New insights on persistent megathrust asperity and fault friction revealed by the 2021 Kermadec Mw8.1 earthquake sequence

Wei Shengji, Associate Professor

The Earth Observatory of Singapore

The Asian School of the Environment, NTU

2023.07.07（Friday）15:00, Science Building II, 2821

Abstract:

To what degree large earthquakes are ‘characteristic’ and what frictional fault property determines seismic wave radiation are critical questions in understanding the fundamentals of earthquake physics and the seismic cycle. Here we study the ruptures of the 2021 Mw7.4 and Mw8.1 doublet earthquake sequence in the Kermadec subduction zone through a holistic seismological analysis, including finite fault inversions, focal mechanism inversion, surface wave earthquake relocation and path-calibrated back-projection. The finite fault model of the Mw8.1 event shows a slip patch at 20-45km depth, elongating ~100 km along strike, while high frequency (HF) sources are distributed along the up-dip edge of the slip, coinciding with the forearc Moho depth and bursting within a belt of background events, aftershocks, mainshock epicenter and triggered HF sources from the M7.4 foreshock. Strong heterogenous friction promoting the up-dip HF radiation is likely caused by the beginning of serpentinization in the overriding mantle wedge.

The comparison with the 1976 doublet that occurred at the same area suggested the 2021 mainshock likely re-ruptured the asperity of the 1976 Mw7.9 event, although the detailed slip distribution is different and the other ruptures in the doublets differ in character and location. All earthquakes occur in an isolated area of the megathrust, which is bounded by changes in the lithospheric structure of the overriding plate as indicated by bathymetric and gravity data. This high-seismicity region is coincident with an isolated forearc sedimentary basin, possibly formed by basal erosion, suggesting that seismic slip has persisted here for several million years. Refined up-dip aftershock and background seismicity focal mechanisms show steeper dip angle than slab interface, suggesting these events are located within the subducting oceanic slab, forming a rougher plate interface that facilitates basal erosion.

We conclude that the stress heterogeneity within this bounded seismogenic zone is long-lived and has produced a rich spectrum of earthquake ruptures, and large seismic slip on the serpentinized plate interface urges a re-evaluation of seismic hazards in conventionally considered aseismic area.

Bio:

        Wei Shengji is a PI at the Earth Observatory of Singapore and an associate professor and assistant Chair at the Asian School of the Environment, NTU. He obtained his bachelor and PhD degrees from USTC and had been a postdoc at Caltech SeismoLab before working at NTU. He is an associate editor of BSSA and Geoscience Letters, and an editorial board member of Tectonophysics.  His research focusses on earthquake source and the properties of earth structures to better understand earthquake physics, tectonic and geodynamic processes. His approach to resolving the kinematic nature of earthquakes involves a combination of geodesy, geology, and seismology to better constrain the spatial-temporal evolution of seismic rupture properties. His team develops techniques that allow waveform inversion/modelling to be extended to a higher frequency (>1Hz) range of relevance for damage assessment.