Scaling and Stress Release in the Darfield-Christchurch, New Zealand Earthquake Sequence

Abstract:

The Canterbury earthquake sequence began with the M7.1 Darfield earthquake in 2010, and includes the devastating M6.2 Christchurch earthquake in 2011. The high ground accelerations and damage in Christchurch suggested that the larger eartthquakes may be high stress drop events. This is consistent with the hypothesis that faults in low-strain rate regions with long inter-event times rupture in higher stress drop earthquakes. The wide magnitude range of this prolific sequence, and the high-quality recording enable us to test this. The spatial migration of the sequence, from Darfield through Christchurch and then offshore, enables us to investigate whether we can resolve any spatial or temporal variation in earthquake stress drop. An independent study of 500 aftershocks (Oth & Kaiser, 2014) found no magnitude dependence, and identified spatially varying stress drop. Such patterns can be more confidently interpreted if observed by independent studies using different approaches.

We use a direct wave, empirical Green’s function (EGF) approach that includes measurement uncertainties, and objective criteria for assessing the quality of each spectral ratio (Abercrombie, 2013). The large number of earthquakes in the sequence enables us to apply the same approach to a wide range of magnitudes (M~2-6) recorded at the same stations, and so minimize the effects of any systematic biases in results. In our preliminary study, we include 2500 earthquakes recorded at a number of strong motion and broadband stations. We use multiple EGFs for each event, and find 300 earthquakes with well-resolved ratios at 5 or more stations. The stress drops are magnitude independent and there is broad correlation with the results of Oth & Kaiser. We apply the same approach to a much larger data set and compare our results to those of Oth & Kaiser, and also to other regions studied using our EGF method.