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A SEISMIC MICROZONING TECHNIQUE BASED ON MICROTREMOR DATA

Takeyasu Suzuki

Technical Research & Development Institute, Kumagai Gumi Co., Ltd., JapanAB STRACT

A seismic microzonlng technique based on microtremor observation is introduced and anexample of its application is demonstrated il一 this paper･ A什equency amplification function ina limited area is necessary ln the actual utilization of a seismic microzonlng map･ A shear wave

veloclty Profiles is identified uslng an OPtlmization method at the strong motion observationsite･ Microtremor measurement is also carl･ied out at the same site･ Then, the spectral ratio, aratio of Fourier amplitude spectrum of the horizontal component of a microtremor to the verticalone, is compared with the frequency amplification function calculated from the identified soil

profile of the observation site･ Then, a technique for microzonlng based on microtremor data isverified･ Finally, this technique is applied to Kushiro clty, Hokkaido Island of Japan where alarge earthquake of M=7.8 attacked in 1993.
INTRODU CTION

Fourier amplitude spectrum of the horizontal component of amicrotremor is frequently usedin the englneenng field to detect the naturalfrequency of a surface soil deposit･ However, notonly a natural frequency but a frequency amplification factor at a site is indispensable forearthquake englneerS tO evaluate the extent of ground motions al‑d damage due to an expectedearthquake･ Recent advances in the research of a microtremor glVe uS the possibility to detectboth of theml),2).

VERIFICATION OF A MICROZONING TECHNIQUE
Strong motion observation has been conducted since 1984 in the northern‑east part of Japan

by Kumagai Gumi Co･, Ltd･ The system is called HKASSEMH3)･ At the center array sites
which are located on an alluvial deposit, microtremol. Was Observed･ Fourier amplitude spectraof three directional components of a microtremor were calculated and Parzen's window

operation was applied with bandwidth of 0.8. Then, the following spectral ratio SR(a ) was
calculated:sR(o)‑3 ‑‑‑‑ (.)

in which, Fj/o) ) and Fv(o) ) denote Fourier.amplitude spectrum of the horizontal componentof a microtremor and the vertical one, respectlVely･ The author hypothesized that two times ofSR(a ) is a.ppro.xim.ately ident.icalwith frequency. response funct.ion of a surface deposit･ If
this approxlmatlOn lS Valid, mlCrOtremOr ObservatlOnS lead to a simple and effective way forseismic microzonlng.
In order to show the appropnateness of this microtremol･ technique, above‑mentioned ratio
was compared with the frequency response function of an earthquake observation site

constituting the center array of HKASSEMM･ In the first step, identification analysis for a shear
wave velocity profile was carried out at the site, uslng the optlmization theoI･y･ The targetfunction used in the analysis Was the transfer function at the site, which was calculated as theratio of Fourier amplitude spectrum of accelerogram recorded at su血ce ground to that in the

diluvial gravel and sand layer･ Fig･l illustrates the soil profile, the result of a standard
penetration test and shear wave veloclty profiles obtained什om PS‑10gglng and theidentification analysis4)･ Using the identified shear wave veloclty Profile in the figure, thefrequency response function, which means the ampliflCation factors at ground surface when anincident wave with unit amplitude inputted at the bedrock, was determined based on theuo!l〇utlJ aSuOds〇J血lanba･tJ aLll pu12 0!ltu ltZ･1133ds ･toua･T)OJ〇!u aql uaa叫3q uOS!112dLuO〇 Z･富!d(zH)八〇u9nbaJd()168L9;

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乱‑0.tlS aLt1 1t! atTJO.]d Jlt〇OP^ aAt紬.陀aqS Pa!J!1uaP! aLIl put! Sisal nl!S‑u!JO SltnSaⅦ l･宕!d(u) S^.,(,ど (〇aS((.T)
ウS^o'(fEPT3(,Cza^℃Jh.o'ア9qSO()E()()Z 00 1

anlt!^‑N()();り()V()Ot:()()乙()OL ()(); (け()C OZ ()t ()0･印 俳 托 拡 ･5: o ･; =llllIll‑♯監盟中豊董卓;撃茸讐一
DepthbetowGroundSurface,d(m)DepthbelowGroundSurface,d(m)0 ⊂一 〇 'Jt ウ tJ Cレ 7しrJ 0 日日H HHU

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Fig･3 Contour map fわr tlle tllicklleSS Oralluvial deposits in KLlShil･O City

(Freq･(Hz), Amp･ 戸ac.)■ : Sites of Microtremor Measurement

Fig.4 Natural frequencies and amplification faclol.S in the KushiI.0 plain
estimated by the p1‑OPOSed 111etllOd

APPLICATION OF TO THE SEISMIC MICROZONG OF KUSHIRO CITY

Microtremor observation was conducted in Kushiro City two moths after the Kushir0‑0ki
earthquake of 1993 took place. The maximum acceleration of 922 gals was recorded on thediluvial hill in Kushiro clty, but little severe damage was occurred in the central part of the clty
where alluvial soft soil is deeply sedimented･ The author is interested in this strange
phenomenon, which motivated this microtremor observation･Fig･3 illustrates the contour map of the thickness of alluvial deposits in Kushiro clty･ 20
sites were selected for microtremor observations, Covenng three different ground conditions in

the city aS Shown in Fig･45)･ In the figure, natural frequencies and amplification factors

derived from themicrotremor observations are listed. Companng Fig.4with Fig.3, It is clearthat the depth of alluvial deposits in Fig･3 col･relates well with the natural什equency in Fig･4･,

which validates this approach.

CONCLUSIONS

Themicrotremor spectral ratio proposed in this paper is a simple and effective method toevaluate seismicrisks in terms of both natural frequency and amplification factor for seismic
microzonlng･ In addition, this technique can be applied to any place in the world, because it is
economical and easy to measure microtremors.

REFERENCES

1. Ohmachi, T. and Nakamura, Y., 1992, "Local site effects detected by microtremor

measurements on the damage due to the I 990 Philipplne earthquakeH, Proc･ 10WCEE, Vol･2,pp.997‑ 1002
2. Tokimatsu, K. et a1., 1992,日Use of Short‑period Microtremors fわr Vs ProfilingH, Proc･ASCE, Jour. Geotech. Engrg., Vol.1 18, No.10, pp.1544‑1559
3. Abe, K. et a1., 1991, "Estimation of Amplification Spectra for P and SV Waves", Proc･ 2ndtnt. Conf. Recent Ad. Geotech. Earthq. Engrg. Soil Dyn., St. Louis, Missourl, pp･1229‑1235
4. Suzuki, T. and Tanaka M., 1994, "An Approach to Evaluate Microtremor Data Based on
the Result of Identification Analysis for S‑wave Veloclty Profile of Surface Soil Deposits",
Proc. the 9th Japan Earthq. Engrg. Symp. (submitted)

5. Suzuki, T. et a1., 1994, "Application of Microtremor MeasurelmentS tO the Estimation ofEarthquake Ground Motions in Kushiro City during the Kushir0‑0ki Earthquake of January
15, 1993", Earthq. Engrg. & Struc. Dyn･ (to be published)