HR: 1340h
AN: S23A-0290    [Abstracts]
TI: Aftershock Number for Forecasting Short-Term Earthquake Probabilities
AU: * Christophersen, A
EM: annemarie\_christophersen@yahoo.co.nz
AF: School of Earth Sciences Victoria University of Wellington, PO Box 600, Wellington, 6000 New Zealand
AU: Smith, E G
EM: euan.smith@vuw.ac.nz
AF: School of Earth Sciences Victoria University of Wellington, PO Box 600, Wellington, 6000 New Zealand
AB: Data from earthquakes worldwide with depths shallower than 70 km were combined from the International Seismological Centre, the US National Earthquake Information Center, Blacknest, and Harvard. An extensive magnitude and catalogue completeness study defined a `best' magnitude using the Harvard moment as a reference. The catalogue covers the period 1964 to 1995 and is effectively complete for earthquakes of magnitude 5.0 and above. The data were divided into six tectonic settings, and searched for related events using a simple window in space and time. An objective method was developed to define an elliptical aftershock area. The database of aftershock sequences has about 28,000 mainshocks of which about 2,400 have a magnitude $M \ge 6.0$, and these were followed by a total of about 7,000 aftershocks. The database was analyzed in space, time, magnitude, and in the number of aftershocks in a sequence, hereafter called abundance. The aftershock decay in time and the magnitude-frequency distribution follow well- established empirical laws, Omori's law and the Gutenberg and Richter relationship. These relationships were analyzed by stacking data from various sequences within the same tectonic setting. The $p$-value for the aftershock decay in time was found to be 1.0 for subduction and collision zones, and for regions of mixed tectonic character like New Zealand. For mid-ocean ridges the $p$-value of the present dataset is $1.19 \pm 0.08$ and for intracontinental zones $0.86 \pm 0.14$. The $b$-value of the magnitude-frequency relation is 1.0 for aftershock sequences in all settings. No variation of the $b$-value with time was observed. The abundance varies greatly from sequence to sequence. It can be modeled by a geometric distribution, where the mean abundance $N$ grows exponentially with mainshock magnitude, $M$ i.e. log $N$ is proportional to $M$. The distribution parameters for time, magnitude and abundance can be combined to probabilistically predict the number of aftershocks in a given time and magnitude interval for a given mainshock. This study was funded by the New Zealand Earthquake Commission.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting