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Deep seismic reflection profiling across the Ou Backbone range, northern Honshu Island, Japan
Tectonophysics 355 (2002) 41 – 52 www.elsevier.com/locate/tecto
Deep seismic reflection profiling across the Ou Backbone range, northern Honshu Island, Japan Hiroshi Sato a,*, Naoshi Hirata a,1, Takaya Iwasaki a,1, Makoto Matsubara a,2, Takeshi Ikawa b,3 a
Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan b Japex Geoscience Institute Inc., 1-5-21, Otsuka, Tokyo 112-0012, Japan
Received 30 January 2001; received in revised form 24 September 2001; accepted 17 October 2001
Abstract Knowledge of the crustal structure, especially the geometry of seismogenic faults, is key to understanding active tectonic processes and assessing the size and frequency of future earthquakes. To reveal the relationship between crustal structure and earthquake activity in northern Honshu Island, common midpoint (CMP) deep reflection profiling and earthquake observations by densely deployed seismic stations were carried out across the active reverse faults that bound the Ou Backbone range. The 40-km-long CMP profiles portray a relatively simple fault geometry within the seismogenic layer. The reverse faults merge at a midcrustal detachment just below the base of the seismogenic layer, producing a pop-up structure that forms the Ou Backbone range. The top of the reflective middle to lower crust (4.5 s in travel time (TWT)) nearly coincides with the bottom of seismogenic layer. The P-wave velocity structure and surface geology suggest that the bounding faults are Miocene normal faults that have been reactivated as reverse faults. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Deep seismic reflection profiling; Active reverse fault; Intraplate earthquake; Seismogenic layer; Ou Backbone range; Northern Japan
1. Introduction The deep geometry of active faults is important for understanding on-going active tectonic processes and evaluating the risk of destructive earthquakes. *
Corresponding author. Fax: +81-3-5841-5737. E-mail addresses: [email protected] (H. Sato), [email protected] (N. Hirata), [email protected] (T. Iwasaki), [email protected] (M. Matsubara), [email protected] (T. Ikawa). 1 Fax: + 81-3-5689-7234. 2 Fax: + 81-3-5841-8265. 3 Fax: + 81-3-5461-7430.
Together with monitoring seismicity and surface deformation, delineating the geometry of seismogenic faults allows us to construct quantitative models of crustal deformation (e.g., Beavan et al., 1999). Deep, common midpoint (CMP) reflection profiling is a powerful tool to discern the deep geometry of faults, as demonstrated by deep seismic profiles in active deformation zones (e.g., Hauck et al., 1998). Knowing the deep structure of active faults contributes to estimates of the source parameters of scenario earthquakes, such as the fault geometry, co-seismic displacement and possible asperities. Investigations of on-going tectonic process in active deformation zones
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 1 3 3 - 6
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will also help us to better understand past orogens and crustal evolution. To reveal the relationship between crustal structure and earthquake activity in northern Japan, a new multidisciplinary investigation was started in 1997 under a new program of study and observation for earthquake forecasting. The first target area was the central part of northern Honshu Island because of its simple tectonic setting as a compressive volcanic arc (Hasegawa et al., 2000; Sato, 1994). In 1997 –1998, a group of Japanese universities undertook a multicomponent geophysical study of the area using common midpoint (CMP) deep reflection profiles (Hirata et al., 1999a; Ohguchi et al., 1998), wide-angle reflection/ refraction profiles (Iwasaki et al., 2001), earthquake monitoring (Hirata et al., 1999b; Hasegawa et al.,
2000) and crustal deformation measurements by GPS and magnetotelluric methods (Ogawa et al., 2001). The seismic profiles across the Ou Backbone range of northern Honshu are the first CMP deep reflection profiles to be acquired over a volcanic island arc on land. This paper describes these reflection profiles across the backbone range and explores the implications for our understanding of on-going tectonic processes and evolution.
2. Geologic setting Northern Honshu is a classic example of a trench arc –back arc system (Yoshii, 1979). The Pacific plate is being subducted beneath northern Honshu. The
Fig. 1. Topographic map of the central part of northern Honshu, showing the location of the 1997 and 1998 seismic lines. The distribution of active faults is from the Research Group for Active Faults (1991). QVF: quaternary volcanic front. X – XV: location of the tomographic profile shown in Fig. 6.
H. Sato et al. / Tectonophysics 355 (2002) 41–52
crust of northern Honshu was produced through Paleozoic to Mesozoic accretionary processes and the voluminous intrusion of granitic rocks in the Late Cretaceous (e.g., Minoura and Hasegawa, 1992). Northern Honshu was rifted away from the Eurasian continent at 25– 15 Ma, accompanied by the formation of grabens and half grabens and volcanic erup-
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tions in the back arc (Otofuji, 1996; Yamaji, 1990). Due to shortening since the Pliocene, folds and narrow uplifts bounded by reverse faults were formed on the back arc of northern Honshu (Sato, 1994). In the central part of northern Honshu (Fig. 1), the Ou Backbone range is a representative uplift between the Senya fault on the west and Uwandaira fault on
Fig. 2. Generalized geological map of the central part of northern Honshu (modified from Yamazaki et al., 1983).
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the east. The rocks of the Ou Backbone range were once part of a normal-faulted rift basin formed by crustal stretching during Miocene back arc spreading and subsequent lithospheric cooling (Yamaji, 1990; Sato, 1994). The eastern edge of the Miocene rift system now lies beneath the Kitakami Lowland, and pre-Neogene rocks are widely distributed in the Kitakami massif (Fig. 2). The Neogene sequence in the Ou Backbone range shows a single sedimentary cycle from terrestrial to deep marine and back to terrestrial sediments (Sato and Amano, 1991). The uplift of the Ou Backbone range since late Miocene was mainly due to voluminous magmatic intrusions forming many felsic, Valles-type calderas along the backbone range. Reverse faulting since late Pliocene elevated the backbone range up to 1000 m above the sea level. The volcanic front has been located near the eastern part of the range since 10 Ma (Ohguchi et al., 1989). Quaternary andesitic stratovolcanoes distributed along the Ou Backbone range form a volcanic chain whose rate of effusion shows its maximum at the volcanic front. The Riku-u earthquake (Mj 7.2, Usami, 1987) occurred on 17 January, 1896 beneath the west-central part of the Ou Backbone range (Fig. 1). Surface breaks associated with the earthquake were thrusts along the preexisting Senya fault, a west-dipping reverse fault, and the Kawafune fault, located in the center of the backbone range along the eastern edge of the Mahiru range (Fig. 1; Yamasaki, 1896; Matsuda et al., 1980). The Senya fault was reactivated during the 1896 earthquake with a maximum vertical displacement of 3.5 m (Matsuda et al., 1980). A 1.3 mm/year rate of shortening along the fault is interpreted from analyses of drill hole data (Imaizumi et al., 1997). A
shallow seismic reflection profile shows the Senya fault to be an emergent thrust with a flat and ramp geometry (Sato et al., 1997). From the surface to the depth of 800 m, the Senya fault dips at an angle of about 30j to the east until it becomes flat in a horizon of late Miocene mudstone. Beneath the main topographic range front, it becomes steeper again as it merges with the main strand of the fault. The Uwandaira fault, which bounds the eastern edge of the Ou Backbone range, is an active reverse fault that also shows a flat and ramp geometry on shallow seismic reflection profiles (Kurashimo et al., 1999). The shallow detachment fault lies in the late Miocene lake deposits at depth of 300 m. The latest paleoseismic event along this fault is about 4000 years BP, and a 0.2 mm/year of vertical slip rate is estimated for the fault (Nakata, 1976; Watanabe et al., 1993).
3. Trans-Ou Backbone range CMP line 3.1. Data acquisition The two deep reflection profiles were acquired perpendicular to the trend of the Ou Backbone range (Fig. 1). Data acquisition over the western side of the backbone range (OBR97) was carried out in October 1997 as a collaborative project between the Earthquake Research Institute (ERI) of the University of Tokyo and the Akita prefectural government (Ohguchi et al., 1998). The profile over the eastern backbone range (OBR98) was carried out in August 1998 by the ERI. Between OBR97 and OBR98 is a 10-km gap along a NNE direction. All seismic profiles were acquired by JAPEX Geoscience Institute (JGI). The
Table 1 Data acquisition parameters in the Trans-Ou Backbone range CMP reflection profiling Seismic line Length of seismic line (km) Source (type) Sweep No. of sweeps No. of shot points Receiver No. of channels Grouping Recording Standard CMP fold
OBR97-W OBR97-E 12.65 11.2 four vibrators (IVI, Y2400) 6 – 45 Hz (or 40 Hz), 20-s sweep length, 200 – 300-m shot interval 24 – 30 24 – 30 41 35 10-Hz geophones at 50-m receiver interval 254 225 2.78 m 18 2.78 m 18 24-bit recording at 4 ms sample rate, 20 s listen 25 25
OBR98 14.5
30 – 40 60 300 2.78 m 9 45
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combined length of the two seismic profiles is about 38.4 km. The data acquisition parameters are shown in Table 1. The seismic sources were four vibroseis trucks. The signal was recorded with JGI’s GDAPS-4 digital telemetry system with fixed channels. Explosive sour-
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ces ( < 500 kg) at nine locations for Trans-northern Honshu wide-angle reflection and refraction experiments (Iwasaki et al., 2001) were also recorded by the CMP recording lines. The processing was performed using standard CMP processing steps, including poststack migration and depth conversion. As the seismic
Fig. 3. Shot gather M1 recorded on receiver array OBR98 with normal moveout correction applied. The location is shown in Fig. 1. The charge size was 100 kg.
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lines were very crooked, stacking line was defined referring to the distribution map of common midpoints. Static correction for weathering layer was applied based on refraction analysis using the timeterm method. Due to a large elevation change (ca. 450 m), the processing on CMP stacking was carried out using floating datum. 3.2. Data interpretation Fig. 3 shows shot point M1 (100-kg charge size) recorded on the OBR98 receiver spread (Fig. 1). The shot gather has had automatic gain control (AGC), band pass filter and normal moveout correction applied based on the stacking velocity obtained by velocity analysis. Between 4.5 and 9 s travel time (TWT), coherent horizontal reflections suggest the presence of a laminated lower crust like those found in many continental areas (e.g., Mooney and Meissner, 1992). The laminated lower crust is also visible on shot gather L4 (500-kg shot size) recorded on OBR97 (Fig. 4). Between 5 and 10 s TWT, several reflectors, including dipping reflectors, are visible. On the L4 record, the Senya fault is marked by a rapid change in the first arrival times. A reflector extending eastward from near the surface trace of the Senya fault to between 4 and 4.5 s TWT under the center of the backbone range is interpreted to be a fault plane reflection. The reflector from 10 s TWT is interpreted as a reflection from Moho discontinuity based on the velocity model obtained by the refraction/wide angle reflection profiling (Iwasaki et al., 2001). The CMP reflection profiles and geologic interpretation are shown in Fig. 5. The two CMP lines are projected onto a line perpendicular to the general trend of the Ou Backbone range. Due to mainly crooked geometry of the seismic lines and elevation changes, the reflections can be easily identified in the stacked, unmigrated sections (Fig. 5A) rather than in the migrated sections by conventional method (Fig. 5B). Thus, the identification of reflections is mainly based on the unmigrated sections, and geologic interpretation (Fig. 5C) is carried out after the migrated, depth-converted sections. Based on the systematic change in the pattern of reflectors, an east-dipping line of discontinuity is identified from the surface trace of the Senya fault to about 3 s TWT beneath the Mahiru range (Fig. 5A).
The line of discontinuity can be traced to an eastdipping reflector at 4– 4.5 s TWT at the eastern end of OBR97, implying that the reflector is the deeper extension of the Senya fault. In OBR98, a strong reflection at 4.5 s TWT is interpreted as a possible extension of the Senya fault. The geometry of the Senya fault was tested using a ray-tracing method to model shot L4 recorded on OBR97 (Fig. 4); the modeled fault geometry compares well with that from the CMP stacked section. The Senya fault is listric with the dip angle becoming subhorizontal at 4.5– 5 s TWT. A line of discontinuity and west-dipping reflectors on the seismic profile also marks the subsurface extension of the Kawafune fault (Fig. 5A). Both faults merge beneath the Mahiru range at 2.5 s TWT ( f 6 km depth). In OBR98, the possible deeper extension of the Uwandaira fault is recognized as a west-dipping series of reflectors down to about 3 s TWT at the western end of OBR98 (Fig. 5A). The deeper extension of these faults shows a simple geometry forming pop-up structures. The Senya and Uwandaira faults form a pop-up structure that uplifted the f 40-km-wide Ou Backbone range. The Senya and Kawafune faults show a small-scale pop-up structure forming the Mahiru range. Continuing uplift of the Mahiru range was observed in the coseismic slip during the Riku-u earthquake of 1896 (Matsuda et al., 1980).
4. Seismic activity and seismic tomography across the Ou Backbone range In the central part of northern Honshu, intensified seismic observation using 50 temporary stations was carried out by the Japanese university groups from October 1997 to July 1999 (Hasegawa and Hirata, 1999; Hasegawa et al., 2000). To obtain dense tomographic data and recordings of microearthquake activity, 43 off-line seismic stations were deployed in the profile area (Fig. 1). The 1726 microearthquakes were recorded from October 1997 to July 1999 (Hirata et al., 1999b). Hypocenters were determined using a velocity model derived from a wide-angle reflection/ refraction experiment across northern Honshu (Iwasaki et al., 2001). The resulting distribution of hypocenters is shown in Fig. 6. On this figure, the hypocenters within 40 km in width along the section
H. Sato et al. / Tectonophysics 355 (2002) 41–52 Fig. 4. Shot gather L4 recorded on receiver array (A) OBR97 and ray diagram modeling the shot at (B) L4. The location of the shot point is shown in Fig. 1. The charge size of L4 is 500 kg.
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Fig. 5. CMP deep reflection profiles across the Ou Backbone range, and the geologic interpretation of the profiles. (A) Unmigrated stacked section; (B) post-stack migrated section; (C) geologic interpretation.
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Fig. 6. Tomographic image and distribution of earthquakes across the Ou Backbone range. The P-wave velocity perturbation is indicated by gray scale shading and contour lines (in %). The background velocity model is shown on the right. The white circles are hypocenters of earthquakes recorded from October 1997 to July 1999. The hypocenters within 40 km in width along the profile are plotted.
are plotted. Most of the crustal earthquakes occur above about 13 km in this section. The microearthquake hypocenters form clusters beneath both the flanks of the Ou Backbone range. Using the 756 microearthquakes, tomographic images have been determined around the Ou Backbone range (Hirata et al., 1999b). The grid spacing used for the calculation is 5 km in the vertical and 3 km in the horizontal. The three-dimensional velocity structure has been determined using the three-dimensional, iterative tomographic method (Zhao et al., 1992). The resultant cross-sectional view of the three-dimensional velocity structure is shown in Fig. 6 as deviations from the horizontally averaged veloc-
ity distribution in percentage. The discontinuity at 14 – 21- and 29 – 35-km depth in the tomographic profile is caused by an artificial effect from the initial velocity structure. On the tomographic image, lowvelocity material in the upper most crust deepens to the west from the Kitakami Lowland to the Yokote Basin. This thickening is in accordance with the distribution of thick Tertiary sediments in the backbone range and Yokote Basin. An east-dipping lowvelocity zone occurs beneath the western part of the backbone range along and below the deeper extension of the Senya fault. Along the estimated deeper extension of the Uwandaira fault, a west-dipping, lowvelocity zone is also observed.
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5. Discussion 5.1. Relationship between seismic activity and crustal structure Through CMP reflection profiling, the deep geometry of active reverse faults bounding the Ou Backbone range has been revealed. In the upper part of the crust, the faults show flat and ramp geometry with a shallow flat occurring in late Miocene mudstones (Sato et al., 1997; Kurashimo et al., 1999). The deeper extension of the Senya fault becomes subhorizontal at a depth of about 13 km (4.5 s TWT), and the deeper extension of the Uwandaira fault probably merges with the Senya fault at that depth. The depth of 13 km coincides with the lower limit of microearthquake activity. Thus, the Senya and Uwandaira faults form a ramp structure in the seismogenic layer, with the midcrustal detachment coinciding with the bottom of seismogenic layer. The similar feature under extensional stress regime is already reported in the southern basin and range province, western United States (Brocher et al., 1998). Microearthquakes occur primarily beneath the flanks of the backbone range. However, due to the low seismicity along the fault planes, the locations of clusters of microearthquakes do not show the precise coincidence with the fault planes estimated from the CMP profiling. The observed focal mechanisms are thrust-type and are consistent with the estimated fault geometry (Hirata et al., 1999b). The geometry of the dipping, low-velocity zones from the tomographic image shows good agreement with the fault geometry estimated from reflection profiles. The reflective lower to middle crust is clearly seen in the shot gathers from explosive sources (Figs. 3 and 4). Similar reflections from the lower to middle crust are also reported beneath the Kitakami massif (Iwasaki et al., 1994; Takeda et al., 1999). This area has been free from the Miocene back arc spreading and also intense magmatic activity, and has been a stable continental block since Tertiary. Thus, as a primary interpretation for the reflective lower crust beneath the Ou Backbone range, it is considered to represent an old feature of continental crust formed in Late Cretaceous. Since the early Miocene, the backbone range has been located in volcanic field and experienced extensional deformation associated with the back arc
opening. Actually, a large number of S-wave reflectors in the lower crust are reported beneath the backbone range (Hori et al., 1999; Hasegawa et al., 2000). There must have been some contribution of magmatic activity and extensional deformation to produce the reflective lower crust beneath the backbone range. 5.2. Fault reactivation in the Ou Backbone range The Kawafune and Uwandaira faults are the late Quaternary reverse faults (Research Group for Active Faults, 1991). In spite of the reverse faulting of these faults, the P-wave velocity structure deduced from seismic tomography shows the thick low-velocity layer on their hanging wall rather than on the footwall (Fig. 6). The low-velocity layer is also marked by the stacking velocity from CMP reflection, the velocity structure from wide-angle reflection/refraction study (Iwasaki et al., 2001) and Bouguer anomaly (Hiroshima et al., 1990). Judging from the surface geology, this low-velocity layer corresponds to the Tertiary sediments. Based on the sedimentary facies of the Tertiary sediments in the backbone range, a deep sedimentary basin was formed along the present backbone range during the middle Miocene (Kitamura, 1963; Sato, 1994). This Miocene rift graben was most likely bounded on the east by a system of west-dipping normal faults. To explain the abovementioned features, the fault reactivation from normal to reverse faulting is most probable. Namely, it is interpreted that the Kawafune and Uwandaira faults were formed as west-dipping normal faults during early Miocene and reactivated as reverse faults since the late Tertiary. For the Senya fault, there is no evidence suggesting the reactivation of a preexisting normal fault. The Senya fault is a younger reverse fault formed at f 2.4 Ma (Sato et al., 1997).
6. Conclusions The deep CMP reflection profile across the Ou Backbone range portrays a relatively simple fault geometry in the seismogenic upper crust. The active reverse faults that bound the range merge at a midcrustal detachment just below the base of seismogenic layer and produced a pop-up structure that forms the mountain range.
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The existence of a laminated lower crust beneath the volcanic arc is clearly demonstrated by the seismic profiles. The top of the horizontally laminated middle to lower crust is at 4.5 s TWT, nearly coincident with the bottom of the seismogenic layer. The tomographic image obtained from the earthquake observations shows a low-velocity zone along the deeper extension of the active faults that bound the backbone range.
Acknowledgements We are grateful to Akira Hasegawa, Tanio Ito, Norihito Umino and Thomas Pratt for helpful discussions. We also thank Hans Thybo, Tom Brocher and Lars Ole Bolreel for careful reviewing the manuscript. Their effort helped to improve our manuscript. The Ministry of Education, Science, Culture and Sports funded the seismic experiment as a project of the promotion of new programs on study and observation for earthquake forecast.
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Deep seismic reflection profiling across the Ou Backbone range, northern Honshu Island, Japan Hiroshi Sato a,*, Naoshi Hirata a,1, Takaya Iwasaki a,1, Makoto Matsubara a,2, Takeshi Ikawa b,3 a
Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan b Japex Geoscience Institute Inc., 1-5-21, Otsuka, Tokyo 112-0012, Japan
Received 30 January 2001; received in revised form 24 September 2001; accepted 17 October 2001
Abstract Knowledge of the crustal structure, especially the geometry of seismogenic faults, is key to understanding active tectonic processes and assessing the size and frequency of future earthquakes. To reveal the relationship between crustal structure and earthquake activity in northern Honshu Island, common midpoint (CMP) deep reflection profiling and earthquake observations by densely deployed seismic stations were carried out across the active reverse faults that bound the Ou Backbone range. The 40-km-long CMP profiles portray a relatively simple fault geometry within the seismogenic layer. The reverse faults merge at a midcrustal detachment just below the base of the seismogenic layer, producing a pop-up structure that forms the Ou Backbone range. The top of the reflective middle to lower crust (4.5 s in travel time (TWT)) nearly coincides with the bottom of seismogenic layer. The P-wave velocity structure and surface geology suggest that the bounding faults are Miocene normal faults that have been reactivated as reverse faults. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Deep seismic reflection profiling; Active reverse fault; Intraplate earthquake; Seismogenic layer; Ou Backbone range; Northern Japan
1. Introduction The deep geometry of active faults is important for understanding on-going active tectonic processes and evaluating the risk of destructive earthquakes. *
Corresponding author. Fax: +81-3-5841-5737. E-mail addresses: [email protected] (H. Sato), [email protected] (N. Hirata), [email protected] (T. Iwasaki), [email protected] (M. Matsubara), [email protected] (T. Ikawa). 1 Fax: + 81-3-5689-7234. 2 Fax: + 81-3-5841-8265. 3 Fax: + 81-3-5461-7430.
Together with monitoring seismicity and surface deformation, delineating the geometry of seismogenic faults allows us to construct quantitative models of crustal deformation (e.g., Beavan et al., 1999). Deep, common midpoint (CMP) reflection profiling is a powerful tool to discern the deep geometry of faults, as demonstrated by deep seismic profiles in active deformation zones (e.g., Hauck et al., 1998). Knowing the deep structure of active faults contributes to estimates of the source parameters of scenario earthquakes, such as the fault geometry, co-seismic displacement and possible asperities. Investigations of on-going tectonic process in active deformation zones
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 1 3 3 - 6
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will also help us to better understand past orogens and crustal evolution. To reveal the relationship between crustal structure and earthquake activity in northern Japan, a new multidisciplinary investigation was started in 1997 under a new program of study and observation for earthquake forecasting. The first target area was the central part of northern Honshu Island because of its simple tectonic setting as a compressive volcanic arc (Hasegawa et al., 2000; Sato, 1994). In 1997 –1998, a group of Japanese universities undertook a multicomponent geophysical study of the area using common midpoint (CMP) deep reflection profiles (Hirata et al., 1999a; Ohguchi et al., 1998), wide-angle reflection/ refraction profiles (Iwasaki et al., 2001), earthquake monitoring (Hirata et al., 1999b; Hasegawa et al.,
2000) and crustal deformation measurements by GPS and magnetotelluric methods (Ogawa et al., 2001). The seismic profiles across the Ou Backbone range of northern Honshu are the first CMP deep reflection profiles to be acquired over a volcanic island arc on land. This paper describes these reflection profiles across the backbone range and explores the implications for our understanding of on-going tectonic processes and evolution.
2. Geologic setting Northern Honshu is a classic example of a trench arc –back arc system (Yoshii, 1979). The Pacific plate is being subducted beneath northern Honshu. The
Fig. 1. Topographic map of the central part of northern Honshu, showing the location of the 1997 and 1998 seismic lines. The distribution of active faults is from the Research Group for Active Faults (1991). QVF: quaternary volcanic front. X – XV: location of the tomographic profile shown in Fig. 6.
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crust of northern Honshu was produced through Paleozoic to Mesozoic accretionary processes and the voluminous intrusion of granitic rocks in the Late Cretaceous (e.g., Minoura and Hasegawa, 1992). Northern Honshu was rifted away from the Eurasian continent at 25– 15 Ma, accompanied by the formation of grabens and half grabens and volcanic erup-
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tions in the back arc (Otofuji, 1996; Yamaji, 1990). Due to shortening since the Pliocene, folds and narrow uplifts bounded by reverse faults were formed on the back arc of northern Honshu (Sato, 1994). In the central part of northern Honshu (Fig. 1), the Ou Backbone range is a representative uplift between the Senya fault on the west and Uwandaira fault on
Fig. 2. Generalized geological map of the central part of northern Honshu (modified from Yamazaki et al., 1983).
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the east. The rocks of the Ou Backbone range were once part of a normal-faulted rift basin formed by crustal stretching during Miocene back arc spreading and subsequent lithospheric cooling (Yamaji, 1990; Sato, 1994). The eastern edge of the Miocene rift system now lies beneath the Kitakami Lowland, and pre-Neogene rocks are widely distributed in the Kitakami massif (Fig. 2). The Neogene sequence in the Ou Backbone range shows a single sedimentary cycle from terrestrial to deep marine and back to terrestrial sediments (Sato and Amano, 1991). The uplift of the Ou Backbone range since late Miocene was mainly due to voluminous magmatic intrusions forming many felsic, Valles-type calderas along the backbone range. Reverse faulting since late Pliocene elevated the backbone range up to 1000 m above the sea level. The volcanic front has been located near the eastern part of the range since 10 Ma (Ohguchi et al., 1989). Quaternary andesitic stratovolcanoes distributed along the Ou Backbone range form a volcanic chain whose rate of effusion shows its maximum at the volcanic front. The Riku-u earthquake (Mj 7.2, Usami, 1987) occurred on 17 January, 1896 beneath the west-central part of the Ou Backbone range (Fig. 1). Surface breaks associated with the earthquake were thrusts along the preexisting Senya fault, a west-dipping reverse fault, and the Kawafune fault, located in the center of the backbone range along the eastern edge of the Mahiru range (Fig. 1; Yamasaki, 1896; Matsuda et al., 1980). The Senya fault was reactivated during the 1896 earthquake with a maximum vertical displacement of 3.5 m (Matsuda et al., 1980). A 1.3 mm/year rate of shortening along the fault is interpreted from analyses of drill hole data (Imaizumi et al., 1997). A
shallow seismic reflection profile shows the Senya fault to be an emergent thrust with a flat and ramp geometry (Sato et al., 1997). From the surface to the depth of 800 m, the Senya fault dips at an angle of about 30j to the east until it becomes flat in a horizon of late Miocene mudstone. Beneath the main topographic range front, it becomes steeper again as it merges with the main strand of the fault. The Uwandaira fault, which bounds the eastern edge of the Ou Backbone range, is an active reverse fault that also shows a flat and ramp geometry on shallow seismic reflection profiles (Kurashimo et al., 1999). The shallow detachment fault lies in the late Miocene lake deposits at depth of 300 m. The latest paleoseismic event along this fault is about 4000 years BP, and a 0.2 mm/year of vertical slip rate is estimated for the fault (Nakata, 1976; Watanabe et al., 1993).
3. Trans-Ou Backbone range CMP line 3.1. Data acquisition The two deep reflection profiles were acquired perpendicular to the trend of the Ou Backbone range (Fig. 1). Data acquisition over the western side of the backbone range (OBR97) was carried out in October 1997 as a collaborative project between the Earthquake Research Institute (ERI) of the University of Tokyo and the Akita prefectural government (Ohguchi et al., 1998). The profile over the eastern backbone range (OBR98) was carried out in August 1998 by the ERI. Between OBR97 and OBR98 is a 10-km gap along a NNE direction. All seismic profiles were acquired by JAPEX Geoscience Institute (JGI). The
Table 1 Data acquisition parameters in the Trans-Ou Backbone range CMP reflection profiling Seismic line Length of seismic line (km) Source (type) Sweep No. of sweeps No. of shot points Receiver No. of channels Grouping Recording Standard CMP fold
OBR97-W OBR97-E 12.65 11.2 four vibrators (IVI, Y2400) 6 – 45 Hz (or 40 Hz), 20-s sweep length, 200 – 300-m shot interval 24 – 30 24 – 30 41 35 10-Hz geophones at 50-m receiver interval 254 225 2.78 m 18 2.78 m 18 24-bit recording at 4 ms sample rate, 20 s listen 25 25
OBR98 14.5
30 – 40 60 300 2.78 m 9 45
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combined length of the two seismic profiles is about 38.4 km. The data acquisition parameters are shown in Table 1. The seismic sources were four vibroseis trucks. The signal was recorded with JGI’s GDAPS-4 digital telemetry system with fixed channels. Explosive sour-
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ces ( < 500 kg) at nine locations for Trans-northern Honshu wide-angle reflection and refraction experiments (Iwasaki et al., 2001) were also recorded by the CMP recording lines. The processing was performed using standard CMP processing steps, including poststack migration and depth conversion. As the seismic
Fig. 3. Shot gather M1 recorded on receiver array OBR98 with normal moveout correction applied. The location is shown in Fig. 1. The charge size was 100 kg.
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lines were very crooked, stacking line was defined referring to the distribution map of common midpoints. Static correction for weathering layer was applied based on refraction analysis using the timeterm method. Due to a large elevation change (ca. 450 m), the processing on CMP stacking was carried out using floating datum. 3.2. Data interpretation Fig. 3 shows shot point M1 (100-kg charge size) recorded on the OBR98 receiver spread (Fig. 1). The shot gather has had automatic gain control (AGC), band pass filter and normal moveout correction applied based on the stacking velocity obtained by velocity analysis. Between 4.5 and 9 s travel time (TWT), coherent horizontal reflections suggest the presence of a laminated lower crust like those found in many continental areas (e.g., Mooney and Meissner, 1992). The laminated lower crust is also visible on shot gather L4 (500-kg shot size) recorded on OBR97 (Fig. 4). Between 5 and 10 s TWT, several reflectors, including dipping reflectors, are visible. On the L4 record, the Senya fault is marked by a rapid change in the first arrival times. A reflector extending eastward from near the surface trace of the Senya fault to between 4 and 4.5 s TWT under the center of the backbone range is interpreted to be a fault plane reflection. The reflector from 10 s TWT is interpreted as a reflection from Moho discontinuity based on the velocity model obtained by the refraction/wide angle reflection profiling (Iwasaki et al., 2001). The CMP reflection profiles and geologic interpretation are shown in Fig. 5. The two CMP lines are projected onto a line perpendicular to the general trend of the Ou Backbone range. Due to mainly crooked geometry of the seismic lines and elevation changes, the reflections can be easily identified in the stacked, unmigrated sections (Fig. 5A) rather than in the migrated sections by conventional method (Fig. 5B). Thus, the identification of reflections is mainly based on the unmigrated sections, and geologic interpretation (Fig. 5C) is carried out after the migrated, depth-converted sections. Based on the systematic change in the pattern of reflectors, an east-dipping line of discontinuity is identified from the surface trace of the Senya fault to about 3 s TWT beneath the Mahiru range (Fig. 5A).
The line of discontinuity can be traced to an eastdipping reflector at 4– 4.5 s TWT at the eastern end of OBR97, implying that the reflector is the deeper extension of the Senya fault. In OBR98, a strong reflection at 4.5 s TWT is interpreted as a possible extension of the Senya fault. The geometry of the Senya fault was tested using a ray-tracing method to model shot L4 recorded on OBR97 (Fig. 4); the modeled fault geometry compares well with that from the CMP stacked section. The Senya fault is listric with the dip angle becoming subhorizontal at 4.5– 5 s TWT. A line of discontinuity and west-dipping reflectors on the seismic profile also marks the subsurface extension of the Kawafune fault (Fig. 5A). Both faults merge beneath the Mahiru range at 2.5 s TWT ( f 6 km depth). In OBR98, the possible deeper extension of the Uwandaira fault is recognized as a west-dipping series of reflectors down to about 3 s TWT at the western end of OBR98 (Fig. 5A). The deeper extension of these faults shows a simple geometry forming pop-up structures. The Senya and Uwandaira faults form a pop-up structure that uplifted the f 40-km-wide Ou Backbone range. The Senya and Kawafune faults show a small-scale pop-up structure forming the Mahiru range. Continuing uplift of the Mahiru range was observed in the coseismic slip during the Riku-u earthquake of 1896 (Matsuda et al., 1980).
4. Seismic activity and seismic tomography across the Ou Backbone range In the central part of northern Honshu, intensified seismic observation using 50 temporary stations was carried out by the Japanese university groups from October 1997 to July 1999 (Hasegawa and Hirata, 1999; Hasegawa et al., 2000). To obtain dense tomographic data and recordings of microearthquake activity, 43 off-line seismic stations were deployed in the profile area (Fig. 1). The 1726 microearthquakes were recorded from October 1997 to July 1999 (Hirata et al., 1999b). Hypocenters were determined using a velocity model derived from a wide-angle reflection/ refraction experiment across northern Honshu (Iwasaki et al., 2001). The resulting distribution of hypocenters is shown in Fig. 6. On this figure, the hypocenters within 40 km in width along the section
H. Sato et al. / Tectonophysics 355 (2002) 41–52 Fig. 4. Shot gather L4 recorded on receiver array (A) OBR97 and ray diagram modeling the shot at (B) L4. The location of the shot point is shown in Fig. 1. The charge size of L4 is 500 kg.
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Fig. 5. CMP deep reflection profiles across the Ou Backbone range, and the geologic interpretation of the profiles. (A) Unmigrated stacked section; (B) post-stack migrated section; (C) geologic interpretation.
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Fig. 6. Tomographic image and distribution of earthquakes across the Ou Backbone range. The P-wave velocity perturbation is indicated by gray scale shading and contour lines (in %). The background velocity model is shown on the right. The white circles are hypocenters of earthquakes recorded from October 1997 to July 1999. The hypocenters within 40 km in width along the profile are plotted.
are plotted. Most of the crustal earthquakes occur above about 13 km in this section. The microearthquake hypocenters form clusters beneath both the flanks of the Ou Backbone range. Using the 756 microearthquakes, tomographic images have been determined around the Ou Backbone range (Hirata et al., 1999b). The grid spacing used for the calculation is 5 km in the vertical and 3 km in the horizontal. The three-dimensional velocity structure has been determined using the three-dimensional, iterative tomographic method (Zhao et al., 1992). The resultant cross-sectional view of the three-dimensional velocity structure is shown in Fig. 6 as deviations from the horizontally averaged veloc-
ity distribution in percentage. The discontinuity at 14 – 21- and 29 – 35-km depth in the tomographic profile is caused by an artificial effect from the initial velocity structure. On the tomographic image, lowvelocity material in the upper most crust deepens to the west from the Kitakami Lowland to the Yokote Basin. This thickening is in accordance with the distribution of thick Tertiary sediments in the backbone range and Yokote Basin. An east-dipping lowvelocity zone occurs beneath the western part of the backbone range along and below the deeper extension of the Senya fault. Along the estimated deeper extension of the Uwandaira fault, a west-dipping, lowvelocity zone is also observed.
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5. Discussion 5.1. Relationship between seismic activity and crustal structure Through CMP reflection profiling, the deep geometry of active reverse faults bounding the Ou Backbone range has been revealed. In the upper part of the crust, the faults show flat and ramp geometry with a shallow flat occurring in late Miocene mudstones (Sato et al., 1997; Kurashimo et al., 1999). The deeper extension of the Senya fault becomes subhorizontal at a depth of about 13 km (4.5 s TWT), and the deeper extension of the Uwandaira fault probably merges with the Senya fault at that depth. The depth of 13 km coincides with the lower limit of microearthquake activity. Thus, the Senya and Uwandaira faults form a ramp structure in the seismogenic layer, with the midcrustal detachment coinciding with the bottom of seismogenic layer. The similar feature under extensional stress regime is already reported in the southern basin and range province, western United States (Brocher et al., 1998). Microearthquakes occur primarily beneath the flanks of the backbone range. However, due to the low seismicity along the fault planes, the locations of clusters of microearthquakes do not show the precise coincidence with the fault planes estimated from the CMP profiling. The observed focal mechanisms are thrust-type and are consistent with the estimated fault geometry (Hirata et al., 1999b). The geometry of the dipping, low-velocity zones from the tomographic image shows good agreement with the fault geometry estimated from reflection profiles. The reflective lower to middle crust is clearly seen in the shot gathers from explosive sources (Figs. 3 and 4). Similar reflections from the lower to middle crust are also reported beneath the Kitakami massif (Iwasaki et al., 1994; Takeda et al., 1999). This area has been free from the Miocene back arc spreading and also intense magmatic activity, and has been a stable continental block since Tertiary. Thus, as a primary interpretation for the reflective lower crust beneath the Ou Backbone range, it is considered to represent an old feature of continental crust formed in Late Cretaceous. Since the early Miocene, the backbone range has been located in volcanic field and experienced extensional deformation associated with the back arc
opening. Actually, a large number of S-wave reflectors in the lower crust are reported beneath the backbone range (Hori et al., 1999; Hasegawa et al., 2000). There must have been some contribution of magmatic activity and extensional deformation to produce the reflective lower crust beneath the backbone range. 5.2. Fault reactivation in the Ou Backbone range The Kawafune and Uwandaira faults are the late Quaternary reverse faults (Research Group for Active Faults, 1991). In spite of the reverse faulting of these faults, the P-wave velocity structure deduced from seismic tomography shows the thick low-velocity layer on their hanging wall rather than on the footwall (Fig. 6). The low-velocity layer is also marked by the stacking velocity from CMP reflection, the velocity structure from wide-angle reflection/refraction study (Iwasaki et al., 2001) and Bouguer anomaly (Hiroshima et al., 1990). Judging from the surface geology, this low-velocity layer corresponds to the Tertiary sediments. Based on the sedimentary facies of the Tertiary sediments in the backbone range, a deep sedimentary basin was formed along the present backbone range during the middle Miocene (Kitamura, 1963; Sato, 1994). This Miocene rift graben was most likely bounded on the east by a system of west-dipping normal faults. To explain the abovementioned features, the fault reactivation from normal to reverse faulting is most probable. Namely, it is interpreted that the Kawafune and Uwandaira faults were formed as west-dipping normal faults during early Miocene and reactivated as reverse faults since the late Tertiary. For the Senya fault, there is no evidence suggesting the reactivation of a preexisting normal fault. The Senya fault is a younger reverse fault formed at f 2.4 Ma (Sato et al., 1997).
6. Conclusions The deep CMP reflection profile across the Ou Backbone range portrays a relatively simple fault geometry in the seismogenic upper crust. The active reverse faults that bound the range merge at a midcrustal detachment just below the base of seismogenic layer and produced a pop-up structure that forms the mountain range.
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The existence of a laminated lower crust beneath the volcanic arc is clearly demonstrated by the seismic profiles. The top of the horizontally laminated middle to lower crust is at 4.5 s TWT, nearly coincident with the bottom of the seismogenic layer. The tomographic image obtained from the earthquake observations shows a low-velocity zone along the deeper extension of the active faults that bound the backbone range.
Acknowledgements We are grateful to Akira Hasegawa, Tanio Ito, Norihito Umino and Thomas Pratt for helpful discussions. We also thank Hans Thybo, Tom Brocher and Lars Ole Bolreel for careful reviewing the manuscript. Their effort helped to improve our manuscript. The Ministry of Education, Science, Culture and Sports funded the seismic experiment as a project of the promotion of new programs on study and observation for earthquake forecast.
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