Seismological Research Letters Volume 83, Number 1 January/February 2012 77 doi: 10.1785/gssrl.83.1.77 E Characterization of Regional Seismic Source Zones in and around India Sreevalsa Kolathayar and T. G. Sitharam Sreevalsa Kolathayar and T. G. Sitharam Indian Institute of Science Online material: Earthquake catalog; coordinates of source zones; list of events in each zone; frequency magnitude distri- bution plots. INTRODUCTION Identiication and characterization of seismic sources are essen- tial input for seismic hazard analysis. A complete and consis- tent catalog of earthquakes in a region can ofer good data for studying the distribution of earthquakes with respect to space, time, and magnitude. Earthquakes are known to have occurred in the region of the Indian subcontinent from historic times. Most of the activities, including many great earthquakes, have occurred in the northern subcontinent and in the Andaman and Nicobar regions. In the southern peninsula, damaging earthquakes have occurred, but their frequency was less and magnitudes were lower than the plate boundary earthquakes. he tectonic framework of the Indian subcontinent—an area of about 3.2 million square kilometers—is complex and varied spatially. his necessitates identifying di ferent regions of simi- lar seismicity. In this study we attempt to identify and charac- terize regional seismic source zones based on a homogeneous catalog of earthquakes in and around India from 250 BC until 2010. We identiied diferent regional seismic source zones and created separate catalogs for each respective zone. Most earthquake catalogs do not report earthquake mag- nitudes consistently over time, which may hinder deining seis- micity patterns or assessing seismic hazards. he magnitude scales used for earthquake catalogs in India are not homoge- neous. Compiling a complete catalog of earthquakes with uniform magnitudes is essential for deining seismicity pat- terns and assessing seismic hazards for a region (Chen and Tsai 2008). Hence the original magnitudes of earthquakes have to be converted to a common and reliable magnitude scale using appropriate magnitude correlations. In the raw catalog, many events can be dependent events that occurred in association with a mainshock in a cluster. hese atershocks and foreshocks have to be removed from the catalog using a declustering algo- rithm to assure a Poissonian distribution of earthquake events. In the present study, we have identiied and characterized regional seismic source zones across the whole of India and the adjoining area (5°–40°N and 65°–100°E). We compiled our earthquake database from various sources and prepared a com- plete catalog of Indian earthquakes from 250 BC until 2010. We used diferent region-speciic earthquake magnitude scal- ing relations correlating diferent magnitude scales to develop a homogenous earthquake catalog for the region in uniied moment magnitude (M W ) scale. We removed the dependent events in the raw catalog to ensure the Poissonian distribution of earthquakes. We identiied regional seismic source zones of homogeneous seismicity from previous works and modi- ied these based on the seismic event distribution, geology, and fault alignment. We created separate catalogs for each zone, and based on the threshold magnitude, estimated the seismic- ity parameters for all the source zones based on the maximum likelihood method. he earthquake catalog, the coordinates of source zones, the list of events in each zone, and frequency mag- nitude distribution plots for all the source zones are uploaded in the Web site http://civil.iisc.ernet.in/~sreevals/resource.htm. DATA USED IN THE ANALYSIS We started with a larger data set provided by various national and international agencies for our analysis. We used two types of earthquake catalogs in this study: historical and instrumen- tal. We compiled the historical part of the catalog from various literature. Except for an event (5 December 1063) reported by the Indian Meteorological Department, we obtained the details of earthquake events for the period from 250 B.C. to A.D. 1505 from Dunbar et al. (1992), which lists historic earthquakes worldwide from 2150 B.C. to A.D. 1991. We compiled the later portion of historic earthquakes from the work of various researchers such as Oldham (1883); Basu (1964); Kelkar (1968); Tandon and Srivastava (1974); Rastogi (1974); Chandra (1977), 1978; Kaila and Sarkar (1978); Rau and Rau (1984); Srivastava and Ramachandran (1985); Biswas and Dasgupta (1986); Guha and Basu (1993); and Bilham (2004). he major portion of the instrumental catalog was compiled from national and inter- national agencies. he national agencies include the Bhabha Atomic Research Centre (BARC), Indian Meteorological Department (IMD), Indira Gandhi Centre for Atomic Research (IGCAR) Kalpakkam, and National Geophysical Research Institute (NGRI) Hyderabad. International agencies include the International Seismological Centre (ISC) data ile