Earthquake maximum magnitude estimation considering regional seismotectonic parameters

Transcription

1 Southern Cross University 23rd Australasian Conference on the Mechanics of Structures and Materials 2014 Earthquake maximum magnitude estimation considering regional seismotectonic parameters P Anbazhagan King Saud University Nairwita Dutta Indian Institute of Science Ketan Bajaj Indian Institute of Science Sayed S. Moustafa National Research Institute of Astronomy and Geophysics Nassir S. Al-Arifi King Saud University Publication details Anbazhagan, P, Dutta, N, Bajaj, K, Moustafa, SSR, Al-Arifi, NS 2014, 'Earthquake maximum magnitude estimation considering regional seismotectonic parameters', in ST Smith (ed.), 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), vol. II, Byron Bay, NSW, 9-12 December, Southern Cross University, Lismore, NSW, pp ISBN: epublications@scu is an electronic repository administered by Southern Cross University Library. Its goal is to capture and preserve the intellectual output of Southern Cross University authors and researchers, and to increase visibility and impact through open access to researchers around the world. For further information please contact epubs@scu.edu.au.

2 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23) Byron Bay, Australia, 9-12 December 2014, S.T. Smith (Ed.) EARTHQUAKE MAXIMUM MAGNITUDE ESTIMATION CONSIDERING REGIONAL SEISMOTECTONIC PARAMETERS P. Anbazhagan* Visiting Professor, King Saud University, Riyadh 11451, Department of Civil Engineering, Indian Institute of Science, Bangalore, India. (Corresponding author) Nairwita Dutta Department of Civil Engineering, Indian Institute of Science, Bangalore, India. Ketan Bajaj Department of Civil Engineering, Indian Institute of Science, Bangalore, India. Sayed SR Moustafa King Saud University, Riyadh 11451, SAUDI ARABIA, National Research Institute of Astronomy and Geophysics (NRIAG), Cairo 11421, Egypt. Nassir S. Al-Arifi King Saud University, Riyadh 11451, Saudi Arabia. ABSTRACT The objective of this paper is to estimate the maximum magnitude (M max ) which is defined as the upper limit of earthquake magnitude for a given region by a new approach considering regional rupture characteristics. The proposed method has been explained in details and examined for an active seismic region. Seismic study area (SSA) has been generated by dividing into radii of 200 km and 500 km, based on the seismicity and seismotectonics. The regional rupture character has been established by considering Percentage Fault Rupture (PFR), which is the ratio of subsurface rupture length (RLD) to Total Fault Length (TFL) expressed in percentage. PFR is used for determining RLD, which is further used for estimating the maximum magnitude for each source. Maximum magnitude for SSA of Kanpur region has been estimated and compared with the M max values from existing methods. It is erved from the study that the existing deterministic and probabilistic M max estimation methods are sensitive to SSA radius, magnitude of completeness (M c ), a and b parameters and M max values. However, M max from the proposed method is a function of the rupture character and is irrespective of the SSA and M c, a and b parameters and M max values. KEYWORDS Maximum earthquake magnitude (M max ), regional Rupture Characteristics, a and b G-R parameters, magnitude of completeness, maximum erved magnitude (M max ). This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit 979

3 INTRODUCTION Seismic hazard analysis describes the potential of earthquake induced ground shaking at a site. The maximum possible earthquake magnitude (M max ) calculation is indispensable in many seismic and engineering applications viz. earthquake engineering community, disaster management agencies and the insurance industry. However, there is no universally accepted practice for estimating the value of M max (Kijko and Singh 2011). M max is defined as the upper limit of earthquake magnitude for a given region and is synonymous with the magnitude of the largest credible earthquake (EERI Committee on Seismic Risk 1984; Working Group on California Earthquake Probabilities (WGCEP 1995). It assumes a sharp cutoff magnitude at a maximum magnitude but by definition, no earthquakes are to be expected with magnitude exceeding M max (Joshi and Sharma 2008). In this present study, a newly proposed method has been compared using two extensively used methods. These conventional approaches are the maximum erved magnitude (M max ) with an increment of 0.5 and Kijko method (Kijko and Sellevoll 1989). M max is widely estimated by taking up the largest erved earthquake magnitude with or without an increment. Anbazhagan et al. (2013) highlighted that 5.62 times energy increment is less and should be region specific to estimate the maximum possible earthquake. Kijko and Sellevoll (1989) have given a method to estimate the maximum magnitude considering doubly truncated Gutenberg-Richter relation. The objective of this paper is to establish an alternate method for estimating the largest possible magnitude considering the regional rupture character. Kanpur (moderate to high seismicity region), lying in the highly fertile Indo Gangetic Basin (IGB), is selected as SSA. A seismic study area has been defined considering a radius of 500 km from the Kanpur city center. This is based on the maximum damage distance taking into account past earthquakes in this region (Anbazhagan et al. 2013). A detailed earthquake catalogue has been generated for the SSA. Seismic parameters such as M c, a and b parameters and maximum reported earthquake magnitude have been identified. These data are used to estimate the maximum magnitude for Kanpur SSA. In the present study, seismic study area has been divided into two radii of considerations i.e. 200 km and 500 km, based on seismotectonic and seismicity of the Kanpur region. All the earthquake moment magnitudes of magnitude 4.8 and above are identified and RLD has been estimated by the well recognized relation given by Wells and Coppersmith (1994). The RLD is then divided by the respective total length of seismic source for calculating the normalized subsurface length. The plot of the normalized rupture factor with TFL shows a unique trend; this is called as a regional rupture character (Anbazhagan et al. 2014a) and has been estimated separately for each study region. Based on the erved trend, maximum possible subsurface rupture in terms of percentage of the TFL is determined and called subsurface rupture character. Subsurface rupture character is further used to estimate the M max for each identified source. It is found that the maximum magnitude from regional rupture character is a unique value which represents the rupture phenomenon of the region. M max has been estimated by considering the regional rupture character which does not vary with seismic study area and is based on the length of each seismic source. The proposed method is relevant for the region where seismic sources are well identified and known. SEISMIC STUDY AREA ESTIMATION The study area of Kanpur lies in the central part of the Indo-Gangetic Plain and is surrounded by two main rivers of India i.e. Ganges in the north-east and Yamuna in the South. Kanpur city covers an area of over 605 km 2 with its centre point having latitude o N and longitude o S. The study area of Kanpur lies in the Seismic Zone III as per the current Seismic zonation map of India (IS 1893, 2002) and is having a zone factor of Kanpur city being situated in the IGB is surrounded by many inhomogeneities in the form of faults and ridges like the Delhi-Haridwar Ridge, Faizabad Ridge and the Lucknow Fault. In addition to that, Kanpur lies within 100 km from the Lucknow-Faizabad Fault which is under a high risk of earthquake hazard. Besides being surrounded by several faults and ridges, Kanpur also lies within a distance of 350 km from the two active thrusts - Main Boundary Thrust (MBT) and the Main Central Trust (MCT), which are present along the entire Himalayan tectonic belt and have a history of reported earthquakes. ACMSM

4 In the present study, for preparation of seismotectonic map, linear sources are taken from SEISAT for the whole India. These maps were scanned using a high resolution scanner and digitized for identification of the linear sources considering 500 km radius from Kanpur city centre. Data of all the past earthquake events around 500 km radial distance of Kanpur city center have been collected from different sources such as National Earthquake Information Centre (NEIC), International Seismological Centre, Indian Meteorological Department (IMD), United State Geological Survey (USGS), Northern California Earthquake Data Centre (NCEDC) and the Geological Survey of India (GSI). A total number of 3140 events were collected and homogenized to moment magnitude (M w ) for catalogue consistency. Out of 3140 events, 49.8% were found to be depended events; a total of 1564 events have been acknowledged as main shock. So, homogenized and declustered catalogue contains 918 events having M w < 4 and 646 more than 4. The region consists of about 90 seismic sources including Main Boundary Thrust (MBT) and Main Central Thrust (MCT); the shortest length of source is about 6.3 km and the longest length is about km. Maximum erved magnitude is found to be 7.1 for MBT, having a length of A total of 47 seismic sources have been erved with a moment magnitude between 4 to 5 and 43 sources with a moment magnitude above 5. A combined seismotectonic map of Kanpur with markings of the two seismic study areas in circles is shown in Figure 1. Figure 1. Seismotectonic Map of Kanpur SSA ESTIMATION OF RUPTURE CHARACTER OF THE REGION It is well known that the amount of energy released, i.e. magnitude is related to the rupture phenomena of the region. Most of the methods presently used do not take the regional rupture phenomena into account (Anbazhagan et al. 2014b). M max depends on the tectonic features where future seismicity is supposed to occur. So the incorporation of regional tectonic features in the form of rupture character is attempted in this study for the estimation of M max. The source criterion which influenced the fault rupture is the density and shear wave velocity of the crustal rock at rupture. These parameters are directly related to the shear strength of ruptured rock. These parameters are considered to be uniform throughout the region in many seismological models based on the geology and deep geophysical data. Rupture character of the region is derived by moment magnitude of 4.8 and above and the associated RLD by considering the validity of Wells and Coppersmith 1994 correlation between RLD and M w. ACMSM

5 RLD of each damaging earthquake is estimated by using a well known M w and source parameter relation presented by Wells and Coppersmith (1994). This relation is applicable to all types of faults, shallow earthquakes, and interplate or intraplate earthquakes (Wells and Coppersmith 1994). RLD values obtained from past earthquakes are divided by the TFL of the associated source and plotted against the total length of the source. The ratio of RLD to TFL expressed in percentage is defined as Percentage Fault Rupture (PFR). PFR follows a unique trend with TFL and is found to be similar for a particular SSA; this is called as the rupture character of the region. Anbazhagan et al. (2014a) found that regional rupture character is unique and do not change with seismic study area. Based on the erved unique trend, the typical curve can be divided into different segments, considering the maximum percentage of fault rupture and TFL. The rupture values of these segments can be considered as an average rupture of a region. The average/maximum rupture values can be increased based on the structural type and can be used to estimate the maximum magnitude of a particular region. This unique trend needs to be established for each SSA. In this study, moderate to high seismicity region of Kanpur in the IGB has been selected to estimate the maximum magnitude using the approach discussed above. M max ESTIMATION FOR SSA For the estimation of M max from the new proposed method, region specific rupture character has been established for the SSA. Sources having M w > 4.8 have been identified and RLD due to past earthquake has been estimated using Wells and Coppersmith (1994) relation. The rupture character of the region has been established by PFR versus TFL by dividing the SSA into two radii of consideration of km and km based on the seismicity of SSA. From Figure 2a and 2b it can be noticed that PFR follows a unique trend with TFL. It can be erved from figure 2 that the percentage of the fault ruptured for shorter faults is more when compared to that of longer faults and showing a decreasing trend with an increase in the fault length. Based on the erved unique trend for both radii of consideration of SSA, the curve is divided into three segments, considering PFR and TFL of the faults considered. This PFR can be used to estimate potential RLD of each source, which can be again converted as the maximum M w for respective source. Possible worst scenario PFR is established by considering minimum, maximum and average PFR in three lengths bins for both radius of consideration. In case of km, Segment-I consist of faults having TFL less than 150 km; segment II has TFL between 150 km to 250 km and segment-iii has TFL greater than 250 km. The respective worst scenarios PFR for these three segments are 12%, 5% and 2%. In case of km SSA, the Segment-I consist of faults having TFL less than 150 km, segment II has TFL between 150 km to 350 km and segment-iii has TFL greater than 350 km. The respective maximum PFR for the worst case scenario for these three segments are 33%, 15% and 9%. The respective rupture character is further used to estimate the M max for each source in the region. The range of the maximum moment magnitude estimated from regional rupture characteristic for Kanpur site for two different radii of consideration are 5.5 to 6.2 for 200 km and 4.7 to 7.3 for km respectively. The seismic parameters - M c, a and b parameters and M max estimation is a prerequisite for M max estimation by the existing method. a and b parameters can be estimated by the standard Gutenberg- Richter (G-R) recurrence relationship (Gutenberg and Richter, 1956) using data from completed period or using the magnitude of completeness. a and b parameters for the present SSA are found to be 5.69 and The maximum magnitude has also been estimated by the conventional approach of M max with an increment of 0.5 and Kijko method (Kijko and Sellevoll 1989). The incremental method of M max estimation considers Gutenberg Richter b-value (Wheeler 2009). The b-value was found to be close to 1 for both radii of consideration and so an increment of 0.5 is done to M max, resulting in M max of 7.6 M w. This method is widely practiced in India without bothering to increment according to the regional 'b- value' which should be taken care of since 0.5 increment equals to one increment of earthquake intensity value. For the estimation of M max using Kijko method (Kijko and Sellevoll 1989), magnitude of completeness has been calculated using entire magnitude method (EMR) proposed by Woessner and Stefen, Detailed description of calculating M c and a and b value for Kanpur Region was presented in Anbazhagan et al., 2014b. The M c calculated from EMR method is 4.5, ACMSM

6 Percent Faut Rupture (%) Percent Faut Rupture (%) which has been further used for calculating M max using Kijko and Sellvoll (1989) method. This method for calculating M max has been used by various researchers worldwide as well as in India. For the present study area, regional values are used as per Kijko and Sellevoll (1989) corresponding to each source having M w > 6.0 as shown in Table 1. Typical calculation for fault having M w > 6.0 has been shown in Table 1 for all the three methods Distance 200 km Earthquake Events 乘幂 (Earthquake Events) Total Fault Length (TFL) (km) Figure 2a and 2b. Regional rupture character for subsurface rupture length in terms of percent of total length of fault versus total length of fault for Kanpur for 200 km and km respectively Table 1. Estimation of M max for each seismic source used in the study considering three approaches Fault Name Rupture Characteristics M max by Kijko and M max by RLD (% M TFL (km) max by Sellevon +0.5 TFL) Rupture MBT MCT F F F F F F F F F CONCLUSION Distance > 200 km Total Fault Length (TFL) (km) Earthquake Events 乘幂 (Earthquake Events) In this study, a new approach is proposed for estimating the maximum earthquake magnitude considering the regional rupture characteristics of seismic study area. The regional rupture characteristic is established by taking into account the rupture length from the past earthquakes and associated fault/source length. The study area of Kanpur is considered to show the maximum ACMSM

7 magnitude estimated by the newly proposed method discussed here and also from two widely used methods i.e. incremental to maximum erved method and Kijko method. This study shows that in most of the existing methods, maximum magnitude values are mainly dependent on seismic study area around source, cut-off magnitude and a and b values of the region. The proposed method depends on the seismic sources and magnitude associated to the seismic source of the seismic study area and follows the same trend irrespective of the maximum seismic event close to source. As this method depends on regional rupture characteristic, it can be applied to the region where seismic sources are well defined. This method can be used as an alternate method to estimate the maximum magnitude for seismic hazard analysis. REFERENCES Anbazhagan P, Smitha CV, Kumar A, Chandran D (2013) Seismic hazard Assessment of NPP site at Kalpakkam, Tamil Nadu, India, Nuclear Engineering and design, Vol. 259, pp Anbazhagan P, Smitha CV, Kumar A (2014a) Representative Seismic Hazard map of Coimbatore, India, Engineering Geology, Vol. 171, pp Anbazhagan P, Bajaj K, Moustafa SSR, Al-Arifi NS (2014b) Maximum Magnitude Estimation Considering the regional rupture characteristics Journal of Seismology (Submitted) EERI committee on seismic risk (1984) Glossary of Terms for Probabilistic Seismic Risk and Hazard Analysis, Earthquake Spectra, Vol. 1, pp Gutenberg B, Richter CF (1956) Earthquake magnitude, intensity, energy and acceleration, Bulletin Seismology Society of America, Vol. 46, pp IS 1893 (2002) Indian standard criteria for earthquake resistant design of structures, part 1-general provisions and buildings, Bureau of Indian Standards, New Delhi Joshi GC, Sharma ML (2008) Uncertainties in the estimation of Mmax, Journal of Earth System Science, Vol. 117(S2), pp Kijko A, Sellevoll MA (1989) Estimation of earthquake hazard parameters from incomplete data files. Part I, Utilization of extreme and complete catalogues with different threshold magnitudes, Bulletin Seismology Society of America, Vol. 79, pp Kijko A, Singh M (2011) Statistical tool for Maximum possible earthquake magnitude estimation Acta Geophysica, Vol. 59, pp Wells DL, Coppersmith KJ (1994) New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bulletin Seismology Society of America 4 (84): Wheeler RL (2009) Methods of Mmax estimation east of the Rocky Mountains U. S. Geological Survey, Open-File Report ( Accessed 27 February 2013) WGCEP (Working Group on Central California Earthquake Probabilities) (1995) Seismic Hazard in Southern California: Probable Earthquakes 1994 to 2024, Bulletin Seismology Society of America, Vol. 85, pp Woessner J, Stefan W (2005) Assessing the quality of earthquake catalogues: Estimating the magnitude of Completeness and its uncertainty, Bulletin Seismology Society of America, Vol. 95, No. 2, pp ACMSM