Application of Microtremor HVSR Method for Assessing Site Effect

International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 04 73

118804-9393 IJBAS-IJENS © August 2011 IJENS I J E N S

Abstract— This paper investigates the local site effect of the

earthquake induced slope instabilities using microtremor

horizontal- to-vertical ratio (HVSR) method. It is accepted that

soil effects (soil thickness and variation of surface soil parameter)

and topographic effects namely local site effects, is considered

having a significant effect on the seismic amplifications. However

the understanding of seismic slope response is still limited. The

research was carried out in a residual soil slope near Batu town,

Malang District–Indonesia. The microtremor investigation had

been conducted on 54 free-field measurements having 20 x 20 m

dense grid. The HVSR analysis has been carried out using

Geopsy Software. The predominant frequency (f0) ranges

between 1 and 5.5 Hz and amplification factor (Am) varies from

2.5 to 10 though most of the areas having 4 to 5 Am value. The

topographic patterns are identified by the fo value as related to

bedrock depth. Variations of both parameters are indicated as a

result of variations in surface soil parameters. Surface soil

parameters are considered having more significant effect

comparing to those of topographic effects. The vulnerability

index (Kg) is indicated the soil damage level due to ground

motions. The weak zone, failed during earthquake on the

Southern slopes was identified by the highest Kg value.

Index Term— Site effects, the predominant frequency,

amplification factor, vulnerability index, microtremor HVSR,

residual soil slope, Malang.

I. INTRODUCTION It is now well known that local site characteristics may

produce large ground motion amplifications during

earthquakes. This issue can be investigated by means of the

analysis of actual seismic records and the study of synthetic

seismogram as well. By last century‟s middle years, effects of

local soil and geological condition were studied mainly in

terms of peak accelerations or peak velocities and the effects

of topography on surface ground motion have been observed

and studied from field experiments [1]. In last decade the

microtremor method has been widely used for site effect

studies (e.g. [2], [3], [4]).

Dwa Desa Warnana is with Civil engineering department, Institut

Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia. Formerly is Physics Department, Institut Teknologi Sepuluh Nopember (ITS), Surabaya,

Indonesia- 6011. (E-mail: dwa_desa@ physiscs.its.ac.id).

Ria Asih Aryani Soemitro is with Civil engineering department, Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia - 6011.

(E-mail: [email protected]).

Widya Utama is with Physics Department, Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia- 6011.

(E-mail: [email protected]).

In fact, even though the knowledge on local site effects have

been historically improved, the understanding of seismic slope

response is still limited due to of the scarcity of ground motion

recordings on landslide-prone slopes. Furthermore, numerical

modeling of slope behavior under earthquake shaking is not

easy because the acquisition of relevant geotechnical

parameters of slope materials is difficult in sites characterized

by rough topography and sharp lateral lithological and/or

physical heterogeneities. The assessment of subsurface

geology through borehole or “active” geophysical surveying is

expensive and is typically limited to post-factum (post-failure)

local scale investigations. Then, exploring the capability of

microtremor is interesting as it is considered as cheaper and

quicker geophysical.

The present paper is attempted to investigate the local site

effects of the earthquake induced slope instabilities in residual

slope at site near Batu Town, malang District – Indonesia

(Figure 1). This site is located in the Southern part of Java

Island, a region undergoing a recent increase in seismic

activity. According to the seismic hazard map of java,

Indonesia for a 475-year return period a design ground

acceleration value for a rock site in site location ranges from

0.2 g to 0.25 g [5].

The Nakamura technique has been adopted for the

microtremor measurements analysis (HVSR) (e.g. [6], [7]) to

determine the predominant frequency (f0), amplification factor

(Am). Reference [7] also proposed the vulnerability index „Kg

value‟ (Kg = A2/F) as a means to determine the extent of

seismic hazard. In this study, we determined the site

characteristics of the slope area using the HVSR of

microtremors and the Kg values to predict the potential for

slope instabilities at the sites.

.

II. LOCATION AND GEOLOGICAL SETTING

The research site is located on the hills of Tunggangan

Mountain, Sumber berantas village, Batu Town, Malang

District (S70 45‟ 18.80‟‟ and E112

0 31‟ 45.83‟‟). The site is at a

distance of approximately 15 km from Batu Town (Figure 1).

The topography in the site is of moderately steep in the range

20-25o. The elevation ranges between 1650 m and 1730 m.

Based on the Malang geological map, the location of site

investigation is a part of Young Anjasmara volcanic

sedimentary [8]. The rock units are dominated by the volcanic

breccias, lava, tuff and lahars (Figure 2). The geotechnical

investigation were drilling to 5 m depth. The drilling data

indicated that the soil is brownish black and mainly consisted

of sandy silt. Because no deep boreholes have been drilled at

site, the thickness of residual soil is completely unknown.

Dwa Desa Warnana, Ria Asih Aryani Soemitro, and Widya Utama

Application of Microtremor HVSR Method for

Assessing Site Effect in Residual Soil Slope



Based on general geological knowledge of the area and the

climate changes are intense with a predominance of chemical

weathering over other process of weathering, thus resulting in

deep weathering profiles and soil thickness often exceeding 30

m.

Fig. 1. Location map of study area

Fig. 2. Simplified geological map of Batu town (Site area is shown by black

box)

III. MICROTREMOR MEASUREMENT AND ANALYSIS

Measurement microtremor data was conducted by 20 x 20 m

in grid arround of slope area with the number of points are 55

points on August 2010 (Figure 3). For each point of

measurement 15 minutes of ambient noise were recorded at

the sampling rate of 100 Hz. Their locations were carefully

selected to avoid the influence of trees, sources of

monochromatic noise and strong topographic features (edges

of terraces).

The data processing to obtain the HVSR at each site was

performed in the following way: the data was filtered between

0.2 and 25 Hz by a band-pass 4 poles Butterworth filter after

the mean and a linear trend were removed; then each

component of the recorded signal was windowed in a time

series of 20 sec length (cosine taper 5%) and for each time

window an FFT was calculated and smoothed using the Konno

and Ohmachi logarithmic window function. For each time

window the spectral ratio between the root-mean square

average spectrums of the horizontal components over the

spectrum of the vertical component was calculated and,

finally, the average HVSR and the standard deviation were

computed. Overall HVSR analysis performed using GEOPSY

Software [9].

HVSR analyses of 55 free-field microtremor measurements

in the site showed that most of them (90%) fulfill the criteria

defined by SESAME project (Table 1) for reliable

measurements [10]. Three criteria for a reliable HVSR curve

are based on the relation of a peak frequency to the window

length, number of significant cycles and standard deviation of

a peak amplitude. Six criteria for a clear peak are based on the

relation of the peak amplitude to the level of the HVSR curve

elsewhere, and standard deviations of the peak frequency and

of its amplitude (the amplitude should decrease rapidly on

each side). If all three criteria for reliable curve and at least

five criteria for a clear peak are fulfilled, the frequency of the

peak is considered to be the fundamental frequency of

sediments.



Fig. 3. Microtremor measurement layout

TABLE 1

CRITERIA FOR REALIBLE HVSR CURVE AND CLEAR HVSR PEAK DEFINED BY SESAME PROJECT (SESAME,2004)

IV. RESULTS AND INTERPRETATION

A. Distribution of predominant frequency (f0) and

Amplification factor (Am)

Figure 4 shows the distribution of f0 where the distribution

of predominant frequencies is relatively uniform, ranging from

1-5.5 Hz. The topographic patterrn is associated with the fo

value. The residual soil thicknes (h) is possibly estimated

using the formula f0 = Vs/4h, where Vs is shear wave velocity

[11]. It could be noted that the f0 is associated with the the

depth of bedrock. The smaler of f0 value, the greater of depth

of bedrock.



Fig. 4. Distribution map of the fo value, a predominant frequency of a site, at

the 55 measured points.

Figure 5 represents the amplification factor (Am) or peak

ratio HVSR spectrum in investigation sites ranging from 2.5 to

10. High amplification factor (Am>4) was found almost all

area and the highest amplification factor just found at a top of

the hill (point B1). Distribution of the high amplification

factor in whole area measurements showed that the

topographic effect is not the only one factor controlling the

amplification factor.

Different value of Am might be found in the same value of

the predominant frequency (Figure 6). No correlation between

predominant frequency and its amplitude (Am) was

established. It can be noted that the variation of Am value is

not strongly effected by the soil depth. Reference [12]

explained that the variation of soil parameters (shear modulus,

damping ratio and density) influenced the amplification factor.

Reference [13] explained that the influence of the saturation

state of the bedrock is insignificant; a change of the saturation

state of the soil layer may have a marked impact on the

amplification factor. It can be clearly stated that the geological

factors are more dominant to the Am variation.

Fig. 5. Distribution map of the Am value, a amplification factor of a site, at the

55 measured points.

Fig. 6. Peak ratio vs. predominant frequency graph of HVSR peak.

At present, using the Am as site amplification parameter is

still a hot debate among the experts [14]. There is no ultimate

correlation between the Am and the maximum spectral

amplification of the site from strong motion. There might be

some local relationships for a limited area and should only be

regarded as a relative indicator of local site amplification since

applied instruments or instrumental settings may also exert

influence [11]. Reference [15] explained that amplitude/

amplification factor depends mainly on the impedance contrast

and HVSR also does not provide any estimate of the actual

bandwidth over which the ground motion is amplified. On the

other hand, it is widely accepted in the scientific community

that the predominant frequency (f0) reflects the fundamental

frequency of the site [11], [16], [17], [6].



B. Distribution of Soil vulnerability index (Kg)

Reference [7] introduced a vulnerability index parameter

(Kg), which combined Am and fo to identify areas where

greater seismic hazards and damage may be expected using

HVSR technique. Previous studies ([18], [19], [20]) had

showed that distribution form of Kg; very well correspond

with damage due to more recent earthquake. Thus, Kg value

reflect local site effect and can be considered as an indicator

which might be useful in selecting weak point of ground

especially in slope areas.

To estimate soil vulnerability index (Kg), the value of shear

strain () need to be considered [14]. According to Ishihara

[21] ground soil becomes plastic state at about 1000 x 10-6

; and for > 10,000 x 10-6

catastrophic landslide or very large

deformation will be occured. Reference [7] had outlined the

formulation in detail, but in summary it can be written as

follows:

(1)

In this equation, (Am)2/f0 is called soil vulnerability index (Kg),

a is the ground acceleration and vb is the shear wave velocity

of bedrock.

Figure 7, shows the distribution of vulnerability index (kg)

having values ranging from 2 to 32. Assuming a = 0.2 g and vb

= 600 m/s, for >1000 x 10-6

then the kg value > 3; and for

> 10,000 x 10-6

the kg value > 30. Based on the above criteria,

for the value of Kg> 3 was found spread throughout research

areas and for the value of Kg> 30 was found only at several

points the southern hills. Reference [20] reported that Kg> 14

in Central Java was caused deformation of the soil and

destroyed buildings on it. Thus, the higher Kg value (Kg> 14)

in the site areas spread over the southern area. These zones

were considered as weak zones which may fail during the

earthquake.

Fig. 7. Contour map of the Kg value, refer to (1), in the site areas. The higher Kg values appear in the southern area.

V. DISCUSION AND CONCLUSION

Geotechnical investigation (boreholes, penetrometic tests,

etc), local instrumentation placed in boreholes (piezometers,

inclinometers) are generally employed for assessing site effect

in slope instabilities. Even though these studies provide direct

information on the slope material, except by multiplying the

number of tests, these methods are not able to image the

lateral variability of slope characteristics [22].

Microtremor investigations have proved to be an effective

tool for assessment of site effects in the case of residual soil

slope. In such conditions, the microtremor HVSR method is

very useful for quantitative seismic microzonation and

assessment of earthquake induced slope instabilities. At the

same time, the costs of measurements and processing are kept

low, because no active source is needed.

Microtremor performed in residual soil slope at site near

Batu town, showed that the predominant frequencies are

concentrated at about 1- 5.5 Hz (Figure 4) and amplification

factor varied from 2.5 -10 (Figure 5). An important remark

should be noticed that the geological factors are more

dominant rather than topographic effects to the Am variation.

Based on the predominant frequencies and their

amplification factors, one important parameter, Kg, could be

determined to assess the local site effects. In this study, the Kg

values in the southern areas are higher than the northern areas,

as indicated as weak zones which may fail during earthquake.

Nevertheless, before more general conclusion can be made

and to find out the origin of the difference between the

southern and the northern area, microtremor modeling which

inverts observed HVSR to find soil model should be

performed, including analysis of their dynamic physical

behavior.

ACKNOWLEDGMENT

The authors are indebted to R. kiswanto, Irma Novalita, S.Si

and Yedi Darmadi, S.Si for their help in field measurements.

They also extended their gratitude to the Meteorological,

Climatological and Geophysical Agency (BMKG) for

providing the microtremor equipment.

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Application of Microtremor HVSR Method for Assessing Site Effect