Fractal Dimension of the 1999 Chamoli Earthquake from Aftershock Studies in Garhwal Himalaya RICHA JAIN, 1 B. K. RASTOGI 1 and V. P. DIMRI 1 Abstract — The Aftershock sequence of Chamoli earthquake (M w 6.4) of 29 March 1999 is analyzed to study the fractal structure in space, time and magnitude distribution. The b value is found to be 0.63 less than which is usually observed worldwide and in the Himalayas. This indicates that the numbers of smaller earthquakes are relatively less than the larger ones. The spatial correlation is 1.64, indicating that events are approaching a two-dimensional region meaning that the aftershocks are uniformly distributed along the trend of the aftershock zone. Temporal correlation is 0.86 for aftershocks of M ‡ 1, indicating a nearly continuous aftershock activity. However, it is 0.5 for aftershocks of M ‡ 1.75, indicating a non continuous aftershock activity. From the assessment of slip on different faults it is inferred that 70% displacement is accommodated on the primary fault and the remainder on secondary faults. Key words: Aftershocks, earthquake clustering, fractals, Himalayan earthquakes. 1. Introduction In recent years fractal concepts have been used widely to study earthquake dynamics. Understanding the physical mechanism underlying the scaling laws in seismology is a fascinating research topic. Its study can be applied for seismic hazard analysis. Many complex natural processes can be described and interpreted with the help of scale-invariant properties (MANDLEBROT, 1982; TURCOTTE, 1986). The term fractal is used to explain the self-similarity seen in many natural phenomena of different scales. It has been known that an earthquake size distribution follows a power-law distribution, which can be expressed in terms of a Gutenberg-Richter frequency magnitude relation. The analysis of the Gutenberg-Richter law for the earthquake generation is given by Log N ¼ a  bM ð1Þ where N is the total number of earthquakes with magnitude greater than M, a and b are parameters of correlation. Coefficient b varies for different tectonic regimes and depends on stress regime and structural heterogeneity. The b value is calculated using both the least square and maximum likelihood method. There is a relation given by 1 National Geophysical Research Institute, Hyderabad-500 007, India. E-mails: postmast@csngri.ren.nic.in; director@ngri.wipro.net.in Pure appl. geophys. 160 (2003) 2329–2341 0033 – 4553/03/122329 – 13 DOI 10.1007/s00024-003-2405-1 Ó Birkha ¨ user Verlag, Basel, 2003 Pure and Applied Geophysics