SEISMIC MICROZONATION STUDY BYCOMPARING MEASURED MICROTREMORSWITH OBSERVED EARTHQUAKE MOTIONS-A Case Study in the Damaged Area during the 2007 Noto Hanto Earthquake, JapanHussam Eldein Zaineh*
MEE06024

Supervisors: Kazuoh Seo** and Kentaro Motoki**

ABSTRACTThe influence of local geology and soil conditions on the intensity of ground shaking and earthquake
damage has been known for many years. Therefore, a significant part of damage observed in destructiveearthquakes around the world is associated with seismic wave amplification due to local site effects.
Microtremor measurements were carried out for 55 sites in Monzen-machi, Wajima city, where thedamage had been concentrated during the 2007 Noto Hanto Earthquake. The site response (transferfunctions) had been calculated by using individual site to reference spectral ratio of microtremors as well
as horizontal to vertical spectral ratio. These resulted transfer functions are then compared with thetransfer functions obtained from aftershock records for 7 sites where we have aftershock observation
points. Also, the dynamic characteristics of two public reinforced concrete buildings have been estimatedby using microtremors and then compared with dynamic characteristics which were calculated fromaftershock records inside these buildings. The results show high amplification factors in the heavily
damaged area due to site effects which can be estimated by microtremors as well as by aftershock records.
The comparison between microtremors and observed earthquake motions demonstrate that microtremors
can be considered as a powerful tool for estimating the earthquake site response as well as for evaluating
the dynamic characteristics of building.
Keywords: Microzonation, Microtremors, Site Effect, Transfer Function, Dynamic Characteristics of Buildings

INTRODUCTION
A significant part of damage observed in destructive earthquakes around the world is associated withseismic wave amplification due to local site effects. Local site effect is generally summarized as theamplification factors of strong ground motions resulting from responses of local soils relative to incident
seismic wave in the bedrock. Site transfer function, which has been extensively used to evaluate the local
site effects, is defined as the ratios of Fourier amplitude spectra of earthquake recorded on soft soil to
Fourier amplitude spectra of incident earthquake waves in the bedrock. Since deep borehole records (inputmotions at bed rock) are not so easy to be obtained, a reference stiff site which is supposed to be free ofsite effects or with minimum site effects is used as an alternative of in put motion at bed rock. Using
earthquake data to calculate site transfer function gives very reliable results, but earthquake data are notoften available. Site transfer function also can be estimated by boring exploration method that gives very
accurate results. But, boring exploration method demands considerable manpower and substantial time aswell as very high cost. On the other hand, using microtremors to evaluate the seismic site response
becomes very popular in seismic microzonation studies due to its easiness and inexpensive application of*

National Earthquake Center (NEC), Damascus, Syria ** Tokyo Institute of Technology, Tokyo, Japan1

microtremors. In 25th of March 2007, an earthquake with magnitude 6.9 according to JMA (JapaneseMeteorological Agency) shook the offshore Noto peninsula, Ishikawa Prefecture, Japan. Intensity 6+ on
JMA scale with maximum PGA 1.3g was observed at the seismic station in the Monzen Branch office inMonzenmachi, Wajima city during the main shock (National Earthquake for Land and InfrastructureManagement and Building Research Institute, 2007). The aftershock observation had been carried out by
Tokyo Institute of Technology (TITech) by installing 11 seismic observation stations in the heavily
damaged area (Yamanaka et. al. 2007). Also, aftershock records were collected from Building ResearchInstitute (BRI) seismic stations which had been installed in the buildings of Wajima City Hall Office andMonzen Branch Office. Regarding the geological settings of Wajima city and the damage distribution,
microtremor measurements had been carried out in Hashiride and Touge areas. Also, microtremor
measurements have been carried out for Wajima City Hall Office and Monzen Branch Office, where we
obtained aftershock records, in Wajima city.
The main goals of this study are:–Earthquake site response estimation by using microtremors.
–
Estimation of dynamic characteristics of building by using microtremors.
–
Verifying the accuracy and the applicability of microtremors in microzonation studies
by comparing them with observed earthquake motions.

BACKGROUND THEORY
Site transfer function has been extensively used to evaluate the local site effects (Borcherdt 1970).
Earthquake data as well as microtremors have been widely used to estimate site effects. Since earthquake
data are not often available, using microtremors to estimate local amplification has attracted many
researchers for a long time ago. Microtremors are the background vibration of the earth from artificialsources such as traffic, industrial machines and so on. The period of Microtremors ranges from 0.05 sec to
2 sec and their amplitude is less than several microns (Kanai 1961). Kobayashi et al. (1985) compared theacceleration response spectra (for 5% damping) of the main shock of Mexico City earthquake with theFourier velocity spectra of the microtremors. The comparison showed good agreements in spectral shape.
They considered empirically that the Fourier velocity spectra of microtremors reflect the transfer functionof S-wave in layered soil ground. They considered that spectral ratio of acceleration response spectrum of
the main shock and Fourier velocity spectrum of microtremors would be equivalent to the acceleration
response spectrum of the incident waves from seismic bedrock during the main shock. Then the product of
this spectral ratio with Fourier spectrum of microtremors will indicate the inferred acceleration response
spectrum at the ground surface during the main shock. This method had been used in Mexico City topredict strong ground motions due to the Michoacan, Mexico earthquake of Sep. 19th, 1985. On the otherhand, Nakamura (1989) popularized the method of recording microtremors with a three-component
seismometer and producing an estimation of site geological conditions from horizontal-over-vertical
spectral ratio. He assumed that H-over-V spectral ratio is similar to transfer function for horizontal motionof surface layer with no need for any time restriction on microtremor measurements.

OBSERVATIONS
Aftershock Observation
Aftershock observation was conducted in the heavily damaged areas (Hashiride, Touge and Kuroshima)by installing 11 seismic accelerometers. The heavily damaged areas are located in a narrow valley along
Hakka River with a width of 1 to 2 km. The three seismic observation stations (L1, L2 and L3) as well as2

the five stations (L4, L5, L6, L7 and D1) were installed in Hashiride and Touge areas, respectively.
Hashiride and Touge areas are sandwiched by the mountains in the northern and southern sides of thevalley along Hakka River where the Quaternary alluvial layers deposited over Tertiary rock. The thicknessof Quaternary alluvial deposits becomes less towards the mountains where the Tertiary rocks become
closer to the surface. L1 and L4 stations are located near the mountain foot where Tertiary tuff andTertiary sandstone are placed, respectively. L2, L3, L5, L6 and L7 stations are located in the Quaternary
alluvial layers, while D1 station is located on the Terrace deposits which is classified into different
geological conditions from the others in Touge area. K1, K2 and K3 seismic observation stations wereinstalled in Kuroshima area which is characterized by its steep slope from the coast. Figure 1 shows thesurface geological map (Editing Committee of Engineering Geological Map in Hokuriku District, 1990)
with the location of aftershock observation stations. Three hundreds and twelve aftershocks were recordedby these stations. Also, 9 aftershocks in Wajima City Hall Office and 34 aftershocks in Monzen Branch
Office building were recorded by Building Research Institute (BRI) seismic observation stations.

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Figure1. Surface geological maps ofMonzenmachi town (after, Editing
Committee of Engineering Geological
Map in Hokuriku District, 1990) withthe location of aftershock observation
stations

Microtremors Observation
Microtremor measurements were carried out using very high sensitive seismometers with servo system.
The sensor can observe ground motion for the period ranging from 0.1 Hz to 100 Hz. The resolution is 1.5* 10-6 m/sec.
Regarding the geological settings of Wajima city and the damage distribution during 2007 Noto
Hanto earthquake, 55 microtremor measurements were carried out in Monzenmachi town where the heavy
damage was observed including the sites of aftershock observation stations. Twenty-eight microtremor
measurements were carried out in Hashiride area (including L1, L2 and L3) and 27 microtremor
measurements in Touge area (including L4, L5, L6, L7 and D1). In order to obtain simultaneous
microtremor records, continues measurements were carried out in L3 and L6 sites for Hashiride area andTouge area, respectively. While other microtremor measurements have a length of 300 sec. Figure 2
shows the locations of microtremor measurements in Monzenmachi town. Also, microtremor
measurements were carried out for Wajima City Hall Office and Monzen Branch Office where we haveaftershock records (recorded by BRI seismic stations). Three horizontal sensors have been used to measure3

microtremors on the building in order to obtain the natural period for normal and torsion modes, while
damping factors of the building were calculated neither from microtremors nor from aftershocks.

Figure 2. Points of Microtremor
measurements in Monzen-machi
area, circles refer to microtremor
measurements in the site ofaftershock observation, red andblue rectangular show Hashiride
and Touge regions, respectively

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COMPARISON OF MICROTREMORS & EARTHQUAKE SITE RESPONSE
Earthquake site to reference spectral ratios (L4 was defined as a reference site) have been calculated andthe average spectrum has considered as a site transfer function. Regarding Nakamura’s method,
microtremor H/V spectral ratios represent earthquake transfer function for horizontal motion of surface
layers. Therefore, microtremor H/V spectral ratios have been calculated and compared with earthquakesite transfer functions obtained from aftershocks as shown in figure 3. Regarding microtremor site toreference method, horizontal spectral ratios of rover mobile site to reference site ( H rov. H ref . ) represent
the relative transfer function between these two sites. Then, the product of this ratio with observed spectraof earthquake motion in the reference site ( E ref . ) represents the estimated spectra of earthquake motion in
the rover site. Figure 4 shows the comparison of estimated earthquake motions (based on microtremor site
to reference spectral ratios) with observed earthquake motions.

0.1
1
Period (sec)

10L610.1
0.01

Spectral Ratio

Spectral Ratio

10101

1Period (sec)

10EQ

1010.10.01

0.1

10.1
0.01

0.11
Period (sec)

10EQ

1010.1
0.01

0.1

1

Period (sec)Figure 3. Comparison of microtremor H/Vspectral Ratios with earthquake transferfunctions, similar fundamental frequencies
are found. The peak amplitudes of H/V for
soft sites (L3) are higher than them in stiffsites while the predominant periods of softsites are longer.

L7
MTEQ

0.1

0.1
1
Period (sec)

100

MT

0.1
0.01

10Spc. Ratio

Spc. Ratio

Spc. Ratio

1MTMT

10L5100

EQEQ

EQ

100

L3100MT

100.10.01

L2100MT

Spectral Ratio

L1

100

1

10

Period (sec)

4

10

10.1
Obs.

10.1
Obs.

Est.

0.01
0.01

0.1
1
Period (sec)L1

10
Spc. Amp.(gal*sec)

10

10.1
Obs.
Est.

0.01
0.01

0.1
1
Period (sec)100.1
1Period (sec)

10

10

10.1
Obs.
Est.

Est.

0.01
0.01

L2

L4

10

Spc. Amp. (gal*sec)

Spc. Amp. (gal*sec)

Spc. Amp. (Gal * sec)

L5

10

Spc. Amp. (gal*sec)

L7

100.01
0.01

10.1
Obs.
Est.

0.1
1
Period (sec)100.010.01

0.1
1Period (sec)

10Figure 4. Comparison of estimated earthquake motions (based onmicrotremors) with observed earthquake motions, a good agreement has
been observed especially when both sites (reference & rover) are locatedon soft deposit layers in the same basin, while the agreement becomesless for stiffer sites located out of the basin or at the basin edge
(L1&L4). The less agreement for these sites is due to the quite big
difference in the excitation of microtremors in these sites compared withsoft sites.

COMPARISON OF MICROTREMORS & EARTHQUAKE BUILDING RESPONSE

Using aftershock records, Fourier spectra are calculated for Monzen Branch Office and Wajima City Hall
Office in both directions and for top and first floors. Building transfer function, which is the spectral ratioof top floor to first floor, is calculated for each building in both directions. By using microtremors, Fourier
spectra are calculated for Wajima City Hall office and Monzen Branch Office in both directions.
By comparing the Fourier spectra obtained from microtremors with building transfer function obtained
from aftershock records, a good agreement in fundamental frequencies was observed as shown in figure 5.
MNZ

WJM
10Y (EQ)

X (EQ)X (EQ)X (MT)Y (EQ)Y(MT)Y (MT)

EQ: Spc. Amp. Ratio
MT: (cm/sec*sec)*E-03

EQ: Spectral Ratios
MT: (cm/sec*sec)*E-03

101

0.1

0.01
0.01

0.1

1

10

X(MT)

1Figure 5. Microtremor Fourier
spectrawithearthquake
transfer function of Monzen
Branch Office (left) andWajima City Hall Office

0.1

0.01
0.01

0.1

110Period

PeriodCONCLUSIONS AND RECOMMENDATIONSThe site transfer functions as well as building transfer functions had been estimated by using microtremors.
Then compared with earthquake transfer functions and good agreements were found that confirmed thevalidity of microtremors in microzonation studies. As a result, following conclusions have been drawn:5

Microtremor H/V spectral ratio, which is a simple technique and easy to be applied, gives reliable
results for predominant period estimation while for amplification factors, we don’t obtain a goodagreement with earthquake amplification factor for all sites. But, it was remarkable that the peak
amplitude of H/V ratios increased when the site effect appeared very clearly, which means the peak
amplitude of H/V ratios gives a good indicator to identify significant site effects.
Microtremor site to reference spectral ratio, which requires simultaneous measurements inreference and rover sites, gives good agreements with observed earthquake records. But, a great attention
must be paid for selecting the reference site which is supposed to have similar microtremor excitationconditions and located on soft sediments in the same basin. That means this method can not be applied toestimate the ground motions in stiff sites. But such stiff sites don’t have significant site effects and weneed not to worry about their site effects.
By comparing the microtremor transfer functions of two public RC buildings with earthquake
transfer functions; we can verify the reliability of microtremors as a tool for evaluating building transferfunctions.
More than one technique is recommended to estimate site response in order to obtain more reliableresults. Comparison and complementation of different techniques are more probable to provide moreaccurate and adequate results.

ACKNOWLEDGMENT

We are very thankful to Prof. Dr. Yamanaka from Tokyo Institute of Technology as well as Dr.
Kashima from Building Research Institute for their help and their support.
REFERENCES
Borcherdt, RD., 1970, Effects of local geology on ground motion near San Francisco Bay, Bulletin ofSeismological Society of America, 60, 29-61.
Editing Committee of Engineering Geological Map in Hokuriku District, 1990, Engineering geologicalmap of Hokuriku district 1:200,000.
Kanai, K. and Tanaka, T., 1961, On Microtremors VIII, Bulletin of the Earthquake Research Institute,
University of Tokyo, 39, 97-114.
Kobayashi, H., Seo, K., and Midorikawa, S., 1985, Estimated strong ground motions in Mexico City due
to the Michoacan, Mexico Earthquake of Sep. 19, 1985, Report, Tokyo Institute of Technology.
Nakamura, Y., 1989, A Method of Dynamic Characteristics Estimations of Subsurface Using
Microtremors on the Ground Surface, Quarterly Report of Railway Technical Research Institute(RTRI), 30, 25-33.
National Institute of Land and Infrastructure Management and Building Research Institute, Quick report ofsurvey for the 2007 Noto Hanto Earthquake,07.3., 26-27.
Seo, K., 1991, Microtremor measurement in the San Francisco Bay area, Report, Tokyo Institute ofTechnology.

Yamanaka, H., Motoki, K., Yamada, N., Sugawara, T., Mabuchi, Y., and Seo, K., 2007,
Observation of aftershocks of 2007 Noto Hanto Earthquake in Monzenmachi, Wajima city,
for estimation of site amplification, submitted to E.P.S.

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