Ocean bottom seismological observation of slow earthquake activity in the northern part of the Hikurangi subduction margin, New Zealand

Abstract:

The northern Hikurangi trough off the North Island, New Zealand, where the Pacific Plate subducts beneath the Australian Plate at a rate of ~5 cm/year, is known as a region where the seismicity of both ordinary and slow earthquakes are very high. In particular, occurrence of slow slip events (SSEs) is relatively frequent with a temporal interval of 1-2 years [e.g., Wallace et al., 2010]. This frequent occurrence offers an excellent chance to capture seismicity accompanying SSE using temporary deployments of ocean bottom instruments directly above the shallow SSE source.

From May 2014, through June 2015, an international collaborative observation was carried out using 15 ocean bottom seismometers (OBSs) and 24 ocean bottom pressure gauges (OBPs). This observation was successful in capturing an SSE directly beneath the network, and a precise slip distribution of the event was obtained [Wallace et al., 2016]. The slip reached near the trough axis while it was reduced in the area of subducted seamounts. The results of seismological observations of long-lasting tremor activity [Todd et al., 2018], temporal variation of focal mechanisms [Warren-Smith et al., 2019] and cracks in tandem with the occurrence of SSEs [Zal et al., 2020] suggest a potential role of fault-valving in the generation of slow earthquakes.

We conducted another temporary OBS deployment from Oct. 2018 through Oct. 2019 using 5 OBSs in the same region of the 2014-2015 observation spanning the subducted seamount where we recorded offshore tremor activity previously. A large SSE occurred during the observation period in April-May 2019 around our OBS network, and we were successful in capturing seismic activity accompanying the SSE. The seismicity increased around the start of the SSE and continued throughout the event. Shallow low-frequency tremor activity was found again in the area around the seamount. Shallow tremor have a relatively short duration compared to deep tremor and have an isolated shape, which is similar to the shallow tremor observed in other subduction zones [e.g., Yamashita et al., 2015; Ohta et al., 2019]. The shallow tremors appear to be localized at subducted seamounts and accompanied by a narrower range of migration phenomena. We are conducting further investigations and will discuss the characteristics of these seismicity.