The Second International Conference on Earthquake ...

The Second International Conference on Earthquake Engineering and Disaster Mitigation (ICEEDM-2): “Seismic Disaster Risk Reduction and Damage Mitigation for Advancing Earthquake Safety of Structures,” Surabaya, Indonesia, 19-20 July 2011

LOCAL SITE EFFECT OF A LANDSLIDE IN JEMBER BASED ON MICROTREMOR MEASUREMENT Dwa Desa warnana1,3*†, Ria Asih Aryani Soemitro2, Widya Utama3 and Alain Tabbagh4 1.

2 3 4

Doctoral student, Civil Engineering Departement, Institut Teknologi Sepuluh Nopember, Surabaya – Indonesia and UMR 7619, SISYPHE Paris, France Civil Engineering Departement, Institut Teknologi Sepuluh Nopember, Surabaya – Indonesia Physics Department, Institut Teknologi Sepuluh Nopember, Surabaya – Indonesia UMR 7619, SISYPHE Case 105, 4 Place Jussieu, 75252 Paris, France

ABSTRACT This paper investigates the local site effect of the earthquake induced landslide using microtremor method. It is widely assumed that soil effects and topographic effects – are considered under the general denomination of local site effects - have a significant effect on the ground motion amplifications. The research was carried out at Kemuning Lor village, Jember –Indonesia, which experiences deep landslide almost every year. The objective of this present paper is to locate the area of failure within the landslide during periods of active movement. The microtremor investigation had been conducted on 82 free-field measurements having 20 x 20 m dense grid. Overall HVSR analysis performed using GEOPSY Software. In general, amplification factor (Am) varied from 2 to 7 and the predominant frequency (f0) ranges between 1 and 3 Hz. It had been found that maximum Am happened at main scarp and slope area that have a high topographic gradient. However, change of f0 followed topographic pattern. Variations of both parameters are indicated as a result of variations in surface soil parameters and in general the soil effects are more important than topographic effects which are controlled by the contrast of impedance between the surface materials and the bedrock. The level of soil damage due to the influence of local site effects at the study area is shown with vulnerability index (kg). The vulnerability index ranges between 2.6 and 34.6. From vulnerability index can be seen that at main scrap area become plastic and failed during the earthquake. Thus earthquake also contribute as a trigger landslide in this research area. KEYWORDS: Local site effects, Amplification factor, vulnerability index, landslide, microtremor

* †

Corresponding author:[email protected] Presenter:[email protected]


1.

INTRODUCTIONS

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 (Sanchez-Sesma et.al., 2002). In last decade the microtremor method has been widely used for site effect studies (e.g. Nakamura et al. 2000; Gosar 2007; Herak et al. 2009). 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 landslide in the village of Kemuning Lor, Jember Regency, East Java Indonesia. According to the seismic hazard map of java, Indonesia for a 475-year return period (Asrurifak et.al., 2009) a design ground acceleration value for a rock site in site location ranges from 0.2 g to 0.25 g. During the period from 2001 to 2008, a large number of houses were destructed by five large sized mass movements in the research area; though a delayed indirect seismic effect might be responsible for the landslides. The microtremor investigation was use to determine the characteristics of surface ground in the landslide areas. The Nakamura technique has been adopted for the microtremor measurements analysis (HVSR) (e.g. Nakamura, 1989; 1997) to estimated the natural frequency (f0), amplification factor (Am) and vulnerability index (kg = A2/F). The parameters were combined to determine the relationship between the local site effects and the landslides. 2. GEOLOGICAL SETTING The research site is located near kemuning Lor village, Jember regency (S80 06.461’ and E1130 42.472’). During the period from 2001 to 2008, five large sized mass movements occurred in the research area causing the destruction of a large number of houses on the slope. Most landslides triggered during this period were due to climatic factors, but an indirect seismic influence with delayed triggering could also be proved for some of them. Based on the Jember Regency geological map, the location of site investigation is an area of Argopuro breccias sedimentary. The surface ground is dominated by the volcanic breccias


distribution with the insertion of lava. Generally,

volcanic breccias is mild-high weathering, gray,

with the base rock consisted of andesite and tuff. The rocks of this formation generally have a low to high rock strength (Sapei, et.al., 1999). The geotechnical investigation were drilling and standard penetration test (SPT) to 30 m depth. The drilling data indicated that the soil is brown and mainly consisted of sandy silt-clayey and silty sand-clayey. There was no ground water level to 30 m depth. The soil is relatively soft indicated by the SPT value (N) from 10 to 30 and the relatively density from 40 to 60. The stiff soil was found at the depth below 26 m having SPT value greater than 50. 3. FREE-FIELD MICROTREMOR MEASUREMENT Measurement microtremor data was conducted by 20 x 20 m in grid arround of the main landslide area with the number of points are 82 points on December 24 to 28 October 2009. For each point of measurement 15 minutes of ambient noise were recorded at the sampling rate of 100 Hz. 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 (1998) window (b=40). 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 (2007). 4.

RESULTS AND DISCUSSION

4.1. Distribution of soil natural frequency (f0) and amplification factor (Am) Figure 1a shows the distribution of f0 where the distribution of natural frequencies is relatively uniform, ranging from 1-3 Hz. The topographic patterrn is associated with the fo value. The soil thicknes (h) is possibly estimated using the formula f0 = V/4h, where V is shear wave velocity (Bard, PY, 1999). Ghalandarzadeh (2006) in Towhata Ikuo (2008) is also empirically determined the sandy soil layer thickness using the formula of (h) = 96 x f0-1.388. 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. Figure 1b represents the amplification factor (Am) or peak ratio HVSR spectrum in investigation sites ranging from 2 to 7. High amplification factor (Am>4) are found in several main scarp areas and in slope areas having high topographic gradients. The previous investigators (Bovckovalas & A.G Papadimitriov, 2005) using the numerical analysis of seismic wave propagation due to topographic effects found the similar results. An important remark should be made upon similar findings of the site investigation results and numerical analysis results.


(a)

(b)

Figure 1. (a) Distribution of soil natural frequency (f0). (b) Distribution of amplification factor (Am) on study area The large amplification factor (Am) was found not only on the high topographic gradient slope but also on the terrain topographic slope. On the other hand, small amplification factor was found on the part of main scarp. Thus the topographic effects is not the only one factor controlling the amplification factor. Different value of Am might be found in the same value of the natural frequency. It can be noted that the variation of Am value is not strongly effected by the soil depth. She Wang and Hong Hao (2002) explained that the variation of soil parameters (shear modulus, damping ratio and density) influenced the amplification factor. Yang, Jun (2006) 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. At present, using the Am as site amplification parameter is still a hot debate among the experts (Nguyen, et.al., 2004). 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 (Gosar 2010). On the other hand, it is widely accepted that the natural frequency (f0) reflects the fundamental frequency of the sediments. 4.2. Distribution of Soil Vulnerability Index (kg) Nakamura (1997) introduced a Vulnerability Index Parameter (kg), which combined Am and fo to determine soil damage level due to the local site effects. Thus, Kg can be considered as an index to indicate easiness of deformation of measured points which is expected useful to detect weak points


of the ground. To estimate soil vulnerability index (kg), the value of shear strain () need to be considered (Nguyen, et.al., 2004). According to Ishihara (1978) 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. Nakamura (1997) had outlined the formulation in detail, but in summary it can be written as follows: 𝛾=

𝐴2𝑚

𝑎

(1)

𝑓0 𝜋 2 𝑣𝑏

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 2, shows the distribution of vulnerability index (kg) having values ranging from 2.6 to 34.6. Assuming a = 0.25 g and vb = 800 m/s, for  >1000 x 10-6 then the kg value > 3.2; and for  > 10,000 x 10-6 the kg value > 32.2. Large values of kg (kg > 32.2) were found all along the main scrap; these zones were considered as weak zones which may fail during the earthquake.

Figure 2. Distribution of soil vulnerability index (kg) on study area 5.

CONCLUSIONS

The iso-f0 map shows a distribution is relatively uniform, ranging from 1-3 Hz. The observed frequencies can be related total thickness of soil. The amplification factor (Am) is ranging from 2 to 7. High amplification factor (Am>4) are found in several main scarp areas and in slope areas having high topographic gradients, indicating high impedance contrast between soils and the bedrock. Although the amplification factor gives no reliable indication of the amplification of seismic ground and the interpretation of HVSR is limited to fundamental soil frequency, vulnerability index (kg) values can be calculated to estimate the vulnerabilities of the slope. The results showed larger value along main scrap.


REFERENCE Asrurifak M., Irsyam M., Budiono B., Triyoso W., Hendriyawan Hendriyawan, 2010. Development of Spectral Hazard Map for Indonesia with a Return Period of 2500 Years using Probabilistic Method, Civil Engineering Dimension, 12(1),2010: 52 – 62. Bard P.Y. 1999. Microtremor Measurements: A tool for site effect estimation?., The effects of Surface Geology on Seismic Motion, Vol 3, 1251 – 1279., Balkema. Gosar, A. 2007. Microtremor HVSR study for assessing site effects in the Bovec basin (NW Slovenia) related to 1998 Mw5.6 and 2004 Mw5.2 earthquakes. Engineering Geology 91 (2007) 178–193. Gosar, A. 2010. Site effects and soil-structure resonance study in the Kobarid basin (NW Slovenia) using microtremors. Nat. Hazards Earth Syst. Sci., 10, 761–772. Herak, M,, Allegretti, I.,

Herak, D., Kuk, K., Kuk, V., Marić, K., Markušić, S., Stipˇcević, J..2009. HVSR of

ambient noise in Ston (Croatia): comparison with theoretical spectra and with the damage distribution after the 1996 Ston-Slano earthquake Bull Earthquake Eng .DOI 10.1007/s10518-009-9121-x Ikuo Towhata, 2008. Geotechnical Earthquake Engineering, Springer Series in Geomechanics & Geoengineering, Verleg Berlin. Ishihara,Kenji. 1978. Introduction to Dnyamic Soil Mechanism, January. Konno, K. and Ohmachi, T. 1998, .Ground-Motion Characteristics Estimated from Spectral Ratio between Horizontal and Vertical Components of Microtremor., Bull. Seism. Soc. Am., Vol. 88, N.1, 228-241. Nakamura, Y. 1997. Seismic vulnerability indices for ground and structures Using microtremor. World congress on railway research, florence, nov. 1997 Nakamura, Y., Gurler, E.D., Saita, J.,

Rovelli, A., Donati, S. 2000. Vulnerability investigation of roman colosseum

using Microtremor. Prepared for 12th WCEE 2000 in Auckland, NZ 2660/6/A Nguyen, F., Teerlynck, H., Van Rompaey, G., Van Camp, M., Jongmans, D. and Camelbeeck, T. 2004. Use of microtremor measurement for assessing site effects in Northern Belgium-interpretation of the observed intensity during the Ms5.0, June 11, 1938 Earthquake. Journal of Seismology, 8(1) 41-56, 20. Sanchez-Sesma, F.J, Victor J. Palenzia, Francisco Luzon (2002). Estimation of Local Site Effects During earthquake: An overview. ISET Journal of EarthquakeTtechnology, vol. 39, no.3, pp. 167 -193. Sapei.T, A.H Suganda, K.A.S Astadiredja dan Suharsono., Peta Geologi Lembar Jember, Jawa, Pusat Penelitian dan Pengembangan Geologi,Bandung, 1992. Sheng Wang and Hong Hao. 2002. Effects of Random variations of Soil Properties on Site Amplification of seismic Ground Motion, Soil Dynamics and Earthquake Engineering, Vol 22, issue 7, September 2002, pp. 551-564 Yang Jun. 2006. Frequency-Dependent Amplification of Unsaturated surface Soil Layer, Journal Geotech and Geoenvironmental Engineering, Vol 132, issue 4, April 2006, pp. 526-531.