KEYWORDS: Seismic hazard, sensitivity analysis, uncertainty, Sudan

Transcription

1 Journl of Science nd Technology 12 (02) Decemer 2011 ISSN X Sudn University of Science nd Technology Sensitivity Anlysis of Prmeters for Proilistic Seismic Hzrd for Sudn A. E. Hssll 1, A. Rhmn E. Mohmed 1, nd Mohmed Soih 2 1 Dept. of Civil Engineering, Sudn University of Science nd Technology 2 Structurl Engineering Dept., Fculty of Engineering, Ciro University, Giz, Egypt. ABSTRACT: The sensitivity nlysis used in the proilistic seismic hzrd ssessment for Sudn ws investigted using lterntive mgnitude sets, re source ptterns, ssumption of ccelertion stndrd devition, nd estimtion of erthquke focl depths. The ctive fults pssing through Sudn were modeled s re sources. An updted ctlogue covering the period ws used. The clcultions were mde for pek ground ccelertion (PGA) t Khrtoum city for 10% proility of exceednce of PGA for exposure time periods of 50, 100, 200 nd 500 yers. The pper shows tht the ccelertion stndrd devition nd the focl depth were the most sensitive prmeters to seismic hzrd level. On the other hnd, it ws oserved tht smll mgnitudes hve smll effect upon the hzrd level. KEYWORDS: Seismic hzrd, sensitivity nlysis, uncertinty, Sudn INTRODUCTION Sensitivity nlysis (SA) is the study of how the vrition (uncertinty) in the output of mthemticl model cn e pportioned, qulittively or quntittively, to different sources of vrition in the input of model. In more generl terms uncertinty nd sensitivity nlyses investigte the roustness of study when the study includes some form of mthemticl modeling [1]. Sensitivity nlysis tries to identify the source of uncertinty tht weights more on the study's conclusion. Seismic hzrd ssessment for Sudn nd its vicinity ws crried out for 10% proility of exceednce of pek ground ccelertion (PGA) for exposure time periods of 50, 100, 200 nd 500 yers using the modified EQRISK softwre [2] with other preprocessing nd post processing progrms. EQRISK softwre ws coded y McGuire in 1976 [3]. In this pper, the sensitivity study ws mde for four min fctors tht influence the finl results of the hzrd nlysis in order to select the most resonle comintion of these fctors. The fctors re the choice of the re source ptterns, the choice of the mgnitude lower ound limit, the ssumption of the ccelertion stndrd devition, nd the estimtion of the erthquke focl depth. For ese of comprison, ll the hzrd nlyses in the sensitivity study were mde under four fixed study ssumptions [4] including, i) events with unknown mgnitude vlues in the ctlogue were neglected, the upper ound limit of the mgnitude inside ech re source ws picked up from the instrumentl records only, iii) no ckground seismicity ws ssumed other thn the re sources nd iv) the site of interest ws Khrtoum with coordintes (32.75 E, N). The present study is the first of its kind to present sensitivity study for Sudn sed on proilistic pproch. 105

2 Modeling of Source Pttern: The ctive fult source cn e used for seismic hzrd evlution in the regions where seismic sources re reltively well defined long the plte oundries or fults [5]. However, for Sudn the seismicity is diffused over lrge re s shown in Fig. (1). Thus tectonic province pproch is used to delinete different provinces s re sources. Due to the lck of sufficient historicl nd geologicl evidence, there is no unique nd consistent wy to delinete the provinces [6]. A comintion etween the lrge numer of seismic records [6] nd tectonic province nd the fults pttern shown in Fig. (2) [7] my result in the rtionl source modeling. Fig. (1): Regionl Seismicity of Sudn nd Vicinity Fig. (2): Seismic Rift Systems in Sudn Where i) CARS: Centrl Africn Rift System ii) ARRS: Atr River Rift System nd iii) EARS: Estern Africn Rift System. Three ptterns were chosen to represent the possile source pttern oundries in Sudn. Pttern 1: ws chosen y tking fult lines nd trends to represent oundries etween different re sources, s shown in Fig. (3). pttern 2: ws chosen y tking fult lines nd trends to represent oundries etween different re sources, nd then sudivisions were mde ccording to the concentrtion of the records s shown in Fig. (4), Pttern 3: ws chosen ccording to the sptil nd temporl distriution of the events together with the tectonic chrcteristics of the regions s shown in Fig. (5). For ll ssumed source ptterns, the updted ctlogue [8] ws used to otin the prmeters of the cumultive normlized frequency mgnitude reltionships [9] s given in tle (1). 106

3 LONGITUDE LATITUDE LATITUDE Fig. (3): Pttern 1 for Are Source Divisions Fig. (4): Pttern 2 for Are Source Divisions LONGITUDE Fig. (5): Pttern 3 for Are Source Divisions Where : Mximum Mgnitude in Richter scle, : Minimum Mgnitude in Richter scle, : constnt depends on the level of seismicity of the region nd : constnt descries the reltive distriution of smll nd lrge mgnitude events; lrger vlues of imply reltively fewer lrge shocks. The hzrds in res of ech pttern re shown in Figs 6-8. From these Figures, re source numer five shows the highest hzrd level, so it ws chosen to represent the different source ptterns shown in Fig. (9). It is worth mentioning tht seismic hzrd is generlly defined s the predicted level of ground motion ccelertion with certin proility of exceednce t the site under considertion during specific time intervls. On the hnd, the ojective of hzrd ssessment is to evlute, for certin plce, how frequent nd how strong erthquke will e felt, in order to tke mesure to reduce the possile dmges. 107

4 Tle (1): Sttisticl Dt of the Cumultive Normlized Frequency Mgnitude Reltionship for Different Source Ptterns Pttern Prmeters Pttern 1 Pttern 2 Pttern 3 Are Are Are Are Are

5 Tle (1) continued Are Are The methodology used in the proilistic seismic hzrd nlysis (PSHA) for Sudn [10], on which this pper is sed, contins four steps: i) the definition of erthquke sources. Sources my rnge from smll fults to lrge seismotectonic provinces with uniform seismicity, ii) the definition of seismicity recurrence chrcteristic for the sources. A recurrence reltionship indictes the chnce of n erthquke of given size to occur nywhere inside the source during specified period of time, iii), the estimtion of the erthquke effects relting to ground motion prmeter, such s pek ccelertion, iv) distnce for n erthquke of given size nd iv) the determintion of the hzrd t the site. Fig. (6): Hzrd Curves for Ares in Pttern 1 Fig. (7): Hzrd Curves for Ares in Pttern 2 109

6 Fig. (8): Hzrd Curves for Ares in Pttern 3 Fig. (9): Hzrd Curves for Different Source Ptterns Using the pre-mentioned vlues with the minimum mgnitude in ech re s strict lower ound mgnitude nd ssuming the vlues 0.8 nd 20 Km for the stndrd devition of the nturl logrithm of ccelertion nd the erthquke focl depth, respectively, the hzrd ws clculted for ech t eight vlues of PGA s shown in tle 2. The hzrd curves re shown in Fig. (9). This Figure shows tht: Pttern 2 results in the mximum hzrd level due to the decrese in - vlue, i.e., the hzrd increses for decresed β-vlue (β = ln10). Tle 2 : Hzrd Vlues of Different Source Ptterns Accelertions (cm/sq.sec) Pttern 1 Pttern 2 Pttern

7 Mgnitude Lower Bound Limit Two options of lower ound mgnitude were investigted. First, strict lower ound which indictes tht no erthqukes re ssumed to occur with lesser mgnitude. Second, loose lower ound which indictes tht erthqukes of lesser mgnitude re ssumed to occur t rte consistent with tht for mgnitudes greter thn the lower ound. Stte 1: strict lower ound mgnitude ws chosen y tking the minimum mgnitude inside ech re. Stte 2: loose lower ound mgnitude ws chosen for ll res. For cses, the hzrd vlues were clculted using pttern 2 nd ssuming the vlues (0.8) nd (20 Km) for the stndrd devition of the nturl logrithm of the ccelertion nd the erthquke focl depth, respectively. The results t eight vlues of PGA re given in tle 3. Tle 3 : Hzrd Vlues of the Two Lower Bound Mgnitudes Accelertions (cm/sq.sec) Stte 1 Stte The hzrd curves were drwn in Fig. (10), which indictes tht choosing loose ound mgnitude will result in slightly higher hzrd level. The result meets the fct tht smll events hve smll effect upon the hzrd level. Fig. (10): Hzrd Curves for the Two Lower Bound Mgnitudes 111

8 Stndrd Devition It is necessry to study the influence of stndrd devition on the sensitivity of seismic hzrd ssessment nd to ssume vlue for the logrithmic stndrd devition to crry out the hzrd nlysis with the ville dt. From the list of the widely used ttenution functions nd their uncertinties [3], two vlues were ssumed s follows: Cse 1: using (0.8) s the vlue of the logrithmic stndrd devition. Cse 2: using (0.5) s the vlue of the logrithmic stndrd devition. For oth cses, the hzrd vlues were clculted using pttern 2 nd ssuming the vlue 20 Km for the erthquke focl depth nd considering the strict lower ound mgnitude. The results t eight vlues of PGA re given in tle (4). Tle (4): Hzrd Vlues of the Two Assumptions of the Stndrd Devition Accelertions (cm/sq.sec) Cse 1 Cse The hzrd curves were drwn in Fig. (11), which indictes tht the choice of the stndrd devition hs gret influence upon the result of the hzrd nlysis. A higher vlue results in high hzrd level, nd my results in overestimting the hzrd level. On the other hnd, lower vlue my result in underestimting tht vlue. Accelertion (cm/sq.sec) Fig. (11): Hzrd Curves for Different Stndrd Devition 112

9 Focl Depth: Focl depth of erthqukes is not ccurte for most of the records in the ctlogue (12). In Sudn it ws given s 20 Km on the verge. However, in some cses it ws given etween 5 Km nd 60 Km. In this study, two vlues, 20 Km nd 40 Km, were used to test the sensitivity of erthquke focl depths on the hzrd level. Cse 1: Tking 20 Km s the vlue of the focl depth. Cse 2: Tking 40 Km s the vlue of the focl depth. For oth cses, the hzrd vlues were clculted using pttern 2 nd ssuming the vlue 0.8 for the stndrd devition of the nturl logrithm of the ccelertion nd considering the strict lower ound mgnitude. The results t eight vlues of PGA re given in tle (5). Tle 5 : Hzrd Vlues of the Two Estimtes of the Focl Depth Accelertions (cm/sq.sec) Cse 1 Cse The hzrd curves were drwn in Fig. (12), which indictes tht the choice of the focl depth hs gret influence upon the result of the hzrd nlysis. The lesser focl depth displys higher hzrd level; nd my results in overestimting the hzrd level. On the other hnd, the higher vlue of the focl depth my result in underestimting tht level. Fig. (12): Hzrd Curves for Different Focl Depths 113

10 The percentge difference in hzrd prmeters used in this study is shown in tle (6) to demonstrte nlyticlly which prmeter ws most sensitive to ground motion ccelertions. This concept is shown grphiclly in fig. (13). Tle 6 : Differences in Hzrd Prmeters (%) Accelertions (cm/sq.sec) Diff. Between Lower Mg. Diff. Between Std. Dev. Diff. Between F. Depths Averge Diff Fig. (13): Percent differences in Hzrd Prmeters 114

11 CONCLUSIONS The sensitivity nlysis used in the proilistic seismic hzrd ssessment for Sudn ws investigted using different hzrd prmeters. The site of interest ws Khrtoum, the cpitl, t longitude º E nd ltitude º N. From this study it cn e concluded tht: 1. The hzrd of certin source increses due to the increse in numer of records inside the sme geometricl re or the decrese of geometricl re with the sme numer of records. 2. The lrgest contriutor to seismic hzrd vriility is uncertinty in the choice of stndrd devition. This is followed y the vriility of the focl depth nd the mgnitude lower ound limit. The percentge differences in the uncertinties of these prmeters were found to e 21%, 17% nd 3%, respectively. 3. It ws oserved tht smll mgnitudes hve smll effect upon the hzrd level. RECOMMENDATIONS From this pper nd the results otined, the following recommendtions cn e drwn: 1. Updte the dtse whenever new dt is ville. 2. Accurte ttenution reltionships for different loctions in Sudn should e prepred. These reltionships should e otined sed on instlling detiled seismogrph network ll over the country. 3. Becuse of the sensitivity of the prmeters investigted in this pper, it is recommended tht those using seismic hzrd techniques to give creful ttention to the choice of input prmeters s well s exmining the sensitivity of results s function of these choices. REFERENCES 1. Sltelli, A., Rtto, M., Andres, T., Cmpolongo, F., Crioni, J., Gtelli, D., Sisn, M., nd Trntol, S., "Glol sensitivity nlysis", the Primer, John Wiley & Sons. 2. Mohmed E. Soih, Rshd M. Keesy nd Khlid A. Ahmed, "Development of seismic hzrd mps for Egypt", Interntionl Journl of Erthquke Engineering, Guire, Mc. R.K., 1976," Fortrn Computer progrm for seismic risk nlysis", U.S.G.S., Open File Report Khled A. Ahmed, Mohmed E. Soih, nd Rshd M. Keesy, (1992)."Sensitivity nlysis of uncertinty in estimting seismic hzrd for Egypt", Interntionl Journl of Erthquke Engineering,1, 1 32,. 5. Shh, H. C. nd Dong, W. M., (1984). "A Re-evlution of the Current Seismic hzrd ssessment methodologies", proceedings of the eight world conference on erthquke engineering, Sn Frnsisco, PP Armouti, N.S. (2003). " Response of structures to synthetic erthqukes", proceedings of the ninth Ar structurl engineering conf., Au Dhi, UAE, Browne S.E. nd Firhed J.D., (1994). "Grvity study of the white nile rift system, Sudn, nd its regionl tectonic setting", Tectonophysics, 113, Anon, (1996)."Ctlog of the Seismicity of Sudn for the Period ", Compiled y the Seismologicl Reserch Unit, Ntionl Center for Reserch. 115

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