformation, and is determined by temperature. The northern edge (Mt. Yakeishi-dake) and southern edge (the Onikobe and Naruko volcanoes) of this region would be at higher temperature than the central part, resulting in shallower lower limit of seismicity in these regions.
4. Preceding Seismic Activity
4.1 Comparison of the aftershock distribution with the seismic velocity structure
This observations means that the fault plane of the present earthquake is possibly correspond to the velocity changing zone and hanging wall has lower velocity than the footwall. This is the same as the previous compressional earthquakes as the 2003 northern Miyagi earthquake and others recently occurred in NE Japan (e.g. Okada et al., 2005, 2007a).
We can also see the eastward-dipping aftershock alignment. Crustal deformation detected by InSAR suggests the possible eastward-dipping fault plane which seem to almost correspond to this eastward-dipping aftershock alignment (e.g. Takada et al., 2009). The hanging wall for these eastward-dipping alignments do not have a lower seismic velocity than the footwall (Figs. 8(g), (h) and (i)). This is the same as for the conjugate M 6 aftershock of the 2004 Mid Niigata (Chuetsu) earthquake (e.g. Okada et al., 2005). This might suggest eastward-dipping alignment may means relatively newly developed fault or the fault which is less developed in Miocene in comparison with the major westward-dipping fault plane.
5.1 Comparison with fault model and coseismic slip
As discussed above, we relocated hypocenters of the foreshocks, main shock and aftershocks of the 2008 earthquake. The source fault model and slip distribution for this earthquake is estimated by seismic waveforms and geodetic data (e.g. Asano and Iwata, 2008; Ohta et al., 2008). For example, GPS data indicate that the fault plane is westward dipping and the length of the major slip region is approximately 30 km (Ohta et al., 2008). According to the detailed slip distribution on the fault plane, the major slip area was south of the main shock in a shallow area and the estimated maximum slip is as large as 5 m.
5.2 Relation with volcanoes and deep crustal fluid
Note that we can see with the shallow low-velocity and high-V p /V s regions at and around the surface trace of faults. This region can be interpreted as the highly-fractured region includes high amount of fluid, which would be originated from deeper part.
Such low-velocity regions beneath the focal area of inland earthquakes in Japan can be commonly seen and interpreted as the region with overpressurized fluid, which promote the occurrence of inland earthquake; for example, the 1995 southern Hyogo (M 7.2) (Zhao et al., 1996; Zhao and Negishi, 1998), the 2000 western Tottori (M 7.3) (Zhao et al., 2004), the 1962 northern Miyagi (M 6.2) (Nakajima and Hasegawa, 2003), the 2004 Niigata-Chuetsu (M 6.8) (Okada et al., 2006; Wang and Zhao, 2006) and the 2007 Noto-Hanto (M 7.3) and Niigata-Chuetsu-Oki (M 6.8) (Nakajima and Hasegawa, 2008). This suggests that crustal fluid was involved also in the occurrence of the 2008 earthquake. As shown before, the seismic velocity structure also suggests the present earthquake would be a compressional inversion earthquake. The over-pressurized fluid imaged as low-velocity zone is the plausible cause of the reactivation of such a compressional inversion fault (Sibson, 1990).
This low-velocity zone in the lower crust would correspond to region of partial melting. High temperature partial melting zone and fluid released from it would promote the local crustal deformation and causes stress concentration in the surrounding area to occur the present earthquake.
Dip of the westward dipping fault plane is estimated to be about 40–50 degrees from aftershock alignment near the mainshock hypocenter. “Eastward dipping” aftershock alignment can be seen in the central part. Low seismic velocity hanging wall for the westward dipping aftershock alignment can be imaged near the mainshock hypocenter. Low velocity zone is imaged beneath the focal area (the hypocenter of mainshock, in particular). Lower limit of aftershock varies spatially and becomes shallower near the volcanoes, where distinct low-velocity areas are distributed in the shallower part. The aftershocks tend not to occur in low-velocity regions, where the temperature would be relatively high. Large coseismic slip areas are also not distributed in low-velocity regions but can be seen in the high velocity areas. The present earthquake is possibly a fluid-driven compressional inversion earthquake but strongly influenced by the recent volcanic activity, thermal structure and complex fault system.
We used data from JMA, Hi-net/NIED, National Astronomical Observatory of Japan (Mizusawa) and Tokyo Institute of Technology. We also used data from JNES (Japan Nuclear Energy Safety Organization). We thank Prof. Cliff Thurber and Dr. Haijiang Zhang for providing their programs and valuable discussion. We also thank Prof. R. Sibson for fruitful discussion. We would like to thank the editor, Satoshi Kaneshima and the reviewers for helpful comments. This work was conducted under the support of Grant-in-Aid for Special Purposes, MEXT, Japan.
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