Geodetic resolution of the interplay between earthquakes and slow slip in the Hikurangi margin

Abstract

Interactions between slow slip and earthquakes remain poorly understood. We examine such an interaction in the central Hikurangi subduction zone where several moderate (Mw 4–5+) earthquakes occurred during a deep, 2-year M7 slow slip event that started in 2021. We apply three different geodetic inversion methods to 6 years (2018–2024) of continuous GNSS data from 97 stations across New Zealand’s North Island to retrieve the time-dependent evolution of fault slip at depth: (a) a fixed-window (1, 3, 6 months) static offset approach; (b) an Independent Component Analysis-based inversion Method (ICAIM); and (c) an elastic block modeling approach utilizing TDefnode. All three methods yield a consistent, high-resolution slip history in time and space. The 2021–2024 long-term, deep slow slip event (equivalent 𝑀𝑤 7.1–7.2) evolved with five to six distinct sub-events, three of which represent an increase in slow slip event moment release within weeks after 𝑀𝑤 4+ normal mechanisms (intraslab) earthquakes. In contrast, plate interface events (𝑀𝑤 > 5) during this period do not seem to impact the slow slip event evolution. Static Coulomb modeling shows that cumulative slow slip increased stress by 40–60 kPa at the hypocenters of two plate-interface earthquakes, suggesting that slow slip may have contributed to bringing these faults closer to failure. Our results reveal an asymmetric interplay between slow slip and seismicity: intraslab earthquakes modulate ongoing slow slip, while slow slip loading may trigger interface events, demonstrating two-way interactions at considerably lower magnitudes than typically documented at other subduction zones.