A SEISMIC-REFRACTION PROFILE IN AND AROUND NAGANO PREFECTURE, CENTRAL JAPAN

J. Phys. Earth, 34, 457-474, 1986 A SEISMIC-REFRACTION PROFILE IN AND AROUND NAGANO PREFECTURE, CENTRAL JAPAN Akira IKAMI,1 Toshikatsu YOSHII,2 Susumu KUBOTA,3,* Yoshimi SASAKI,4 Akiko HASEMI,5 Takeo MORIYA,6 Hiroki MIYAMACHI,6 Ritsuko Segawa MATSU'URA,2 and Kazushige WADA7,** 1 School of Science, Nagoya University, Nagoya, Japan 2 Earthquake Research Institute, The University of Tokyo, Tokyo, Japan 3 Meteorological College, Japan Meteorological Agency, Kashiwa, Japan 4 Faculty of Education, Gifu University, Gifu, Japan 5 Faculty of Science, Yamagata University, Yamagata, Japan Faculty of Science, Hokkaido University, Sapporo, Japan 6 Faculty of Marine Science and Technology, 7 Tokai University, Shimizu, Japan (Received November 1, 1985; Revised October 23, 1986) In 1981, the Research Group for Explosion Seismology recorded a seismicrefraction profile across northern and central Nagano Prefecture, central Japan, to study the velocity structure of the upper crust. The 220-km-long profile, which consisted of six shot points and 82 temporary stations, extended northeastward from near Mt. Ontake to the southwestern part of Niigata Prefecture, crossing the Itoigawa-Shizuoka tectonic line. The entire region is characterized by the highly heterogeneous upper crust. In the northeastern part of the profile, three thick folded surface layers with P-wave velocities of 2.0, 3.1, and 4.7 km/s overlay the basement with a velocity of 6.0 km/s, that slopes down northeastward. In the Nagano basin, the basement lies at about 1 km depth below the surface. An abrupt change of the basement depth by 4 km is found on the east of the Matsumoto basin. The associated subsurface fault is inferred. to a reverse type to satisfy the constraints from both the travel time and the Bouguer gravity anomalies. This fault may be a part of the Tsunan-Matsumoto tectonic line, which has been geologically recognized. The Itoigawa-Shizuoka tectonic line, which is one of the major tectonic lines in Japan, strikes from north to south in the Matsumoto basin. It is, however, not marked by any distinctive seismic velocity features. Southwest of the * Present address: Osaka District Meteorological Observatory, Japan Meteorological Agency, Osaka, Japan ** Mitsui Mineral Development and Engineering Co., Ltd., Tokyo, Japan 457

458 A. IKAMI et al. Matsumoto basin, Paleozoic-Mesozoic rocks are recognizable, and a basement with a velocity of 5.9 km/s lies at about 1 km depth. 1. Introduction Geophysical and geological studies of the upper crustal structure in Nagano Prefecture are specifically important to understand the past and the present tectonics of Honshu, Japan. Figure 1 shows the study area and a map of tectonic lines in central Honshu. The Itoigawa-Shizuoka tectonic line (ISTL), a major fault in Honshu, extends from Itoigawa on the coast of the Sea of Japan to Shizuoka on the Pacific coast. It marks the western margin of the Fossa Magna which is known as the greatest depression in central Honshu. Following the earlier geological inference by YAMASHITA (1970), its eastern margin might be the Kashiwazaki- Choshi tectonic line (KCTL) extending from Kashiwazaki on the coast of the Sea of Japan to Choshi on the Pacific coast. By taking these geological features into account, Japanese Islands are geologically divided in two regions, northeastern Japan and southwestern Japan. In recent studies on the tectonics of Japanese Islands, it is a point in dispute whether ISTL is the boundary between the North American plate and the Asian plate (KOBAYASHI and NAKAMURA, 1983; KOBAYASHI, 1983; NAKAMURA, 1983; NAKAMURA and KOBAYASHI, 1983; SENO, 1983). Seismic activity has been also high as verified by historical to recent earthquakes such as the 1874 Zenkoji earthquake (M 7.4), the 1918 Ohmachi earthquake (M = 6.1) and the Fig. 1. Tectonic lines in central Japan (broken lines) and the study profile (thick solid line). MTL, Median tectonic line; ATL, Akaishi tectonic line; ISTL, Itoigawa-Shizuoka tectonic line; KCTL, Kashiwazaki-Choshi tectonic line.

A Seismic-Refraction Profile in and around Nagano Prefecture 459 Fig. 2. Shot and observation points. Cross marks in dicate shot points and solid circles, observation sites. Solid triangles indicate some of the summits of the mountains in this region. 1965 Matsushiro earthquake swarm near Nagano. It is noted that the 1984 Western Nagano Prefecture earthquake (M = 6.8) occurred at the southwest end of our profile after the experiment. Therefore the investigation of the crustal structure in this region is an important step to the further study of the tectonics of central Japan. The site of a 1981 seismic-refraction experiment from southwestern Niigata Prefecture to western Nagano Prefecture was chosen as shown in Fig. 1. Figure 2 shows the location of shot and observation sites. The Matsushiro earthquake swarm was very active and lasted in the central part of this region for more than three years following August 1965, which motivated the first seismic-refraction investigation of this area in 1967 (ASANO et al., 1969 a, b, c; OKADA et al., 1970). The experiment in 1981 was planned to extend the 1967 profile to the important

460 A. IKAMI et al. area in Fossa Magna. Three of six shots and 16 of 82 observation stations were located at the same points as in the 1967 experiment and the profile was extended farther northeastward and southwestward to determine the detailed and large scaled crustal structure, which was needed to obtain accurate hypocentral locations and to study this seismogenic region from a geophysical and geological viewpoint. We describe here the interpretation of the P-wave velocity structure in this area based on the data for a 220-km-long northeast-southwest seismic refraction profile. The fundamental data such as location of shot and observation sites, travel times, record sections, etc. are presented in a separate paper (RESEARCH GROUP FOR EXPLOSION SEISMOLOGY, 1985). 2. Geologic Setting Figure 3 shows the geological map (HIROKAWA et al., 1978) and tectonic lines (KOSAKA, 1984) in Nagano Prefecture together with the location of the profile. ISTL, striking from north to south in central Nagano Prefecture, crosses our profile just west of Matsumoto. West of ISTL, the Mesozoic and Paleozoic rocks crop out, and east, the clastic and pyroclastic rocks in the Neogene. We can also find a remarkable difference in landform. In the northern area along ISTL, it is recognized that several faults trend from north to south, though the boundary between the Neogene rocks and older ones are not so clear. Near the northeastern end of the profile, KCTL crosses the profile. This tectonic line, however, does not show any clear geological evidence on the surface in the southern part of Niigata Prefecture, but a possibility of subsurface faults with northwest-southeast strikes was suggested in this area by several geologists (e.g., CHIHARA, 1976). ISTL and KCTL cross Honshu with high angles to its extension axis. The study area is also characterized with another fault group with strikes parallel to the trend of Honshu. Thus the profile traverses a complex geological setting. From northeast to southwest, the profile extends from a southern extension of the Niigata plain, across the Nagano and the Matsumoto basins, to the southern flank of Mt. Ontake as shown in Fig. 3. In the northeastern end, the profile traverses the Neogene and the Quaternary sediment areas. Northeast of Nagano Prefecture, there are pyroclastic rocks. In the Nagano and the Matsumoto basins, alluvial and diluvial deposits are spreading widely and Tertiary sedimentary rocks are partly exposed. The Matsumoto basin, where ISTL runs, is filled with the Quaternary sediments, so the tectonic line is hardly detected by a surface survey. Paleozoic and the Mesozoic rocks are distributed west of ISTL. As shown in Fig. 3, there exist a few tectonic lines striking northeast to southwest. One of the them is the Tsunan-Matsumoto tectonic line, a middle Miocene fault with northwest side depression (KOSAKA, 1984). It crosses our profile at two points; one is on the west of Matsumoto and the other, near the border

A Seismic-Refraction Profile in and around Nagano Prefecture 461 Fig. 3. Geologic map of Nagano Prefecture after HIROKAWA et al. (1978) together with the profile. Broken lines indicate tectonic lines after KOSAKA (1984). A, Itoigawa-Shizuoka tectonic line; B, Kashiwazaki-Choshi tectonic line; C, Arai- Kotani tectonic line; D, Tsunan-Matsumoto tectonic line; E, Shirane-Fujimi tectonic line. A thick solid line indicates the profile and solid circles, shot points. between Nagano and Niigata Prefectures; and is running on the northwestern side of the profile between the two points. 3. Previous Seismic Works Three seismic experiments were previously conducted in this region. The first explosion seismic experiment was conducted by YAMADA et al. (1976) along a 16- km-long profile in the north of Matsumoto. They detected a low velocity zone about

462 A. IKAMI et al. Fig. 4. Locations of the 1967 explosion seismic experiments and the derived crustal structure in profile B (ASANO et al., 1969 a). 600 m wide beneath the Quaternary gravel beds about 360 m thick, and claimed its identity with the fracture zone along ISTL. This low velocity zone was only recognized at depths deeper than about 360 m and not in the shallower layers. Therefore they interpreted the fault movement within the Matsumoto basin as less active after the Diluvium. The second explosion seismic experiment was conducted in 1967, when the seismic activity of the Matsushiro earthquake swarm was declining (ASANO et al., 1969a, b, c; OKADA et al., 1970). As shown in Fig. 4, two profiles A and B were chosen to obtain the velocity structure around the earthquake swarm area for the accurate hypocenter determination and to understand the relation between the underground structure and the occurrence of the earthquake swarm. In Fig. 4, the derived crustal structure along the profile B is shown (ASANO et al., 1969a). The structure along the profile A is simple; the velocity of the basement varies from 6.0 km/s in the northeast to 5.9 km/s in the southwest, and the depth is about 1.5 km from the earth's surface even at the deepest. On the other hand, the structure along the profile B is fairly complicated. An abrupt northwestward deepening of the basement by about 3 km or a fault-like structure is obvious near Nagano. In 1969, the third explosion seismic experiment was conducted along the same profile to that of the 1967 experiment to detect the relative travel-time changes during two years from the former experiment (ASANO et al., 1970).

A Seismic-Refraction Profile in and around Nagano Prefecture 463 4. Field Procedures The total profile length is approximately 220 km. There are five shot points in the profile. The profile has reversed coverage over 180 km of its length but not for 40 km in the southwestern part. An additional shot point is located at about 20 km southeast of the profile for a fan shooting. Dynamite from 240 kg to 800 kg was detonated in bore holes of 30 m to 60 m deep. The seismic data were recorded on about 80 portable cassette-recording seismographs, each equipped with a 2-Hz vertical component seismometer. The seismic signal is recorded in FM or PCM formats. At each station, appropriate gain level was selected, allowing for recording with a high signal-to-noise ratio. Every chronometer was checked against JJY, a Japanese standard time signal broadcast by radio, before and after the recording time of the seismic signals. When the time signal could be clearly received during the recording time, it was recorded together with a seismic signal on a separate channel. Therefore, a time accuracy was kept better than 0.01 s. All stations were located using 1: 25,000 topographic maps to an accuracy of 20 m. The average station spacing along the profile is about 3 km and much closer along the 1967 profile. Shot point locations, shot times and charge weights are listed in Table 1. In the following discussion, we call six shots A-1, A-2, A-3, A-4, A-5, and B-1. A-1 to A-5 are on the main profile from northeast to southwest and B-1 on the fan profile. 5. Data Analysis The record section for the shot point, A-4, is presented in Fig. 5 with a reduction velocity of 6.0 km/s. Impulsive first arrivals are visible on the majority of the records. Figure 6 shows the travel-time plots of the first and/or later arrivals with a reduction velocity of 6.0 km/s, where symbols denote grades in reading arrival time; A, B, and C mean "very good" (for first arrivals only), "good" and "fairly good," respectively. "L" means the safe reading of the arrival, that is, its Table 1. List of shot points and fundamental data.

464 A. IKAMI et at. Fig. 5. Record section for A-4. The time axis is reduced by a velocity of 6.0 km/s. All records are not shown on this record section to avoid the overlapping of records. very onset was difficult to identify and might have been earlier than the reading time. Arrivals from the deeper crust were too weak to discuss the Conrad and the

A Seismic-Refraction Profile in and around Nagano Prefecture 465 Mohorovicic discontinuities. Seismic amplitudes have not been modeled quantitatively due to two reasons; one is the dissimilar geological conditions along the profile, which affect the signal amplitude at each station, and the other is the complex crustal structure for which it is difficult to calculate synthetic seismograms. The final model was partly derived using a time-term method and partly, a trial-and-error method on the basis of applying the method by AOKI (1971), in which a model was modified until it gave the best fit to the observed data. It can be estimated that there exists a fault-like structure near A-4. The Bouguer gravity anomaly data were used to determine a fault type, whether it is normal or reverse, and the vertical offset of the fault. In discussing velocity structure, the term "basement" refers to "seismic basement" which is defined here as the layers with a velocity greater than 5.9 km/s. The nomenclature of the profiles is by shot point (Ai), where i is from 1 to 5, and direction; thus, A-1SW refers to the data from shot point 1 recorded at stations to the southwest. 6. Result Although the profile length is as long as 220 km, we were not successful in recording clear phases from the lower crust, so the interpretation is restricted to a P- wave velocity structure in the crust shallower than 6 km. The seismic data indicate considerable horizontal variations in the velocity of the surface layer. We have set up several geophones with a spacing of about 100 m to determine the velocities of the uppermost surface layers at A-1 and A-5. The travel-time curves at these two shot points are shown in Fig. 7. In addition to these velocities, earlier explosion seismic references (ASANO et al., 1969 a, b, c; YAMADA et al., 1976) were adopted to obtain the velocities of surface layers over the full range of the profile. The travel-time curves shown in Fig. 6 indicate the following features: 1) A large intercept time of over 2.0 s and a high apparent velocity for A-1SW. 2) A small intercept time and a slow apparent velocity for A-2NE. It is therefore inferred that the basement dips to the northeast and lies at about 6 km deep at the northeastern end of the profile. 3) Slight but significant travel-time advances, which occur at distances around 40 km from A-1 for the shots A-1, A-2, A-3, and A-4 mean local upheaval of the basement. 4) Beginning at about 20 km southwest of A-4, a traveltime delay of 0.5 s at its maximum occurs for the shots A-1, A-2, and A-3. Traveltime curves of A-4NE and A-4SW show a remarkable difference of intercept time by about 0.8 s. Such a travel-time delay and the intercept-time difference can be interpreted by a fault-like model for which the observed travel times near A-4 were examined by the calculated ones as shown in Fig. 8. These observed data suggest a thick sedimentary basin to the southwest of A-4. The seismic data analysis for the southern part of the profile indicates that the upper boundary of the basement becomes shallow to the southwest with a slightly folded structure. The P-wave velocity structure was first derived using a time-term method and then forward modeling was applied using a trial-and-error method until the best fit

466 A. IKAMI et al, Fig. 6. Travel time curves for six shots. The data are reduced by a velocity of described in the text.

A Seismic-Refraction Profile in and around Nagano Prefecture 467 6.0 km/s. A, B, C, and L mean a grade in reading arrival time, which is

468 A. IKAMi et al. Fig. 7. Travel time curves near shot points A-1 and A-5. The time axes are reduced by a velocity of 6.0 km/s. Left diagrams, the data at less than 600 m shot distances, right diagrams, a close-up view of the travel time curves shown in Fig. 6. Fig. 8. A close-up view of the travel-time curves near A-4. Open circle, A-3; cross, A-4; solid circle, A-5. Lines show the travel-time curves calculated from the time-term solutions. was attained. The final structure was thus obtained. The time-terms are separately calculated for profiles A-4NE and A-4SW due to the existence of the fault-like structure near A-4 and are plotted in Fig. 9 to represent the basement along the

A Seismic-Refraction Profile in and around Nagano Prefecture 469 Fig. 9. Time-terms of the basement along the profile. Bars attached to data point mean standard deviations. The parenthesized data are determined without a reverse shot. A velocity gradient with a depth in the basement layer, which is expressed by V= 5.941+ 0.00119D, where V is a velocity in km/s at a shot distance D in km, was modeled in a northeastern half of the profile, but not in its southern half. Open triangles show shot points. Fig. 10. A velocity-depth relation along the A-4NE together with that in profile A of the 1967 explosion seismic experiment. Depths are taken from the top of the basement layer. profile. Along A-4NE, the time-term calculation was made by assuming the continuously increasing velocity with depth. A velocity-depth relation calculated from this relation is given in Fig. 10 together with that obtained in the 1967 experiment in the Matsushiro area (YOSHII and ASANO, 1972). In a trial-and-error

470 A. IKAMI et al. Fig. 11. Crustal structure along the profile. Numerals indicate P-wave velocities in km/s. Vertical dotted line shows the location of a velocity boundary modeled in a trial-and-error method. Broken line shows the fault-like structure referred to Bouguer gravity anomaly. method, we adopted two velocities for the basement layer as 6.0 km/s on A-4NE and 5.9 km/s on A-4SW. The ray paths for initial phases for all shots are restricted shallower than 6 km. The deepest point, however, is located near A-1. Below A-4, the ray paths for initial phases cross the fault, and we could not recognize any travel-time curves of later phases propagated below the fault. Therefore, the discussion by explosion seismic data was restricted within a crustal structure shallower than 6 km. The final crustal structure as shown in Fig. 11 indicates the following characteristics. In the northeastern profile, there are surface layers with velocities of 2.0, 3.1, and 4.7 km/s, which dip to the northeast. In order to minimize travel-time residuals, we have to introduce a concave basement structure beneath the Kijimadaira in the midpoint between A-1 and A-2. In the Nagano basin, the velocities of surface layers are 2.3 and 4.75 km/s. The basement lies at 1 km or more below the earth's surface. To the southwest from A-3, the depth of the basement decreases and lies above sea level. Near the shot A-4, a fault-like structure is suggested. It is impossible to construct the best model with a time-term method only. The offset should be located just northeast of A-4, but the fault-type structure could not be determined from the present travel-time data alone, except for the location of the fault. For forward modeling, Bouguer gravity anomaly data by KONO et al. (1982) as shown in Fig. 12 were therefore referred to estimate the fault-type and the dimension. It seems better to assume a reverse fault just northeast of A-4 to fit the model into the observed Bouguer gravity anomalies. Assuming a density difference of 0.3 g/cm3, the offset of the basement is estimated at about 4 km, with depression on the southwestern side. In the Matsumoto basin, southwest of A-4, there are thick surface layers with P-wave velocities of 2.6, 3.0, and 3.8 km/s. The depth of the basement decreases to

A Seismic-Refraction Profile in and around Nagano Prefecture 471 Fig. 12. Topography and Bouguer gravity anomaly (KONO et al., 1982) along the profile. the southwest. Just southwest of the Matsumoto basin, the basement indicates a folded structure. The P-wave velocity of the basement from A-4 to A-5 was determined as 5.9 km/s. Near the A-5, the surface layers, with P-wave velocities of 2.8 and 3.9 km/s, are overlying the basement, the depth of which is about at sea level. 7. Discussion and Conclusion Explosion seismic investigations in and around Nagano Prefecture define the velocity structure to a depth of 6 km using refracted arrivals. Although our profile is as long as 220 km, the velocity structure below 6 km is not determined because of no clear arrivals from the lower crust. The composite feature of the upper crust is shown in Fig. 11. The velocities of surface layers have not been determined all over the profile, so the P-wave velocity was assumed where no other velocity data are available. In addition to this, we assumed that, considering the geological features in the Nagano and Matsumoto basin areas, the near-surface layers comprise a basinlike structure, that is, in mountain areas, surface soil is thin, and in basin areas, surficial deposits are thick. One of the significant features of the velocity structure along this profile is the fault near A-4. It is likely that, referring to the Bouguer gravity anomaly data, this fault is a reverse type and has a remarkably large 4 km vertical displacement with the depression of the southwest side. In the middle of the Matsumoto basin, it has been believed that the Itoigawa- Shizuoka tectonic line strikes from north to south, though its accurate location is not known owing to the thick sediments in the Matsumoto basin. In the 1967 explosion seismic experiment of a 16 km long profile, a low velocity zone about

472 A. IKAMI et al. 600 m wide was found by YAMADA et al. (1976), who suggested that this might be the Itoigawa-Shizuoka tectonic line. The fault location that we have derived is about 10 km east of the Itoigawa-Shizuoka tectonic line inferred from the surface geology. It is not clear whether this fault is the Itoigawa-Shizuoka tectonic line or not. KOSAKA (1984) compiled the geological structure in northern Nagano Prefecture and concluded that there are two groups of tectonic lines; one is nearly parallel to the Itoigawa-Shizuoka tectonic line, and the other is striking at high angle against the tectonic line. A tectonic line of the latter group is traced from Matsumoto to the northeast nearly in parallel with our profile, and then crosses the profile at the northeastern part of the profile. This line was named "the Tsunan- Matsumoto tectonic line" by KOSAKA (1984). It divides the area into two regions. The southeastern region is characterized by the so-called "Green Tuff" of the early and middle Miocene and the intrusive plutonic and hypabyssal rocks. This region was also characterized by the upheaval and erosion after the middle and later Miocene, but the Neogene strata were not intensely folded. On the contrary, the northwestern region is characterized by well-developed and remarkably folded marine deposits. This tectonic line crosses the 1967 seismic profile near Nagano where a vertical offset of the basement of about 4 km is pointed out by ASANO et al. (1969a) as shown in Fig. 4. AKABANE (1981), from his investigation of the faults and topographic features in this area, named this line as "Nagano-bonchi-toendansou," which meant the tectonic line located at the eastern margin of the Nagano basin. Considering these earlier geological and geophysical references, the fault near A-4 may be identified as the Tsunan-Matsumoto tectonic line, which runs to the northwestward and cuts the B-profile of the former explosion experiment by ASANO et al. (1969a). The thick surface layer obtained also in the northern part of the profile positively suggests the existence of this tectonic line. Geologically, however, this tectonic line can be traced farther northeast into Niigata Prefecture. If the fault near A-4 is the Tsunan-Matsumoto tectonic line, the Itoigawa- Shizuoka tectonic line was not detected by our present travel-time analysis. The change of structure near the Itoigawa-Shizuoka tectonic line may be gradual and is not so sharp, as to be detected by the explosion seismic experiments of such a scale of observation as in this case. The observation of Bouguer gravity anomaly was densely carried out in this area by YAMAMOTO et al. (1982). However, if we correct the effect of thick sediments in the area with a reasonable density difference for the sediment, we cannot identify the existence of the Itoigawa-Shizuoka tectonic line in the Bouguer gravity anomaly map in the Matsumoto basin. Therefore the crustal structure along the Itoigawa-Shizuoka tectonic line may not have a remarkable characteristic velocity feature beneath the Matsumoto basin. This results shows the same feature of the velocity structure as in the case of the San Andreas fault in North America, which is not marked by any distinctive seismic velocity feature (MOONEY and COLBURN, 1985). Therefore, we must set up geophones with smaller spacings to derive the fine velocity structure near the Itoigawa-Shizuoka tectonic line.

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