[S-CG45] Science of slow-to-fast earthquakes

[SCG45-22] When a Slow Slip Event Plays with the Seismicity: The Case of the 2021-2023 Manawatu-Kapiti Events and Their Impact on New Zealand's Seismic Hazard

*Louise Maubant¹、Laura Wallace²、William Frank³、Charles Williams⁴、Sylvain Michel⁵ (1.Earthquake Research Institute, the University of Tokyo、2.GEOMAR, Kiel, Germany、3.Massachusetts Institute of Technology, Cambridge, USA、4.GNS Sciences, New Zealand、5.ENS de Paris, France)

キーワード：Slow slip events, Hikurangi subduction zone, Geodesy

Between 2021 and 2023, a deep slow slip event (SSE) occurred in the deeper regions of the Hikurangi subduction zone. This event exhibited a complex temporal evolution characterized by varying slip rates and intermittent periods of quiescence. Two earthquakes happened in the slow slip event region during this period. The first one, in February 2023, a Mw 5.7 occurred on the deep part of the interface, in the slow slip region during the event. In April 2023, a Mw 5.4 earthquake occurred on the subduction plate interface, followed by a vigorous aftershock sequence. The mainshock and aftershocks occurred immediately up-dip of the ongoing Manawatu Kapiti SSE, which may have triggered this earthquake. By applying Independent Component Analysis (ICA) to a dense dataset of GNSS displacements, we were able to examine the slow slip event's temporal evolution. The ICA method allowed us to decompose the GNSS data from 2020 to 2024, isolating the SSE signal from external contributions, such as common mode noise or seasonal signals. It shows the spatio-temporal complexity of the slow slip event, and its interaction with the deep seismicity. This decomposition revealed the migration of the SSE from the deeper western part of the subduction zone to a shallower eastern part in the months prior to the April 2023 earthquake sequence. Using Coulomb stress change modeling, we demonstrate that the SSE may have triggered the April 2023 earthquake. These results highlight the importance of monitoring SSEs to capture the interplay between slow slip and earthquakes, key to our understanding of seismic hazard in subduction zones. We also show that the 2021-2023 SSE occurred in the same location as past slow slip events and that this and previous events accommodate the majority of the plate motion budget in the deep SSE source region at Hikurangi.