SOILS AND FOUNDATIONSJapanese Geotechnical SocietyVol. 51, No. 2, 215–226, Apr. 2011

MICROZONATION FOR SEISMIC GEOTECHNICALHAZARDS AND ACTUAL DAMAGE DURING
THE 2005 FUKUOKA-KEN SEIHO-OKI EARTHQUAKESUSUMU YASUDAi), HIDEO NAGASEii) and YUTAKA TANOUEiii)
ABSTRACTThe 2005 Fukuoka-ken Seiho-oki earthquake caused damage to structures due to liquefaction, slope failure andstrong shaking in Fukuoka City. One of the authors had conducted microzonation for liquefaction, slope failure andstrong shaking about 17 years before the earthquake. After the earthquake, the authors compared the zoning map withthe actual damage. The results showed that liqueˆed sites coincided fairly well with predicted zones in the reclaimed
lands. However, they were slightly diŠerent in a big sand spit named Uminonakamichi. Failure occurred at several
slopes inside the predicted areas, while damage to buildings occurred due to strong shaking along the Kego Fault. The
damaged area was slightly diŠerent from the predicted area. Furthermore, a liquefaction analysis was conducted andthe results of this analysis were compared with the results of microzonation in 1988. In this paper, the authors discuss
the validity of microzonation based on this comparison.
Key words: earthquake damage, geotechnical hazard, liquefaction, microzonation, slope failure, strong shaking(IGC: E8/D7)

authors conducted site investigations just after the earthquake and compared the actual geotechnical hazards withthose projected in the zoning map. Furthermore, a liquefaction analysis was performed and the results werecompared with the results of microzonation in 1988.
Here, the authors discuss the accuracy and problems of
microzoning by comparing the actual damage and thatprojected in the microzoning map.

INTRODUCTION
The seismicity around Fukuoka City is of the lowest
class in Japan. However, microzonation for probable
earthquake eŠects was needed because Fukuoka City is
the biggest and most important city in southern Japan.
Microzonation for liquefaction, strong shaking and slope
failure was conducted based on several analyses in 1988
(Fukuoka City Waterworks Bureau, 1988 and 1989,
Yasuda and Matsumura, 1991). The base acceleration forthese analyses was assumed to be 150 cm/s2, although the
maximum acceleration at the ground surface was assumed to be 200 cm/s2 in the microzonation for liquefaction and slope failure, considering the results of theanalysis for strong motion. According to the results of
the analyses, liquefaction was predicted in almost all of
the artiˆcially reclaimed lands. Slopes considered susceptible to failure were located along several mountains andhills. Areas considered susceptible to strong shaking were
distributed along the Kego Fault and central lowlands ofthe city.
Seventeen years later, in 2005, a big earthquake with amagnitude of 7.0 occurred near Fukuoka City. The maximum surface acceleration in the downtown of Fukuoka
was 276 cm/s2 which was almost the same as the maximum acceleration assumed for the microzoning map. The
i)
ii)
iii)

MICROZONATION CONDUCTED IN 1988

Geotechnical, Geomorphological and Seismological Conditions of Fukuoka City
The seismicity in and around Fukuoka City is low,compared with that in other cities Japan, because Fukuoka City is almost 300 km from the subduction zone of thePhilippine Sea Plate, where huge earthquakes occur
periodically. Only small earthquakes, rarely close to thecity, have caused damage. An earthquake occurred in
1898 at the southwest boundary of Fukuoka City. A survey of literature about this earthquake revealed that liquefaction was induced at several sites in the western partof the city. However, the damage to structures due to theliquefaction was light. Though the seismicity in Fukuoka
City is low, microzonation for probable earthquake
eŠects was needed because Fukuoka City is the biggest

Professor, Department of Civil and Environmental Engineering, Tokyo Denki University, Japan (yasuda＠g.dendai.ac.jp).
Professor, Department of Civil and Architectural Engineering, Kyushu Institute of Technology, Japan.
Senior Engineer, Kiso-jiban Consultants Co., Ltd., Japan.
The manuscript for this paper was received for review on May 24, 2010; approved on December 7, 2010.
Written discussions on this paper should be submitted before November 1, 2011 to the Japanese Geotechnical Society, 4-38-2, Sengoku,
Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month.
215

YASUDA ET AL.

216

Fig. 1.

Photo 1

Topography in Fukuoka City

Bird's-eye view of Fukuoka Cityand most important city in southern Japan, with a population of about 1.4 million.
The north side of the city faces Hakata Bay, and othersides are surrounded by mountains of 300 to 1000 m inheight, as shown in Fig. 1 and Photo 1. From the mountains, several small rivers carry soil to Hakata Bay, forming an alluvial plain. Along the foot of the mountains arehills. A fairly large spit, named Uminonakamichi, lies between Hakata Bay and the Sea of Japan. Along Hakata
Bay, a small sand dune runs continuously. In front of thissmall sand dune are several areas of artiˆcially reclaimed
lands. A big fault named Kego Fault crosses the central
part of the city from northwest to southeast. Therefore,
structures in the city may be damaged during an earthquake which produces: (1) the liquefaction of reclaimed
lands and the sand dunes along the coast, (2) the failureof mountain or hill slopes, and (3) strong shaking near
the fault and on the soft ground.

Fig. 2. Selected sites for 1988 soil cross sections (After Fukuoka City
Waterworks Bureau, 1988)

Estimation of Soil Layers
As the ˆrst stage of the 1988 microzoning, almost 1,200
sets of borehole data were collected. Based on this data,soil cross sections at the 13 areas shown in Fig. 2 were estimated. These sites were selected to cover the whole alluvial plain. Figure 3 shows a typical soil cross section,that in area (7)–(7?). A layer of artiˆcially ˆlled sand orsilt, with a thickness of 1 to 13 m, is formed near thecoast of Hakata Bay. Behind this area is a sand dune layer with a thickness of 3 to 6 mm which changes to the deltaic alluvial sand inland. Therefore, the sand layers inFukuoka City are classiˆed into the following three layers:(1) an alluvial sand layer which covers almost all of the
alluvial plain(2) a dune sand layer which runs along the natural coast(3) an artiˆcially reclaimed sand layer
Microzonation for Liquefaction
As the ˆrst step, undisturbed samples of the three sandlayers were taken at the ˆve sites shown in Fig. 1, and undrained cyclic triaxial tests were conducted to measure theliquefaction strength. Sampling methods at the ˆve sites
were as follows:(1) No. 1, No. 2 and No. 3: Undisturbed samples weretaken by the block sampling method. Thin wall tubesof 75 mm in inner diameter, 79 mm in outer diameterand 15 cm in length were pushed into the groundfrom the bottom of the excavated ditches, as shownin Photo 2.
(2) No. 4 and No. 5: Undisturbed samples were takenfrom bore holes by stationary piston samplers. Thin
walled tubes of 75 mm in inner diameter, 79 mm inouter diameter and 100 cm in length were used.
The reclaimed lands had been formed several years prior
to the tests. Tests on reconstituted samples of thereclaimed sands with the same density were also carriedout. Figure 4 shows the typical grain-size distribution

MICROZONATION IN FUKUOKA

Fig. 3.

217

Soil cross section in the area 7–7? and predicted liqueˆable layers for 1988 microzonation (After Fukuoka City Waterworks Bureau, 1988)

Photo 2Block sampling at site No. 1

Fig. 5. Relationships between liquefaction strength and ˆne content ofsoils in Fukuoka City

Fig. 4. Grain-size distribution curves of tested sands (After Fukuoka
City Waterworks Bureau, 1988)

curves of the alluvial, dune and reclaimed sands. Dunesand and artiˆcially reclaimed sand have few ˆne particles, but alluvial sand has a considerable amount of ˆneparticles. The ˆnes content, Fc, of the alluvial sand was
15z. The relationships between Fc and the liquefaction
strength (Undrained cyclic shear stress ratio), RL (N＝20,

DA＝5z) are plotted in Fig. 5. As shown in the ˆgure,the liquefaction strength of alluvial sand was higher thanthose of the reclaimed sand, the dune sand and the reconstituted alluvial sand.
In Japan, the method of estimating liquefaction usedin the speciˆcation for highway bridges (Japan Road Association, 1980), which was derived from Iwasaki et al.'s
formula (1978) with slight modiˆcation, and which uses
SPT N-values, was the one commonly used at the time.
Therefore, after comparing the cyclic triaxial test results,
shown in Fig. 5, with the formula, the following modiˆed
formula was derived and applied to soil in Fukuoka City:R ＝ R 1＋ R 2＋ R 3

(1)

218

YASUDA ET AL.

where, R: liquefaction strength (undrained cyclic shear
strength ratio)

R1＝0.0882 N/((s?n/98)＋0.7)
R2＝0.19 for 0.02 mm≦D50º0.05 mm
0.225 log (0.35/D50) for 0.05 mm≦D50º0.6 mm－0.05 for 0.6 mm≦D50º2.0 mm
R3＝0 for dune sand and reclaimed sand
0.187 for alluvial sandA value of 0.187 was added to the liquefaction strength
ratio R of alluvial sand, on the basis of the results shownin Fig. 5. The alluvial sand has much ˆnes though its
mean diameter is fairly large, as shown in Fig. 4. This is
the reason why the value of 0.187 must be added to theformula used in the speciˆcation for highway bridges.
Assuming a maximum surface acceleration of 200
cm/s2, liqueˆable layers were predicted at all soil crosssections. The hatched zones in Fig. 3(b) show the predicted liqueˆable layers along section (7)–(7?). The reclaimed
sand layer and dune sand layer were judged to be liqueˆable along this section.
It was necessary not only to predict the liqueˆable layer
but also to predict the possibility of damage to structuresdue to liquefaction. The amount of possible damage wasjudged based on the relationship between the thickness of
the liqueˆable layer, H2, and the thickness of the upper
non-liqueˆable layer, H1, proposed by Ishihara (1985).
Figure 6 shows the 1988 microzoning map for liquefaction thus determined. In general, structures on artiˆcially reclaimed land or partly on the sand dune wereconsidered susceptible to damage due to liquefaction. Liquefaction was predicted to occur throughout most of thesand dune, but damage due to liquefaction was not expected. Various soils have been used for the ˆll soils in theFig. 6. Microzoning map of Fukuoka City (After Fukuoka City
Waterworks Bureau, 1989)artiˆcially reclaimed lands. In general, sandy soils andclayey soils have been ˆlled in old and new reclaimed
lands, respectively. Therefore, it was judged that liquefaction would not occur in newly reclaimed lands.
These reclaimed lands were located in Meino-hama, Odo,
Atago-hama and Kashii-hama.

Microzonation for Slope Failure
Microzonation for slope failure was conducted by thefollowing two methods:(1) Slopes susceptible to failure during heavy rain had
been investigated in Fukuoka City. Assuming theseslopes had the potential to slide during earthquakes
also, detailed site investigations of the slopes wereconducted. Determining which slopes were susceptible to failure during an earthquake was based on theslope angle, hardness of the slope and several otherfactors, as shown in Fig. 6.
(2) A simple method to predict the susceptibility to slope
failure had been developed by Kanagawa Prefecture(Kanagawa Prefecture, 1986) and introduced in theseismic zoning manual prepared by TC4 (TC4, 1994).
This method was derived from the main factors
which aŠected slope failures during four earthquakes
in Japan. The weightings of these factors in one grid(about 500 m times 500 m) were also clariˆed in thismethod. The factors identiˆed were: (a) maximum
surface acceleration, (b) the length of slope in one
grid, (c) the variation of level in one grid, (d) the
length of the fault in one grid, (e) the length of the cut
slope, and (f) the angularity of slope. Based on thismethod, grids of slopes susceptible to failure wereconstructed. Figure 7 shows the predicted zoningmap.
As shown in Fig. 7, slopes susceptible to failure areprevalent in several mountains and hills. Most failure susceptible slopes predicted by the ˆrst method shown in
Fig. 6 are in the grids of slopes susceptible to failure
predicted by the second method.
Relating to slope failure, the locations of artiˆciallyˆlled ponds were found by comparing a recent map with a

Fig. 7. Microzoning map for slope instability predicted by Kanagawa
method (After Fukuoka City Waterworks Bureau, 1988)

MICROZONATION IN FUKUOKA

219

map of about 100 years ago, as shown in Fig. 6. The stability of these ponds could not be evaluated quantitatively. However, these sites were pointed out as being possibly susceptible to failure during an earthquake.

Microzonation for Strong Shaking
Microzonation for strong shaking was conductedbased on seismic response analyses. In addition to thecollection of boring data mentioned before, PS logging
data were collected. By comparing these data, the following relationships between SPT N-values and shear velocity, Vs (m/s), were derived:
Vs＝80 N 0.333
Vs＝144 N 0.159
Vs＝100 N 0.333
Vs＝157 N 0.180

for dune sand and reclaimed sand for other sands
(2)for reclaimed clayfor other clays
Fig. 8.

Coe‹cients of Equation (3) (Yasuda and Yamaguchi,1985)

Data on the relationships between the shear strain, g, thedynamic shear modulus ratio, G/G0, and the damping ratio, h, could not be collected for soil in Fukuoka City.
Therefore, the following relationships, derived from cyclic torsional shear tests of almost 100 undisturbed samples in Japan (Yasuda and Yamaguchi, 1985), were used
for the analyses.

G/G0＝(A1＋A2 log D50)×( p?/98)(B ＋B log D )h＝(C1＋C2 log D50)×( p?/98)(D ＋D log D )
1

1

2

2

50

50

}

(3)

where,
D50: mean diameter of soil (mm)A1 to D2: coe‹cients shown in Fig. 8
p?:eŠective mean conˆning pressure (kN/m2)
Two kinds of seismic response analysis were conducted: (1) one dimensional analysis, and (2) two dimensional
analysis. One dimensional seismic response analysis wasapplied to 37 typical borehole sites. The dynamic shear
modulus at low strain level, G0, was determined from theSPT N-value using Eq. (2). The relationships between g
and G/G0, and g and h, were estimated using Eq. (3). Assuming that a big earthquake would not hit Fukuoka Cityin the near future, as mentioned before, a seismic wave
which had been recorded during a past medium earthquake was used for an input base seismic motion. The
wave used was recorded at the Institute of Industrial
Science of the University of Tokyo, Chiba Campus, during the 1987 Chibakentoho-oki earthquake. Moreover, asa representative seismic wave obtained during a big earthquake, the wave recorded in Hachinohe Harbour during
the 1968 Tokachi-oki earthquake was also applied. The
peak acceleration of these waves was adjusted to 150
cm/s2 at outcrop motion. The computer program usedwas ``SHAKE'' (Schnabel et al., 1972).
Figure 9 shows the relationships between the analyzed
ampliˆcation ratio of acceleration, Amax, s/Amax, b, thepredominant period of the ground, TG, and the maximum
surface displacement, Dmax. TG (sec.) was calculated using
the following equation:n

TG＝S (4Hi/VSi)
i＝ 1

(4)

Fig. 9 Relationship between ampliˆcation ratio of acceleration, maximum displacement and predominant period of the ground for 1988
microzonation (After Fukuoka City Waterworks Bureau, 1989)

where,
i: number of the soil layers
Hi: thickness of the (i)th soil layer (m)Vsi: shear wave velocity of the (i)th soil layer (m/s)
The dotted points in Fig. 9 are scattered to some extent. However, assuming that there are comparatively
unique relationships between TG and Amax, s/Amax, b, andTG and Dmax, Amax, s and Dmax in all boring sites along the
13 soil cross sections were predicted.
As mentioned before, a fault, named the Kego Fault,
crosses the central part of the city. Moreover, there aremany quay walls along the coast. Therefore, two dimensional seismic response analyses across the Kego Fault
and a quay wall at Hakozaki reclaimed land were conducted to ascertain the eŠect of the fault and the quay
wall on the seismic response. The computer program usedwas ``FLUSH'' (Lysmer et al., 1975). Figures 10 and 11
show the cross sections across the Kego Fault and thequay wall at Hakozaki, respectively, and the relationships between g and G/G0 or h were estimated using Eqs.
(2) and (3), respectively. The input wave was the recorded
wave at the Institute of Industrial Science of the University of Tokyo, and the peak acceleration of this wave wasadjusted to 150 cm/s2. The quay wall in Fig. 11 is a sheet
pile type wall.

220

YASUDA ET AL.

Fig. 10. Soil cross section and analyzed peak acceleration across the
Kego Fault (After Fukuoka City Waterworks Bureau, 1989)

Fig. 12. Hazard map for liquefaction and liqueˆed sites during the
Fukuoka-ken Seiho-oki earthquake

the failure of retaining walls on a tiny island named
Genkai-jima which is only about 10 km from the epicenter. A lot of tiled roofs of timber houses were damaged
due to shaking over a wide area. Several buildings
suŠered damage in the central area of Fukuoka City. Liquefaction occurred in many reclaimed lands and caused
damage to quay walls, tanks and sheds. Slope failures occurred at several sites on Shikano-shima Island, which islocated about 20 km from the epicenter.

Fig. 11. Soil cross section and analyzed peak acceleration across aquay wall at Hakozaki (After Fukuoka City Waterworks Bureau,
1989)

The contour lines in Figs. 10 and 11 show the distributions of analyzed peak acceleration in the grounds. Fairly
strong shaking was likely to occur at the ground surfaces
about 1 km east of the Kego Fault and the ground behindthe quay wall. Then the hatched zones shown in Fig. 6
were judged as the areas susceptible to strong shaking.
DAMAGE DURING THE 2005 FUKUOKA-KENSEIHO-OKI EARTHQUAKE AND COMPARISONWITH PREDICTED HAZARDS

Outline of Damage
The Fukuoka-ken Seiho-oki earthquake, with a magnitude of 7.0 (Mj), occurred in Japan on March 20, 2005.
The epicenter of the earthquake was about 30 km northwest of downtown Fukuoka City. The direction of thefault is estimated as NW-SE. The maximum horizontal
surface acceleration recorded in the center of Fukuoka
City was 276 cm/s2. The seismic intensity in Fukuoka
City, according to the JMA scale, was 5 Lower to 6 Lower, which was almost the same as the assumed seismic intensities for the microzoning. Therefore, it was considered appropriate to compare the actual geotechnicalhazards that occurred during the earthquake with thepredicted ones.
The earthquake caused severe damage to houses due toDamage to Structures due to Liquefaction
The closed circles plotted in Fig. 12 indicate the liqueˆed sites. Almost all the sites are located in the reclaimed
lands along Hakata Bay. No liquefaction occurred on the
alluvial low land. One site in the Uminonakamichi sandspit liqueˆed. However, the liqueˆed site was newly ˆlled
land on a pond. Therefore, it can be said that liquefaction
occurred in only artiˆcially reclaimed lands.
Among the reclaimed lands, Oki-hama, Nanotsu,
Higashi-hama, Aratsu, Hakozaki and Momochi-hamawere constructed before 1990. These reclaimed lands wereˆlled mainly with sandy soil. Other reclaimed lands,
Meino-hama, Odo, Atago-hama, Kashii-hama and Island City were newly ˆlled with clayey soil. Therefore,
few boiled sands were observed in the latter newly
reclaimed lands. Liquefaction did not occur in Meino-hama and Odo. A few places with boiled sands were observed on pavements in Kashii-hama and Island City, but
no structures were damaged. In Atago-hama, liquefaction occurred at two school grounds, a ˆshing port
and along roads. A shed settled, quay walls tilted and aroad subsided due to the liquefaction.
In contrast, in the former reclaimed land, liquefaction
occurred at many sites. In Oki-hama, a quay wall tilted,
as shown in Photo 3. The horizontal displacement of thequay wall was about 1 m. The ground behind the quay
wall ‰owed toward the bay and subsided. The ‰ow zone
extended about 10 m from the quay wall. Several quay
walls tilted a little in Nanotsu, Higashi-hama and Aratsu.
Two small tanks settled about 10 to 20 cm due to liquefaction in Aratsu. In Momochi-hama, sand boils wereobserved at many sites, as shown in Photo 4. However,MICROZONATION IN FUKUOKA

Photo 3 Displacement of a quay wall and subsidence of the ground in
Oki-hama

Photo 6

221

Liquefaction-induced ‰ow at Uminonakamichi Seaside Park

shallow at both beaches.
Photo 6 shows liquefaction in Uminonakamichi. This
site was newly developed for a park, named
Uminonakamichi Seaside Park, by ˆlling part of a pondwith sand. Liquefaction occurred at this site and induced
ground ‰ow toward the remaining pond. The maximum
horizontal displacement due to the ‰ow was about 10 m.
Simple piers and promenades were damaged due to the‰ow.
In Itoshima Peninsula, except for the playground of
Shima Junior High School, little liquefaction occurred
during the 2005 Fukuoka-ken Seiho-oki earthquake,though liquefaction occurred during the earthquake in
1898.
Photo 4

Photo 5

Boiled sand in Momochi-hamaLiquefaction at the man-made beach in Atago-hamano damage to buildings and timber houses occurred andobvious ground subsidence did not occur at the liqueˆedsites. Therefore, it is estimated that the liqueˆed layer was
thin.
There are two man-made beaches at the front of
Momochi-hama and Atago-hama. Many large sand volcanoes were observed on the beaches, as shown in Photo
5. The ground was very loose and the water table wasComparison of Liqueˆed Sites with Predicted Liquefaction Areas
The shadowed zones in Fig. 12 show the areas where
structures had been predicted to be susceptible to damage
due to liquefaction. These areas compare with the liqueˆed sites as follows:(1) Almost all the liqueˆed sites are located in the areas
where structures had been deemed susceptible todamage. Therefore, roughly speaking, the microzonation for liquefaction was fairly valid.
(2) Liquefaction occurred at three sites in Atago-hama,though it was predicted that liquefaction would notoccur there because this land was ˆlled with clayeysoil. The liqueˆed sites are located along old shore
protections. In general, sandy soil is ˆlled just behindshore protections during reclamation work. As such,it could be that there was sandy soil at the sites whichliqueˆed during the earthquake.
(3) In the Uminonakamichi sand spit, liquefaction did
not occur in natural ground, though some areas at thetoes of sand dunes had been predicted to liquefy.
There was little borehole data from this sand spit because there were no heavy structures there. Therefore, in the microzoning, the grounds at the toes ofsand dunes were judged to be liqueˆable, based on experience with past earthquakes. For example, liquefaction occurred in the grounds at the toes of sanddunes during the 1964 Niigata and 1983 Nihonkai-

YASUDA ET AL.

222

Photo 7

Slope failure in Shikano-shima island

Photo 8A damaged apartment above the Kego Fault

chubu earthquakes. The diŠerence in the ground conditions
betweentheliqueˆedsites
andUminonakamichi is not clear. Further study is necessary.

Damage to Slopes
Comparatively large slope failures occurred at threesites in Shikano-shima Island, as shown in Fig. 7. Photo
7 shows the biggest slide, which occurred on the east sideof the island. Two other slides occurred on the west sideof the island. Shadowed grids in Fig. 7 show the zones
where several of the slopes were predicted to be unstable.
The three sites in Shikano-shima are located mainly inside the landslide susceptible grids. Therefore, it can besaid that the microzonation by the Kanagawa method
was fairly valid for Shikano-shima island. However, inthe east and south of Fukuoka City, no slope failure occured in the grids where unstable slopes had been predicted. As the epicenter was located in the northwest, seismic
intensity in the eastern and southern areas might havebeen too low to cause slope failure.
In Fig. 6, the sites of slopes that had been predicted tobe susceptible to failure based on site investigations andthe sites where damage of artiˆcially ˆlled ponds had
been predicted are plotted. These sites were not damaged
during the 2005 Fukuoka-ken Seiho-oki earthquake. Seismic intensity at these sites was low because these sites arelocated mainly in eastern and southern areas also.
Damage to Buildings
As shown in Photo 8, several buildings suŠered
damage near the Kego Fault in the downtown area of
Fukuoka City. The closed circles in Fig. 13 show the locations of the damaged buildings, investigated by the Seibu
Branch of the Japan Society of Civil Engineers (2005).
The area predicted to be susceptible to strong shaking is
also drawn in the ˆgure. Four damaged buildings are located inside the predicted area. However, other damagedbuildings are located slightly outside the predicted area.
The reason why the damaged area was slightly diŠerent
from the predicted area is not clear. However, one of theauthors found a small diŠerential settlement on a roadwhere buildings were damaged, as shown in Photo 9. The

Fig. 13. Hazard map for strong shaking and locations of buildings
damaged during the Fukuoka-ken Seiho-oki earthquake

Photo 9DiŠerential settlement observed above the Kego Fault

site is just above the Kego Fault. One possibility is, therefore, that some vertical diŠerential displacement occurred along the fault and caused the damage to thebuildings. More study on the mechanism of the damage

MICROZONATION IN FUKUOKA

to buildings is necessary in this area.
LIQUEFACTION ANALYSES CONDUCTED AFTERTHE EARTHQUAKE

Analytical Procedure
The sites liqueˆed during the 2005 Fukuoka-ken Seihooki earthquake were mostly restricted to the reclaimed
lands mentioned before. In order to clarify the reason forthis peculiarity in the liquefaction phenomenon liquefaction analyses were performed based on the actual
acceleration during the earthquake. In the analyses, sevenboring logs along cross section 7–7?, shown in Fig. 3,
were selected. These boring data are the same as the boring data used for the 1988 microzoning. Two borings near
Hakata Bay at Sites 7–1 and 7–2 are located in reclaimed
land. Other boring logs are located in alluvial plain.
Liquefaction-induced sand boil and damage to structures
occurred near Site 7–1 and Site 7–2, but no trace of liquefaction was observed near other sites during the 2005
Fukuoka-ken Seiho-oki earthquake. The analyticalprocedure is similar to the one used by the Fukuoka City
Bureau (1988), with some slight diŠerences, as follows:
1) A program for a one-dimensional seismic response
analysis, ``SHAKE,'' was used to obtain the maximum
accelerations on the ground surface at the selected sevensites during the 2005 Fukuoka-ken Seihou-oki Earthquake.
2) The input seismic waveform from the basement layer
at the seven sites for the analysis was estimated from theseismic record of the EW component measured on the
ground surface at Fukuoka public hall, by reconverting
into the basement layer. Figure 14 shows the input seismic waveform for the analysis, which was utilized by
Nagase et al. (2006). The maximum basement acceleration is 174.9 cm/s2.
3) The shear wave velocities of the soil layers at the sevensites were obtained from the empirical formulae shown in
Eq. (2).
4) The strain dependencies of the dynamic shear modulus ratio, G/G0, and damping ratio, h, were expressed using the empirical formulae shown in Eq. (3).
5) The soil boring logs used in the analysis are the sevenboring logs shown in Fig. 3. Two borings near Hakata
Bay are located in reclaimed land. Other boring logs arelocated in alluvial plain.

2236) The shear stress ratio during the earthquake, L, can be
estimated directly from the analysis using the ``SHAKE''
program. However, for comparison with the previous estimate by the Fukuoka Waterworks (1988), the equation
used by the Fukuoka Waterworks was used in this study.
That equation is:

L＝(1－0.015 Z )・khc・sv/s?v

Z and khc denote the depth from the ground surface
and the ratio of the maximum acceleration on the groundsurface to the gravity acceleration, respectively. sv and s?v
denote the vertical total and eŠective stress, respectively.
In the estimate by the Fukuoka Waterworks, khc was assumed to be 200 cm/s2, as mentioned before. On the contrary, khc during the 2005 Fukoka-ken Seiho-oki earthquake was evaluated from the seismic response analysis
using the ``SHAKE'' program. The evaluated khc for theseven boring sites ranged from 0.181 to 0.220.
7) The liquefaction strength ratio R, was estimated by
two simple methods. The ˆrst method is the above mentioned Eq. (1), which was derived in 1988 based on thetest data taken in Fukuoka. The other method was introduced in the Speciˆcation for Highway Bridges after
the 1995 Hyogoken-nambu earthquake based on the test
data from many sites in Japan. These methods werecalled the Fukuoka method and the JRA method, respectively. The equation for the JRA method is as follows(JRA, 1996):



R＝CwRL
RL＝0.0882 Na/1.7(Naº14)RL＝0.0882 Na/1.7＋1.6×10－6(Na－14)4.5(NaÆ14)



(6)




where,(RLÃ0.1)
1.0
Cw＝3.3RL＋0.67 (0.1ÃRLÃ0.4)


2.0

(0.4ÃRL)
for a near-ˆeld earthquake

Cw＝1.0for a plate boundary earthquake

Na＝c1N1＋c2

for sandy soil

N1＝1.7N/((s?v/98)＋0.7)(0zÃFcÃ10z)1

c1＝(Fc＋40)/50
Fc/20－1
c2＝{

0(Fc－10)/18(10zÃFcÃ60z)(60zÃFc)(0zÃFcÃ10z)(10zÃFc)

Na＝[1－0.36 log10 (D50/2)]×N1

Fig. 14. Waveform used for one-dimensional seismic response
analysis(5)for gravelly soil

In the JRA method, CW and FC must be determined.
CW was decided as 1.0 because the epicentral distance wasabout 25 km. As no soil tests at the seven sites were conducted, two sets of FC for the soil layers at these sites wereassumed based on Fig. 4. Then, the three cases of analysis shown in Table 1 were analyzed as follows:
Case 1: The JRA method was used. The ˆne content Fc

YASUDA ET AL.

224

Table 1.

Estimation method for L and R and assumed grain size of soil layers in liquefaction analyses

Estimation method for L and R
Case

L

R

Assumed grain size
Alluvial and artiˆcially reclaimed soils
Sand

Case 1Case 2
Case 3Eq. (5)

Silty or clayey sandArtiˆcially reclaimed
soil

Dune sandGravelly sand

JRA method

FC＝20z

FC＝60zD50＝2.0mm

FC＝0z

JRA method

FC＝40z

FC＝80zD50＝2.0 mm

FC＝0z

Fukuoka method

D50＝0.35 mm

D50＝0.025 mm

D50＝2.0 mm

D50＝0.35 mm

Fig. 15.

Results of liquefaction analysis

was assumed to be 20z and 60z for the sand and silty or
clayey sand, respectively, in the artiˆcially reclaimed andalluvial soil layers. The value of 20z for the alluvial soil
layer was decided from the grain size distribution curve
shown in Fig. 4. The value of 60z for the alluvial silty or
clayey sand layer was determined on the basis of the grain
size distribution curves estimated by the Fukuoka City
Waterworks Bureau (1988). The Fc values of the artiˆcially reclaimed soil layer were assumed to be the sameas those of the alluvial soil layer. The Fc value of the dunesand was assumed to be 0z, according to the data shownin Fig. 4. D50 was assumed to be 2.0 mm for the gravelly
sand in the artiˆcially reclaimed soil layer.
Case 2: The JRA method was used. The Fc values wereassumed to be 40z and 80z. According to the Fukuoka
City Waterworks Bureau (1988), the grain size distribution curves of the alluvial soil layer varied exceedingly, as
the Fc values ranged form 20z to 80z. Therefore, inCase 2, the Fc values were assumed to be 40z and 80z
for the sand layer and the silty or clayey sand layer, respectively. The Fc value of the dune sand was assumed to
be 0z, as in Case 1. The D50 of the gravelly sand in the

MICROZONATION IN FUKUOKA
Table 2.
Case

Case 1Case 2

Case 3

Liqueˆed or
non-liqueˆed layer

225

Estimated thicknesses of liqueˆed and surface non-liqueˆed layers
Site 7–1

Site 7–2

Site 7–3

Site 7–4

Site 7–5

Site 7–6

Site 7–7

Liqueˆed layer

9.3 m

4.3 m

3.3 m

3.2 m

2.2 m

2.0 m

1.0 m

Non-liqueˆed layer

2.3 m

2.5 m

5.8 m

2.3 m

1.6 m

1.8 m

3.0 m

Liqueˆed layer

9.3 m

3.0 m

3.3 m

3.2 m

2.2 m

0m

0m

Non-liqueˆed layer

2.3 m

3.8 m

5.8 m

2.3 m

1.6 m

3.8 m

4.0 m

Liqueˆed layer

9.9 m

4.3 m

3.3 m

3.2 m

0m

0m

0m

Non-liqueˆed layer

1.8 m

2.5 m

5.8 m

2.3 m

3.8 m

3.8 m

4.0 m

artiˆcially reclaimed soil layer was 2.0 mm.
Case 3: The Fukuoka method was used. The D50 valueof the sand, silty or clayey sand and gravelly sand in theartiˆcially reclaimed and alluvial soil layer were assumed
to be 0.35 mm, 0.025 mm and 2.0 mm, respectively. The
values of 0.35 mm and 0.025 mm were almost equal tothe average values decided from the grain size distribution curves with Fc values from 20z to 40z and from60z to 80z, respectively, on the basis of the data
gathered by the Fukuoka City Waterworks Bureau(1988). The D50 value was assumed to be 0.35 mm for thedune sand layer based on Fig. 4.
8) The ratio of FL, a safety factor against liquefaction,
was obtained from the following equation:

FL＝R/L(7)

Results of Analysis
Figure 15 shows the results of the liquefaction analysis
at the seven sites for Cases 1–3. The thicknesses of the liqueˆed layers and the non-liqueˆed layers in the threecases are estimated from this ˆgure and summarized inTable 2. Moreover, the relationships between the thicknesses of the liqueˆed layers and the non-liqueˆed layers
are plotted in Fig. 16 together with the boundary to cause
sand boil and/or liquefaction-induced damage proposedby Ishihara (1985). The FL-values for the reclaimed sandsat Site 7–1 and Site 7–2 are mostly less than 1.0 in Cases
1, 2 and 3. The thicknesses of the liqueˆed and surface
non-liqueˆed layers at Site 7–1 are about 9.3 to 9.9 m and1.8 to 2.3 m, respectively. The liquefaction that occurredand caused sand boils and damage to structures at Site
7–1 can be explained in every case. The thicknesses of theliqueˆed and non-liqueˆed layers are about 3.0 to 4.3 m
and 2.5 to 3.8 m, respectively at Site 7–2. Liquefactioninduced damage can be explained in Cases 1 and 3, where
the estimated thickness of the liqueˆed layer is the maximum (4.3 m) and the estimated thickness of the nonliqueˆed surface layer is the minimum (2.5 m).
At Site 7–3 and Site 7–4, it is estimated that liquefaction occurred in the dune sand layers in Cases 1, 2
and 3, because the dune sand does not have ˆnes.
However, it can be estimated that sand boils could not be
spouted out onto the ground surface at Site 7–3 because
the surface non-liqueˆed layer is thick, at about 5.8 m. At

Fig. 16. Relationships between the thicknesses of liqueˆed layers andthe non-liqueˆed layers estimated in three cases

226

YASUDA ET AL.

Site 7–4, sand boil could be explained because the thicknesses of the liqueˆed and non-liqueˆed layers are near
the boundary to cause sand boil, as shown in Fig. 16.
At Site 7–5 and Site 7–6, the estimated FL of the alluvial soil layer are greater than 1.0 in Case 3. However, theFL in Case 1 of the alluvial soil layer are less than 1.0.
And the thicknesses of liqueˆed and surface non-liqueˆedlayers are about 2.0 to 2.2 m and 1.6 to 1.8 m, respectively. Therefore, sand boil should have occurred at thesesites under Case 1. There was, however, no trace of sandboil or liquefaction-induced damage at these sites. The
apparent lack of liquefaction at these sites cannot be explained in Case 1 but can be explained in Case 3. In Case2, liquefaction cannot be explained at Site 7–6 but can be
explained at Site 7–5. Therefore, Case 3 was shown to be
the most suitable for estimating liquefaction. This implies
that the hazard map shown in Fig. 12 is appropriate under 177 to 216 cm/s2 of maximum surface acceleration.
CONCLUSIONS
Microzonation for liquefaction, slope failure, andstrong shaking during earthquakes in Fukuoka City,
Japan was conducted in 1988 based on several analyses.
Liquefaction was predicted in almost all of the artiˆcially
reclaimed land. Slopes susceptible to failure were locatedalong several mountains and hills. Areas susceptible tostrong shaking were distributed in the artiˆcially
reclaimed land and the central part of the city.
Seventeen years after the microzonation, the 2005
Fukuoka-ken Seiho-oki earthquake hit Fukuoka City. As
the seismic intensity during the earthquake was similar to
the intensity assumed for microzoning, actual damage
was compared with the predicted damage. Liqueˆed
areas coincided fairly well with the predicted areas. Slope
failure occurred near the predicted zones, while damage
to buildings occurred in slightly diŠerent zones.
Liquefaction analysis was performed on the same soil
layer in which liquefaction had been predicted in the
microzoning, using the waveform observed during theearthquake, and the results of liquefaction analysis werecompared with the results of microzonation. The occurrence of liquefaction observed during the earthquake wassuitably predicted by the microzonation.
ACKNOWLEDGMENTS
The analyzed results were discussed by a technical com-mittee of Fukuoka City. Analyses for microzoning wereconducted by Mr. Y. Yamamoto and Mr. S. Nakamura
et al., former students at the Kyushu Institute of Technology. Site investigation on the damage due to the earthquake was conducted with Mr. K. Yamagata and Mr. S.
Itoh, of Kiso-jiban Consultants Co., Ltd., and by Prof.
S. Goto of Yamanashi University. The authors are grateful for this assistance.
REFERENCES
1) Fukuoka City Waterworks Bureau (1988): Evaluation on seismic
behavior of water supply facilities and grounds, Part 1 (inJapanese).
2) Fukuoka City Waterworks Bureau (1989): Evaluation on seismic
behavior of water supply facilities and grounds, Part 2 (inJapanese).
3) Ishihara, K. (1985): Stability of natural deposits during earthquakes, Proc. 11th ICSMFE, 1, 321–376.
4) Iwasaki, T., Tokida, K., Tatsuoka, F., Watanabe, S., Yasuda, S.
and Sato, H. (1982): Microzonation for soil liquefaction potential
using simpliˆed methods, Proc. 3rd Int. Earthquake Microzonation
Conf., 3, 1319–1330.
5) Japan Road Association (1980): Speciˆcation for Highway Bridges(in Japanese).
6) Japan Road Association (1996): Speciˆcation for Highway Bridges(in Japanese).
7) Kanagawa Prefecture (1986): Prediction of Seismic Damage in
Kanagawa Prefecture (in Japanese).
8) Lysmer, L., Udaka, T., Tsai, C. F. and Seed, H. B. (1975):
FLUSH-A computer program for approximate 3-D analysis of soilstructure interaction problems, Report No. EERC75–30, Universityof California, Berkeley.
9) Nagase, H., Zen, K., Hirooka, A., Yasufuku, N., Kasama, K.,
Kobayashi, T., Maeda, Y., Uno, K., Hashimura, K. and Chen, G.
(2006): Zoning for liquefaction and damage to port and harbor
facilities and others during the 2005 Fukuoka-ken Seiho-oki Earthquake, Soils and Foundations, 46(6), 805–816.
10) Schnabel, P. B., Lysmer, J. and Seed, H. B. (1972): SHAKE- A
computer program for earthquake response analysis of horizontal
layered sites, Report No. EERC72–12, University of California,
Berkeley.
11) Seibu Branch of JSCE (2005): Reconnaissance Report on the 2005
Fukuoka-ken Seiho-oki Earthquake (in Japanese).
12) TC4, ISSMFE (1994): Manual for Zonation on Seismic Geotechnical Hazards, The Japanese Geotechnical Society.
13) Yasuda, S. and Yamaguchi, I. (1985): Dynamic soil properties of
undisturbed samples, Proc. 20th Japan National Conf. on SMFE,
539–542 (in Japanese).
14) Yasuda, S. and Matsumura, S. (1991). Microzonation for liquefaction, slope failure and ground response during earthquakes inFukuoka City, Proc. 4th International Conference on Seismic Zonation, 3, 3–10.