The Focal Process of the Kita-Mino Earthquake 229

The Focal Process of the Kita-Mino Earthquake 229 field have been advanced. MARUYAMA (1963), HASKELL (1964, 1969) and SATO (1969) presented the mathematical formulae of the seismic waves due to a shear faulting model in an infinite medium. On the basis of the synthetic seismograms calculated by the above formulae, AKI (1968, 1972), KANAMORI (1971, 1972), KANAMORI and ANDO (1973), ABE (1974a, b) and MIKUMO (1973a, b) have successfully developed source studies on the basis of nearfield data, especially from the viewpoint of a fault motion. KAWASAKI et al. (1972, 1973) and SATO (1972) derived the complete expressions of the seismic waves including refracted SP wave and surface Rayleigh wave due to a shear faulting model in a semi-infinite medium. In papers by KAWASAKI et al. (1972, 1973), characteristics of each phase of the seismic wave are described and discussed in detail. For example, they pointed out that a refracted SP wave is distinct and has a large amplitude comparable to P wave and cannot be neglected when the focal mechanism is discussed by theoretical seismograms. KAWASAKI and SUZUKI (1974) synthesized theoretical seismograms due to moving source models in a semi-infinite and an infinite media for the Sanriku earthquake of 1933 and compared them with the actual records. As a result, they proved that a careful examination of the refracted SP wave can yield an estimation of 5sec for a rise time of the source time function. This result is based on the fact that the SP phase can be discriminated as a later phase from the initial P motion for the rise times shorter than about 5sec. The purposes of this work are (1) to study the focal process of the Kita- Mino earthquake in relation to the Hatogayu-Koike fault and (2) to discuss some properties of the seismic waves in the near-field, with a particular attention to the SP wave refracted at the free surface. 2. Hypocenter Re-location

4. Spatial Distribution of Aftershocks Closed circles plotted in Fig. 7 are the epicenters of the aftershocks which are listed in the Seismological Bulletin of JMA with the seismological data in the period of August 19-31, 1961. Open circles are epicenters of the same aftershocks re-located in this study by the data of the stations within the epicentral distance of 200km. A bold solid line trending in NE-SW direction indicates the Hatogayu-Koike fault (MORIMOTO and MATSUDA, 1961). A rectangle of the broken line shows the horizontal projection of the fault which is determined in the following sections. Parameters of the re-located hypocenters of the aftershocks are given in Table 1. Standard deviations are within 3km for the epicenter and within 10km for the focal depth for each after- Fig. 7. Spatial distribution of aftershocks. Closed circles and open circles indicate epicenters of the aftershocks determined by JMA and the present study, respectively. +JMA, +M1 and +M2 are epicenters of main shock determined by JMA, by this work with the data of all of the Japanese stations and with the data of the stations within epicentral distance of 200km, respectively. KT, Katsuyama; ON, Ohno; KD, Kadohara; IT, Itoshiro; SH, Shiratori. Bold solid line is the Hatogayu-Koike fault. A rectangle of broken line is a horizontal projection of assumed fault.

238 I. KAWASAKI Fig. 12. Fault geometry determined in the present work. for the slip angle. A fault length of 12km seems to be small for the large magnitude 7.0. However, the southwest end of the fault will certainly not extend more because, if the fault is assumed to extend more several kilometers toward southwest, relative calculated displacement across BC in Fig. 10 becomes about 10cm or more. We also considered that the northeast end of the fault would probably not extend more, taking into account the relative situation of the aftershock area, the assumed fault and the Hatogayu-Koike fault. It is not clear why faulting motion did not reach the free surface at the time of the earthquake. In connection with this fact, it is of interest that the thickness of the Tetori group of the younger Mesozoic sedimentary rocks in this area is estimated to be 0.5-2.0km (MAEDA, 1957). The highly metamorphosed rocks (Hida gneiss complex) of the Paleozoic age underlie the Tetori group as quoted from the paper of MORIMOTO and MATSUDA (1961) in section 4. The broken nodal line in Fig. 6 shows the fault plane solution obtained in (vi). This nodal line is not unfavourable for the distribution of the first motions. 6. Long Period Seismograms in the Near-field The long-period seismograms (Fig. 13) were obtained by the seismographs

The Focal Process of the Kita-Mino Earthquake 239 Fig. 13. Long period seismograms obtained by the seismographs installed at Abuyama Seismological Observatory. T0 pendulum period is 28sec, h damping coefficient 0.4 and V magnification about 1.2. Fig. 14. Locations of the main shock (+) Hand stations of Abuyama (AB), Fukui (FUK), Takayama (TKM) and Gifu (GIF). (T0, pendulum period is 28sec; V, magnification is 1.2; h, damping coefficient is 0.4) operated at Abuyama Seismological Observatory (designated by AB in In order to determine the rupture velocity Vc and the rise time t0, we synthesized seismograms including the instrumental response for various combinations of rupture velocities and rise times. In the calculation of the seismograms, the rupture is assumed to have been initiated at the northeast end of the fault area, to have taken place simultaneously over the width (10km) and to have

I 240. KAWASAKI propagated unilaterally to the southwest with a constant velocity Vc. P and S wave velocities are taken to be 6.0km/sec and 3.5km/sec, respectively. The synthesized seismograms are obtained by convolving the theoretical ground displacements with the instrumental response. The synthesized seismograms which show better agreement with the records are illustrated in Fig. 15. The best agreement of the synthesized seismograms (V) of Fig. 15 with the records yields estimates of Vc=3.0km/sec and t0=2sec. When we interpret the actual records at Abuyama, it is very important that the initial recorded cycle is not due to P wave but due to SP wave refracted at the free surface. This SP wave is generated as an S wave at the source, and is incident onto the free surface with the critical angle and is propagated with P wave velocity along the free surface as illustrated in Fig. 16-(I). We must interpret this cycle as an SP wave for the following reasons: Fig. 15. Synthesized seismograms for the various rupture velocities (V c) and rise times (t0) are compared with the records. Fig. 16. (I) Ray path of SP wave. (II) Ray path of SPn, wave.

244 I. KAWASAKI 7. Strong-motion Seismograms The seismograms in Fig. 19 are the strong-motion records obtained at Takayama (designated by TKM in Fig. 14) and Fukui (FUK), which are located at the similar epicentral distances (49km, 44km) in the opposite direction from the epicenter as illustrated in Fig. 14. A significant agreement Fig. 21. Strong motion seismograms at Gifu (GIF) with synthesized ones. Fig. 22. Strong motion seismograms at Takayama (TKM) with synthesized ones.

246 I. KAWASAKI connected with a fact that the earthquake had occurred in the hard basement rocks of the Hida gneiss complex of Pre-Cambrian age. 9. Relationship to the Atotsugawa Fault The Hida earthquake of magnitude 7.9 occurred on June 18, 1586 in the same region as the Kita-Mino earthquake (USAMI, 1966). However, according to detailed records of Dai-Nippon-Zishin-Shiryo edited by MUSHA (1941), severe damage was concentrated in the area along the Shirakawa valley, which is about 20km eastward from the Hatogayu-Koike fault. No descriptions of the damage in the Ohno and Katsuyama regions nearer the Hatogayu- Koike fault is given. This fact suggests that the source of the Hida earthquake of 1586 was not the Hatogayu-Koike fault but was situated at the Shirakawa Valley (hatched area in Fig. 23). Since any other remarkable earthquake in this region is not described, we may conclude that the Hatogayu-Koike fault has moved in the last 1000 years. MORIMOTO and MATSUDA (1961) interpreted the discontinuity of the each terrain on both sides of the fault as the fault displacement as much as 2000m if it is a strike slip fault. If occurrence of the earthquake of the same type at the rate of one time per 1000 years is assumed, a time duration of 1.25m.y. will explain the discontinuity of 2000m of the each terrain on the both sides. In the same way, if it is a dip slip fault, a time duration of 0.3m.y. will explain the 400m uplift of the NW side over the SE side. If this evaluation of 0.3-1.25m.y. is correct, it is estimated that the age of the opening of the fault is the Quaternary at any rate. Now, the Hatogayu-Koike fault is situated on an extension line toward SW direction of the Atotsugawa fault (MATSUDA, 1966) (designated by A) as illustrated in Fig. 23. The both faults denote the fault motion of the same type-right lateral motion in strike component and uplift of the northwest side Fig. 23. Locations of the major faults near the Hatogayu-Koike fault in the central Japan. A, the Atotsugawa fault; B, the Atera fault; C, the fault of the Nobi earthquake of 1891; D, the seismological fault of the Fukui earthquake of 1948; E, the Hatogayu-Koike fault; F, the source region of the Hida earthquake of 1586; G, the seismological fault of the Gifu earthquake of 1968; H, the Itoigawa-Shizuoka tectonic time.

The Focal Process of the Kita-Mino Earthquake 247 over the southeast side. These facts strongly reveal one prospect that both faults constitute one fault system mediated by the Shirakawa Valley of the epicentral region of the Hida earthquake of 1586. However, any indication of a fault trace connecting both faults were not found through a topographic re-examination executed recently by Matsuda (personal communication, 1974). Irrespective of this, the both faults may be made up of one fault system in the basement rocks. 10. Conclusions It has been corroborated that the seismological data in the near-field of the Kita-Mino earthquake of 1961 can be interpreted as the result of the rightlateral and reverse faulting of the Hatogayu-Koike fault (a Quaternary fault). Focal parameters determined in the present work are listed in Table 2. For an analysis of the first motion data, the following questions are important: Careful examination is particularly required for a shallow focus earthquake, like this earthquake. For example, if the focal depth is within 2km, the difference of travel times of P and SP waves is within about 0.4sec for all Japanese stations. Dense distribution of the dilatations on the focal hemisphere which raised an interruption in the determination of the fault-plane solution in section 3 might arise from misinterpretation of the first motion as mentioned above. Table 2. Source parameters of the Kita-Mino earthquake determined in this study.

248 I. KAWASAKI When a seismographic station is located out of the critical distance, seismic waves except that of SH type are effected very much by the free surface: A seismic wave of the SV type suffers a phase change, SP wave of a comparatively large amplitude appears and surface Rayleigh wave develops. For the direction of the anti-node of P-wave radiation, doubling of horizontal components of displacements obtained by whole-space calculation may be a good correction for the free surface effect. However, for the direction near the node of P wave radiation, the whole-space calculation requires great care because SP wave of a relatively large amplitude to P wave arrives at the station after P-wave arrival with a small time lag. Free surface effect on the wave form is striking for the vertical component among others. When a seismographic station is located within the critical distance, the doubling of displacements obtained by the whole-space calculation is an excellent correction for the free surface effect. A detail of problems of the whole-space calculation and the half-space calculation is presented in papers by KAWASAKI et al. (1972, 1973, 1975) and KAWASAKI and SUZUKI (1974). I wish to express my sincere thanks to Prof. Ryosuke Sato, Mr. Yasunori Suzuki and Mr. Katsuhiko Ishibashi for stimulating suggestions, discussions and critical reading of the manuscript. I am very grateful to Prof. Tokihiko Matsuda for helpful discussions on the geological aspects of this work and critical reading of the manuscript. My sincere thanks are also due to Prof. Kennosuke Okano, Dr. Masaji Ichikawa, Mr. Nobuo Hamada, Prof. Tatsuo Usami and Dr. Masaharu Sakata for kindly providing me with the seismological and geodetic data. Prof. Takeshi Mikumo kindly lent me the original film of the figure he used in a paper in 1973. I used the computer program for a hypocenter determination written by Prof. Kenshiro Tsumura and Prof. Hiroo Kanamori. I wish also to express my sincere thanks to the referees for their helpful comments. Numerical computations were done by HITAC 8800/8700 of Computer Centre of the University of Tokyo. REFERENCES ABE, K., Fault parameters determined by near- and far-field data: The Wakasa Bay earthquake of March 26, 1963, Bull. Seism. Soc. Amer., 64, 1369-1382, 1974a. ABE, K., Seismic displacement and ground motion near a fault: The Saitama earthquake of September 21, 1931, J. Geophys. Res., 79, 4393-4399, 1974b. AKI, K., Study of earthquake waves by a seismometer array: Part 1. Aftershocks of the Kita- Mino earthquake of Aug. 19, 1961, Bull. Earthq. Res. Inst., 40, 371-389, 1962. AKI, K., Seismic displacements near a fault, J. Geophys. Res., 73, 5359-5376, 1968. AKI, K., Earthquake mechanism, Tectonophysics, 13, 423-446, 1972. EWING, W.M., W.S. JARDETZKY, and F. PRESS, Elastic Waves in Layered Media, McGraw B Hill ook Company, New York, 1957. GEOGRAPHICAL SURVEY INSTITUTE, Vertical movements in Chubu district (I), Report of the C oordinating Committee for Earthquake Prediction, IX, 74-78, 1973. HASKELL, N.A., Total energy and energy spectral density of elastic wave radiation from propagating faults, Bull. Seism. Soc. Amer., 54, 1811-1841, 1964.

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