Predicting the liquefaction phenomena from shear

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Procedia Earth and Planetary Science

ISEDM 2013 3rd International Symposium on Earthquake and Disaster Mitigation

Predicting the liquefaction phenomena from shear velocity profiling: empirical approach to 6.3Mw, May 2006 Yogyakarta earthquake Eddy Hartantyoa,b, Kirbani S. Brotopuspitob, Sismantob, Waluyob* a

PhD student, Physics Department, FMIPA, UGM. Sekip Utara Yogyakarta 55281 Indonesia Geophysics Laboratory, FMIPA, Universitas Gadjah Mada, Sekip Utara Yogyakarta 55281, Indonesia.

b

Abstract The liquefactions phenomena have recorded while shocking 6.5Mw earthquake hit Yogyakarta province in the morning of 27 May 2006. Several researchers have reported the damages, casualties, and soil failures due to the quake, including the mapping and analyzing the liquefaction phenomena. Most of them based on SPT test. In this paper, we try to draw the liquefaction susceptibility by means the shear velocity profiling using modified Multichannel Analysis of Surface Waves (MASW). This paper is preliminary reported by using only several measured MASW points. We built 8-channel seismic data logger with 4.5 Hz geophones for this purpose. Several different offsets used to record the high and low frequencies of surface waves. We stack these phase-velocity diagrams in the frequency domain rather than in time domain, for a clearer and easier dispersion curve picking. All codes are implementing in Matlab. From these procedures, we have shear velocity profiling beneath each geophone’s spread. By mapping the minimum depth of shallow water table, calculating PGA with soil classification, using empirical formula for saturated soil weight from shear velocity profile, and calculating CRR and CSR at every depth, the liquefaction characteristic can be identify in every layer. From several acquired data, we have a liquefiable potential at some depth below water table. Keywords: liquefaction, VS profiling, MASW

1. Introduction Liquefaction is one of the secondary effect due to high magnitude and shallow earthquake event (Jefferies and Been, 2006; Huang and Yu, 2012). At least, three conditions are needed to generate liquefaction; i.e. lithological conditions, water-table position, and magnitude (as well as duration) of shaking (Anderson et al, 1982; Jefferies and Been, 2006). The south part of Yogyakarta province (Bantul regency, Southeast part of Sleman Regency and Yogyakarta city, See Fig 1) are dominated by volcanic-clastic sediments on the upmost layer (Rahardjo, et al 1995; Hendrayana, 1993). This sediment was transported by flood and avalanche in the body of river and consists of gravel, sand, silt, tuff, agglomerate and clay (Rahardjo, et al 1995). The grain size of liquefied sand is dominated by sand with finest FC 35% (Elnashai et al, 2007). The grain size of measured sands are in the range of liquefiable sand. The volcanic-clastic sediments is dominated by sand and clay figured from 99 wells lithology log. The second condition is the depth of water table. Upmost water-table in Southern areas of Yogyakarta was very shallow (Hartantyo et al, 2013). This condition will improve the liquefaction occurrences due to increasing pore pressures of saturated soil. The third condition is the existence of active faults (the main fault was known as Opak fault) and it believed as a line position of main earthquake fault at May 2006 (Soebowo et al, 2007; Sarah and Soebowo, 2013). Other researchers indicate that the main shocks, deduced from aftershock epicentres (Walter et al, 2008), and from INSAR analysis (Tsuji et al, 2008),

* Corresponding author. Tel.: +62-274545185; fax: +62-274-545183. E-mail address: [email protected]

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was located 15 km eastward parallel with Opak fault. These three conditions are fulfilling for the Southern areas of Yogyakarta for having liquefaction hazard while earthquake strucks.

Fig. 1. Location of Yogyakarta special province (grey area in inset figure) and volcanic-clastic sediment (white dash) drawn on topographic map of this area

Fig. 2. Compiled location of liquefaction occurrences (blue dot) and fractures (green dot) with their respective faults (compiled from Soebowo et al, 2007, Lee et al 2006, Adawiyah, 2008, Nolte 2007).

Fig. 3. (a) Map of water table depth from surface. Black dot represents 493 measured points (re-mapped from Hartantyo et al, 2013). (b) Qualitative correlation with position of liquefaction occurrences and fractures

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When an earthquake hit Yogyakarta in the morning of May 27th 2006 (USGS), several areas were reported to employ liquefactions (see Fig 2), in the form of sand boils, cracks, water ejects, and loss water at wells (Lee et al, 2006; Adawiyah, 2008; Soebowo et al, 2007; Nolte, 2006; Sarah and Soebowo, 2013; Pramumidjoyo, 2009). These report shows that the area of southern Yogyakarta, especially volcanic-clastic area, is vulnerable to liquefy. Soebowo et al (2007) has conducted several SPT and CPT test around Pundong, Patalan, and Jetis (sub-districts directly to the east of Bantul city). By analysing the liquefaction safety with Liqit software, they show that the depth of liquefaction was found from near surface (0.2m) up to 12m depth. Similar with Soebowo et al (2007), Sarah and Soebowo (2013) also using the same data to calculate the thickness of liquefiable sands at the same area. They show that the thickest (around 5m thick) liquefiable sand is found in the Opak fault area, at Pundong sub district. This paper will, as a compliment to these works, find the ‘depth’ of liquefaction reported by using simplified liquefaction analysis procedure (Seed and Idriss, 1971) from elastic (small shear modulus) condition i.e. velocity (V S) profiles. Survey area is limited only in the southern part of volcanic-clastic sediments, especially where the locations of shallow water table were exist and there are evidences of liquefaction occurrences

2. Data preparation and parameter estimation 2.1 Water table map Water table measurements were conducted at the peak of rainy season in third week of March 2012 (Hartantyo 2013). It was assumed that in that piece of time, the condition of water table was in shallowest depth. Simple measurements were used to simplify the method, just by measuring the depth of water by meter-tape in open wells (dig well without closed lid at the top), from surface of water up to ground surface. Positions of each point were measured by handy GPS. Shallow water table was found in central and southern part of Bantul regency (see Fig 3a), creating the ‘Y’ shape towards Northeast and Northwest. This figure shows that in Bantul regency, we have very shallow water tables (0-3m). This map of correlate well with recorded liquefaction and fracture occurrences (see Fig 3b). These water levels are used for liquefaction calculation. It is assumed that from these levels down to 12m depth, soils were fully saturated with water. 2.2 Horizontal Peak Ground Acceleration (PGA) estimation There are some papers calculate the PGA value in Yogyakarta and its vicinity due to May 2006 earthquake (Burton et al, 2008; Kirbani, et al, 2006; ). In this paper, the PGA at has been calculated by Ambraseis et al (2005) attenuation formula. Even this attenuation is not set locally in Yogyakarta area; it shows a high correlation with measured PGA in some areas. The horizontal PGA formulation is;

(1) with; = peak horizontal ground acceleration (m/s2); = closest distance from point of interest to line segment (faults) in km; = magnitude; = existing soft or stiff soil; and = existing type of fault (Normal, Transverse, or Unknown faults).The soil classes were deduced from VS30 (USGS). The earthquake magnitude was set to 6.3Mw and the type of fault was set as sinistral (right transverse) fault (interpret from Tsuji et al, 2008). Last, the distance from points of interests to segmented fault was calculated by assuming that the fault was a line rather than a curve. We use a segmented fault interpreted from INSAR and aftershock analysis (Tsuji et al, 2008).The horizontal PGA map which is use in liquefaction analysis is shown in Fig 4. It shows that the range of PGA in Bantul area is around (1 - 3) m/s2 or 0.1-0.3g. The highest PGA of course is lying at where the fault exist. 3. MASW measurement and processing We calculate V S profiles by using multichannel analysis of surface waves (MASW), originally introduced by Park et al (1999). This method is still continue developing and has been used in several areas (Miller et al., 199a; Miller et al., 199b; Park and Miller, 2004; Park and Miller, 2005; Park and Miller, 2006; Hartantyo, 2009; Hartantyo et al., 2008; Hartantyo, 2010; Srbulov, 2008). We made some improvements, while other schemes were stack recorded traces data in space-time (x-t), but we do this in frequency-phase velocity (f-v) domain. By using f-v stack, we can use the variation of MASW acquisition geometries to provide f-v stack. MASW data was acquired by eight 4.5Hz geophones recorded with own-constructed data logger. Data record was set to 7200 data for 8 channels per second, and geophone spacing was set to 3m. Five kg hammer and wood plate were using for

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seismic source. Both forward and reverse shots were acquired, each by 3m and 5m near offsets. Fig 5 shows the position of measured MASW data (a) with some photographs during acquisition (b).

Fig. 4. Horizontal PGA values in Yogyakarta Province inferred from interpreted fault by Tsuji et al. (2008), calculated by Ambrayseis (2005) attenuation formula

Fig. 5. (a) MASW measurement points; (b) some photographs during acquisition. Data logger system was packed using lunchbox (b; top right)

Example of measured MASW data is taken at point number A226, especially at four shot gathers recorded with source at 5m offset. These shot gather then were Butterworthband pass filtered (3-45) Hz especially for removing power frequency 50Hz, which dominantly influence the data (see Fig. 6a). Surface waves are clearly identified for this site,from longer wavelength seen at far offset. Consistency of each gather looks good, shows by similar shape, even data were snap with different recording length. Data then bring to f-v, via remapping them in f-k (frequency-wavenumber) domain, (see Fig.6b). In the f-v domain, all shot seem consistent for each data set. These four f-v diagrams on Fig.6b are then stacked manually to increase the data quality and suppress the inconsistent noises closures (see Fig. 7a). By using this stack f-v data, we then easily pick the data, and let the program do interpolation and smoothing. The picked data then automatically inverted by using ‘Seisimager’ module (WaveEq) to produce V S profile, (see Fig.7b). We set a model with depth up to 30m, divided in fixed 15 layers. The synthetic f-v data from optimum model then plot back to f-v diagram for picking to see the misfit with picked f-v data. All procedures, except inversion, were written in Matlab. 4. VS based liquefaction analysis The procedures and analysis of liquefaction by means of VS profile firstly simplified by Seed &Idriss, 1971. Several researchers then modified and applied to judge the liquefied area (e.g. Dobryet al., 1981, 1982; Seed et al., 1983; Bierschwale

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and Stokoe, 1984; de Alba et al., 1984; Hynes, 1988; Stokoe et al., 1988; Tokimatsu and Uchida, 1990; Robertson et al., 1992; and Andrus et al., 1999). Following the procedure compiled by Idriss and Boulanger (2004) and Noutash et al (2012), we calculate the Cyclic Resistance Ratio (CRR); (2) with;

and m/s for finest content >= 35%; 0.5 –where soil age was unknown.

for uncemented Holocene soil,

= air pressure at surface,

=

Fig. 6. Data recorded at point A226: (a) Filtered shot gather records; (b) f-v diagram for each data in (a)

For Cyclic Shear Stress Ratio (CSR), we calculate as; (3) with: = gravitational acceleration (m/s2), = constant correction due to depth, = Total vertical stress, and = effective stress. Both CRR and CSR are calculated in every depth of question. We use an empirical equation (Mayne, 2010) to find the unit weight of saturated soil from VS profile, which have good fit with all type of soil database. This value is used as a parameter to calculate the total overburden stress. For dry soil, especially for dry soil in the top of water level, we assume that its soil porosity is 0.2.

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Fig. 7. (a) Stacked data from Fig.6b shows normalized correlation (red high) and picked f-v (white dot), also synthetic data (black line) from optimum VS model (b) Shear velocity profile for A226

Factor of safety (FS) from the occurrence of liquefaction is calculated by FS = CRR/CSR. If FS 1, which means that CRR (resistance of soil due to cyclic loading) is lower than CSR (induce shear stress due to cyclic mobility of an earthquake). This means that the layer in question is liquefied. Otherwise, if FS > 1, it means that the soil have sufficient resistance to hold the cyclic shear shaking from earthquake. We write this procedures in excel worksheet. Due to limitation of liquefaction possibilities at depth, and cementation to soil on deeper depth by soil ages, we delineate the analysis only from surface down to 9m depth.

Fig. 8. Correlation of FS (Safety) to lithology log.

5. Result and Discussion 5.1 Correlation with lithology log From several lithology logs, we choose logs with nearestposition to the MASW points. One of these log example can be seen in fig. 8 (point E24), located at Pundong Bantul. The area is covered by thin soil (1 meter), followed by silt-sand up to 8m deep, then sand up to 23m and followed by breccia. Unfortunately the lithology log was less in detail. Water table was measured at 3m depth. By defining the FS value, liquefaction might occur at just below water table (3m) down to 15 meter. This point also has a liquefaction report. This mean that probably the liquefaction shows in this area was triggered at 3m depth.

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Figure 9. Liquefaction occurrences depth based on VSprofiles. Red colour tends to liquefy and transparent tend to not liquefy for (a) at 0-1m, (b) at 1-2.3m, (c) at 2.3-3.7m, (d) at 3.7-5.3m, (e) at 5.3-7m and (f) at 7-8.9m

5.2 VS based on liquefaction depth The FS value of each MASW point can be used as indication of the liquefaction occurrences in adjacent depths. Map of VS-based liquefaction occurrence at every depth can be seen in Fig.9. It can be seen that there are no liquefaction occurred at depth 0-1m (see fig 9a). The liquefaction recorded seems are not occur from very near surface layer. It might be true because usually sediments transported by surface water are covered by silty or clayey sediments at their surfaces. The water level less than 1m depth are exist only on near Bantul town, especially at the south, east, and west parts. But the topmost VS are found relatively high in this area. High VS, shows top soil was tends to more rigid than the lower one. It will have higher shear modulus at the same density. At the level 1-2.3m depth from surface (Fig. 9b), the liquefaction occurs at three points around Bantul city. This data coincide with the recorded of liquefaction from previous papers. The depth of 1-2.3m was relatively critical for 1-2 storey

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building, because this depth level were used for base of foundation. This will be dangerous if foundations are laid on this liquefied potential layer. Liquefaction potential at 2.3-3.7m depth level (Fig. 9c) still seen at the south part of Bantul and Berbah subdistrict. At this depth level, liquefactions potential are likely still in a cluster mode, whereas only small area are probably liquefied when the earthquake struck. At more depth level from surface, from 3.7m down (See Fig 9d-f), almost Bantul area and vicinity has a potential to occur liquefaction. The potential are spread at west part, south, and north up to the Yogyakarta city. This potential are dangerous due to the increasing of more than 4 storey building (especially hotels) develop nowadays. At Bantul area, the liquefaction possibility are decreased at depth more than 7m depth. At the area with non-volcanic-clastic sediments, especially at the east part of this sediment,see Fig. 4, even in area where the fault is inferred, there were no liquefaction occurred. This phenomenon is caused by the Formation of highland which is dominated by massive limestone with the limited sand covered at the topmost soil. Also, the measured evidences from several researcher due to earthquake hazards are focused at volcanic-clastic area, where the huge damage exists. Conclusions At Bantul regency, there are high potential of liquefaction due to the very shallow water table, the existence of saturated sand and likely liquefy from grain size parameter, and near relatively large and shallow earthquake. By only 6.3Mw earthquake, at several places in volcanic-clastic area are reported occur liquefaction. No liquefiable potential found at depth of 0-1m, and at Bantul city, and there are found liquefiable potential at depth 1-2.3m in Bantul city. Due to existing reported data and information of liquefaction depth, it might be useful information for any construction especially whose foundations reach the liquefiable layers.

Acknowledgements We thank for valuable discussion to Dr.SubagyoPramumidjoyo and Dr.rer. nat. WiwitSuryanto. Also for students who contributing in data measurements, both water table and MASW campaigns. Thankyou for assisting the measurement instruments construction (data logger, cables, etc.) to Suparwoto, M.Sc and Yan Hendra.

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