ESTIMATION OF PEAK GROUND ACCELERATION MAP OF SABAH BASED ON DETERMINISTIC SEISMIC HAZARD ANALYSIS

Transcription

1 ESTIMATION OF PEAK GROUND ACCELERATION MAP OF SABAH BASED ON DETERMINISTIC SEISMIC HAZARD ANALYSIS Noor Sheena Herayani Binti Harith 1,, Azlan Adnan 1,*, Abdollah Vaez Shoushtari 1 1 Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310, Johor Bahru, Malaysia. School of Engineering and Information Technology, Universiti Malaysia Sabah, Kota Kinabalu, Sabah, Malaysia * Corresponding author: azelan_fka_utm@yahoo.com, (+6) , (+6) Abstract: Major natural hazards such as earthquake are the biggest threat for urbanisation. The consequences due to these events may significantly cost loss of life and money. East Malaysia has known as a low to moderate seismicity region as the most of the active seismic sources are far from the region. However, large magnitude earthquakes, occurring several hundred kilometres away, are capable of causing extensive damages due to amplification of seismic waves by the soft soil on the ground surface. The state of Sabah could be affected by tremors originating from large earthquakes of Southern Philippines, Sulu and Northern Sulawesi. Thus, it is reasonable to conduct with the seismic hazard of the region to apply appropriate seismic designs for the existing and future structures. In this study the peak ground acceleration (PGA) map on rock site of Sabah due to strike slip and subduction faults located around the region are estimated using the deterministic method. The selection of appropriate ground motion attenuation laws is the key factor that could affect the results of the analysis to achieve more reliable PGA values due to both type of faults. The trends of PGA reflected by contours map shows the values are within a range of 0 to 100 cm/s. Keywords: Sabah, deterministic, strike slip, subduction 1.0 Introduction Sabah is located on the Island of Borneo is at the middle of the Indonesian Archipelago bordered by Brunei and Sarawak to the West and Kalimantan on its southern part. As referring to the seismic database history in Sabah, the earthquake situation that occurs can be categorized as low-to-moderate earthquake. In particular, southern Sabah has been most affected by earthquakes in past due to surrounding area is being affected by low seismic active level from the Sunda tectonic plate and moderately active seismic level from Kalimantan and Sulawesi. Sabah is within the location at remote distance from tectonic plate boundaries, however surrounds by major active faults and subduction. The ground shaking caused by earthquakes and attenuates with further distance provide damage in much greater causes for most moderate seismic regions than in high seismic regions (Nordenson and Bell, 000). This region recently has local faults that capable of producing earthquake, unfortunately the source is less well understood. Therefore, the hazards assessments are more difficult to conduct. In other studies stated that, the rate of earthquake is increasing from past few decades might due to building development and number of people escalating throughout the year (Pan and Megawati, 00) and (Nordenson and Bell, 000). In order to predict the size of future earthquake, analysis on seismic activity for past few decades is need to be further studied by performing seismic hazard analysis to predict earthquake activities.

2 Seismic hazard analysis (SHA) is the most widely used approach for estimating seismic-design loads by knowing the characteristics of ground motions related to earthquakes at a site that can potentially affect the structure. In recent past, the hazard analysis towards lowto-moderate seismicity region has been in the subject by numerous studies. In one study by (Anbazhagan et al., 009) investigated on hazard analysis of south India that once predominantly considered as stable and seismic shield region. Gaspar (007) also performed study on the low-to-moderate seismicity region in Spain by doing theoretical model from other region. SHA study for low-to-moderate seismic also been study by several researcher such as (Pan and Megawati, 00), (de Vos, 010) and (Kumar et al., 01). It is important to perform the seismic hazard analysis since it has been a concern by few researchers (Grünthal and Bosse, 1996); (Nordenson and Bell, 000) and (Kracke and Heinrich, 004) which they mentioned on the risk that may higher in developing countries with low to moderate seismicity region..0 Deterministic Seismic Hazard Analysis (DSHA) Seismic hazard analysis is the process to evaluate the design parameters of earthquake ground motion at the particular site, quantitatively. The ground motion parameters that become consideration in this analysis is peak ground acceleration. Generally, there are two methods to conduct seismic hazard analysis, i.e. deterministic seismic hazard analysis (DSHA) and probabilistic seismic hazard analysis (PSHA). DSHA procedure tends to be conservative where the process uses the known or assumption seismic sources that capable of generating earthquake sufficiently near the site. Typically one or more earthquakes are specified and estimated deterministically by magnitude and location which determines earthquake that have the most significant impacts. DSHA focus is generally on determining the maximum credible earthquake (MCE) motion at the site while PSHA combines the probabilistic characteristics (i.e. location, magnitude, attenuation relationship) of a suite of earthquakes (Huang and Wang, 01). Deterministic events can be compared with a probabilistic analysis to ensure that the event is reliable (McGuire, 001). The disadvantages of PSHA is the requirement of comprehensive databases such as seismicity, geology, tectonic environment as well as attenuation function characteristics with many earthquakes. On the other hand, DSHA considers just one maximum magnitude-distance have the most significant impacts (A. Deif et al., 013). Present study, the DSHA approach is carried out to evaluate the seismic hazard.

3 DSHA preceded PSHA as the prevalent form of hazard assessment for maximum (worst case) earthquake shaking. It involves development of a seismic scenario and characterization of that scenario. The scenario consists of the postulated occurrence of an earthquake of specified size occurring at a specified location. The analysis of MCE by deterministic methods can be illustrated in Figure 1. Figure 1: Flow chart of deterministic analysis The first step in deterministic analysis is to collect the input data and dividing the seismotectonic model into seismotectonic provinces corresponding to zones of diffuse seismicity and seismogenic structures. After that, identifying the maximum potential earthquake associated with closest distance. Lastly, compute the design with an appropriate attenuation relation should be used to determine the ground motion that each of these earthquakes would cause at the site. The description of each steps can be seen in the following topics..1 Earthquake Catalogue The geometrics of the seismotectonic model were constructed based on earthquake spatial distributions and regional tectonic setting of Sabah. The historical earthquake data were compiled from local and international institutions such as MMD, USGS, ISC and individual catalogues such as (Pacheco and Sykes, 199). The seismicity data catalogues of earthquake

4 epicentre cover areas from E to 15 0 E longitude and from 10 0 S to 10 0 N latitude (Figure ). The catalogues include more than 3000 earthquake events occurred during 113 years period of observation ( ) having an assigned magnitude of more or equal to MW 5.0. Figure : Distribution of epicenters around East Malaysia According to the Map published by Mineral and Geoscience Department Malaysia (MGDM, 006), the description of observed damages around the proposed location ranges VII on MMI scale (Figure 3). The range of intensity approximately correlate with peak ground acceleration in the range of 0.15 g (150 cm/s ) to 0.16 g (160 cm/s ). However, the MMI scale was increased to VIII of MMI scale where the PGA predicted between 340 to 650 cm/s as illustrated in Figure 4.

5 Figure 3: Modified Mercalli intensities in Malaysia. Source: Malaysian Meteorological Department in (Yan and Saim Suratman, 006) Figure 4: Maximum Modified Intensity (MMI scale) of Sabah (MMD, 007). Seismic sources of possible impact to Sabah The definition of seismic sources involves the parameterized of earthquake from past seismicity at their preferred location (Reiter, 1991). In this stage, seismic source zones are

6 identified including all potential seismic sources capable of generating significant ground motion at the site. A seismic source represents a region of the earth s crust where the characteristics of earthquake activity are recognized to be relatively different than those of the adjacent crust. Fault sources are modelled in the hazard analysis in terms of geometry, maximum magnitude, and earthquake recurrence. The geometric source parameters include fault location, segmentation model, dip, and depth of the seismogenic zone. The source geometries were constructed based on earthquake spatial distributions and regional tectonic setting of Sabah. Generally, the seismic source model can be divided into two classifications, i.e. North Sulawesi subduction zone and shallow crustal zone. The seismic source models are divided into 15 shallow crustal fault sources (Figure 5). The fault sources are North Sulawesi Subduction zone (Megathrust and Benioff), Palu-Koro (zone 1,, and 3), Tarakan Basin, Kutai, Melange (zone 1and ), Mamuju, Matano and Sangkulirang. The fault source zones within radius of more than 00 km from Sabah are considered in the analysis. Figure 5: Seismic Source Models of East Malaysia

7 This model is different from a seismotectonic model proposed by (Gibson, 003). (Gibson, 003) used a very simple seismotectonic model with a single source zone covering most of Borneo and a second zone covering the more active area from East Malaysia to the south along the east coast of Borneo. This model does not include any specific active faults. The maximum and minimum magnitude of each source zone can be given in Table 1. Table 1: Magnitude details for shallow crustal and subduction zone Segments Magnitude Max Mw Min Mw Tarakan Kutai Walanae Walanae Walanae Melange Melange Palu Koro Palu Koro Palu Koro Mamuju Matano Sangkulirang Sangkulirang Megatrust Benioff Attenuation Function Relationship The seismic hazard at a site needs attenuation function that probable of capturing the various ground motion parameters with distance, earthquake size and the geological conditions. However, the scarcity and lack of attenuation function equation of Sabah makes it a must to apply already-developed attenuation that able to represent characteristics of hazard in the region. The attenuation function are selected to predict the seismic hazard within the various considered tectonic environments in order to account for the epistemic uncertainty. There has been a number of attenuation relations derived in the last two decades since the record of ground motions becomes more available. In general, they are categorized according to tectonic environment (i.e. subduction zone and shallow crustal earthquakes) and site condition. There

8 are several attenuation relationships derived for subduction zone earthquake, and shallow crustal earthquakes that summarized by (Douglas, 004, Douglas et al., 010, Douglas, 011) that developed from various countries with different type of parameters used. Most of the attenuation functions were developed using empirical method. Therefore, the limitation of the function will depend on the quality of strong motion data such as quantity and the distribution of parameters of attenuation function such as magnitude, depth, distance and peak acceleration. Usually attenuation relationships are derived for near source earthquakes; consequently, most of the attenuation relationships have a distance limit. Three different ground motion prediction relationships are selected for this study. The deterministic hazard and its associated uncertainty for Sabah is assessed using the method proposed by (Campbell, 003), (Zhao et al., 006) and (Atkinson, 003). These relationships have been widely used in the seismic hazard assessment from all over the world. These attenuation functions were used due to the criteria that specifies the condition in Sabah. The three attenuation function are dominated by the rock site condition with a magnitude type of moment magnitude (Mw). The limitation in terms of distance and range of magnitude are described in the following Table. Logic trees are used as a tool to capture the epistemic uncertainty in estimating the hazard (Grünthal and Wahlström, 001, Bommer et al., 005, Sabetta et al., 005, Scherbaum et al., 005). Epistemic uncertainty in hazard definition has been tackled to select and rank models by weights within a logic-tree framework considering three attenuation models for each fault. Table : Selected Attenuation Function Parameters No. Model Name Magnitude Range (MW) Distance Range (km) Weight Background Seismicity and Shallow Crustal 1 Campbell (003) Subduction Zhao et al. (006) Atkinson (003) Campbell (003) The attenuations proposed by Campbell (003) was used for ground motions from shallow crustal earthquake events. The Campbell s attenuation was derived using a hybrid method to develop ground motion relations for eastern North America (ENA), for rock sites. Attenuation relation for rock is given as follows:

9 lny c1 cmw c3(8.5 M f ( r ) ( c c M 9 10 W W ) ) r c 4 ln f ( M 1 W, r ) (3.) c exp( c M 1 ( MW, r ) r 5 6 W ) f f ( r ) c 7 (lnr c (lnr 7 lnr ) c 1 0 lnr ) 8 1 (lnr lnr ) r r 1 r r r 1 r r Y is the geometric mean of the two horizontal components of PGA or PSA in g, Mw is moment magnitude, r is closest distance to fault ture in km, r1 = 70 km, and r = 130 km, and C1 to C10 are the coefficient used to Campbell (003). The equation has been extended to large distance using stochastic ground motion estimates..3. Zhao et al. (006) The attenuation function was developed using strong ground motion in Japan and was done for analysis on interface and in-slab events. The magnitude varies between Mw 5.0 and 8.0 with the focal depth distribution ranges between 15 and 15 km. The equation considers five site classes including Hard rock, rock, Hard soil, Medium soil and Soft soil. The model can be expressed as below; Log ( y e i, j ) am wi bxi, j Log e ( ri, j ) e( h hc ) h FR S I SS SSLLog e ( xi, j ) C k i, j i r x cexp( dm i, j i, j wi ) where y is geometric mean of two horizontal component in cm/s, Mw is moment magnitude, x is the source distance (km), h is focal depth, FR is reverse fault parameter for crustal events only, SI is for interface events only, Ss for slab events, SSL is a magnitude dependent path modification term for slab events, Ck is site class, hc is the depth constant (15 km), δh is dummy variable, and i denotes event number and j denotes record number from event i.

10 .3.3 Atkinson (003) This attenuation model was derived for subduction zone considering different types of NEHRP site class. The classification for subduction mechanism is based on focal mechanism where intraslab is consider when the depths is > 50km and interface for less than 50 km. The relations used a broad range of magnitudes and distances. Attenuation function model is logy c 0.507M 0.507M 1 cm c3h c4 D fault ( ) g log D fault ( ) c5slsc c6slsd c7 sls E where M is moment magnitude, h is focal depth, D is distance at fault ture, the value for SC, SD and SE for site class B is equal to 0 and c1 to c7 are the coefficient. 3.0 Seismic Hazard Map By estimating the parameter describing by a variety of parameters of ground shaking, zoning maps can be developed by contouring Sabah region which is useful for city planning development (Figure 6). The production of seismic hazard maps can be considered as the first step towards taking counter measures against earthquake threat. The aim of such maps is to show the variation in seismic hazard within a region and to provide guidance as to the expected levels of ground motion. The ground motion parameters that become consideration in current study is peak ground acceleration since it have been popular instrumental measurements of ground motion. The DSHA maps delineate the relatively higher seismic hazard regions at the southern part of Sabah compared from the remaining regions that relatively lower hazard levels. The mean PGA across Sabah ranges from 0 cm/s in the North and South parts up to about 100 cm/s.

11 Figure 6: Seismic hazard map of Sabah due to subduction and shallow crustal faults 4.0 Discussion The maximum value of ground motion parameter resulting from all considered magnitude and distance was finally assigned to the site. The MCE is the largest earthquake that appears to cause the most severe consequences at the site. Shortest distance from source to Sabah region has been measured from the seismotectonic map. The deterministic seismic hazard maps with controlling distance and moment magnitude, the peak ground acceleration is calculated at bedrock level. The total of 15 sources have generated PGA values shows that least of PGA values is 0 cm/s where large PGA value is caused from South and East of Sabah with 100 cm/s. The comparison of present PGA values with those of (MGDM, 006), indicates that the results of the deterministic seismic hazard analysis for most locations are lower than the MMI presented above. The result to the East of Sabah shows significant different where this study produced at about 80 to 100 cm/s whereas the MMI scale shown double of current values which are 340 to 650 cm/s. The zonation to the West of Sabah showed the result agree fairly well with PGA in current study that between 30 to 100 cm/s. The differences of results in some places around Sabah is expected since it is probably due to the use of another zonation model.

12 5.0 Conclusion Seismic hazard analysis of Sabah based on deterministic approach has been performed using different type of sources and attenuation model so that future maximum earthquakes can be predicted. The currently available database was used in the interpretation by considering logic tree to explicitly account for epistemic uncertainties. The representing contour hazard map shows that the seismic hazard is within low-to-moderate earthquake from North to South of the region. The ground motion predicted from the present study provide the engineers and planners with complete information in safe design of structures. This is due to if a failure occurs on an important structures such as large industrial projects, the consequences will be very serious and dramatic to human and economic. Therefore, the effects of the earthquakes should be anticipated in order to mitigate the catastrophic failure of the structure. References A. DEIF, I. EL-HUSSAIN, K. AL-JABRI, N. TOKSOZ, S. EL-HADY, S. AL-HASHMI, K. AL-TOUBI, Y. AL-SHIJBI & AL-SAIFI, M Deterministic seismic hazard assessment for Sultanate of Oman. Arabian Journal of Geosciences, 6, ANBAZHAGAN, P., VINOD, J. S. & SITHARAM, T. G Probabilistic seismic hazard analysis for Bangalore. Natural Hazards, 48, ATKINSON, G. M Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions. Bulletin of the Seismological Society of America, 93, BOMMER, J. J., SCHERBAUM, F., BUNGUM, H., COTTON, F., SABETTA, F. & ABRAHAMSON, N. A On the use of logic trees for ground-motion prediction equations in seismic-hazard analysis. Bulletin of the Seismological Society of America, 95, CAMPBELL, K. W Prediction of Strong Ground Motion Using the Hybrid Empirical Method and Its Use in the Development of Ground-Motion (Attenuation) Relations in Eastern North America. Bulletin of the Seismological Society of America, 93, DE VOS, D Probabilistic Seismic Hazard Assessment for the Southern part of the Netherlands. DOUGLAS, J Ground motion estimation equations London: Imperial College. DOUGLAS, J Ground-motion prediction equations DOUGLAS, J., FACCIOLI, E., COTTON, F. & CAUZZI, C Selection of ground-motion prediction equations for GEM1. GEM Technical Report 010-E1, GEM Foundation, Pavia, Italy. www. globalquakemodel. org. GIBSON, G Bakun Hydroelectric Project Seismic Confirmatory Study. Report No.: R-801C-SS Seismological Research Center. GRÜNTHAL, G. & BOSSE, C Seismic hazard assessment for low-seismicity areas Case study: Northern Germany. Natural Hazards, 14, GRÜNTHAL, G. & WAHLSTRÖM, R Sensitivity of parameters for probabilistic seismic hazard analysis using a logic tree approach. Journal of Earthquake Engineering, 5, HUANG, D. & WANG, J.-P. 01. Seismic hazard map around Taiwan through a catalog-based deterministic approach. 15th World Conference on Earthquake Engineering. Lisbon, Portugal. KRACKE, D. W. & HEINRICH, R Local seismic hazard assessment in areas of weak to moderate seismicity case study from Eastern Germany. Tectonophysics, 390,

13 KUMAR, B. L., RAO, G. R. & RAO, K. S. 01. Seismic Hazard Analysis of Low Seismic Regions, Visakhapatnam: Probabilistic Approach. J. Ind. Geophys. Union (January 01), 16, MCGUIRE, R. K Deterministic vs. probabilistic earthquake hazards and risks. Soil Dynamics and Earthquake Engineering, 1, NORDENSON, G. J. & BELL, G. R Seismic design requirements for regions of moderate seismicity. Earthquake Spectra, 16, PACHECO, J. F. & SYKES, L. R Seismic moment catalog of Large Shallow Earthquakes, 1900 to Bulletin of the Seismological Society of America, 8, PAN, T.-C. & MEGAWATI, K. 00. Estimation of peak ground accelerations of the Malay Peninsula due to distant Sumatra earthquakes. Bulletin of the Seismological Society of America, 9, REITER, L Earthquake Hazard Analysis: Issues and Insights. New York, NY: Columbia University Press. SABETTA, F., LUCANTONI, A., BUNGUM, H. & BOMMER, J. J Sensitivity of PSHA results to ground motion prediction relations and logic-tree weights. Soil Dynamics and Earthquake Engineering, 5, SCHERBAUM, F., BOMMER, J. J., BUNGUM, H., COTTON, F. & ABRAHAMSON, N. A Composite ground-motion models and logic trees: methodology, sensitivities, and uncertainties. Bulletin of the Seismological Society of America, 95, YAN, A. S. W. & SAIM SURATMAN, A. L Study on the Seismic and Tsunami Hazards and Risks in Malaysia. Report on the Geological and Seismotectonic Information of Malaysia, Mineral and Geoscience Department Malaysia ZHAO, J. X., ZHANG, J., ASANO, A., OHNO, Y., OOUCHI, T., TAKAHASHI, T., OGAWA, H., IRIKURA, K., THIO, H. K. & SOMERVILLE, P. G Attenuation relations of strong ground motion in Japan using site classification based on predominant period. Bulletin of the Seismological Society of America, 96, WEBSITE: International Seismological Center (ISC), United Kingdom. Website: Laman Web Rasmi Jabatan Mineral dan Geosains Malaysia (JMG). Website: Malaysian Meteorological Department (MMD). Website: National Earthquake Information Center (NEIC). Website: United State Geological Survey (USGS). Website: