Source process of the 2021 M_W6.6 outer rise earthquake off the west coast of northern Sumatra

The complexity of the seismicity pattern for the subduction zone along the oceanic plate triggered the outer rise events and revealed cyclic tectonic deformation conditions along the plate subduction zones. The outer rise earthquakes have been observed along the Sunda arc, following the estimated rupture area of the 2005 M_W8.6 Nias earthquakes. Here, we used Kinematic Waveform Inversion (KIWI) to obtain the source parameters of the 14 May 2021 M_W6.6 event off the west coast of northern Sumatra and to define the fault plane that triggered this outer rise event. The KIWI algorithm allows two types of seismic source to be configured: the Moment Tensor model to describe the type of shear with six moment tensor components and the Eikonal model for the rupture of pure double-couple sources. This method was chosen for its flexibility to be applied for different sources of seismicity and also for the automated full-moment tensor solution with real-time monitoring. We used full waveform traces from 8 broadband seismic stations within 1000 km epicentral distances sourced from the Incorporated Research Institutions for Seismology (IRIS-IDA) and Geofon GFZ seismic record databases. The initial origin time and hypocenter values are obtained from the IRIS-IDA. The synthetic seismograms used in the inversion process are based on the existing regional green function database model and were accessed from the KIWI Tools Green's Function Database. The obtained scalar seismic moment value is 1.18x10¹⁹ Nm, equivalent to a moment magnitude M_W6.6. The source parameters are 140°, 44°, and −99° for the strike, dip, and rake values at a centroid depth of 10.2 km, indicating that this event is a normal fault earthquake that occurred in the outer rise area. The outer rise events with shallow normal faults typically occur at the shallow part of the plate, with nodal-plane dips predominantly in the range of 30°-60° on the weak oceanic lithosphere due to hydrothermal alteration. The stress regime around the plate subduction zone varies both temporally and spatially due to the cyclic influences of megathrust earthquakes. Tensional outer rise earthquakes tend to occur after the megathrust events. The relative timing of these events is not known due to the viscous relaxation of the lower part of the slab and poroelastic response in the trench slope region. The occurrence of the 14 May 2021 earthquake shows the seismicity in the outer rise region in the strongly coupled Sunda arc subduction zone due to elastic bending stress within the duration of the seismic cycle.