Long-period coda waves were recorded on the vertical-component seismograms of aftershocks of the Hyogo-ken Nanbu earthquake, 1995. We identify the long-period coda waves as Rayleigh wave, because they appear after the S-arrival times and exhibit the normal dispersion that propagation velocity of the coda waves increases with an increase in period. By applying the moving window analysis to the coda waves from nine aftershocks, the group velocities are determined as a function of period within the range of 2 to 8 s. The group velocity dispersion data are inverted to investigate the S-wave velocity structure of the upper crust. The S-wave velocity structure is consistent with those obtained in previous studies using traval time analysis of bood waves

Ito, Aki

Oda, Hitoshi

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S-wave velocity structure estimated from long-period coda waves

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OKAYAMA UniversityEarth Science Reports,Vol. 1, No. 1, 15~18, (2000)S-wave velocity structure estimated from long-period coda wavesHitoshi Oda*and AId Ito***Department of Earth Sciences, Okayama University,Okayama 700-8530, Japan**Observation Center for Prediction ofEarthquakes andVolcanic Eruptions, Tohoku University,Sendai980-8578,JapanLong-period coda waves were recorded on the vertical-component seismograms of aftershocks oftheHyogo-ken Nanbu earthquake, 1995. We identifY the long-period coda waves as Rayleigh wave, becausethey appear after the S-arrival times and exhibit the normal dispersion that propagation velocity of the codawaves increases with an increase in period. By applying the moving window analysis to the coda waves fromnine aftershocks, the group velocities are determined as a function of period within the range of 2 to 8 s. Thegroup velocity dispersion data are inverted to investigate the S-wave velocity structure of the upper crust.The S-wave velocity structure is consistent with those obtained in previous studies using travel time analysisof body waves.Keywords: surface wave dispersion, coda wave, S-wave velocity structure, upper crust, Hyogo-kenNanbu earthquake,..3~.01~.0134.0--- 30KIILona i tude (del)Fig. I. Epicentral distribution ofaftershocks (cross) andstation location (Plus). The numbers correspond to theevent numbers listed in Table I.34.0 '--_--' --J,.. ...l...._~_--l2. Waveform data and Rayleigh wave group velocitiesearthquake in 1995. In this study, the coda waves areidentified as Rayleigh wave and their group velocities aredetermined by the moving window method (Landisman etal., 1969) as a function ofperiod within the range of2 to 8s The group velocity data are inverted to estimate theS-wave structure of the upper crust, and the structure iscompared with those reported in previous studies.1. IntroductionThe coda wave that appears after the arrival time ofS-wave from near local earthquake provides us withinformation ofthe earthquake source mechanism, seismicvelocity structure and attenuation of the crust (e.g. Aki andChouet, 1975, Sato and Fehler, 1998). The long-periodcoda wave has been recognized to be a kind of surfacewaves, while the short-period coda wave is considered toarise from scattering of the S-wave due to structuralheterogeneity. If the group velocity of the long-period codawaves is measured as a function of period, the dispersioncurve is useful for estimating the seismic velocity structurebeneath a region including station and earthquakehypocenter. However the excitation of the long-periodcoda waves by small local earthquakes is usually so weakthat it has been difficult to measure the dispersion curve ofsurface wave. An advantage in using surface waves forstudying the seismic velocity structure is that the structurecan be determined only from seismic records at a singlestation, whereas a number of seismic stations are requiredfor the study of velocity structure by travel time analysis ofthe P- and S-waves. In addition, the short period surfacewaves are suitable for estimating the S-wave structure inshallow depth, because the surface wave group velocity issensitive to the S-wave velocity structure.We found long-period coda waves on theseismograms of aftershocks ofthe Hyogo-ken Nanbu88764 5Per iod (s)3 4 5Per i od (s)Fig. 3. Wave energy arrival time chart of #3 event bythe moving window analysis. The contours with intervalof 20 dB correspond to the isolines of the amplitude level.Thick line represents the group velocity dispersion curve.1. 5--J:2..~~:;::::::::r=~--'----T'==;:=4q::==;::::>..=J2(i)~...... 3.0>-+-'oo~ 2.5Q.:::Jo...~ 2. O~liijj~~Fig. 4. Dispersion curves of Rayleigh wave groupvelocity for nine events listed in Table I.3.5Seismic waves from aftershocks ofthe Hyogo-kenNanbu earthquake were recorded by three componentseismometers with natural period of 5 s at OkayamaUniversity. Only the vertical-component seismograms wereanalyzed because of trouble in the recording system ofhorizontal components. We selected records of nine eventsofwhich amplitudes were not scaled off The sourceparameters of the events are listed in Table 1. Theepicentral distances are less than 100 km, and the focaldepths fall between 5 and 20 km. The earthquakemagnitudes are between 4.0 and 4.5. The epicenters of thenine events and seismic station are shown in Fig. 1. Figure2 shows the vertical-component seismograms of#3 and #9events. Long-period coda waves appear after the S-wavearrival in the vertical component and they exhibit thecharacteristic of normal dispersion that propagationvelocity becomes higher for longer period, though the codawaves are contaminated with short-period seismic waves.Therefore we identifY the long-period coda waves asRayleigh wave.The moving window method (Landisman et al., 1969)was applied to the long period coda waves in order todetermine the group velocities as a function of period. In16Table 1. Source parameters and magnitudes of aftershocksNo. Date Latitude Longitude M Dep. (km)3 Jan. 17 34.57 0 N 134.960 E 4.2 116 Jan. 17 34.590 N 135.000 E 4.5 197 Jan. 17 34.53 0 N 134.900 E 4.2 109 Jan. 17 34.57 0 N 134.990 E 4.3 1212 Jan. 18 34.440 N 134.900 E 4.0 513 Jan. 18 34.68 0 N 135.180 E 4.3 1615 Jan. 18 34.68 0 N 135.17 0 E 4.3 1316 Jan. 18 34.45 0 N 134.870 E 4.2 817 Jan. 18 34.46 0 N 134.83 0 E 4.1 10After the Bulletin of lM.A.#910.Fig. 2. Vertical-component seismograms of#3 and #9events.the method, the width of time window was set so as to bethree times a given period. The group velocity or waveenergy arrival time was determined at every 0.5 s in theperiod range of 2 to 8 s. The group velocity dispersioncurve for #3 event is shown in Fig. 3, where the waveenergy is displayed in dB as a time series at every 0.14 s fora given period and the contours corresponding to theisolines ofthe amplitude level are drawn with 20 dBinterval. The group velocity dispersion curve is obtained bytracing the peak wave energy against period. Figure 4shows the dispersion curves measured for the nine events.Although the group velocities are scattered and roughlychange with respect to period, they systematically increase3.2 ,..--T""-"---,----.,-----,--,'U 2.8....!,.,...8 2.4~~..Cl 2.01. 8 l...-_.L- -'-- ..J.- -'-_-'~o 4.0 60 ~oPer i od (sec)Fig. 5. Average group velocity of the dispersion curves of#03, #09, #13, and #16 events. Bar denotes the standarddeviation. Theoretical group velocity calculated for the fmalvelocity model in Table 2 is shown by the solid curve.Table 2. Seismic velocity structure estimatedby the group velocity inversion.Layer No. Vp Vs Density Thickness(km/s) (km/s) (g/cm3) (km)3.10 2.03 2.15 0.92 5.50 3.17 2.27 l.l3 5.90 3.45 2.65 4.04 6.25 3.54 2.70 7.05 6.86 3.88 2.90 19.06 8.10 4.20 3.100.034 ,---,---,----,----r----,r----,--.0.033,...i 0.032I0.0310.0300.029 L..-...1....__L-_-.l...__..L-_---l"--_-L.---l11.0 12.0 13.0 14.0 15.0 18.0Depth (kllI)Fig. 6. RMS variation of group velocity residuals againstdepth of the Conrad discontinuity.17with an increase in period. The scatter of the group velocitymay arise from lateral heterogeneity of seismic wavevelocities. Further the rough change of the group velocitywith respect to period may be due to contamination withrefracted waves, reflected waves and short period codawaves.3. Result and discussionIn order to smooth the group velocities roughlychanging with respect to period, we further choose fourdispersion curves that are between 85 and 115% of themean value of group velocities measured for the nineevents. The average group velocity of the four dispersioncurves is plotted as a function ofperiod (see Fig. 5). Thegroup velocity drastically increases up to a period of4 sand then slowly increases. The drastic increase at shortperiods is interpreted as being due to rapid increase inseismic wave velocities with increasing depth.The group velocity data are inverted to investigate theS-wave velocity structure beneath the region betweenOkayama and Kobe. Because the Rayleigh wave groupvelocity is more sensitive to S-wave velocity than toP-wave velocity although it depends on both the velocitystructures, we keep the P-wave velocities invariant duringthe group velocity inversion. We adopted the P-wavevelocity model determined by explosion experiments in theChugoku and Shikoku districts (e.g., Aoki and Muramatsu,1974; Piao et al., 1996). Let VC be the group velocitycalculated for a given model of horizontally layered velocitystructure. The difference of the calculated group velocityfrom observed one, VO, at a period Tk is expressed bywhere N is the number oflayers and tSf3t is the correctionfor the S-wave velocity ofthe i-th layer. Since VO,VC and CUe / Of3; are known for the given velocitymodel, the unknown parameter tSp; is determined bysolving, in a least squares sense, Eqs. (l) formed for a setof group velocity data. The S-wave velocity structure isobtained by adding the velocity correction tSf3t to the initialvalue of the S-wave velocity in each layer. We used thecomputer program 'DISPER80' (Saito, 1988) in order tocalculate the group velocity and its velocity derivatives.The group velocity data may have ability to estimate thevelocity structure in the upper crust, because thewavelength of the surface wave is shorter than about 25 kmin the present period range. The S-wave velocity modelobtained by the group velocity inversion is shown in Table2. Theoretical group velocities calculated for the velocitymodel is illustrated as a function of period in Fig. 5.Agreement between the calculated and observed groupvelocities is excellent. The boundary interface between thefourth and fifth layers corresponds to the Conraddiscontinuity.18aftershocks of the Hyogo-ken Nanbu earthquakes in 1995.We identified the coda waves as the Rayleigh wave,because they exhibited a characteristic of surface wave,normal dispersion, and appeared in the vertical component.By applying the moving window analysis to the long periodcoda waves, the Rayleigh wave group velocities weredetermined for nine aftershocks within the period range of2 to 8 s. The group velocity dispersion data were invertedto investigate the S-wave velocity structure of the uppercrust. The S-wave velocity drastically increases down to 2km depth, and below the depth it gradually increases. Thedepth of the Conrad discontinuity was determined to be 13km in the region between Okayarna and Kobe. The S-wavevelocity is 3.54 kmls above the discontinuity and 3.88 kmlsbelow it. The obtained S-wave velocity structure wasconsistent with those obtained in previous studies usingtravel time analysis ofbody waves. The long period codawaves can be used for the study ofthe seismic velocitystructure.S-wave velocity Ckm/s)2. 5 3. 0 3. 5 4. 0 4. 5--present study.--- Ichikawa & Mochizuki-- - -- Tanaka..... Ichinose et al....•••••• Aok i & Mur8lR8tsu2.0ore=~I::::!::::~ri'-..,--.--r1~ 20Q.Q>o3010Fig. 7. Comparison between the S-wave velocity structuresby group velocity inversion and travel time analysis(Ichikawa and Mochizuki ,1971; Tanaka ,1987). TheS-wave velocities ofIchinose et a1. (1982) and Aoki andMuramatsu (1974) are estimated from the P-wave velocityby assuming VpNs= l.73.Figure 6 shows the RMS variation of the group velocityresiduals against depth ofthe discontinuity. Because theRMS has the minimum value when the Conraddiscontinuity is at 13 km depth, we assign the depth to theConrad discontinuity. The S-wave velocity structure by thegroup velocity inversion is compared with those of theprevious studies using travel time analysis ofP- andS-waves (see Fig. 7). The S-wave velocity falls within arange of scatter of those reported in the previous studies.This result indicates justification of our velocity model andidentification of the long-period coda waves.The effect ofa liquid layer on Rayleigh wave groupvelocity is briefly examined, because the ray paths frommost events cross over the Seto Inland Sea (see Fig. 1).The depth of sea bottom along the ray paths is less than 0.2km. Assuming that a liquid layer with a thickness of 0.1 kmis on the uppermost layer of the velocity model of Table 2,we calculated group velocity dispersion curve of theRayleigh wave. The liquid layer decreases the groupvelocity by about 0.1 kmls or less at the period rangingfrom 2 to 8 s, and it is less influential on the group velocitydispersion curve. Thus the effect of the Seto Inland Seadoes not have to be taken into account for the groupvelocity inversion for estimating the seismic velocitystructure.4. ConclusionsLong-period coda waves after the S-wave arrival wererecorded on the vertical-component seismograms ofAcknowledgmentsThe authors thank all members ofgeophysical laboratoryfor providing useful comments on this study.ReferencesAki, K. and B. Chouet, Origin of coda waves: Source, attenuationand scattering effects, J. Geophys. Res., 80, 3322-3342,1975.Aoki, H. and I. Muramatsu, Crusta! structure in the profile acrossKink.i and Shikoku, Japan, as derived from the Miboro andthe Toyama Explosions, 1. Seism. Soc. Japan, 27, 104-109,1974.Ichikawa, S. and E. Mochizuki, On travel time tables for localearthquakes, Papers in Meteorology and Geophysics, 22,229-290, 1971.Ichinose, Y., S. Asano, M. Saka, K. Miura, Y. Inoue, and R. Miura,Crustal structure in the Sakaide-Siraki profile line byexplosion seismic observations, Programme and Abstracts,Seism. Soc. Japan, No. 1, A 16, 1982.Landisman, M., A. M. Dziewonski, and Y. Sato, Recentimprovement in the analysis of surface wave observations,Geophys.1. Roy. astr. Soc., 17,369-403,1969.Piao. C, T. Yoshii, T. Iwasaki, S. Sakai, T. Takeda, 1. Moriya, T.Ohmura, K. Miyashita, F. Yamazaki, K. Ito, Y. Shimada, K.Tashiro, and H. Miyamachi, Crustal structure in and aroundthe source region of the 1995 southern Hyogo prefectureearthquake deduced from seismic refraction experiment(Keihoku-Seidan profile), Programme and Abstracts, Seism.Soc. Japan, No. 2,873, 1996.Saito, M., Disper80, A subroutine package for calculation ofseismic normal-mode solutions, in Seismological Algorithmsedited by D. 1. Doornbos, Academic Press, New York, pp.293-319,1988.Sato, H. and M. C. FeWer, Seismic wave propagation andscattering in the heterogeneous earth, Springer Press, pp. 308,1998.Tanaka, T., Seismic structure of the crust and upper mantle in theShikoku and Chugoku districts, Master thesis, OkayamaUniversity, 1987.

S-wave velocity structure estimated from long-period coda waves

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