Propagation Characteristics of Intermediate-Period (1-10 Seconds) Surface Waves in the Kanto Plain, Japan

Transcription

1 J. Phys. Earth, 40, , 1992 Propagation Characteristics of Intermediate-Period (1-10 Seconds) Surface Waves in the Kanto Plain, Japan Kensuke Yamazaki,1 Masashige Minamishima,2 and Kazuyoshi Kudo3 Tokyo Gakugei University, Koganei, 1 Tokyo 184, Japan 2Shimura Senior High -School, Itabashi-ku, Tokyo 174, Japan Earthquake Research Institute, The University of Tokyo, 3 Bunkyo-ku, Tokyo 113, Japan Propagation characteristics of the intermediate-period (1-10 s) surface waves predominated in strong motion records are analyzed on the basis of an array observation deployed in metropolitan Tokyo area, Japan. Seismic energy source is the main shock of the 1984 western Nagano earthquake (M 6.8) with a focal depth of 2 km; epicentral distances (ƒ ) for the stations are about km. Phase analyses using the sub-array at KOG (ƒ à7175 km) indicate that dominant phases in the period range 1-10 s are Love and Rayleigh waves with clear dispersion. Apparent velocities of their phases are lying in the range of 800-2,100 m/s. They are matched with the fundamental mode of Love waves for transverse motion but are stretching over the two modes of Rayleigh waves (M11 and M21) for radial component; the crustal model, on which the normal mode solution is based, includes thick ( `2,000 m) sedimentary layers lying on the Pre-Tertiary bedrock. The above results in conjunction with direction of wave propagation and particle orbit of the ground motion suggest that the intermediate-period surface waves are perturbed by other phases such as diffracted surface waves which are owed to lateral heterogeneities in the uppermost crust. Correspondence of major phases in the strong motion records of the long-span array gives rise to "inter-station group velocities" of the surface waves of the intermediate-period. Thus derived group velocities enable us to interpret that the well-dispersed later phases are on a branch near the Airy phase of which propagation velocity is m/s.. Introduction 1 Rapid growth and development of urban areas like Tokyo in the recent few decades inevitably requires the construction of high-rise buildings and large-scale structures of which eigen-periods are lying in the range of 1-10 s. Thus there is a growing awareness of the importance of the intermediate-period range of strong ground motion in the field of earthquake engineering (e.g., Kudo, 1980). Most big cities have been developed on the Quaternary sedimentary layers, e.g., Tokyo, Los Angeles, Mexico City and so on. They had frequently suffered big earthquakes caused by re-activation of tectonic zones; Received November 20, 1990; Accepted September 30, 1991 * To whom correspondence should be addressed. 117

2 118 K. Yarnazaki et al. the 1923 Kanto (Japan) earthquake (M=7.9), the 1971 San Fernando (California) earthquake (MS =6.5), and the 1985 Michoacan (Mexico) earthquake (MS= 8.1) were the most recent events in the respective regions. In the case of the 1985 Michoacan earthquake, recent analyses (e.g., Singh et al., 1988) are elucidating the amplification of strong ground motion with a period of 2-3 s, which was the major factor of the tremendous damage in Mexico City that is situated on the lake bed zone. Array analysis of accelerograms from the 1971 San Fernando earthquake in the area of Los Angeles sedimentary basins (Hanks, 1975; Liu and Heaton, 1984) manifested the intermediateperiod surface waves in strong ground motion; they inferred that the excitation of the surface waves might be owed to the sedimentary basins. As regards to the 1923 Kanto earthquake, we are not sure in detail how the intermediate-period motion contributed to the tragic casualty of the Tokyo area; the urbanization at that time was not so advanced as today and hence there were very few big structures. The metropolitan Tokyo area is still increasing in industries, office buildings and population in a chaotic manner. It is well known that the metropolitan Tokyo area is lying on the thick and soft sedimentary layers of which thickness is 2,000-3,000 m (Shima et al., 1978). Hence, it is natural that some big earthquakes which will occur in and around the Tokyo area may cause unforeseen damages and casualties arising from destruction of huge structures and constructions. Therefore, the prediction of strong ground motion due to earthquakes near the metropolitan area is required for broad-band spectra up to the period of 10 s. In order to make clear the effects of surface sedimentary layers on earthquake ground motion, some works through seismic array observations have been performed at the Kyoto Basin in central Japan (Horike, 1988; Horike et al., 1990) for the frequency range of Hz and at some parts in the Kanto area (Minamishima et al., 1986; Yamanaka et al., 1989; Kinoshita et al., 1992) for the frequency range of Hz. We have deployed array observation since the early 1980's in the area of the Tokyo metropolis: two stations in the downtown area, Bunkyo and Kohtou districts, and one other at Koganei in the western suburbs. A preliminary analysis (Minamishima et al., 1986) using the tripartite sub-array at Koganei showed that coda waves after S-wave onset in strong ground motion records were characterized by waves of intermediate-period, 1-5 s, and of low apparent velocity ( km/s). Such wave groups of low apparent velocity for most events which occurred around the Kanto region did not show a sequence of a unified mode of surface waves for any single event. However, overall plot of apparent or phase velocities for all the events indicated that those phases were interpreted as normal modes of the uppermost crust including soft sedimentary layers in the surface portion. On the other hand, strong motion records at the array from the 1984 western Nagano earthquake (M = 6.8; focal depth = 2 km) depicted normally dispersed wave trains in the coda part after S-wave. In the present paper, we analyze these wave trains to determine the phase velocity at the Koganei sub-array and the group velocity using another large array of large span ( `48 km) deployed along the west-east line in the western part of the Kanto Plain. Yamanaka et al. (1989) using the same event have made clear the excitation and propagation of the intermediate-period Love waves in the southwest of the Kanto Plain; group velocity analysis was utilized in their analysis. J. Phys. Earth

3 Propagation Characteristics of Intermediate-Period Surface Waves 119 The wave modes in the present analysis are both the Love and Rayleigh waves. The main purpose of this paper is to show more confidently that the intermediate-period waves in the coda part are on a branch of normal modes excited and predominated in the sedimentary layers. 2. Observation 2.1 Geology of the observation sites The configuration of the strong motion observation array and the epicenter of the western Nagano earthquake of 1984 are shown in Fig. 1. The station ASK (Asakawa) is located on the bedrock site of Pre-Neogene to Pre-Tertiary basement; weathered deposits of some ten meters cover the bedrock. This station is operated by the Tokyo Institute of Technology, Nagatsuda. The stations KOG (Koganei) and ERI (Earthq. Res. Inst., Bunkyo) are on the Musashino gravel bed of the late Pleistocene, and KOT (Kohtou) is on the Holocene back marsh deposits. KOG has its sub-array of tripartite (Fig. 2), which is operated by the Tokyo Gakugei University. Operation at. ERI and KOT is maintained by the. Earthquake Research Institute, the University of Tokyo. Fig. 1. Map showing the array configuration with structural indices after Tada (1983) and showing the locations of the epicenter (E) of the 1984 western Nagano earthquake and the Kanto Plain in the upper right. (1), Contour line of the Pre-Tertiary basement (unit in meter); (2), river; (3), fault in the basement; (4), fault on the surface outcrop; (5), axis of subsidence in the basement. See the text for the other abbreviations. Vol. 40, No. 1, 1992

4 120 K. Yamazaki et al. Fig. 2. Configuration of the tripartite sub-array at KOG. Fig. 3. Vertical section of sedimentary layer on the uppermost crust along the east-west profile (ASK-YMS). Figure 1 also illustrates the contour lines showing the depth to the Pre-Tertiary basement (Tada, 1983). A vertical section along the west-east or ASK-KOT profile is shown in Fig. 3, which is obtained from refraction survey (Shima et al., 1978) and seismic well-shooting method and geophysical well loggings in the deep ( `2,750 m) borehole at Fuchu (FCH) (Yamamizu et al., 1981; Suzuki and Takahashi, 1985). It is noted that the refraction survey through the experiment of Yumenoshima (YMS) explosion does not make clear the S-wave velocity structure except the Pleistocene surface layer and is not always consistent with the velocity logging at FCH. J. Phys. Earth

5 Propagation Characteristics of Intermediate-Period Surface Waves Instrument The station KOG has its own sub-array of tripartite in the campus of Tokyo Gakugei University at Koganei; configuration of the tripartite is given in Fig. 2. The substations Q and R are located on the surficial Kanto loam of 9-10 m thickness; the surface layer is followed by a more rigid layer of gravels. The main station (P) is situated in the basement of a 3-story building; the sensors of seismometer are set on a platform which reaches the gravel layer. Tri-axial accelerographs of moving-coil type (natural frequency, f0 = 3 Hz; damping constant, h = 21) are set at each point, of which seismic signals are gathered through underground cable to the main station P. Data are stored in a 12-bit digital recorder with sampling interval of 1/60 s. Overall amplitude and phase characteristics are common to the three stations; amplitude response is flat for the range of Hz. Seismic observation at ASK has been performed on the basement floor of a 3-story building; the fact that the floor adjoins the building should be taken into account in the data analysis. The station is equipped with tri-axial strong-motion seismometer with a coupled pendulum (f0= 0.80, h= 35) and with wide frequency range (Muramatsu, 1977). Seismic signals are stored in a 12-bit digital recording system of which characteristic is the same as that of KOG. The amplitude response of the total observation system at ASK is flat in the frequency range Hz with respect to ground motion velocity. However, it is noted that the seismic recordings show sometimes unstable behavior in the initial part and for long-period range. Therefore we deal with the frequency range higher than 0.1 Hz; similar phenomenon is seen for some channels of the KOG recording system. As for the remaining stations, ER1 and KOT, force-balance accelerometers with 12-bit digital recorders of sampling interval of 1/100 s are installed. Frequency characteristics are flat from 0.03 to 30 Hz in amplitude of ground acceleration. We are able to analyze the data for commonly all stations in the frequency range from 0.1 to 20 Hz. 3. Observational Results and Analyses The epicenter of the western Nagano earthquake of 1984 is located almost west from the array. Epicentral distances for ASK, KOG, ERI, and KOT are 157.3, 175.4, 199.6, and km, respectively. In the first part of this chapter, we discuss the phase characteristics of ground motion at KOG sub-array. In the last half, propagation characteristics of the intermediate-period range of wave groups are dealt with using the large array spanned west-east. 3.1 General view of seismograms at KOG and their spectra Accelerograms and velocity seismograms at KOG sub-array (P, Q, R) are shown in Fig. 4. Two horizontal components (NS and EW) at each station are rotated into transverse (T) and radial (R) motions. Velocity seismograms are obtained from acceleration through FFT processing and band-pass filtering ( Hz). Waveforms are normalized by their maximum values of amplitude, which are shown at the end of the traces in Fig. 4. Unstable response of the recording system for longer period ( > 10 s) has made some ghost around the onset time for some channels with low level of signals. Higher frequency waves, say higher than several hertz, seem to behave in a random Vol. 40, No. 1, 1992

6 122 K. Yamazaki et al. Fig. 4. Ground acceleration and velocity seismograms at KOG sub-array (P, Q, R). Time in the top trace indicates lapse time of the records. manner and the maximum accelerations vary among the stations by a factor of 1.5. On the other hand, relatively long-period waves, as are clear in the velocity seismograms, behave in a similar manner among the three stations up to the lapse time about 90 s. This long-period behavior is also seen in the amplitude spectra of Fig. 5. Excepting the point R, where the traffic noise is at a fairly high level, the level of amplitude spectra for horizontal motions at higher frequency than 10 Hz are lower than those at 1-5 Hz by almost one order of magnitude and are lower than those at Hz by almost 2 orders of magnitude for transverse component. High frequency noise arising from J. Phys. Earth

7 Propagation Characteristics of Intermediate-Period Surface Waves 123 Fig. 5. Fourier amplitude spectral density of ground acceleration at KOG. electric circuit of the recording system may also be contaminating the ground motion records. Therefore, the frequency range higher than 10 Hz is excluded from our present discussion. As for the frequency range of Hz, spectral shapes of the two horizontal components are very similar among the three stations. However, certain variances among the three stations are found in the frequency range 1-10 Hz. Specifically, fairly high level of spectral content for the point Q around 5-10 Hz causes the anomalously high value of the ground acceleration in the time domain. It has been always true for other events as well as the present case that the point Q gives the maximum value of the ground acceleration among the three stations (Minamishima et al., 1986). This phenomenon is probably caused by variety of the underground structure beneath each point down to several tens of meters. Vol. 40, No. 1, 1992

8 124 K. Yamazaki et al. 3.2 Phase analyses at KOG In analyzing the phase characteristics of the intermediate-period wave groups, velocity seismograms at KOG are used. The particle orbit (hodogram) at the station Q through a band-pass filter ( Hz) is shown in Fig. 6. Time sequence of the particle motion is illustrated in the lower half of the figure, in which the top row of the hodogram shows a series of plan views and below that are those in the vertical planes of the respective orientations. The top trace of the velocity seismograms, north-south (NS) or almost transverse-component, indicates clearly dispersed wave train around the travel time s. The particle motion of the dispersed waves indicates that these waves are almost transversely polarized in the horizontal plane around the travel time above-mentioned and hence these prominent phases are most probably Love waves. However, detailed inspection suggests a little deviation from purely polarized motion in the SH-direction. This deviation from the linear polarization is more remarkable after the travel time around 85 s; the Rayleigh-type ground motion and other phases such as scattered waves may be coming after this time. Time-varying wave spectra of the ground velocity are obtained (Fig. 7) by means of the moving-window analysis (Dziewonski et al., 1969) for the frequency range Hz and for the prominent phases appearing in the horizontal motions. Thus obtained running spectra show the dispersion of the predominant wave groups more clearly. The predominant wave groups Fig. 6. Particle orbit of the ground motion (velocity) at KOG. Upper three traces are velocity seismograms at Q with the travel time in the top-most scale. Middle part of the figure or the top series of the orbital diagram corresponds to the plan view of orbital motion. Other series of the particle motion correspond to the orbital motions for the respective vertical planes of which orientations are denoted in the left. J. Phys. Earth

9 Propagation Characteristics of Intermediate-Period Surface Waves 125 Fig. 7. Running spectra of the ground velocity for the transverse and the radial motions at KOG. Time in the bottom scales indicates the travel time. Numbered sub-labels are dotted with large symbols. are labeled as IT for transverse component and IR and IIR for radial component. As for the transverse component, spectral ridge of the wave group IT is marked by dots with sub-labels 1-1, 1-2, 1-3, 2, and small crests of the later phases are numbered from 3 to 6. As well, the spectral ridge and small crests of the dominant phases for the radial component are marked and numbered. Numbering of the sub-labels, though they are not always on the spectral peaks, is done to correlate these phases with those for ERI and KOT as are seen in the following sections. In order to understand the propagation characteristics of the predominant wave groups, their phase velocities are obtained using the tripartite array. Because it is very crucial to point out the peaks and troughs precisely both in the time domain and the chart of running spectra, "the method of sums" by Bloch and Hales (1968) is adopted. Assuming the direction of wave propagation, the original "method of sums" is based on two stations. We have extended the method to the tripartite array (Minamishima et al., 1986); that gives us not only the phase velocity but also the direction of wave arrival. In the first step of the method, we assume a set of time-shifts (t1, t2) among the three stations for some wave group; this gives a direction of wave arrival. Then the raw data of horizontal ground motion are rotated into transverse and radial components of tentative. In the next step, the rotated data with time-shifts are summed and processed Vol. 40, No. 1, 1992

10 126 K. Yamazak et al. with FFT which gives the Fourier amplitude at all frequencies of interest. This is done iteratively for all sets of time-shifts of possibles. A set of time-shifts gives the maximum value of the Fourier amplitude for a frequency; this set is chosen as the optimal solution for the frequency. Taking into account the configuration and the span of the tripartite, the validity of this method is limited to the set of time-shifts (t1, t2) `(0.l, 0.1) or the phase velocity 3-4 km/s at the most; actual solutions become unstable when both of the time-shifts lie below this limitation. Further, we reject the solutions which have large deflections (>45 ) of arrival direction from the back azimuth (the horizontal direction from receiver to epicenter), because the mode of waves concerned conflicts with the polarization of the wave mode of assumption when the deflection exceeds 45. Thus obtained results for the transverse and radial components are shown in Fig. 8 with normal mode solutions of Love and Rayleigh waves for the two crustal models of horizontal layering (Fig. 9). Vertical components are omitted in these analyses because of their low level of signal-to-noise ratio. In the case of the transverse component, results for the predominant wave group IT are chained together with the squared symbol at which the spectral amplitude of the summed trace shows the maximum value among the spectral ridges plotted. Though the observed phase velocities for both cases include the higher values, say 4-5 km/s, they are at the very limit of resolution of the array analysis. Observed values of reliance are lying on 700-2,100 m/s for the transverse components and on 1,200-2,100 m/s for the radial ones. Observed azimuths of propagation direction of these wave groups are obtained to ; deflections are settling in somewhat southward direction from the azimuth toward the epicenter (275 ). Thus it is very likely that the wave group IT matches the fundamental mode of Love (a) Fig. 8. Observed and theoretical phase velocities for the transverse (a) and the radial (b) components. Labels attached to the observed values correspond to the wave groups in the frequency-time analysis of Fig. 7. Curves of heavy solid lines and dashed lines correspond to the theoretical values of the model (1) and the model (2), respectively, of the Earth's crust (Fig. 9). (b) J. Phys. Earth

11 Propagation Characteristics of Intermediate-Period Surface Waves 127 wave. Phase velocities of the later phases of less power denoted by open circles are lying in the range below 2,000 m/s. However, the propagation direction of the phases 3 and 6 deviate largely from the epicentral direction by northward. Consequently they may be diffracted waves owed to some heterogeneity in the uppermost crust. In Fig. 8, theoretical phase velocity curves are drawn for two models of the uppermost crust (Fig. 9). The two models of the sedimentary layers of the uppermost crust are based on refraction survey (Shima et al., 1978) and the down-hole measurements at Fuchu (FCH) (Yamamizu et al., 1981; Suzuki and Takahashi, 1985). The model (2) is featured by its thinner sedimentary layers on the bedrock than the model (1). Much deeper structure than the layer of VP=5.60 km/s is based on the model E-3A3 by Mikumo (1966) which is deduced from the seismic and gravimetric data. Observational phase velocities of the wave group I of transverse component show the results unanimously higher than the theoretical values of the fundamental mode of Love waves of horizontal layering. Though this is true for both of the two models, theoretical curve of the model (2) indicates much closer values to the sequence of the observed ones than the model (1). If we get into the problem of model selection for the underground structure, the model (2) rather than model (1) is likely to be a better candidate. Conspicuous differences between the two are the depth and the thickness of the bedrock layer of VP = km/s. Indeed the structure of the sedimentary layers of the model (2) is almost the same as the down-hole measurements at FCH which is located at about 5 km south of KOG sub-array; the measurements were done down to the layer of VP = 4.76 km/s at a depth of 2,750 m. Therefore we take the model (2) to first approximation for the underground structure beneath KOG. However, it should be noted that the surface of the Pre-Tertiary basement beneath KOG may be very irregular as is suggested in Fig. 3. In such a case, we have to be careful to determine Fig. 9. Two crustal models of horizontal layering Vol. 40, No. 1, 1992

12 128 K. Yamazaki et al. the underground structure using the phase velocity on the assumption of horizontal layering (Yamazaki and Ishii, 1973; Yamazaki et al., 1988). As for the radial component (Fig. 8(b)), observed phase velocities of reliance are lying in the range 1,200-2,100 m/s. The wave group IIR has some phases of their velocities exceeding 3 km/s, say 4-5 km/s, in the lower frequency than 0.2 Hz; these are at the very limit of resolution of this array analysis as mentioned before. Observed phase velocities for the wave group IR show a peculiar manner of dispersion; these are not explained by single mode of Rayleigh wave even if we take the model (2). The propagation directions of the main parts which are connected with each other by solid lines are , though the other parts which are connected by dotted lines have normal values ( ` 275 ). Thus the wave group IR consists of some phases which come from various directions. Further, the propagation directions of the phases in the wave group are and deviate more largely southward by This suggests that the phase 3 in the transverse component which comes apparently from the northwest (as mentioned before) may be just the wave group of this radial component which comes from the southwest and arrives at the same time as that of the phase 3. Though it is not clear whether the wave group consists of single mode M21 or includes others such as M12 and M11, the wave group IIR matches a little more the fundamental mode of Rayleigh-type symmetric vibration (M11). 3.3 Wave characteristics at ASK Before analyzing the group velocity through the large array, we glance at the characteristics of seismic waves at ASK. The station ASK with epicentral distance of Fig. 10. Velocity seismograms at ASK in the upper three traces. Notations of small capitals (T, R, V) for each trace correspond to transverse, radial, and vertical components of the ground motion. Fourier amplitude spectral densities for the respective components of the ground velocity are shown in the lower half. J. Phys. Earth

13 Propagation Characteristics of Intermediate-Period Surface Waves km is located at the west end of the large array and on the bedrock of the Pre-Tertiary basement. Therefore, the ground motion due to the present earthquake is considered to be an input signal to the sedimentary basin of the Kanto Plain. Raw data of horizontal motion are rotated into transverse and radial components and the respective Fourier spectra are obtained (Fig. 10). Comparing the velocity seismograms with those at KOG in Fig. 4, less duration and higher frequency contents of the predominant phases than those at KOG are evident. The high frequency contents around 3-5 Hz, as are seen in both the waveforms and the amplitude spectra, are probably due to the effect of thin surficial deposit at the site and/or the effect of the building (Yamanaka, 1990, personal communication). Figure 11 illustrates the running spectra of the ground velocity by means of the moving-window analysis. Spectral peaks, though they include low crests, and some points on spectral ridges of the predominant phases are dotted. It is seen that dispersion of the wave energy is already recognized in the predominant phases more clearly for the transverse component. Predominant phases are labeled and dotted with large symbols so as to correspond to the predominant phases at KOG in Fig. 7. Correspondence of the predominant wave group IT of the transverse component is fairly good, though it is still more dispersed at KOG. However, correspondence for the radial motion between the two stations is still less recognized. Fig. 11. Running spectra of the ground velocity for the transverse and the radial motions at ASK. Dominant phases are dotted and the predominant phases are labeled with large symbols for group velocity analysis. Vol. 40, No. 1, 1992

14 130 K. Yamazaki et al. 3.4 Wave characteristics at ERI and KOT Let us turn to the other two stations, ERI and KOT, of which epicentral distances are greater than KOG by 24 and 30 km, respectively. Figure 12 shows the seismograms of the ground acceleration and velocity at both stations. Corresponding wave spectra of-the accelerograms are shown in Fig. 13. Apparent differences between KOG and these stations are seen in the overall spectral contents and duration of the ground motion. The maximum accelerations are the waves (probably S-waves) of about 1-2 s period at ERI and KOT. This is also recognized in an apparent "cut-off frequency" ( `1 Hz) lower than KOG; over the cut-off frequency the spectral amplitude falls off rapidly. The amplitude spectra of the intermediate-period wave groups are greater than those of KOG by a factor of 1.5; this means mainly the growth of surface waves during propagation. Much more duration of coda waves of the intermediate-period than KOG is seen in both stations; this means the significant dispersion of wave energy during the propagation in the sedimentary layers of the west Kanto Plain. It is also seen that the levels of "background noises" whose spectra lie in the higher frequency than 10 Hz are fairly lower than KOG by at least a factor of 2; we regard the higher frequency contents as "background noise" because our present interest is in the intermediate-period range of wave period. Signal-to-noise ratios of the vertical components for both stations are a little higher than that for KOG. However, wave signals of the vertical component Fig. 12. Ground acceleration and velocity seismograms at ERI and KOT. Time indicates lapse time of the records. Small capitals (T, R, V) correspond to transverse, radial and vertical components of the ground motion. J. Phys. Earth

15 Propagation Characteristics of Intermediate-Period Surface Waves 131 Fig. 13. Fourier amplitude spectral density of the ground acceleration at ERI and KOT. are still weak for the intermediate-period range of signals. Figure 14 shows the moving-window analysis in the frequency-time domain of the velocity seismograms both for transverse and radial components at these two stations. Spectral peaks and some points on spectral ridges are dotted. The predominant phases are labeled and dotted with large symbols as the precedent. Labels are basically numbered in order of arrival time so as to show the corresponding phases among KOG and these stations. However, some ambiguities of the phase correspondence remain: phases 1-3 and 2 in the transverse motions at both the stations, phase 4 in KOT-RADIAL and phase 3 in ERI-RADIAL. Moreover, though it is a second guessing, the phases 4, 5, 6 in radial motion of ERI and KOT possibly correspond to the phases 1, 2, and 2-3 (the phase between 2 and 3), respectively, of the KOG-RADIAL. 3.5 Group velocity analysis through the large array On the basis of the correspondence of these wave groups in these frequency-time domains for the respective stations, we can get the "inter-station group velocities" for these wave groups. The results are shown by solid or semi-solid symbols in Fig. 15 for transverse (a) and radial (b) components. Average group velocities between the epicenter (E) and ASK are also obtained for reference and are dotted by open stars in the figures. Curves with labels (L and M series) are theoretical normal mode solutions of horizontal layering for Love (L) and Rayleigh (M) waves. Curves of solid line Vol. 40, No. 1, 1992

16 132 K. Yamazaki et al. Fig. 14. Running spectra of the ground velocity for the transverse and the radial motions at ERI and KOT. Time in the bottom scales indicates the travel time. correspond to the model (1), and the model (2) is referred by using the long-dashed lines for the fundamental tone of the respective wave modes. It is evident that the group velocities of the predominant phases are becoming lower as they enter the sedimentary basin and they propagate in the layered structure on the basin. Propagation in the wedge-shaped layer between ASK-KOG suggests transient phenomenon of excitation of surface waves. After passing KOG, group velocities of the predominant phases for transverse component settle down to m/s which cluster around the Airy phase of the fundamental mode of Love wave. If we again get into the model selection for the uppermost crustal structure on the basis of Love wave propagation, the present analysis of inter-station group velocity suggests: (i) Average structure between ASK and KOG may also be explained by the model (2); and (ii) The model (1) rather than the model (2) can represent an average structure between ERI and KOT, east end of the array. For radial motion, on the other hand, matching of the predominant phases among the array stations is not good. Separation of predominant wave groups like IR and IIR J. Phys. Earth

17 Propagation Characteristics of Intermediate-Period Surface Waves 133 (a) Fig. 15. Group velocity of the transverse (a) and the radial (b) components for various combinations of the stations and the epicenters. Curves of the theoretical values of normal mode are drawn for the crustal model (1) with solid lines and for the model (2) with dashed lines. at KOG is not seen at ASK. If we correspond the phases 1, 2, 3 at KOG to the phases 1, 2, 3 at ERI, the results are dotted by semi-solid triangles. The results for the case of phase correspondence as 1, 2, 3 at KOG to 4, 5, 6 at ERI are shown by solid triangles, which settle in the fundamental mode (M11) of the Rayleigh wave. As well, ambiguity of phase correspondence of the phases 3 and 4 between ERI and KOT results in scattered data with semi-solid circles around the M11 mode. Even so, very low values of group velocity prefer the model (1) as an underground sedimentary structure. (b) 4. Concluding Remarks Propagation characteristics of intermediate-period ground motion are analyzed on the basis of an array observation deployed on the metropolitan Tokyo area in the Kanto Plain. The array is deployed from the west-end site on a bedrock of the Pre-Tertiary basement to the east or the downtown area on thick (2,000-3,000 m) sedimentary layers. The records from the main shock of the 1984 western Nagano earthquake (M 6.8) is used. Because of extremely shallow depth (2 km) of the seismic focus, dispersed wave trains were observed throughout the array. The main phases dealt with are on this predominant wave groups. Amplitude and phase analyses at KOG tripartite sub-array show the following results. (1) Ground acceleration of the frequency around several hertz varies among the stations which separate about m each other. This is most likely caused by the variety of the very shallow portion of several tens of meters underlying the stations. (2) Predominant wave group of transverse component shows clear dispersion and Vol. 40, No, 1, 1992

18 134 K. Yamazaki et al. its constituents have low phase velocities of several hundreds to 2,200 m per second. Propagation directions of these phases point to the epicenter within 20 southward deflection. Observed phase velocities are concordant with the fundamental mode of Love wave of horizontal layering with thick (2,000-3,000 m) sedimentary layers. Some dominant phases in the coda part show their propagation directions with large deflection from the back azimuth up to 45 northward. Their phase velocities are lying in the range between the fundamental and the first higher mode. (3) Although two prominent wave groups in radial motion show normal dispersion, they contain several phases with different azimuths of propagation directions; deflections from the back azimuth reach up to 40 southward. Phase velocities of 1,200-2,100 m,'s are lying around the fundamental mode of Rayleigh waves, M11, and other higher modes, as M21 and M12. However, each wave group does not coincide with a single mode M11 or M21. These results obtained from phase analyses at the tripartite sub-array suggest that the dominant wave groups both in the transverse and the radial motions include several phases that may be diffracted by some inhomogeneities in the uppermost crust. Alternatively, they may be just the secondary or locally generated surface waves (Horike et al., 1990; Kinoshita et al., 1992). In particular, as is suggested by Kinoshita et al. (1992), the Hachiohji tectonic line along the north-south strike in the west end of the Kanto Basin may play an important role for generating the secondary surface waves. Analyses by using the long-span array (ASK, KOG, ERI, and KOT) give other aspects of propagation characteristics. The following results are obtained. (4) Seismograms at ASK, the west end of the array and on the Pre-Tertiary bedrock, shows already dispersed surface waves, though the duration is short. Growth and dispersion are seen as they pass KOG. Much longer duration of the ground motion and much lower frequency contents of the wave spectra than those at ASK and KOG are found in the accelerograms at ERI and KOT, which are located at longer epicentral distances than KOG by km. This implies much more growth and dispersion of the intermediate-period surface waves during the propagation in the sedimentary layers. (5) Inter-station group velocities on the base of wave group correspondence in the frequency-time domain are obtained. Though the group velocities between ASK-KOG show somewhat higher values, those obtained from the combination of KOG-ERI and ERI-KOT indicate very low values of m/s for transverse component. They are very close to the Airy phase of the fundamental mode of Love wave. (6) As for the radial component, however, poor level of phase correspondence among the array stations gives rise to the group velocities much scattered around M11 mode of Rayleigh wave. To conclude, the excitation of intermediate-period surface waves depends on surficial sedimentary layers as well as the seismic source parameters such as focal depth. In addition, the intermediate-period surface waves are affected by lateral heterogeneity in the uppermost crust. This heterogeneity may cause diffraction and scattering and may also cause other secondary waves like locally generated surface waves. Consequently, we have to take into account 3-dimensionally heterogeneous crustal model as well as seismic source effects for more complete simulation and prediction of strong ground motion. J. Phys. Earth

19 Propagation Characteristics of Intermediate-Period Surface Waves 135 The authors are grateful to Prof. T. Hirasawa (Tohoku University) and Dr. E. Shima (Emeritus Professor of Univ. Tokyo) for their continuous encouragement and comments. We are indebted to Mr. M. Sakaue and Mr. M. Yanagisawa, Earthq. Res. Inst., for providing us the strong motion records at ERI and KOT and their kind assistance in founding the observation stations. Dr. Y. Sawada, Central Research Institute for Electric Power Industry, kindly supplied some transducers for this research. We also thank Prof. K. Seo and Dr. Yamanaka, Tokyo Institute of Technology, for their kind offer of digital data of strong motion records at Asakawa (ASK). This research was partly supported by the Grant-in-Aid for Research in Natural Disaster from the Ministry of Education; Project No (organized by Prof. T. Hirasawa) and Project No (organized by K.Yamazaki). Comments by two anonymous reviewers were helpful for improving the text. Data processing and numerical computations were done at Center for Educational Technology of Tokyo Gakugei University. REFERENCES Bloch, S. and A. L. Hales, New technique for the determination of surface wave phase velocities, Bull. Seismol. Soc. Am., 58, , Dziewonski, A., S. Bloch, and M. Landisman, A technique for the analysis of transient seismic signals, Bull. Seistnol. Soc. Am., 59, , Hanks, T., Strong ground motion of the San Fernando, California, earthquake: Ground displacements, Bull. Seismol. Soc. Am., 65, , Horike, M., Analysis and simulation of seismic ground motions observed by an array in a sedimentary basin, J. Phys. Earth, 36, , Horike, M., Y. Takeuchi, and M. Hoshiba, Analysis of seismic small-scale array data in a sedimentary basin, Zisin (J. Seismol. Soc. Jpn.), Ser. 2, 43, 43-54, 1990 (in Japanese with English abstract). Kinoshita, S., H. Fujiwara, T. Mikoshiba, and T. Hoshino, Secondary Love waves observed by strong-motion array in the Tokyo lowlands, J. Phys. Earth, 40, , Kudo, K., The contribution of Love waves to strong ground motions, Proceedings of 7th World Conference on Earthquake Engineering, Vol. 2, , Liu, H.-L. and T. Heaton, Array analysis of the ground velocities and accelerations from the 1971 San Fernando, California, earthquake, Bull. Seismol. Soc. Am., 74, , Mikumo, T., A study on crustal structure in Japan by the use of seismic and gravity data, Bull. Earthq. Res, Inst., 44, , Minamishima, M., K. Yamazaki, and K. Kudo, A preliminary analysis of wave groups found in the strong ground motion records observed on sedimentary layers, Zisin (J. Seismal. Soc. Jpn.), Ser. 2, 39, , 1986 (in Japanese with English abstract). Muramatu, I., A velocity type strong motion seismograph with wide frequency range, Zisin (J. Seismol. Soc. Jpn.), Ser. 2, 30, , 1977 (in Japanese with English abstract). Shima, E., M. Yanagisawa, K. Kudo, T. Yoshii, K. Seo, and K. Kuroha, On the base of Tokyo III. Observation of seismic waves generated from the 4th and 5th Yumenoshima explosions, Bull. Earthq. Res. Inst., Univ. Tokyo, 53, , 1978 (in Japanese with English abstract). Singh, S. K., E. Mena, and R. Castro, Some aspects of source characteristics of 19 September 1985 Michoacan earthquake and ground motion amplification in and near Mexico City from strong motion data, Bull. Seismol. Sae. Am., 78, , Suzuki, H. and H. Takahashi, Construction and geology of the Fuchu deep borehole observatory, Vol. 40, No. 1, 1992

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