HR: 0800h
AN: S11C-0711    [Abstracts]
TI: The Initiation of Aftershock Sequences: a New Formulation for the Omori Aftershock Law
AU: * Smith, E G
EM: euan.smith@vuw.ac.nz
AF: Victoria U of Wellington, P O Box 600, Wellington, 6012, New Zealand
AU: Christophersen, A
EM: Annemarie@sed.ethz.ch
AF: ETH Zurich ETH Zurich, Schafmattstrasse 30, Zurich, CH-8093, Switzerland
AB: We propose that the onset of an aftershock sequence is controlled by a time constant c that is a simple function of the moment of the initiating earthquake (mainshock). Step changes in a physical system, such as the step change in stress that results from an earthquake rupture, generally result in two responses: a long term response and a transient response. This behaviour is seen in aftershock sequences which are usually modelled by the modified Omori law for aftershock rate of occurrence: dN/dt = A (c + t)^-p. There are several theoretical and practical difficulties with this formulation. Moreover the mainshock and large aftershocks mask small aftershocks that occur early in the sequence. This has led to the speculation that the ‘c' term obtained by fitting the law to aftershock times is really an artefact of this non- detection. However, we argue that a priori there should be a transient process before the full 1/t or 1/t^p aftershock decay is established. We find that a new formulation of the Omori law, dN/dt = Aexp(-t/c)/t fits aftershock data from the southern California catalogue slightly better than the standard law. In this formulation the timescale c is a simple function of seismic moment. The formulation therefore enables the prediction of the evolution of an aftershock sequence once the mainshock moment is known, including the time after which the aftershock rate falls below the background rate. This will have application in aftershock filtering. The formulation also overcomes the practical difficulties that arise if the standard form is used as a probability density function for time-varying hazard models.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting