Plate interface geometry complexity and persistent heterogenous coupling revealed by a high-resolution earthquake focal mechanism catalog in Mentawai, Sumatra

• Published in: Earth and planetary science letters 2024-07, Vol.637, p.118726, Article 118726
• Main Authors: Wang, Xin, Wei, Shengji, Morales-Yáñez, Catalina, Duputel, Zacharie, Chen, Ling, Hao, Tianyao, Zhao, Liang
• Format: Article
• Language: English
• Subjects: Coupling; Earth Sciences; Earthquake source parameters; Ramp structures; Sciences of the Universe; Slab geometry; Sumatra
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2024.118726

•Waveform modeling constrains earthquake source parameters in Mentawai, Sumatra.•Two trench-parallel earthquake belts located at great earthquakes’ rupture areas.•Down-dip belt indicates bending of the megathrust.•Up-dip belt marks the frictional boundary of seismogenic zones.•Lateral variations in up-dip belt indicate diverse seismogenic widths and coupling. Geometry and frictional states of the megathrust in subduction zones play critical roles on the rupture extent, hence the magnitude, of great earthquakes. However, their details are often difficult to obtain, primarily due to limited offshore geophysical observations. Here we overcome this challenge by precisely determining source parameters (i.e., location and focal mechanism) of medium-sized earthquakes using global broadband seismic waveform data. The dip angles of 108 Mw 5.0+ plate interface events from 1990 to 2017 in the central Sumatran subduction zone are tightly constrained by waveform inversion, and their locations are refined with surface-wave relocation and depth phases modeling. Our results reveal compact trench parallel earthquake belts at the up-dip and down-dip boundaries of the ruptures of the Mentawai 2007 Mw 8.4 and Mw 7.9 earthquakes. The down-dip seismicity belt marks ∼8º bending of the plate interface, suggesting that the change of fault geometry may have played a role in halting the megathrust rupture along with thermal effects. The up-dip seismicity belt shows a clear spatial complementarity to the largest events’ slip distribution. This belt is quite narrow (∼20 km) for the Mw 8.4 rupture but much broader (∼50 km) for the Mw 7.9 event. The combination of the up-dip seismicity belt and slip distributions matches well with the positive residual bathymetry and gravity anomalies, suggesting that these events resulted from long-term coupling on the plate boundary. Our findings suggest the importance of considering both up-dip and down-dip seismicity belts, along with residual bathymetry and gravity data, in understanding megathrust rupture behaviors and assessing the associated hazards.