Earthquake triggering in southern Iceland following the June 2000 Ms 6.6 doublet

Abstract

In June 2000, two remarkably similar Ms 6.6 earthquakes stroke southern Iceland in the space of 3 d. Here this doublet is analyzed to test whether aftershock triggering is linear, that is, if the triggering patterns of these two similar main shocks are also similar. Methodologically, the key issue is to separate the two contributions to the overall seismicity patterns. For this purpose, we model the first 3 d (17–21 June) of this aftershock sequence with an Omori‐Utsu law. We then extrapolate it beyond 21 June in order to estimate the variations linked with the second shock. We correct for the large transient changes in catalogue completeness. Our results indicate that instantaneous short‐lived (hours to days) dynamic triggering took place at the Hengill Triple Junction, on the Reykjanes Peninsula, and around the Árnes fault plane that ruptured on 17 June. We observe delayed, long‐lasting (longer than weeks) episodes of quiescence at Hengill and on the Árnes fault plane. At Hengill this quiescence remains significant up to at least the end of 2000. These quiescences are more consistent with Coulomb stress calculations than the preceding episodes of triggering. Comparison between the number of triggered events by the two main shocks reveals that the second one triggered only 2.1% at Hengill and 0.3% at Reykjanes of the number (3.6% and 0.1%, respectively, of the seismic moment) that could be expected with a simple stress threshold model of dynamic triggering. This clear nonlinearity in patterns of dynamic triggering is consistent with a model in which the population of nucleating earthquakes is depleted by the first trigger.