HR: 08:30h
AN: NG31A-03    [Abstracts]
TI: Statistical Analysis of Seismicity in the Sumatra Region
AU: * Bansal, A
EM: abhey.bansal@gmail.com
AF: National Geophysical Research Institute, Uppal Road, Hyderabad, AP 500007, India
AU: Main, I
EM: ian.main@ed.ac.uk
AF: School of Geosciences, The Grant Institute, The University of Edinburgh, The King's Buildings West Mains Road, Edinburgh, EH93JW, United Kingdom
AB: We examine the effect of the great M=9.0 Boxing day 2004 earthquake on the statistics of seismicity in the Sumatra region by dividing data from the NEIC catalogue into two time windows before and after the earthquake. First we determine a completeness threshold of magnitude 4.5 for the whole dataset from the stability of the maximum likelihood b-value with respect to changes in the threshold. The split data sets have similar statistical sampling, with 2563 events before and 3701 after the event. Temporal clustering is first quantified broadly by the fractal dimension of the time series to be respectively 0.137, 0.259 and 0.222 before, after and for the whole dataset, compared to a Poisson null hypothesis of 0, indicating a significant increase in temporal clustering after the event associated with aftershocks. To quantify this further we apply the Epidemic Type Aftershock Sequence (ETAS) model. The background random seismicity rate £g and the coefficient ƒÑ, a measure of an efficiency of a magnitude of an earthquake in generating its aftershocks, do not change significantly when averaged over the two time periods. In contrast the amplitude A of aftershock generation changes by a factor 4 or so, and there is a small but statistically significant increase in the Omori decay exponent p, indicating a faster decay rate of the aftershocks after the Sumatra earthquake. The ETAS model parameters are calculated for different magnitude threshold (i.e. 4.5, 5.0, 5.5) with similar results for the different magnitude thresholds. The ƒÑ values increases from near 1 to near 1.5, possibly reflecting known changes in the scaling exponent between scalar moment and magnitude with increasing magnitude. A simple relation of magnitude and span of aftershock activity indicates that detectable aftershock activity of the Sumatra earthquake may last up to 8.7 years. Earthquakes are predominantly in the depth range 30-40 km before 20-30 km after the mainshock, compared to a CMT centroid depth of the earthquake of 28.6 km.
DE: 4400 NONLINEAR GEOPHYSICS (3200, 6944, 7839)
DE: 4440 Fractals and multifractals
DE: 7200 SEISMOLOGY
SC: Nonlinear Geophysics [NG]
MN: 2007 Fall Meeting