- Dr. Abhey.R. Bansal
(University of Edinburgh, Edinburgh EH9 3JW, Scotland, UK
National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad, India)
- Statistical seismology of Sumatra:
before and after the mega-earthquake of 26 December 2004
- We examine the effect of the Sumatra-Andaman Islands mega-earthquake of 26 December 2004 on Sumatran seismicity.
Irrespective of time window the inter-event time distribution exhibits Omori-Utsu decay at short time intervals,
followed by a gamma distribution at intermediate and long time intervals.
The mean inter-event time drops significantly after the event,
associated with an increase in the fractal dimension of the time series sequence from 0.07 } 0.01 to 0.19 } 0.01,
and a doubling of the coefficient of variation of inter-event time.
Epidemic Type Aftershock Sequence (ETAS) model parameters are used to quantify changes in the statistical properties.
These changes can be significant:
the background random seismicity rate ส drops to be indistinguishable from zero after the mega-earthquake
(indicating a single dominant aftershock sequence) whereas the productivity factor K is ~13.4 times higher.
The magnitude sensitivity parameter ฟ increases after the earthquake,
indicating larger aftershocks are proportionately easier to trigger,
and the Omori-Utsu exponent p decreases (to ~1), consistent with a longer duration of aftershock sequences.
When examined in retrospect, a non-stationary ETAS model provides a better fit to the seismicity data prior to the main shock,
the best fitting model being one with a change point to enhanced rates 4.3 years before the mega-earthquake,
most likely due to a localized swarm at that time.