Physical and statistical models for the seismological properties and a temporal evolution of earthquake sequences (swarms) in the Central Volcanic Region, New Zealand

Abstract

We have investigated earthquake sequences (swarms) in the Central Volcanic Region (CVR), NZ. A new CURATE method was developed to systematically identify these sequences by comparing observed seismicity rates to an average rate. This new method differs from other clustering techniques in that it does not make assumptions about physical processes or the idea that earthquakes are caused by preceding seismicity. The CURATE method allowed us to investigate correlations between sequence parameters (duration, area, number of events, and largest magnitude) and to study temporal evolution of individual sequences. We have found that swarm sequences have a few unique time patterns that are distinct from mainshock-aftershock behaviour. These insights along with further investigations may facilitate future forecasting of swarm sequences as they develop. There are a variety of potential applications for investigations of physical process and hazard assessment. In particular the CURATE method will be a good tool for identifying rate changes and sequences at a range of local and regional scales by providing a context in which to judge ‘normal’ and ‘anomalous’ behaviour, allowing objective investigations of swarm parameters and swarm types that may lead to advances in swarm models and regional earthquake forecast models.

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Technical Abstract

We present a new cumulative rate (CURATE) clustering method to identify earthquake sequences especially in regions with swarm activity. The method identifies sequences by comparing observed rates to an average rate. It is distinct from previous clustering techniques in that no direct assumptions about physical processes relating to temporal decay or earthquake-earthquake interaction are made. Instead these assumptions are replaced by a more general one, that earthquakes occurring within a sequence likely share a common physical trigger, which is manifested by a change in rate. The use of rate as the primary selection parameter emphasizes that temporal proximity is the main commonality among different sequence types.

To investigate catalog-scale earthquake sequence characteristics we apply the method along with four standard (de-)clustering methods to a catalog of 4845 M > 2.45 earthquakes from 1993 through 2007 in the Central Volcanic Region of New Zealand. Despite the distinct focus of the method on sequence formation, the declustered catalog of the CURATE method sits within the suite of declustered catalogs produced by other methods. A stochastic reconstruction based on ETAS parameters is also presented to test the differences between catalogs that exclusively contain mainshock-aftershock sequences and areas that exhibit multiple physical processes. We test the declustered catalogs produced by all methods for Poisson temporal distribution and propose that this be used to ensure reasonable selection parameters. The CURATE method will be especially useful for identifying swarms, creating likelihoods of the size and duration of sequences, and refining earthquake forecasts for swarm regions.