SEISMIC HAZARD ANALYSIS AND SEISMIC INPUT TO TSUNAMI MODELING FOR MICROZONATION OF MEURAXA DISTRICT CITY OF BANDA ACEH

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1 SEISMIC HAZARD ANALYSIS AND SEISMIC INPUT TO TSUNAMI MODELING FOR MICROZONATION OF MEURAXA DISTRICT CITY OF BANDA ACEH I W. Sengara 1, Hendarto 1, P. Sumiartha 1, H. Latief 1, S.B. Kusuma 1 and Munirwansjah 2 1 Center for Disaster Mitigation, Institut Teknologi Bandung, Jl. Ganesha 10, Bandung-40132, Indonesia, iws@geotech.pauir.itb.ac.id 2 Civil Engineering Department, University of Syiah Kuala-Banda Aceh Jl. T Syech Abdul Rauf 7, Banda Aceh, Indonesia ABSTRACT: Earthquake and tsunami disaster of Aceh, December 2004 has caused hundreds of thousands casualties and catastrophic damages to coastal area of city of Banda Aceh. Rehabilitation, reconstruction, and long-term development require seismic and tsunami hazard potential to be used as criteria of risk mitigation strategy from future disaster. This paper presents probabilistic seismic hazard analysis (PSHA) and input to tsunami modeling for microzonation of Meuraxa District, city of Banda Aceh. The PSHA considers both subduction and Sumatra Fault Zones (SFZ). The seismic hazard analysis considering recent geological and seismological data employing 3D sourze zones indicated that the peak baserock acceleration is 0.3g for 475 years return period. De-aggregation analysis indicated that the SFZ is the controlling earthquake under this return period. This level of acceleration is used as a basis for seismic microzonation of the district. Siteresponse analysis for hard, medium, and soft site-class has been conducted and design spectra is recommended. The PSHA was further modified to consisting of subduction seismic source zone only. This hazard analysis was conducted to provide earthquake generation input to tsunami wave propagation modeling. Earthquake generated tsunami hazard curve is developed to correlate earthquake magnitude and associated return periods. Tsunami wave propagation model was then conducted to correlate the earthquake magnitude and tsunami height. The seismic and tsunami microzonation maps are recommended as a basis for disaster mitigation effort in the currently undergoing rehabilitation and reconstruction and future development of the district. 1. INTRODUCTION More than two hundred thousands people had loss their life and many buildings, infrastructures, and lifelines have been completely destroyed due to Aceh December 2004 earthquake (Mw=9.3) and tsunami. Coastal areas of City of Banda Aceh have experienced catastrophic damages due to the tsunami and earthquake. Almost all the residential houses in the coastal area were destroyed by the tsunami. The earthquake had caused many buildings completely collapse and some buildings experienced only slight damage. Moreover, there has been a spatial distribution of damage observed and this is considered due to combination of both distribution of ground shaking contributed by variation in local soil condition and also could be due to the vulnerability of different buildings under different types and building construction qualities. Many roads and embankments were identified to have collapsed and damaged due to liquefaction and lateral spread. Some geotechnical engineering aspects related to Aceh s tsunami and earthquake disaster is presented in Sengara et al., Rehabilitation and reconstruction is currently still undergoing, consisting of construction of thousands of residential houses and many infrastructures and lifelines as well as some critical facilities. Both seismic and tsunami hazard criteria is essential as a basis for rehabilitation, reconstruction, and long-term development of the city. 2. PROBABILISTIC SEISMIC HAZARD ANALYSIS Many seismic building codes including Indonesian seismic building codes suggest its seismic design response spectra be based on probabilistic seismic hazard approach. Most current Back to Table of Contents 299

2 international seismic building codes is still based on seismic hazard criteria of 475 years return period which is equivalent to 10 probability of being exceeded in 50 years. This level of probability is developed from a basic concept that earthquake resistant design of buildings or infrastructures is to provide protection to prevent loss of human life and personal injury in particular, in addition to minimize damage to property and to ensure that vital services such as lifelines and infrastructures are maintained in the event of earthquake. PSHA for city of Banda Aceh has been conducted herein to recommend the level of peak baserock acceleration (PBA). The PSHA considers all earthquake sources within radius of 500 km from city of Banda Aceh. The PSHA also resulted in the associated uniform hazard spectra at the reference base-rock (site-class S B that represents geotechnical profile with shear wave velocity of a value higher than 720 m/s). PSHA methodology as used in Sengara et al. (2008) using EZ-FRISK computer program (Risk Engineering, 2004), that considers 3-dimensional seismic source zones is used herein to estimate peak acceleration and response spectra at baserock in this case study for city of Banda Aceh. Subduction earthquake sources characterization consists of separate identification of megathrust and benioff seismic source zones. In addition, shallow crustal seismic source zone considered to affect the site have also been identified. Recurrence models for each zone and appropriate attenuation functions are adopted in the PSHA 2.1 Tectonic Setting and Seismic Source Model There have been many large earthquakes reported and recorded originated from subduction source zone along south-west of Sumatra Island. Many earthquakes occurred along megathrust of the subduction zone and the rate of this subduction is estimated about mm/year. The latest large earthquake originated from the megathrust was the Great Sumatra 2004 earthquake. Some information on this tectonic setting and earthquake history of this subduction zone is elaborated in Sieh and Natawidjaja (2000) and Kertapati (1999). Due to the in-perpendicular movement direction of Indo-Australian plate to the basin structure then Sumatra Fault Zone (SFZ) was formed along the Sumatra Island. The SFZ activities were indicated by occurrence of earthquakes at shallow depths (< 60 km) along the faults. Compared to subduction earthquake activities, the SFZ activities activity triggered relatively smaller earthquakes. However, the earthquake hypocenters from this SFZ were shallower (usually less than 30 km), therefore earthquakes of the SFZ has often caused large earthquake. Aceh and Simeuleum fault segments are part of the SFZ that are only a few kilometers distance from city of Banda Aceh. Therefore, this shallow crustals are the main faults that is considered potential to generate large earthquakes in the future, since the subduction earthquake would need much longer time to accumulate its energy post 2004 earthquake for another large earthquake. The subduction earthquakes are modeled by area sources with segments in conjunction with the geological input. Likewise, the SFZ earthquakes are modeled by thin area sources with segments also in conjunction with the geological input that referred from Sieh and Natawidjaja (2000) and Natawidjaja (2002). Figure 1 presents seismic source model for the PSHA. Each seismic source zone is assigned seismic parameters representing seismic characteristics of the source. 2.2 Attenuation Functions and Earthquake Recurrence Model Result of PSHA and uniform hazard spectra (UHS) would be significantly affected by attenuation functions adopted in the analysis. Therefore, appropriate attenuation functions need to be used. Since there is no attenuation functions specifically developed yet representing the earthquake characteristics in the region, then available Young s et al. (1997) attenuation function has been adopted to represent the subduction earthquake source. While, Sadigh et al. (1997) and Idriss (2004) attenuation functions have been adopted for shallow crustal seismic sources. Attenuation functions of Sadigh et al. (1997) and Idriss (2004) are assigned a relative likelihood of 0.5, respectively. Recurrence model adopted in the PSHA is in the form of seismic parameters for each seismic source zone. The seismic parameters from use of the recurrence model for each source zone are obtained from both instrumental and historical data, as well as information on its maximum Back to Table of Contents 300

3 moment magnitude (M w ). Maximum magnitude for each source zone is assigned based on both historical and potential magnitude estimated from geometry and slip rate of the plates or faults Banda Aceh Meulaboh edan Pekanbaru Siemelue Nias P_Siantar M Seismicity Source Zoning N E W S Coast Indonesia Volcano_cone Lake Caldera Fold Sum fault_s Sum fault Andaman_f Mentawai_f Zm-CentralJava Zm-West Java Zb-Nias Zm-Nias Zb-AcehSiemelue Zm-AcehSiemelue Zb-Andaman Zm-Andaman Trench Toru Renun Tripa Aceh-seulimeum Enghdal et.al, ML Seismicity Area City Figure 1 Seismic source zoning representing both subduction and shallow crustal faults Maximum magnitude (M max ) assigned for subduction zone is 9.3. Meanwhile, Aceh and Seumeleum faults are assigned a value from its potential to generate maximum magnitude of 7.7 and 7.5, respectively (Natawidjaja, 2002). In this PSHA, earthquake recurrence model parameters have been calculated based on seismicity and slip data to be used in exponential and characteristic recurrence models. There has been limited information and study conducted on two faults (Aceh and Simeuleum faults) that is relatively very close to the city of Banda Aceh. Available information on slip rate of Aceh fault is 5mm/year that was based on GPS monitoring (Genrich et al., 2000) and southern segments of SFZ which has slip rate of 27mm/year (Bellier and Sabrie, 1995). With some of the information that has been collected on the slip-rate, including (Natawidjaja, 2006), there is a tendency of reduced slip-rate to the north to be approximately in the order of 5-11mm/year. There is actually no specific information on slip-rate of the Simeuleum fault. 3. RESULT OF PSHA The PSHA that has been conducted herein has resulted in hazard curve that provides the level of PBA with various probability levels. Results of PSHA provide PBA of 0.18, 0.3, and 0.41g for 200, 475, and 1000 years earthquake return period, respectively. Hazard level recommended is probability hazard level of 475 years earthquake return period, as commonly adopted in many building codes and currently still adopted in Indonesian seismic building codes (SNI ), therefore, PBA recommended for city of Banda Aceh is 0.3g. UHS resulted from the PSHA is presented in Figure 2. This UHS is adopted as target spectra for scaling input ground motion for subduction or shallow crustal earthquake in site-response analysis. It is essential to identify the potential controlling earthquake hazed in the PSHA, therefore deaggregation analysis has been conducted. The de-aggregation analysis determines the dominant earthquake magnitudes and distances. It is indicated in the analysis that for 475 years return period earthquake, the dominant event is originated from shallow crustal SFZ (Aceh and Simeuleum faults), and also from subduction megathrust segment, as presented in Figure 3. There has been sensitivity analysis conducted for the Aceh and Simeuleum shallow crustal faults, providing different slip rates adopted in the PSHA. For this current analysis result, slip rate of 5mm/year for Aceh fault and 11mm/year for Simeuleum fault have been adopted. It is identified that result of PSHA is sensitive to the slip rate adopted in the analysis. Since there are some uncertainties in seismic parameters, particularly for Aceh and Simeuleum fault segments that are still need to be verified, characteristics of these segments still need further detail investigation. Back to Table of Contents 301

4 4. SITE-RESPONSE AND DESIGN SPECTRA Site-response analysis to estimate peak ground surface acceleration and response spectra needs to be performed by considering appropriate input motions and dynamic soil properties of the site. In Target Spectra Mean UHS 475 years RP PBA (g) Period (second) Figure 2 Uniform Hazard Spectra for 475 years return period earthquake Magnitude-Distance Deaggregation Figure 3 De-aggregation curve indicating dominant earthquake that contribute to the seismic hazard the case no strong motion data were available; the simplest and conventional method to generate input motions by scaling available strong motion records from other sites. Strong motion records are commonly scaled to match target PBA of the site of interest. Another method is development of synthetic time histories to match target spectral acceleration through generation of synthetic input motion or through spectral-matching techniques as adopted in the computer program EZFRISK. In this case, spectral-matching techniques to the target spectra are adopted to generate earthquake input motions for the wave propagation analysis from referenced base-rock to the ground surface. For site response analysis, six input motions that represent subduction and shallow crustal fault earthquakes three scaled to to 0.2 second and three scaled to 2 second were generated and then used in site-respons analysis through seismic wave propagation analysis from baserock to the ground surface. Local site condition (site-class) is classified into 3 (three) classifications that is Hard, Medium, and Soft. Each site-class is represented by assigning a soil profile having a value of average shear wave velocity (Vs) in accordance with Indonesian Building Codes, Uniform Building Codes (UBC97), or International Building Codes (IBC2006). Back to Table of Contents 302

5 Results of the wave propagation analysis using NERA computer program (Bardet and Tobita, 2001) shows that there has been significantly different response spectra resulted from each of the soil column representing different shear wave velocity of each site class, as shown in Figure 4 (ac). The approximate design spectra is developed for recommendation to the design of buildings and infrastructures to be reconstructed and developed in the near future for the city of Banda Aceh. Please again note that this response spectra is affected by the Aceh and Simeuleum fault characteristics and the seismic hazard is closely associated with this faults. Futher investigation of this faults and its effect on the seismic hazards is recommended Spectra Acceleration (g) Period ( secon d) Period (sec) (a) (b) Spectral Acceleration (g) Period (second) (c) Figure 4 Result of SSRA for site class (a) Hard; (b) Medium; and (c) Soft 5. PROBABILISTIC TSUNAMI HAZARD ASSESSMENT Rehabilitation, reconstruction, long-term development, and related disaster mitigation strategies of the coastlines destroyed by the great Sumatra 2004 tsunami, demand scientific and engineering input and criteria of future tsunami hazard potential. Tsunami hazard potential post the great 2004 Sumatra earthquake was evaluated herein to recommend tsunami hazard criteria. The earthquake generated tsunami potential to city of Banda Aceh is the subduction zones. This subduction zones are the same source that was used for the PSHA. Therefore, for tsunami hazard potential the subduction source zones and seismic parameters previously used for the PSHA were used for probabilistic tsunami hazard analysis (PTHA). The analysis was made by activating only the subduction seismic source zones (Aceh-Seumelue-Andaman segments). The analysis was done using EZ-FRISK computer program to provide hazard curve correlating earthquake return periods and the potential moment magnitudes considering the seismic parameters adopted in the recurrence models. The analysis was conducted by Sengara and Hendarto (2006). Result of the analysis is presented in Figure 5. It is indicated that, the 2004 earthquake is approximately predicted to be of 520 years return period earthquake. Back to Table of Contents 303

6 Based on seismic hazard curve of Figure 5, several earthquake magnitude scenarios associated with its return periods were developed and these scenarios were adopted for earthquake generating tsunami wave propagation and inundation modeling. 6. TSUNAMI WAVE PROPAGATION ANALYSIS The result of PTHA was used as an input in tsunami wave propagation modeling by Dr. H. Latief, Dr. Kusuma, and team (in Sengara et al., 2006a) to estimate the tsunami height at the shoreline of the site of interest. The earthquake generating tsunami hazard curve (Figure 6) is adopted and five different earthquake scenarios were developed for tsunami modeling. Each scenario with associated earthquake moment magnitude and return period will correspond to particular potential of sea-bed deformation as the tsunami generation input. Results of the tsunami modeling for various scenarios correlating the return period or moment magnitude potential and maximum tsunami height at coastline of the case-study site is summarized in Table 1. Figure 5 Hazard curve correlating earthquake return period, peak acceleration, and earthquake moment magnitude considering Aceh-Seumelue-Andaman segments (Sengara and Hendarto, 2006). Table 1. Summary on results of tsunami wave propagation modeling at Meuraxa District. Scenario Moment Magnitude Return Period (years) Tsunami Height at Case-Study Site (m) CONCLUSIONS AND RECOMMENDATIONS Rehabilitation, reconstruction, long-term development, and disaster risk mitigation strategies of city of Banda Aceh and coastlines of Aceh province demand engineering input as a basis for seismic and tsunami hazard criteria. Probabilistic seismic and tsunami hazard analysis that has been conducted based upon the up-to-date available information and methodologies provide some recommendations on the engineering input as hazard criteria. PSHA for Banda Aceh indicated that PBA of approximately 0.3g is potential to occur in Banda Aceh for probability level of 475 years earthquake return period. This level of PBA is dominant to be originated from shallow crustal fault (Aceh and Siemeulem faults) potential which is of relatively very close distance from the city. Recommendation on design response spectra for three site-class has also been developed. For new construction of buildings and infrastructures in the city, geotechnical site characterization needs to be conducted and site with different local geotechnical conditions should be analyzed and appropriate design spectra need to be used. Since there is limited information and study of the Aceh and Simeuleum faults and there is some uncertainties associated with the seismic parameters Back to Table of Contents 304

7 to the corresponding hazards, then in long-term, further detailed investigation on this faults need to be conducted. Tsunami hazard potential associated with its probabilities and its tsunami height has been developed. This tsunami hazard potential has been developed based on probabilistic hazard assessment with scenario on probability of earthquake generated tsunami potential, that is correlating subduction earthquake magnitudes with return periods and eventually to tsunami height at the coastal area. Results of the probabilistic tsunami hazard analysis provides recommendations that could be used as a basis and criteria for development of new construction and related risk mitigation strategies in the coastal area, particularly in the Meuraxa District of city of Banda Aceh. Probabilistic seismic hazard analysis (PSHA) considering both subduction and SFZ earthquake sources has been conducted. The PSHA indicated that PBA of approximately 0.3g is potential to occur in Banda Aceh for probability level of 475 years return period earthquake. De-agregation analysis shows that this PBA is dominant to be originated from SFZ that is of relatively very close distance from the city. This value of PBA is recommended as criteria for buildings and infrastructures rehabilitation and reconstruction for city of Banda Aceh. The PSHA also provides target spectra further adopted to develop ground motion for both subduction and SFZ earthquakes. The earthquake ground motions were developed by spectralmatching techniques of available strong motion records scaled to the target spectra. Then, frequency-domain site-response analysis, for various site-class or local ground conditions, has been conducted. Finally, design response spectra for various site-classes have been developed as recommendation for rehabilitation and reconstruction for city of Banda Aceh. The methodology adopted in this paper could be used to develop seismic design criteria for other cities and areas as an input to the Indonesian seismic building codes. Since there are some seismic parameters, particularly for Aceh and Simeuleum fault segments, which are still need to be verified, then further investigation of this segments still need to be conducted. 8. ACKNOWLEDGEMENT The authors highly appreciate financial support provided by Rehabilitation and Reconstruction Agency of NAD-Nias and Indonesian Ministry of Research and Technology for this seismic and tsunami hazard analysis. The analysis was conducted at Center for Disaster Mitigation - Institut Teknologi Bandung (CDM-ITB). The authors also thanks the support provided by research assistants and staf at CDM-ITB. Contribution by Dr. Natawidjaja is appreciated for input on geologic setting provided for the analysis. 9. REFERENCES Bardet, J.P. and Tobita, T. (2001). NERA, A Computer Program for Nonlinear Earthquake Site Response Analyses pf layered Soil Deposits, University of Southern California. Indonesian Seismic Building Codes, SNI (2002). Indonesian Department of Public Work. Kertapati, E.K. (1999). Probabilistic estimates of the seismic ground motion hazard in Indonesia, Proceeding of National Conference on Earthquake Engineering, a Conference organized by Indonesian Earthquake Engineering Association, Bandung. Natawidjaja, D.H. (2002). Ph.D Thesis, California Institute of Technology. Natawidjaja, D.H. (2006). Personal Communication. Risk Engineering, Inc., (2004). EZFRISK, Software for In-depth Seismic Hazard Analysis, Boulder, Colorado, USA. Sadigh, K. et al. (1997). Attenuation relations for shallow crustal earthquakes based on california strong motion data, Seismological Research Letters, Seismological Society of America, Volume 68, Number 1, January/February, pp Back to Table of Contents 305

8 International International Conference Conference Earthquake on Earthquake Engineering Engineering and Disaster and Mitigation, Disaster Mitigation Jakarta, April , 2008 Sengara, IW. et al. (2005), Geotechnical engineering aspects related to Aceh s tsunami and earthquake disaster and the need for its mitigation strategy, International Conference on Geotechnical Engineering for Disaster Mitigation & Rehabilitation, Singapore. Sengara, IW., Latief, H., Kusuma, S.B., Hendarto and Sumiartha, P. (2006a). Final report of seismic and tsunami microzonation mapping for input to rehabilitation and reconstruction criteria of Meuraxa District, Banda Aceh, a study sponsored by Rehabilitation and Reconstruction Agency of NAD-Nias, LAPI-Ganeshatama and Center for Disaster Mitigation, Institut Teknologi Bandung. Sengara, IW., Hendarto and Sumiartha, P. (2006b). Earthquake characteristics study of Banda Aceh, Meulaboh, and Gunung Sitoli, (in Indonesian), a study sponsored by Ministry of Research and Technology, Center for Disaster Mitigation, Institut Teknologi Bandung. Sengara, IW. and Hendarto (2006c). Probabilistic seismic and tsunami hazard analysis for city of Banda Aceh, Research Report (unpublished), Geotechnical Engineering Laboratory, Center for Disaster Mitigation, Institut Teknologi Bandung. Sengara, IW., Hendarto, Sumiartha, P., Natawidjaja, D.H., and Triyoso, W. (2008). Probabilistic seismic hazard mapping for Sumatra Island, Proceeding of International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta. Sieh, K. and Natawidjaja, D., (2000), Neotectonic of the Sumatra Fault Indonesia, Journal of Geophysical Research, Vol. 105, No. B12, Youngs, R.R., Chiou, S.J., Silva, W.J. and Humphrey, J.R. (1997). Strong ground motion attenuation relationship for subduction zone earthquake, Bulletin of Seismological Society of America, Vol. 68, No. 01, pp Back to Table of Contents 306