Static stress transfer and earthquake triggering: No lower threshold in sight?

The main objective of this study is to see if a lower threshold for earthquake triggering exists. Resolving this issue is important for the understanding of earthquake mechanics and for the purpose of hazard analysis. We compute the cumulative static stress changes imposed on 63 M greater than or equal to 4.5 earthquakes in central California between 1969 and 1998, by adding the stress changes imposed by all previous M greater than or equal to 4.5 earthquakes as a function of time prior to the events. We find that 85% of the cumulative stress changes at the time of rupture are positive for stress change magnitudes of >10 kPa (>0.1 bar), and 70% are positive for stress changes of <10 kPa (< 0.1 bar) as well as <1 kPa (<0.01 bar). A comparison between these results and those obtained for synthetic catalogs, in which the timing or focal mechanisms of the earthquakes were randomized, shows that this degree of triggering is very unlikely to be found in a random catalog. Thus we conclude that no lower threshold for earthquake triggering in central California has been found. We show that the temporal distribution of stress changes that discourage failure is consistent with the theoretical prediction that the time delay increases with the magnitude of the stress change.