Spatiotemporal characteristics of repeating earthquakes near Porangahau, Hikurangi Margin, New Zealand

Abstract:

The Hikurangi subduction zone beneath the eastern North Island, New Zealand exhibits a variety of fault-slip related phenomena including tsunami earthquakes, non-volcanic tremor, low-frequency earthquakes, episodic slow slip, and repeating earthquakes. The northern Hikurangi margin hosts shallow slow-slip and is weakly coupled to shallow depths on inter-seismic timescales. In contrast, the southern Hikurangi margin is strongly coupled beneath the southern North Island, and only deep slow-slip has been observed. The transition in coupling occurs beneath the township of Porangahau, and is an exemplary focus region for studying how this along-strike change in locking is accommodated. To examine slip processes at this key transition zone, we have constructed and analysed a catalogue of repeating earthquakes that occurred over a period of 14 years between 2004 and 2018.

To build our catalogue of repeating earthquakes we first clustered GeoNet's extensive earthquake catalogue by distance and correlation using weak constraints to identify potentially repeating events. We then used a stronger cross-correlation threshold of at least 0.95 normalised cross-correlation on three or more stations to identify repeating earthquakes from the initial clusters. This threshold was determined by our group's previous work on the northern Hikurangi margin. This analysis yields 234 families of 554 repeating earthquakes, with some of them containing as many as nine events in the span of >10years.

We computed precise absolute locations and focal mechanisms of all 234 families and their constituent earthquakes via manual phase-picking of the youngest earthquake in each family, and cross-correlation pick-correction of older earthquakes. The high waveform-similarities between events within repeating families enables us to compute precise relative magnitudes for all events. This catalogue of 554 repeating earthquakes improves our understanding of how the Hikurangi subduction zone is being loaded at this critical transition zone.