LESSONS LEARNED FROM THE 15 JANUARY 1993 KUSHIRO･OKI,
JAPAN EARTHQUAKE
Takeyasu SUZUKI
Research Engineer, Technical Research & Development Institute, Kumagai Gumi Co･, Ltd･,
1043 Shimoyama, Onigakubo, Tsukuba‑shi, IBARAKI 300‑I 1 , JAPANABSTRACT
An earthquake attacked the southern area of Hokkaido island (North island of Japan) at 8:06
pm･ January 15, 1993･ The magnitude of the earthquake was 7･8 and its focal depth was 107

km. The fault rupture is reported to occurwithin Pacific Platel). Kushiro City is located only
15 km away from the epicenter (see Fig.1) and over 900 gals of acceleration was recorded byKushiro District Meteorological Observatory2). There were a few buildings completely
collapsed and the damage due to this earthquake was not so severe compared with its

magnitude･ However, characteristic feature related to the damage orLglnated from thegeological and landuse conditions･ This paper fわcuses on the relationship between the landuse

condition and earthquake damage in the houslng developments of Kushiro City･GEOLOGICAL CONDITION
Kushiro is famous for the natural marsh which is one of the national park area. The marshdistributeswidely ln the northern area of Kushiro. Fig.2 illustrates the surface geology
classification and the sites where microtremor measurements were conducted. The surface

geology of Kushiro Cityand its vicimity is classified into three typicalgeological conditions: (1 )marsh or fill over marsh which distributes widely in the n.orthern area of Kushiro City : (2)diluvial sand dune area ; (3) diluvial hill which develops/1n the eastern side of old Kushiro
River･ This paper mainly concentrates on the two houslng developments with geologlCalconditions (I ) and (3), where earthquake damages originated from geotechnical conditions
Were COnSplCuOuS･

Mihara is the newly developed housing development, the development of which was carriedout several years ago by filling sand material over the marsh･ The thickness of peat layer

underlying the fill is ranged from 2.0 to 2.5 meters. Underneath the peat layer, alluvial sanddepositwith the thickness from 20 to 30 meters is sedimented over the diluvial volcanic loam.
On the other hand, Midorigaoka is the houslng development located on the diluvial hill･ It wasdeveloped about 30 years ago･ Because the angulation of the hill was complicatedly
distributed, the construction was conducted by cuttlng slopes and filhng valleys･ The materialof the hill is diluvial volcanic loam, which is co汀eSpOndent with the diluvial deposit existlng
under the alluvial deposit in the marsh 弧ea.

TYPICAL EARTHQUAKE DAMAGES
Earthquake damages at Mihara is charac‑terized by the subsidence ranglng from 30 to 40

cm as illustrated in Photo. 1. Hokkaido Island islocated in the northern part of Japan and thetemperature there isalways under 0 degree inCentigrade in winter. Therefore, the surface soil

is frozen even in the day time until May. The
thickness of frozen soil is 150 cm in mid‑winter.

The subsidence was estimated to occur by thedynamic compaction of fill material and peat
including dissipation of pore water pressure built
up by cyclic loading. When the血'ozen soil on thesurface melts in sprlng, Secondary subsidence

will occurs). Moreover, the excess pore waterpressure built up ln Peat layer will be graduallydissipated year to year, which leads to gradualFig.1 Map of Hokkaido and Epicenterincrease in subsidence at Mihara. As shown in Photo.1, the gaps between the concrete platesof the entrance and ground can be seen in many houses･ The fわundation of houses in thisreglOn is deep and its thickness is about 1.0 m. However, such secondary subsidence maylead to tilting of houses.

Earthquake damages at Midorigaoka is characterized by the ground failure. Since
Midorigaoka is located on diluvial hill, various slopes distribute around and on the hill. Photo.

2 illustrates one of the severest damage due to slope failure at Midol.i'gaoka.Asdescribed later,high acceleration of earthquake motionwith high frequencywill llaVe Sufficient ineI●tial force tocause slope failure. The typICal damage due to ground failuI･e Other than slope failures is thelarge local amplification of ground acceleration in the area of fill over drowned valleys), where

the slope on the ground surface is not steep. Surface soil deposit fiIIed usJng the material
moved from cut slopes is irregularly bounded by fresh diluvial loam. The earthquake wave ishighly amplified especially ln the section of inclined bed rock and the surface deposit would

move along the bed rock slope permanently. Due to such permanent displacement of surface
deposit, large number of gas plpe damage took place in the area.

丘肘フ)ilごJ酬i■■i弧臥■t仰Photo. 1 Typical Earthquake damage at Miharai⊆Bl義 sites of Microtrem our Measurement

Fig.2 Classification of Surface Ground Conditions and Locations of Microtremor
Measurement in Kushiro CityPROCEDURE OF MICROTREMOR MEASUREMENTS
Microtremor measurements were conducted in Kushiro City as shown in Fig.2 two months

after the earthquake, for the purpose of clarifying predominant frequencies and amplification
characteristics of various locations in Kushiro City. In this paper, focus was centered on thet㌍o sites: Mihara (fill on marsh); and Midorigaoka (diluvial hill)･ Three components ofmlCrOtremOr (NS, EW, and UD) were measured three times at a site. The number of data
recorded was 8192, the time interval of which was loo Hz.
It was polnted out that predominant ground motion in the horizontal direction became

consplCuOuS When the Fourier spectrum of horizontal motion was divided by that of verticalmotion4). Nakamura suggested that the ratio of the spectra would be correspondent with theamplification ratio from bed rock to ground surface. This idea may not be valid when every

type of surface ground is taken into consideration. However, the ratiowill be greatly relatedwith the amplification factor. Fourier spectra of everymicrotremor data were calculated andwere smoothed using Hannlng Window filter rlrSt. Then, three spectra at each site and in eachdirection were averaged. Finally, the Fourier spectral ratios of two averaged horizontal spectra
divided by a vertical spectrum were calculated at each site.

FOURIER SPECTRAL RATIOSThe Fourier spectral ratio at Mihara‑5 in the NS direction is illustrated in Fig.3. The
predominant frequency around 1.0 Hz can be clearly demonstrated in the figure. The

aforementioned ratio at the peak is 5.0. When Fig.3 means the frequency response function atMihara, One can recognlZe that the fundamental mode of shear vibration is greatly predominant
and that the vibration in high frequency does not amplified by the surface layer. Earthquake

motions recorded at Kushiro District MeteorologlCal Observatory, located on diluvial hiH area
near the sea, shows high frequency of vibration and short duration time. The acceleration at

Mihara due to this earthquake is considered to be comparatively low, because of high damplng
in soft soil and lack of low frequency Input motions. The trace of liquefaction was discoveredin the area. However, the damage was characterized by the subsidence due to the disslpation ofexcess pore water pressure in fill, peat layer and sand underlying peat layer･The Fourier spectral ratio at Midorigaoka in the NS direction is illustrated in Fig･4･ The
figure shows the predominant vibration ranglng Widely from 1.5 to 3.5 Hz. This means thathigh n'equency of vibration lS Predominant and 2 or 3 modes of vibration occur onglnated fromthe irregularly bounded surface ground. The peak value of the ratio is about 4.0, which is low

comparedwith that at Mihara. As mentioned above, over 900 gals of acceleration was recordedon the diluvialhill. The amplification ratio is not so high on this area. However, acceleration

is estimated to be considerably highwith high frequency. Fig.4 describes why earthquake
damage like Photo.2 Occurred at Midorigaoka.
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2

3

4

501

Frq uency (Hz)劔劔2

剪y?｢vﾂ345Frequency (Hz)Fig.3 Fomier Spectral Ratio at Mihara Fig.4 Fourier Spectral Ratio at Midorigaoka
CONCLUDING REMARKSThis paper focussed on earthquake damages of two different houslng developments locatedat sites on two different geologlCal conditions. Bun environment causes various type of earth‑quake damages orlglnated from both the frequency characteristics and the amplification ofearthquake ground motions. Through the earthquake damage investigation and microtremor
measurement, the followlngS are COnCluded as lessons learned from this earthquake:(1) In case of horizontally layered soft soil depPsit, the development should be conducted indeep consideration of both static and dynamlC Subsidence. The approprlate method ofconstruction should be adopted. Furthermore, methods of soil improvement with low cost isnecess∬y to be developed.

(2) In case of irregularly bounded soil deposit, the amplification of earthquake motions at local
area should be considered in terms of earthquake‑resistent design.

(3) On relatively hard diluvial hill, the earthquake motions with high acceleration and highfrequency may predominate. In thisarea, there is a possibility that high inertial force generates.
REFERENCES
l･ Kasahara, M･ et a1., 1993, "Seismic Activities of the January 15 Kushir0‑0ki Earthquakeof 1993 and Those of Before and After the EarthquakeM,Quick Report Qf Earthquake

Englneerlng Committee, JSCE, pp. 1 4
2･ Kushiro District MeteorologlCal Observatory, 1993, Open File on the Kushirol0ki Earth‑quake ofJanuaf7 15, 1993.
3･ Suzuki, T･, 1984, "Settlement of SaturFted Clays under Dynamic Stress History", Jour･ ofthe Japan SocietyofGeological Engineerlng, Vol.25, No.3, pp.21 ‑3 1.
4･ Nakamura et all, 1990,日Relationship between characteristics of earthquake groundmotions estimated什Om microtremor measurements and earthquake damage in Loma Prieta

Earthquake", Proc･ JSCE, Vol.424, No.3‑14, pp.51.
5･ Suzuki, T. and Unami K., 1992, …The Extended (〕uasi‑three‑dimensional Ground Modelfor Irregilarly Bounded Surface Ground, Structural Eng. / Earthquake Eng., JSCE, Vol.9,No.I, 33S‑43S.