Secondary Love Waves Observed by a Strong-Motion Array In the Tokyo Lowlands, Japan

Transcription

1 J. Phys. Earth, 40, , 1992 Secondary Love Waves Observed by a Strong-Motion Array In the Tokyo Lowlands, Japan Shigeo Kinoshita,1,* Hiroyuki Fujiwara,1 Tadashi Mikoshiba,1 and Tsutomu Hoshino2 National Research Institute for Earth 1Science and Disaster Prevention, Tsukuba 305, Japan 2Institute of Civil Engineering of the Tokyo Metropolitan Government, Koto-ku, Tokyo 136, Japan In order to investigate the generation and propagation of Love waves recorded in the Tokyo lowlands, we analyzed array data obtained for six events that occurred around the Kanto district. From wavenumber analysis, the following conclusions are obtained. (1) Love waves are converted from direct S-waves at the edge of a sedimentary basin. The Hachiohji tectonic line is an especially important zone for the generation of Love waves. (2) Apparent velocities of Love waves for periods between 6 and 9 s are concordant with the phase velocities of fundamental mode Love waves calculated from a threelayered model. The first, second, and third layers in the model have S-wave velocities of 0.5, 0.8, and 1.2 km/s, respectively. The S-wave velocity in the pre-tertiary basement is assumed to be 2.6 km/s. 1. Introduction For shallow earthquakes whose epicentral distances are shorter than 150 km, strong ground motions in a period range between 6 and 13 s predominate on seismograms recorded in Tokyo (Tanaka et al., 1979). Kudo (1978) showed that surface waves travel in the sedimentary basin in the Kanto Plain mainly contribute to these long period waves. Surface waves were recorded by many seismographs in the Kanto district for the main and aftershocks of the 1978 Izu-Oshima-Kinkai earthquake (MJMA = 7.0). Seo (1981) proposed that these surface waves were generated secondarily from the basin edge in the Kanto district. Yokota et al. (1986) showed that the arrival direction determined from the particle motion of the recorded Love waves is not always coincident with the direction to the epicenter. They pointed out that these surface waves were generated in the western part of the Kanto Plain for shallow earthquakes whose epicenters were in and around the Izu Peninsula. On the other hand, many numerical analyses have been performed to study the seismic response of two-dimensional sedimentary deposits (Aki and Lamer, 1970; Bard and Bouchon, 1980, 1985; Bard and Gariel, 1986). Their results indicated that surface Received November 20, 1990; Accepted September 30, 1991 * To whom correspondence should be addresseḍ 99

2 100 S. Kinoshita et al. waves are generated secondarily by the basin structure of sediments. More recently, three-dimensional analyses for investigation of such basin-induced seismic waves are performed (Horike et al., 1989; Ohori et al., 1990; Toshinawa and Ohmachi, 1990). These studies showed that it is important to understand the propagation characteristics and origins of surface waves in order to construct a model for the prediction of long period strong ground motions in the Kanto Plain. In this report, using array data obtained in the Tokyo lowlands, we estimate the dispersion of Love waves and construct an S-wave velocity structure model beneath this area. We also investigate the arrival directions of Love waves and then propose locations of secondary source zones, from where Love waves seem to be generated. But this secondary source zone is an apparent zone determined by taking account of the arrival direction of Love waves and the group velocity of the maximum amplitude portion of Love waves. Therefore, the secondary source zones proposed in the present paper may not correspond to geographical boundary zones around the Kanto Plain. However, we can expect that our results offer useful information for numerical simulations of long period strong ground motions. Although our discussion on surface waves is restricted to Love waves in this paper, we will investigate basin-induced Rayleigh waves in near future. 2. Array Observations in the Kanto District Figure 1 shows the strong-motion observation stations of the National Research Institute for Earth Science and Disaster Prevention (NIED) in the Kanto district. Two arrays are now in operation in this area. One is located in the Tokyo lowlands (SAT) Fig. 1. Map showing locations of strong-motion observation sites in and around the Kanto district. Arrays at Fuchu and in the Tokyo lowlands are denoted by SAF and SAT, respectively. J. Phys. Earth

3 Secondary Love Waves Observed by a Strong-Motion Array 101 Fig. 2. Map showing locations of stations in the Tokyo lowlands. Fig. 3. Map showing locations of stations in the Fuchu area_ and the other is located in the Fuchu area (SAF). The Tokyo lowlands strong-motion array consists of 20 stations as shown in Fig. 2. The Fuchu strong-motion array consists of 6 stations including a vertical array at the center site FCH as shown in Fig. 3. Each strong-motion site in these arrays is equipped with an identical tri-axial seismometer. Figure 4 shows the overall amplitude characteristics of the instruments. Differences in overall response between the arrays are due to different data acquisition systems. Vol. 40, No. 1, 1992

4 102 S. Kinoshita et al. Fig. 4. Amplitude characteristics of velocity strong-motion seismographs installed in the Fuchu area and in the Tokyo lowlands. 3. Structure of the Kanto District From the studies of travel time analyses, Mikumo (1966) and Seo (1981) have obtained models for the crust of the Kanto district. The structure of sedimentary deposits in the central Kanto area has been investigated by Shima et al. (1978), Yamamizu et al. (1981), and Komazawa and Hasegawa (1988). Using Bouguer anomaly data, Komazawa and Hasegawa (1988) investigated the basement structure in the Kanto Plain. The difference between ground level and basement depth is assumed to be thickness of the sediments whose thickness is shown in Fig. 5. The sediments form a basin structure and are over 3 km thick at several points. Tada (1976) pointed out that the sediments thin abruptly along the Hachiohji tectonic line, shown as HL in Fig Procedures for Discrimination of Secondary Love Waves Considering the structure described above, two kinds of Love waves can be generated when a shallow earthquake occurs in and around the Kanto Plain. Roughly speaking, one is the direct Love wave which travels in the multilayered structure from the source. We will call this type of Love wave LW 1. Another is the Love wave which is expected to be converted from S-waves by the basin structure of sediments in the Kanto Plain. Strictly speaking, we have to consider the three-dimensional structure of sediments, in order to explain this wave. This type of Love wave will be called LW2. We try to discriminate between these two kinds of Love waves for shallow earthquakes that occurred around the Kanto Plain using the following three procedures. (1) Procedure 1 is based on the difference of arrival time of the Love waves. The Love wave generated at an earthquake source (LW1) is recorded just after an initial S-wave arrival. However, Love wave (LW2) arrives with travel time delay because the sum of the travel time from the hypocenter to the secondary source zone and the travel time from the secondary source zone to an observation site is longer than the travel time from the hypocenter to the observation site. Since the average S-wave velocity in sediments is about 1 km/s (Shima et al., 1978), the travel time from secondary source zone to the observation site in the Tokyo lowlands is long enough so that two kinds J. Phys. Earth

5 Secondary Love Waves Observed by a Strong-Motion Array 103 Fig. 5. Sediment thickness in the Kanto Plain (after Komazawa and Hasegawa(1988)). The dashed line HL shows the Hachiohji tectonic line. Epicenters of the six earthquakes used in this study are also shown in this figure. The arrival direction of secondary Love waves is denoted by an arrow for each event. The solid rectangle shows the Love wave generation zone. of Love waves have different arrival times on a seismogram recorded in SAT. (2) Procedure 2 is based on the difference of arrival directions. In the case of direct Love waves (LW1), the arrival direction almost agrees with the epicentral direction. However, except for a special case wherein the epicenter, the secondary source zone and observation site lie on a line, the arrival direction of Love waves (LW2) is different from the epicentral direction. The difference of arrival directions of the two kinds of Love waves is due to the fact that the generation of LW2 depends upon the three-dimensional structure of the sediment-basement system. (3) Procedure 3 is based on variation in signal strength at stations located throughout the Kanto area. Secondary Love waves (LW2) are not always observed at all stations due to complicated three-dimensional effects between the secondary source zone and the basin structure of sediments, while the direct Love waves (LW1) are usually observed everywhere. However, to justify this procedure, we must solve difficult problems such as a directivity problem of LW2 generated from the secondary source zone. Therefore, procedure (3) is a proposition in the present stage. 5. Discrimination of Secondary Love Waves Using array records of two events, No. 4 and No. 6 events in Table 1, we examine Vol. 40, No. 1, 1992

6 104 S. Kinoshita et al. that the procedures proposed in the previous section are adequate to the discrimination of secondary Love waves. The epicenters used in this paper are shown in Fig. 5 and the source parameters for these earthquakes published by the Japan Meteorological Agency (JMA) are shown in Table 1. First, in order to discriminate between LW1 and LW2 by using procedures 1 and 2, we examine the records obtained by the SAF and SAT arrays for the earthquake east off Izu Peninsula of July 9, 1989 (MJMA = 5.5). Figure 6 shows the transverse components of displacements obtained in SAF. In this figure, Love waves arrive just after the arrival of initial S-waves which are denoted by 'S' in this figure. Therefore, these Love waves are mainly LW1. Figure 7 shows the transverse components of the displacements recorded in SAT. Although Love waves Table 1. Earthquakes analyzed in this study. Fig. 6. Seismograms from the earthquake of July 9, 1989, recorded in SAF. Wavenumber spectra are calculated from the portion denoted by arrows. J. Phys. Earth

7 Secondary Love Waves Observed by a Strong-Motion Array 105 Fig. 7. Seismograms from the earthquake of July 9, 1989, recorded in SAT. Wavenumber spectra are calculated from the portion denoted by arrows. arrive just after the arrival of direct S-waves, the waveforms differ from those recorded in SAF. The epicentral distances to the CHF station in SAF and the TTM station in SAT differ by only 11 km. In order to investigate the difference of the waveforms between the Love waves recorded in the two arrays, we estimate arrival directions and apparent velocities, using Vol. 40, No. 1, 1992

8 106 S. Kinoshita et al. Fig. 8. Apparent velocities of Love waves. Upper and lower results are calculated from array data recorded in SAF and SAT, respectively. frequency-wavenumber spectral analysis. The analyzed sections are shown by arrows in Figs. 6 and 7. The directions and apparent velocities obtained are shown in Fig. 8. In the case of SAF, we can find two kinds of Love waves. The wave trains in a period range between 7.5 and 9.0 s propagate from the direction of the epicenter. These wave trains can be identified mainly as Love waves directly generated by the earthquake source (LW1). The arrival directions of wave trains in a period range between 5.5 and 6.5 s are between N250 E and N260 E. In this period range, secondary Love waves (LW2) mainly predominate. In the case of SAT, the arrival directions of Love waves range between N240 E and N255 E in a period range between 6.0 and 8.5 s. These arrival directions are different from the epicentral direction of N218 E, and the apparent velocities in periods between 7.5 and 8.5 s are smaller than those of SAF. Therefore, in the case of SAT, secondary Love waves (LW2) mainly predominate. Next, we investigate another example. By using the data of Love waves due to the earthquake near Izu-Oshima Island of February 20, 1990 (MJMA = 6.5), we consider procedure 3. Figure 9 shows the transverse component of the seismograms at 11 stations located around the epicenter. The Love waves recorded at the F.IM and CHK stations are direct Love waves (LW1), since these waves appear just after the arrival of S-waves. However, we find gradually growing Love wave energy on the seismograms at the SRG, K MD, KSR, and CYN stations located in the Kanto Plain. In order to investigate these Love waves, we compare with four seismograms recorded at the AKW, CHF, SRG, and KSR stations. The portions denoted by arrows are maybe direct Love waves. Following this portion, Love waves are further predominant on the seismograms at the SRG and KSR stations. However, we cannot find such Love waves on the seismograms at the AKW and CHF stations. Since the AKW station is located on the basement, it is reasonable that Love waves such as LW2 are not identified. The thickness of the sedimentary layer beneath the CHF station in the SAF array is almost equal to those beneath the SRG and KSR stations. Furthermore, the epicentral distance of the CHF station is longer than that of the SRG station and is almost equal to that of the KSR station. Therefore, there can be particular areas where the secondary Love waves do not propagate or cannot be identified even though the sedimentary layer is thick enough. J. Phys. Earth

9 Secondary Love Waves Observed by a Strong-Motion Array 107 Fig. 9. Seismograms of the transverse component at 11 stations for the earthquake near Izu-Oshima Island of February 20, 1990 (MJMA = 6.5). Secondary Love waves (LW2) are not identified for this earthquake at any stations in SAF. More generally, secondary Love waves (LW2) are not predominant on the seismograms recorded at the stations in SAF for earthquakes used in this study. If Vol. 40, No. 1, 1992

10 108 S. Kinoshita et al. Fig. 10. Seismograms at the KMD site in SAT. The upper wave train is transverse to the epicenter direction and the lower one is transverse to the arrival direction of the wave group denoted by B. Fig. 11. Arrival azimuths and apparent velocities of two wave groups denoted by A and B in Fig. 10. Shadow areas show the range of apparent velocities and arrival azimuths in a period range between 9 and 10 s. The direction to the epicenter is indicated by an arrow. proposition (3) is accepted, we may have a possibility to explain this observational fact. In order to justify this proposition, however, we need further studies on the generation of secondary surface waves. We further investigate the characteristics of Love waves recorded at the KMD station in SAT by using procedure 2. Figure 10 shows horizontal seismograms in different components at the KMD station in SAT. The N115 E component corresponds to the transverse direction to the earthquake source. The N145 E component corresponds to the transverse direction to the arrival direction of the wave train denoted by the section B. Figure 11 shows the arrival azimuths and the apparent velocities of two wave trains corresponding to sections A and B in Fig. 10. These results are obtained from the peaks of the wavenumber spectra calculated in a period range between 9 and 10 s. The results J. Phys. Earth

11 Secondary Love Waves Observed by a Strong-Motion Array 109 obtained from the section B show that Love waves are not LW1 since the arrival directions differ from the direction of the epicenter. On the contrary, the wave train denoted by section A is mainly LW1. 6. Dispersion of Secondary Love Waves and Locations of Secondary Source Zones In order to determine accurate phase velocities of secondary Love waves and the Fig. 12. Wavenumber spectra at a period of 7 s. The spectra are calculated from array data recorded in SAT for the N160 E component of the No. 4 event. Fig. 13. Wavenumber spectra at a period of 7 s. The spectra are calculated from array data recorded in SAT for the N255 E component of the No. 5 event. Vol. 40, No. 1, 1992

12 110 S. Kinoshita et al. source zones of these waves, we estimate the arrival directions of Love waves by using the array data obtained in SAT. An arrival direction is determined from the peak point of the frequency-wavenumber spectra calculated from the array data. Wavenumber spectra are calculated by using a complex AR-model (Kinoshita, 1986) in a period range between 7 and 8 s, because Love waves recorded in the Tokyo lowlands are predominantly in this period range. Results are shown by arrows in Fig. 5 and tabulated in Table 1. In the cases of the three events No. 4, No. 5, and No. 6, whose epicenters are located Fig. 14. Wavenumber spectra at a period of 7.5 s. The spectra are calculated from array data recorded in SAT for the N145 E component of the No. 6 event. Fig. 15. Wavenumber spectra at a period of 7 s. The spectra are calculated from array data recorded in SAT for the N160 E component of the No. 1 event. J. Phys. Earth

13 Secondary Love Waves Observed by a Strong-Motion Array 111 Fig. 16. Wavenumber spectra at a period of 7 s. The spectra are calculated from array data recorded in SAT for the N30 E component of the No. 2 event. near the Izu-Oshima Island, the arrival directions of Love waves observed in SAT are distributed in an azimuth range between N235 E and N250 E. Figures 12, 13, and 14 are representative wavenumber spectra for these three earthquakes. Another spectral peak which is common to these wavenumber spectra shows the existence of secondary waves coming from the southeast of the Tokyo lowlands, i.e., the Boso Mountains. For two earthquakes, No. 1 and No. 2, whose epicenters are in the area where basement outcrops, the arrival directions of Love waves almost agree with the epicentral directions as shown in Figs. 15 and 16, or Fig. 5. The No. 3 event, whose epicenter is near Choshi, is a rather deep earthquake which occurred with a depth of about 55 km at the boundary zone where the Philippine Sea plate is colliding with the Pacific plate subducting beneath the Kanto area. Comparing the waveforms of this event with those of other events such as earthquakes that occurred in central Chiba Prefecture and in the south-eastern part of Ibaraki Prefecture, the duration of this event is extremely long. For the No. 3 event, the results of the frequency-wavenumber spectral analysis for the later phase including Love waves, which prolongs the duration, are interesting to us, because the peaks of wavenumber spectra have information on the directions of secondary source zone. As shown by the wavenumber spectra in Fig. 17, there are many wave groups with various arrival azimuths. Among them, two kinds of wave groups are significantly distinct. One is a secondary Love wave with arrival direction between N30 E and N40 E. The other is a wave group with arrival direction between N180 E and N190 E as shown in Fig. 18. This wave group is predominant at periods of about 6 s and is similar to that observed for the 1987 Chibaken-Toho-Oki earthquake (MJMA = 6.7) (Kinoshita et al., 1990). We consider the phase velocities of secondary Love waves. On the basis of arrival directions obtained for six events, we recalculated the wavenumber spectra from the Vol. 40, No. 1, 1992

14 112 S. Kinoshita et al, Fig. 17. Wavenumber spectra at a period of 7.5 s. The spectra are calculated from array data recorded in SAT for the N120 E component of the No. 3 event. Fig. 18. Wavenumber spectra at a period of 6 s. The spectra are calculated from array data recorded in SAT for the N180 E component of the No. 3 event. transverse component data and then determined the apparent velocities in a period range between 6 and 9 s. Figure 19 shows the average apparent velocities for six earthquakes in this period range. These results are compared with the phase velocities of fundamental mode Love waves produced by the sedimentary layer-basement structure under the Tokyo lowlands. Our observed phase velocities are not consistent with the phase velocities calculated for the velocity structure obtained from the Yumenoshima J. Phys. Earth

15 Secondary Love Waves Observed by a Strong-Motion Array 113 Fig. 19. Average apparent velocities of Love waves estimated from array data recorded in SAT. Phase velocities of fundamental mode Love waves for three velocity structure models are also shown for comparison. Fig. 20. The model for determining a secondary source area at which Love waves seem to be generated. explosion (Shima et al., 1978). Our results are fairly well explained by the velocity structure proposed by Yamazaki et al. (1992). The best fit model for our results is obtained by a slight modification of the Yamazaki et al. model as shown in Fig. 19. Finally, we propose the location of the secondary source zones considering the propagation model of seismic waves as shown in Fig. 20. In this figure, S-waves or Love waves emitted from source Po with an origin time of to arrive at the point P1 at the time t1. P1 represents the location of the secondary source. The hypocentral distance of P1 is x1 and the propagation velocity of seismic waves is v1. Secondary Love waves emitted from P1 arrive at an observation site P2 at the time t2. The distance between P1 and P2 is x2 and the propagation velocity between two points is v2. We assume that v2 is approximately equal to the group velocity Vg of the wave packet with the maximum amplitude. Vg is estimated from the array data recorded in SAT. The estimate is stable Vol. 40, No. 1, 1992

16 114 S. Kinoshita et al. Fig. 21. The N 145 E component seismograms recorded in SAT for the earthquake near Izu-Oshima Island of February 20, 1990 (MJMA = 6.5). because the smallest distance across the array is 10 km. This is similar to the wavelength of the predominant secondary Love waves. According to the model in Fig. 20, the following relation can be obtained. ti = t0 + x1/v1 = t2 - x2/v2, where to is the origin time of the event published by the JMA. We read t2 from a seismogram obtained at the center station KMD in SAT. In this paper, v1 can be determined by using the arrival times of S-waves on seismograms recorded at sites in the basement area around the Kanto Plain. We show an example. The arrival direction of secondary Love waves obtained in SAT is N235 E for the No. 6 event. Figure 21 shows the seismograms polarized in a direction of the N145 E component recorded in SAT. This direction is transverse to the direction of propagation. In this figure, the travel time curve of the wave group with the maximum amplitude and the arrival time t2 at the KMD site are indicated. The apparent velocity (Vg) is estimated about 0.53 km/s. We get the values of Vg for five events as shown in Table 1. The values of Vg are from 0.47 to 0.57 km/s. These values correspond to those of group velocities in a period range between 7.5 and 8.0 s calculated from the velocity structure obtained in this study. J. Phys. Earth

17 Secondary Love Waves Observed by a Strong-Motion Array 115 As the values of Vg were estimated in a period range between 7.0 and 8.0 s, the structural model proposed in Fig. 19 is a reliable model. The secondary source zones for the five earthquakes are shown in Fig. 5. Except for the event No. 1, these zones correspond to the boundary zones of the Hachiohji tectonic line, which separates the Kanto Plain from the Kanto Mountainous region. Such a boundary zone between abruptly dipping basement and sedimentary deposit contributes to the generation of secondary Love waves (Fujiwara, 1991; Kinoshita, 1985). Therefore, the Hachiohji tectonic line may be an especially important secondary source zone for the generation of Love waves energy found in the Tokyo lowlands. 7. Conclusions The following results are obtained from our observational study in the Kanto district. (1) For shallow earthquakes whose epicenters are located around the Kanto district, secondary Love waves are more predominant in the Tokyo lowlands than Love waves coming directly from source. (2) It is possible to explain the apparent velocities of Love waves recorded by the array observation in the Tokyo lowlands as the phase velocities of fundamental mode Love waves calculated from a three-layered sediment model. S-wave velocities and thicknesses are (0.5 km/s, 0.5 km), (0.8 km/s, 0.6 km), and (1.2 km/s, 1.0 km). S-wave velocity in the basement is assumed to be 2.6 km/s according to the result obtained by Yamamizu et al. (1983). (3) The wave groups with the maximum amplitude are in a period range between 7 and 8 s, and have group velocities of 0.47 to 0.57 km/s in the Tokyo lowlands. These values are well explained by using the velocity structure proposed in (2). (4) Except for the event No.1, all the estimates of secondary source zone for the generation of Love waves energy found in the Tokyo lowlands are determined around the Hachiohji tectonic line. We thank Hatsuo Kato (Takumi Construction Co.), Kenji Nurishi (Ryodenshya Co.) and Ryuichi Sakai (Tokyo Sokushin Co.) for their help to construct and maintain the Tokyo lowland strong-motion array. We also thank Dr. William Scott Phillips for his helpful suggestions and critical reading of the primary manuscript. The comments and grammatical corrections of two anonymous reviewers were particularly helpful to the improvement of paper. REFERENCES Aki, K. and K. L. Lamer, Surface motion of a layered medium having an irregular interface due to incident plane SH waves, J. Geophys. Res., 75, , Bard, P. Y. and M. Bouchon, The seismic response of sediment-filled valleys, Part I. The case of incident SH waves, Bull. Seismol. Soc. Am., 70, , Bard, P. Y. and M. Bouchon, The two-dimensional response of sediment-filled valleys, Bull. Seismol. Soc. Am., 75, , Vol. 40, No. 1, 1992

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