HR: 0800h
AN: S31A-1017    [Abstracts]
TI: A New Model for Earthquake Occurrence World-Wide and its Implications for Seismic Hazard Assessment
AU: * Smith, E G
EM: euan.smith@vuw.ac.nz
AF: School of Earth Sciences Victoria University of Wellington, P O Box 600, Wellington, 6000 New Zealand
AU: Christophersen, A
EM: annemarie\_christophersen@yahoo.co.nz
AF: School of Earth Sciences Victoria University of Wellington, P O Box 600, Wellington, 6000 New Zealand
AB: We have developed a new model for the time interval between earthquakes greater than a given magnitude. The model allows us to calculate quantities of interest in seismic hazard assessment including the probability distribution of times to the next (large) earthquake in a region given the time since the previous one. We have established a database of earthquakes world-wide that is effectively complete above magnitude 5. The database and a comprehensive model for aftershocks are described elsewhere at this meeting. We use an extension of the aftershock model to define a circular region in which earthquakes are assumed to be causally related. The size of this region scales with magnitude: log Area \propto$ M. From these regions we assemble the distribution of time intervals between earthquakes with magnitudes $>$ some reference magnitude M$_{r}$ e.g. M 7. If earthquakes were independent, the distribution would be a negative exponential (Poisson process) with time constant equal to the mean time between events $>$ M$_{r}$. The short time intervals are well described by Omori's Law 1/t$^{p}$, with p=1.0. However we find that the whole distribution is well modeled as a mixture: a 1/t power law modulated by a negative exponential plus a negative exponential. These two parts may be thought of as comprising the 'aftershocks' and the 'background'. The model has two parameters to be determined: the weights of the two parts (which must sum to 1) and the time constant for the negative exponentials, assumed the same for both parts. To fit the model a correction must be applied for the finite duration of the catalogue, as time intervals longer than this cannot be observed and the frequencies of intervals that are a large fraction of the catalogue duration are biased downwards. We find that the time constant is several years, larger than the mean time between events. An implication of this is that 'aftershocks' persist as a significant contribution to earthquake occurrence for much longer - many years - than is often considered. This study was (part) funded by the EQC Research Foundation.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting