Spatial variation of maximum considered and design basis earthquakes in peninsular India

Spatial variation of maximum considered and design basis earthquakes in peninsular India Kishor Jaiswal and Ravi Sinha* Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai 400 076, India Realistic seismic hazard assessment is essential for carrying out safe and economic design of structures. The zone factors corresponding to seismic hazard in different parts of India that has been specified in the IS code (IS 1893 : 2002) does not fully consider the recent advances in understanding of seismic hazard in peninsular India. The recent damaging earthquakes in this region have indicated that the seismic zonation may not be accurate. The MCE-level peak horizontal accelerations may also be underestimated, considering the damage caused by moderate to large earthquakes in the past. The IS code also assumes a uniform ratio of maximum considered earthquake (MCE) to design basis earthquake (DBE) as 2, which may result in nonuniform margin of safety at MCE level ground motions. These issues have been discussed in this article based on probabilistic seismic hazard assessment of peninsular India. It is shown that uniform level of safety against MCE cannot be achieved in both peninsular India as well as the seismically active regions. It is also shown that the MCE-level ground motion earthquake is significantly underestimated in large parts of peninsular India. Keywords: Earthquake-resistant design, peak ground acceleration, peninsular India, seismic hazard, stable continental region. *For correspondence. (e-mail: rsinha@civil.iitb.ac.in) SOME of the most devastating earthquakes in recent times (e.g. Koyna, 1967; Killari, 1993; Jabalpur, 1999 and Bhuj, 2001) have occurred in peninsular India (10 N 26 N; 68 E 90 E), a region which was predominantly considered as stable and aseismic shield of Indian plate. The seismic hazard in the existing design code of India 1 has been quantified in terms of seismic zoning map, which assigns four levels of seismicity for entire India in terms of different zone factors. The MSK (Medvedev Sponheuer Karnik) intensity broadly associated with the various seismic zones is VI (or less), VII, VIII and IX (and above) for zones-ii, III, IV and V respectively, corresponding to maximum considered earthquake (MCE). The IS code follows a dual design philosophy: (a) under low probability or extreme earthquake events the structure damage should not result in total collapse, and (b) under more frequently occurring earthquake events, the structure should suffer only minor or moderate structural damage. The specifications given in the design code are not based on detailed assessment of maximum ground acceleration in each zone using deterministic or probabilistic approach. Instead, each zone factor represents the effective period peak ground accelerations that may be generated during maximum considered earthquake ground motion in respective zone. A uniform factor 2 has been used to reduce the maximum considered earthquake zone factor to the factor for design basis earthquake 1. It is important to note that the zone factors used in calculation of horizontal seismic force during earthquake-resistant design of structure are dependent on many variable factors and has been estimated empirically based on a combination of engineering judgment and seismic hazard assessment. The main focus of this article is the evaluation of the ratio of peak horizontal accelerations corresponding to MCE-level (2% probability of exceedance in 50 years) and design basis earthquake (DBE)-level (10% probability of exceedance in 50 years) ground motions by carrying out probabilistic seismic hazard analysis for peninsular India. The inherent shortcomings in establishing a wellestimated number corresponding to MCE ground motions for peninsular India are due to the following: (i) incompleteness of the historical catalogue to capture the recurrence of large earthquakes, (ii) poor knowledge of ground motion characteristics due to limited strong motion data in peninsular India and (iii) little or no information about geological and palaeoseismic characteristics of the moderate seismicity regions. The present investigation performs the completeness analysis for estimation of seismicity parameters and uses gridded seismicity approach 2 for seismic hazard estimation. Due to lack of well-defined attenuation characteristics of ground motion in peninsular India, three different attenuation relationships have been used with appropriate weighting scheme. Geologic setting and earthquake database The seismotectonic features of peninsular India, which include numerous faults and fractures (Figure 1) are quite complex and difficult to identify since most of the region is covered under a thick layer of basalt (of about 2 3 km on the west coast and decreases towards the east) 3. Based on the CURRENT SCIENCE, VOL. 92, NO. 5, 10 MARCH 2007 639

tectonic features and the observed seismic activity in the peninsular shield, the regions can be broadly classified as either cratons or palaeorifts. The seismogenic characteristics of palaeorifts suggest that these regions contain large faults and have experienced extensional deformation in their most active phase. In peninsular India, it includes all passive continental margins and inactive grabens such as Cambay, Godavari, Mahanadi graben and active Narmada lineament. The cratons are tectonically stable continental regions that have seismogenic activity typically concentrated in the upper few km of the earth s crust. Historically worldwide data suggests that the maximum earthquake magnitude in cratons is in the range of magnitude 6 to 7. A complete earthquake catalogue with a uniform magnitude scale is a prerequisite for a reliable parameterization of the magnitude distribution essential for hazard analysis. In the present study, a working catalogue prepared by Jaiswal and Sinha 4 has been used. It includes the published catalogue 5 (M w 3.0), after equivalent moment magnitude conversion, up to 1997. The most recent events up to 2002 are included in the catalogue from Preliminary Determination of Epicenters (PDE) records of US National Earthquake Information Center 6. The PDE events are in terms of body wave magnitude and hence converted to equivalent moment magnitude using the relation proposed by Johnston 7. 640 log(m 0 ) = 18.28 + 0.679M b + 0.0077m 2 b. (1) A declustering algorithm based on Seeber et al. 5 has been used for removing dependent events of the entire catalogue. The criteria for identifying foreshocks and aftershocks work on uniform time ( 90 days) and space (radius 50 km) window between the successive events. However, for some of the large events (e.g. Bhuj earthquake, 2001), the aftershock activity had continued for much larger period hence such events have been removed manually from the Figure 1. Prominent seismotectonic features in the Peninsular shield of India. (Adapted from Seismotectonic Atlas of India and its Environs 3.) catalogue in order to have uniform assumptions in the rate modelling. Probabilistic seismic hazard analysis Previous hazard studies, which include Khattri et al. 8, have made direct use of existing catalogue to develop the hazard maps, which shows very low prediction of earthquake hazard in most of the quiet zones of peninsular India. Similarly the probabilistic seismic hazard map prepared by Bhatia et al. 9 has been based on very few small source zones identified in case of peninsular India whereas more than 80% of the peninsular India region has not been considered as a part of potential seismogenic source. Thus, any possible future activity outside these small source zones has not been included by Bhatia et al. 9 in their predicted seismic hazard map for the region. These studies have evaluated the seismic hazard for entire India using a single attenuation model, which was applicable only for (inter-plate) active tectonic environment and may not represent the attenuation characteristics associated with stable continental regions (intra-plate) such as peninsular India. The regional earthquake recurrence activity is commonly expressed in terms of Gutenberg Richter (GR) magnitude frequency relationship represented as log 10 λ(m) = a bm, (2) where λ(m) is the mean annual rate of exceedance of magnitude M, 10 a is the mean annual number of earthquakes of magnitude greater than or equal to zero, and the b-value is the slope of log-linear fit that represents the relative likelihood of larger and smaller earthquakes. For the estimation of decay rate b in the present study, the maximum likelihood estimate (MLE) algorithm proposed by Weichert 10 has been used. The seismic activity rate or mean annual rate of exceedance λ(m) is expressed in terms of a number of events in complete earthquake sub-catalogue. For estimating seismicity parameters of peninsular India, the entire catalogue data between 1842 and 2002 has been used (Figure 2). Considering a criterion of single threshold magnitude level of 4.5 for the entire catalogue data, without accounting for completeness at different magnitude intervals, gives b-value of 0.84. The b-value obtained using this criterion is close to 0.85 obtained by Rao and Rao 11 using historical earthquake data of 170 years. In order to establish the earthquake rate uniformity, the entire catalogue data has been divided based on the cumulative number of events of different magnitude groups (Table 1). As seen from the table, different completeness criteria can be considered for different lengths of catalogue based on observed rate uniformity in different time intervals. Considering this completeness level, the b-value has been found to be equal to 0.92 for the entire catalogue data. This value is more representative of the region s seismicity since it is CURRENT SCIENCE, VOL. 92, NO. 5, 10 MARCH 2007

based on completeness of catalogue data at different magnitude intervals and estimated using maximum likelihood method. Assigning maximum earthquake magnitude M max for most of the shield regions is generally a difficult task either due to short earthquake catalogue history or due to limited understanding of seismotectonic characteristics of the region. Jaiswal 12 describes a convolution scheme for probabilistic seismic hazard assessment of peninsular India to incorporate spatial and temporal variation of seismicity parameters such as b and M max from seismotectonic and geological considerations. In the present study, the upper and lower bounds M max and M min are taken as 8.0 and 4.0 respectively for peninsular India. For the present study, gridded (or zoneless) seismicity approach proposed by Frankel 2 has been used. Based on this approach, the entire peninsular region is divided into smaller grid cells of size 0.1 0.1, i.e. approximately 11 km 11 km area. The total number of earthquakes from catalogue data greater than certain cut-off value are counted in each square grid cell and then corrected in terms of certain magnitude interval estimates using Hermann 13, which represent the a-value for that grid cell. The mean rate or hazard at a particular site is calculated using all the a-values associated with each grid cells that are within the smoothening distance range from the site. Thus for each site, the values of a i are summed in proportion to distance from that site, so that total of a i values are represented as a i values for cells within certain distance increment of the site. The annual rate λ (y > y min ) of exceeding ground motion y min at a specified site is determined from a sum over distance D and magnitude M: [log 10 ( a / T) b( M Mref )] k l λ( y ) λ10 ymin = i k l Py ( y D, M), (3) min where T is duration of catalogue completeness for each reference magnitude M ref, based on the completeness criteria as obtained earlier. The completeness estimate for catalogue data of peninsular India is given in Table 1. The terms P[y > y min D k M l ] represent the probability that the ground motion parameter y will exceed when an earthquake of magnitude M l occurs at D k distance from the site. The first sum in the above equation is taken over k distance range and the second sum is taken over l magnitude range. This probability is dependent on the attenuation relation and the standard deviation of the specified ground motion for any specific distance. The logarithm of ground motion characteristics say peak ground acceleration (PGA) or pseudo spectral acceleration (PSA) are generally assumed to be normally distributed 2 and hence the standard normal variable associated with such probability is given as k l ln PGA (in g) ln ymin (in g) z* =, (4) σ ln PGA Figure 2. Map showing epicentral location of past earthquakes in peninsular India during 1842 2002. Table 1. Distribution of earthquake data for each decade in different magnitude ranges along with completeness period Magnitude Completeness Time interval interval period (in years) [4.0 4.5] 1961 2002 42 [4.5 5.0] 1951 2002 52 [5.0 5.5] 1901 2002 102 [5.5 6.0] 1842 2002 160 [6.0 6.5] 1842 2002 160 where y min is desired level of exceedance probability and σ ln PGA is standard deviation of logarithm of PGA estimation. The probability that the ground motion y is greater than y min can be estimated either using tabular data or using standard error function available for numerical computations. The exceedance level for estimating mean hazard estimates has been taken as 2% probability of occurrence in 50 years, giving average return period of approximately 2500 years or recurrence rate of 4.04 10 4 per year. The earthquake that contributes hazard with this level of probability is termed as MCE in most of the design codes. This level of ground motion is generally considered for estimation of seismic margin so that the structure should not collapse even if badly damaged during this level of earthquake event. This approach has been earlier used for development of seismic hazard maps of the United States (1996 and 2002 versions) under the United States Geological Survey (USGS) mapping project 14,15. For hazard assessment, three attenuation relations have been used with different weights (Figure 3). Out of these three relationships, one relationship has been recently CURRENT SCIENCE, VOL. 92, NO. 5, 10 MARCH 2007 641

Table 2. Comparison of peak ground acceleration with the damage intensities and zone factors as defined by IS 1893 : 2002 IS code seismic zone 1 II III IV V Instrumental intensity 1 (MMI) V or smaller VI VII VIII or larger Perceived shaking 14 Moderate Strong Very strong Severe Potential damage 12 Very light Light Moderate Moderate/heavy Zone factor for MCE 1 (g) 0.1 0.16 0.24 0.36 Peak acceleration 12 (%g) 3.9 9.2 9.2 18 18 34 34 65 Figure 3. Distribution of peak ground acceleration for hard rock conditions for different attenuation relationships. proposed for peninsular India whereas the other two are based on similar seismotectonic conditions to peninsular India and are widely accepted. The Iyengar and Raghukanth 16 relationship for peak ground acceleration is based on recent large earthquakes in peninsular India and is applicable for hard-rock category in the range of 1.5 3.6 km/s. It is considered to be more representative of the attenuation characteristics of the region and hence given double the weightage as compared to the other two relationships, i.e. Atkinson and Boore 17 and Toro et al. 18 Both the relationships have been well-established and rigorously studied ground motion models for hard-rock site conditions (i.e. shear-wave velocity in the range of 2.8 km/s). The hypocentral distance has been estimated for Iyengar and Raghukanth 16 and Atkinson and Boore 17 using a constant hypocentral depth of 10 km, whereas the Joyner Boore distance 19 (which is the closest distance to the rupture surface if this rupture surface is projected to the ground surface) has been directly used for the Toro et al. 18 attenuation relationship. These three relationships have been used in the present study to estimate peak horizontal ground acceleration for assumption of uniform hard-rock site conditions. The hazard assessment has been carried out considering all possible earthquake magnitude and distance ranges and the results have been prepared for MCE level ground shaking. Due to non-availability of well-defined relationship between peak ground acceleration and damage intensity in the literature, the widely used relationship proposed by 642 Wald et al. 20 has been used as a first order approximation (Table 2). The result of MCE-based probabilistic seismic hazard map of peninsular India is shown in Figure 4. Seismic hazard estimates in terms of PGA for MCE level ground motion have been interpreted in terms of zoning parameters specified in the current seismic hazard map of India (Figure 5) using Table 2. The estimated peak ground motion for Koyna region with average return period of 2500 years (MCE) is of the order of 0.34 g to 0.6 g. This level of ground motion is associated with zone-v (zone factor 0.36) of seismic zoning map of India (Table 2 and Figure 5). However, it has been assigned as zone IV (zone factor 0.24) in the IS code 1. The estimated PGA compared with the expected damage intensity according to Wald et al. 20 also shows that this level of maximum acceleration corresponds to intensity IX damage. It may also be noted that the observed seismic activity in certain parts of peninsular India (e.g. Koyna) is high during the last few decades 5. Figure 4 also indicates that for several pockets, such as Killari region of Maharashtra, part of Narmada lineament, southern portion of Tamil Nadu as well as the Northeast region, the estimated peak ground acceleration at MCE level is in the range of 0.34g and greater. Most of these regions including the entire Saurashtra region of Gujarat have been assigned as zone-iii, resulting in much lower seismic zone factor of 0.16 g for the earthquake-resistant design of structures. Similarly, large areas of the Narmada lineament extending up to northern portion of Godavari graben show higher MCE level ground motion and should be included in seismic zone-iv. Bhatia et al. 9 estimated peak ground acceleration of 0.25 g for western Gujarat, 0.20 g for Koyna region and 0.10 g for Narmada lineament for 10% probability of exceedance in 50 years (DBE level). These estimates appear to be on lower side as compared to the present study primarily due to application of intraplate attenuation relation applicable for shield regions used. It is interesting to note that the recent damaging Jabalpur earthquake of 1999 occurred in this region in which the maximum damage corresponded to MSK intensity VIII (or similar to zone-iv). However, the western portions of Bengal, Orissa and southern portion of Madhya Pradesh show very low ground motion estimation and are considered to constitute the main portion of stable shield of peninsular India. CURRENT SCIENCE, VOL. 92, NO. 5, 10 MARCH 2007

Figure 4. Probabilistic seismic hazard map showing peak ground accelerations for maximum considered earthquake in peninsular India. Figure 5. Seismic zoning map of India 1. Spatial variation of seismic hazard and interpretations The return period of maximum magnitude earthquakes (corresponding to MCE) in peninsular India varies widely and the variation is closely associated with the differential seismotectonics separating aseismic cratons from highly active rifting zones 12. IS code 1 states that the maximum seismic ground acceleration in each zone cannot be predicted with accuracy either on a deterministic or on a probabilistic basis. The basic zone factors included in the code have been assumed to be reasonable estimates of effective peak accelerations for MCE level earthquake. From earthquakeresistant design considerations, it is desirable to have uniform margin of safety against collapse for all structures. The IS code attempts this by specifying the MCE in different seismic zones, and providing the factor for conversion of MCE to DBE. The design of structures is based on DBE and the structures are expected to resist the DBE-level earthquakes with minimal structural damage. Under MCElevel earthquakes, the structures are expected to be badly damaged, but in a fail-safe manner wherein the structure failure is prevented due to ductility, arising from inelastic material behaviour and detailing, and overstrength, arising from additional reserve strength in structures over and above the design strength. Since the MCE to DBE ratio is assumed to be two in all seismic zones, the same ductility provisions are assumed to provide the additional safety margin against failure in different seismic zones. It may be noted that the ratio of peak ground acceleration MCE to DBE is about two in the seismically active regions of western United States 15. Evaluation of seismic hazard shows that the zone-based scaling factors MCE to DBE may be lower for more seismically prone regions than low or moderate seismic zones. Frankel et al. 14 have indicated that the difference between the ground motion for DBE-level event and the ground motion for MCE-level event in western United States (WUS) is typically less than the difference observed for central and eastern United States (CEUS). For example in San Francisco, the ratio between the 0.2 second spectral acceleration for the 2% in 50 years and the 10% in 50 years is about 1.5; whereas in other parts of United States, the ratio varies from 2.0 to 5.0 or more. Since the seismic hazard for entire India has been characterized by significant spatial variation mainly due to differential seismotectonics, it is not possible to achieve the same margin of safety against collapse by using single reduction factor from MCE to DBE for the entire country. To understand spatial variation especially for low to moderate seismicity region such as peninsular India, the MCE to DBE ratio for PGA has been estimated as shown in Figure 6. It is seen that except for few locations (which include eastern and small portion of northern craton, most of the eastern craton and parts of southern craton), the MCE to DBE ratio for most of the peninsular India is between 2.0 and 2.5. Similarly, for most pockets of southern craton, parts of Godavari and Mahanadi grabens, and some of the northeastern parts of peninsular India, the MCE to DBE ratio varies from 2.5 to 3.0, whereas for small northern and southern cratons, it is higher than 3.0. Higher MCE to DBE ratio in these zones indicates that the estimated DBE level ground motion is lower (only in limited cases/regions where MCE level shaking is in close match with IS code specification in terms of zone factors). In such cases, if a structural designer wishes to design the structure by directly taking DBE level accelerations, it is difficult to ascertain the ductility demand (safety margin) that may be required within the structural system to withstand MCE ground shaking. As discussed earlier, except few CURRENT SCIENCE, VOL. 92, NO. 5, 10 MARCH 2007 643

Figure 6. Ratio of peak ground accelerations corresponding to MCE and DBE in peninsular India. regions, most of peninsular India faces higher seismic hazard than specified in IS code in terms MCE level shaking which make use of constant MCE to DBE factor questionable for these regions. If the peak ground accelerations specified in the zoning map corresponding to MCE level are considered for design purposes, the use of uniform MCE to DBE factor of 2.0 results in the specified earthquake forces being greater than DBE in most parts of peninsular India since the MCE to DBE ratio is found to be greater than 2.0. On the other hand, if the structures are designed for DBE-level earthquakes in peninsular India using zone factors specified in code, some regions may experience higher MCE-level earthquake accelerations than that assumed in the IS code. The structures in these regions will therefore have lower safety under actual MCE-level earthquake ground motions compared to the IS code specifications 1. Thus the variation of MCE to DBE ratio which seems to be closely related to the prominent seismotectonic zones in peninsular India, has a profound influence on the safety of the structures during DBE- and MCE-level earthquakes. Based on this study, it is recommended that the seismic zoning map must be based on rigorous hazard analysis considering recent advancement of hazard-assessment procedures, knowledge of recent earthquake records and its attenuation characteristics along with geological and palaeoseismic data. It is also recommended that the design accelerations should also be specified for DBE-level earthquakes to ensure uniform margin of safety under the DBE-level earthquake anywhere in India. The DBE level ground motion may be estimated with higher confidence due to known earthquake catalogue history for such period and accurate recurrence pattern assessment of moderate 644 to large earthquakes in each seismotectonic region. If uniform margin of safety under MCE-level earthquake is also required, the ductility provisions will need to be suitably modified in regions with a much higher MCE to DBE ratio. Discussions and conclusions The seismic hazard map for earthquake events with 2% probability of exceedance in 50 years has been presented in this article. The ground accelerations used for earthquakeresistant design of structures have also been evaluated in different seismic zones. These accelerations, which are defined for MCE-level and DBE-level earthquakes are assumed to have a constant ratio by the IS code. However, this study shows that there is considerable variation in the MCE to DBE ratio in different parts of peninsular India. The following are broad conclusions based on the present study: 1. The probabilistic seismic hazard maps in terms of peak ground acceleration for maximum considered earthquake ground motions indicate higher seismic hazard for most regions of peninsular India as compared to the existing specifications of the earthquake-resistant design codes of India. 2. The MCE to DBE ratio seems to be strongly dependent upon the seismotectonic conditions and hence cannot be taken as uniform value for whole peninsular India. It is found that the ratio is higher for low-to-moderate seismicity regions such as most of the cratons and inactive grabens, as compared to high seismicity rifting CURRENT SCIENCE, VOL. 92, NO. 5, 10 MARCH 2007

zones of peninsular India. For most of the peninsular India, the ratio varies between 2 and 2.5, whereas for rifting zones, it is slightly lower. 3. For design purposes, the ground motions corresponding to 10% probability of exceedance in 50 years (DBElevel) should also be directly specified. The use of MCE-level ground accelerations with a constant reduction factor may result in large errors in the specification of DBE-level PGA for design purposes. 4. Uniform safety against MCE-level earthquakes is not possible when designing for DBE-level events in all parts of peninsular India due to the variable ductility demand in structures, since the ratio of MCE to DBE may be much greater than 2.0 in several parts of the region. 5. Various assumptions have been made while developing probabilistic seismic hazard map of peninsular India, e.g. source to site distance definition, constant hypocentral depth, M max or b, uniform hard-rock site condtion without including site effects, etc. therefore, it may be possible to make further improvements in the seismic hazard estimates presented in this article. 1. IS: 1893 (Part-1), Indian Standard Criteria for Earthquake Resistant Design of Structures (Fifth Revision), Bureau of Indian Standards, New Delhi, 2002. 2. Frankel, A. D., Mapping seismic hazard in the Central Eastern United States. Seismol. Res. Lett., 1995, 66, 8 21. 3. Seismotectonic Atlas of India and its Environs, Geological Survey of India, Calcutta, 2000. 4. Jaiswal, K. and Sinha, R., EarthquakeInfo.org: Webportal on earthquake disaster awareness in India, 2005 (http://www.earthquake info.org). 5. Seeber, L., Armbruster, J. G. and Jacob, K. H., Probabilistic assessment of earthquake hazard for the state of Maharashtra, Report to Government of Maharashtra, Earthquake Rehabilitation Cell, Mumbai, 1999. 6. National Earthquake Information Center (USA), Earthquake Hazards Program: NEIC Earthquake Search. http://neic.usgs.gov/neis/epic. 7. Johnston, A. C., Seismic moment assessment of earthquakes in stable continental regions, Part-I. Instrumental Seismicity. Geophys. J. Int., 1996, 124, 381 414. 8. Khattri, K. N., Rogers, A. M. and Algermissen, S. T., A seismic hazard map of India and adjacent areas. Tectonophysics, 1984, 108, 93 134. 9. Bhatia, S. C., Ravi Kumar, M. and Gupta, H. K., A probabilistic seismic hazard map of India and adjoining regions. Ann. Geofisica, 1999, 42, 1153 1166. 10. Weichert, D. H., Estimation of the earthquake recurrence parameters for unequal observation periods for different magnitudes. Bull. Seismol. Soc. Am., 1980, 70, 1337 1346. 11. Rao, B. R. and Rao, P. S., Historical seismicity of peninsular India. Bull. Seismol. Soc. Am., 1984, 74, 2519 2533. 12. Jaiswal, K., Probabilistic seismic hazard estimation methodology for stable continental regions incorporating spatial and temporal uncertainties, Ph D thesis, Indian Institute of Technology Bombay, Mumbai, 2006. 13. Herrmann, R., Recurrence relations. Earthquake Notes, 1977, 48(1 2), 47 49. 14. Frankel, A. et al., National seismic hazard maps: Documentation, US Geol. Surv. Open-File Rept., 1996, 96 532, 69. 15. Frankel, A. et al., Documentation for the 2002 update of the national seismic hazard maps. US Geol. Surv. Open-File Rep., 2002, 02-420. 16. Iyengar, R. N. and Raghukanth, S. T. G., Attenuation of strong ground motion in peninsular India. Seismol. Res. Lett., 2004, 75, 530 540. 17. Atkinson, G. M. and Boore, D. M., Ground motion relations for eastern North America. Bull. Seismol. Soc. Am., 1995, 85, 17 30. 18. Toro, G. N., Abrahamson, N. and Schneider, J., Model of strong ground motions from earthquakes in central and eastern North America: Best estimates and uncertainties. Seismol. Res. Lett., 1997, 68, 41 57. 19. Abrahamson, N. A. and Shedlock, K. M., Overview. Seismol. Res. Lett., 1997, 68, 9 23. 20. Wald, D. J., Quitoriano, V., Heaton, T. H. and Kanamori, H., Relationships between peak ground acceleration, peak ground velocity, and modified Mercalli intensity in California. Earthquake Spectra, 1999, 15, 557 564. Received 20 May 2005; revised accepted 8 January 2007 CURRENT SCIENCE, VOL. 92, NO. 5, 10 MARCH 2007 645